A Specialist Periodical Report
Photochemistry Volume 8
A Review of the Literature published between July 1975 and Jun...
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A Specialist Periodical Report
Photochemistry Volume 8
A Review of the Literature published between July 1975 and June 1976
Senior Reporter D. Bryce-Smith, Department of Chemistry, University of Reading Reporters M. D. Archer, The Royal lnsfifufion, London G. Beddard, The Royal lnsfifufion, London H. A. J. Carless, Birkbeck College, Universify of London A. Gilbert, University of Reading W. M. Horspool, University of Dundee J. M. Kelly, University of Dublin D. Phillips, Universify of Soufhampfon S. T. Reid, Universify ofKent K. Salisbury, University of Soufharnpfon M. A. West, The Royal Institution, London
The Chemical Society Burlington House, London, W i V oBN
ISBN :0 85186 075 3 ISS N :0556-3860 Library of Congress Catalog Card No. 73-17909
Copyright @ 1977 The Chemical Society A II Rights Reserved No part of this book may be reproduced or transmitted in any form or by any means - graphic, electronic including photocopying, recording, taping or information storage and retrieval systems - without written permission from The Chemical Society
Organic formulae composed by Wright's Symbolset method
PRINTED IN GREAT BRITAIN BY JOHN WRIGHT AND SONS LTD., AT THE STONEBRIDGE PRESS, BRISTOL BS4 5NU
Introduction and Review of the Year
Volume 8 of ‘Photochemistry’ undergoes a slight shift towards the biological end of the spectrum with the inclusion for the first time of a section dealing with chemical aspects of photobiology (Part VI). This has been contributed by Dr. Godfrey Beddard, whom we are pleased to welcome to the team of Reporters. Although photobiology has not been totally neglected in previous Volumes, we felt that its growing chemical interest justified the emphasis of separate treatment. We are also glad to welcome back Dr. M. A. West who provides a two-year review of developments in instrumentation and techniques. We may for an experimental period continue to review these aspects on a biennial basis. On the other hand, the spectroscopic and theoretical aspects, which have hitherto been covered on an annual basis, are not included this year, and it is intended that they will form the subject of a biennial review in Volume 9. It is not only photochemists who will have felt that one of the most important reports to appear in the chemical literature during the year has been the preliminary announcement by Whitten and his co-workers of the efficient photoproduction of molecular hydrogen and oxygen from water using near-u.v. and visible light in the presence of monolayer-bound ruthenium(1x) bipyridyl complexes. Although the reported findings appear to be unquestionably authentic, it is necessary to add a cautionary note, for Professor Whitten has informed the writer that he and his colleagues have been unable to reproduce the original results using more highly purified reagents. Some unrecognized impurity may have been involved, so it is greatly to be hoped that further work will lead to the identification of this, and indeed to the development of improved systems which are capable of functioning in homogeneous solution. A successful development along these lines would be of the greatest importance in relation to the ‘energy crisis’. In this connection, it is interesting that [Ru(bipy),12+has been shown to act as a photocatalyst for the air oxidation of Fe2+ to Fe3+, possibly via an 0,-complex (Winterle, Kliger, and Hammond). Schrauzer and Guth have also reported that U.V. irradiation promotes the evolution of hydrogen from ferrous hydroxide gels, apparently via disproportionation to iron metal (see also reports by Crosby, Juris, and their co-workers). Continuing with developments in inorganic photochemistry, we may note that several reports have stressed the importance of electron-transfer as a general mechanism for quenching the excited states of co-ordination compounds. Nonradiative processes usually occur very rapidly in such excited states, and picosecond laser flash photolysis is proving a very useful technique for investigations in this area (Kirk et al.). On the theoretical side, Burdett’s molecular orbital procedure for calculating the course of non-dissociative photoisomerizations of transition-metal compounds appears to have predictive value.
iv
Introductory Review
There is growing evidence that photoaddition reactions of metal-co-ordinated dienes and mono-enes do not generally follow a concerted pathway, so application of orbital symmetry arguments may be less generally justified than has hitherto been believed. Attention is drawn to an important paper by Endicott and Ferraudi on photosubstitution reactions in Co"' and Rh"' complexes which occur via ligand-field excitation. The mechanism of quenching by ferrocene now appears clearer following independent studies by Herkstroeter, and Farmilo and Wilkinson. The production of Si=C compounds has long been a goal in organosilicon chemistry, so it is interesting that Boudjouk and co-workers have obtained evidence for the transitory formation of Me2Si=CH2 by the vacuum U.V. photolysis of Me,SiCMe,. Unstable Si=C intermediates may also be involved in the novel C( 1)C(4)/C(9)C(10) intramolecular dimerization of organosilicon-substituted anthracene rings reported by Felix and co-workers. Parker and Sommer have generated the highly reactive silaimines R,Si=NR by photolysis of the corresponding azides R3SiN3,and have trapped these as t-butanol adducts. Irradiation of ozone co-deposited with sulphur trioxide has led to the first spectroscopic characterization of the species SO4 (Kugel and Taube). On the physical side of the subject, an increasing trend may be noted towards studies of small molecules and atoms of interest in the atmospheric photochemistry of Earth and other planets following developments noted in previous Volumes. Among medium-small molecules, benzene and glyoxal continue to command an enthusiastic following. Formosinho and Dias da Silva have had considerable success in calculating the rate constants for Sl-+s,, and Sl-+ transitions in benzene and 2[H],benzene by application of an empirical tunnelling theory. This treatment indicates that the rate constant for the former process is more energy-dependent than that for the latter. Time- and wavelength-resolved emission spectra are proving very useful in studies of the relaxation of excited aromatic hydrocarbons and other species (D. Phillips and co-workers, among others), and a promising molecular beam technique has also been briefly reported (Sander et a2.): this latter appears to avoid problems from the complexity of emission spectra which result from the normally unavoidable excitation of hot U-ZI bands in the absorption step. See also Smally et al. for a related elegant study of 12. Concern continues to be expressed about possible effects of chlorofluorocarbons (Freons), which are widely used as aerosol propellants, on depletion of the ozone layer in the upper atmosphere. The rate constant for the key reaction C1* O3 C10. + O2has been measured independently by Razumovskii et al., and Kurylo and Braun. The results are in fair agreement, and indicate the possibility that previous estimates of ozone loss may have been too high. Likewise the half-lives of various chlorofluoromethanes in the troposphere have been calculated to be 1-2 years, or less, whereas periods of about 50 years have been estimated by some previous workers. There has been a continued upsurge of academic and other interest in isotope separation by use of high-power i.r. lasers selectively to excite low-lying vibrational transitions by multiphotonic absorption in isotopic mixtures of small molecules. Isotope separation via electronic excitation has also been described.
+
--f
Introductory Review
V
Carroll and Quina have described a new method for determination of intersystem-crossing quantum yields. In methylbenzenes, it is interesting to note that the values show some tendency to increase with the number of methyl groups and with increasing symmetry of the substitution pattern, The vacuum-u.v. irradiation of benzene in argon and nitrogen matrices can lead to thermoluminescence, i.e. phosphorescence after irradiation as the matrix warms up (Hellner and Vermeil). It is well known that the acidity of phenols in the S, state is much higher than in the Sostate, but a previous report of supposed enhancement of a reaction rate due to this phenomenon (nitrosation of 2-naphthol) has been severely questioned by Chandross. Exciplexes and ‘encounter complexes’ continue to attract a good deal of attention, and triple exciplexes (D . . . A . . . D) have been reported (Saltiel et d ; Mimura and Itoh). A study of pyrene and NN-diethylaniline has provided the first direct evidence for solvent-induced changes in the electronic structure of a polar exciplex. In highly polar solvents, the absorption spectrum becomes identical with that of the separated radical-ions (Orbach and Ottolenghi; see also Gupta and Basu, and Mataga et al.). Slifkin and Al-Chalabi have used flash photolysis of donor-acceptor complexes (e.g. perylene-chloranil) to obtain triplet-triplet spectra of the donors. The technique appears promising, but ambiguities may complicate interpretation of the results. The importance of the stereochemical configuration of ground-state and excited-state complexes in determining the stereochemistry of photoaddition reactions is being increasingly recognized. Hochstrasser and King have reported some interesting isotopically selective photochemistry in molecular crystals. Lahav et aZ. have described an ingenious new method for purification of enantiomers based on topochemical control of photodimerization of anthracene units, as with esters of 9-anthroic acid. The presence of cyanobenzenes (which promote charge-transfer processes) can profoundly modify the course of some photoreactions, e.g. dimerization of styrenes This interesting phenomenon merits wider study. Yang and (Asanuma et d,). his co-workers have observed fluorescence emission from a substituted dieneanthracene system which they attribute to an ‘encounter complex’, a species previously more often proposed than identified. The technique of using xenon to detect singlet and triplet processes by efficient enhancement of intersystem crossing rates is becoming more widely employed : see, for example, Morrison, Nylund, and Palensky. The section on Instrumentation and Techniques covers the period July 1974 to June 1976 inclusive. Although 724 references are cited, the review is to some extent restricted to what are considered to be the most significant advances. Some interesting developments in the rather neglected field of U.V. lasers has been stimulated by requirements for isotope separation, as noted above (and many new examples of this have been reported), and thermonuclear fusion. Hoffman, Hays, and Tisone have described devices based on electron-beam pumped halides such as XeBr and KrF: these offer promise as powerful tunable U.V. lasers. Since nitrogen-lasers (at 337 nm) are the most common types now used in photochemistry; it is interesting to note that addition of SF, to a nitrogenlaser can double the power output (Judd).
vi
Introductory Review
Applications of photoacoustic spectroscopy are multiplying rapidly, particularly for the measurement of absorption spectra of biological materials and atmospheric pollutants. Schwarz et al. have described fluorescence techniques whereby sulphur dioxide and nitric oxide can be rapidly and continuously monitored, and Tucker et al. have developed a laser-based procedure for nitrogen dioxide which is even more sensitive. Bradley and Sibbett have described a new type of streak camera which makes possible the achievement of sub-picosecond time resolution from the vacuum-u.v. to the near-i.r. The section on Chemical Aspects of Photobiology deals mostly with photochemical and photophysical aspects of primary processes in photosynthesis and vision. A new examination of highly purified chlorophyll a in EPA has shown that fluorescence previously attributed to a ‘hot’ band is due to an impurity, and that no dimer emission occurs at concentrations up to mol 1-1 (Mau). Among the developments concerning vision processes during the year, one may particularly note Salem and Bruckmann’s proposal of a mechanism whereby twisting about the 11-cis-ethylenic bond in the S 1 m *state of an N-retinylidene chromophore triggers an electrical signal which, inter alia, changes the permeability of the disc membrane to Na+. Downer and Englander have reported hydrogen-exchange studies which indicate that the action of light in vision processes promotes hydrogen exchange which in turn may cause conformational changes in a protein, and thereby distort a ‘plug’ in the cell membrane which opens a channel for passage of an as yet unidentified transmitter molecule. Another ingenious proposal by Warshel invokes a type of ‘bicycle pedal’ motion in photoisomerization of retinal when both ends of the molecule are restrained. In contemplating such developments in our groping attempts to understand photobiological processes, one is left with feelings of profoundest humility. These immensely subtle, complex, efficient, and robust ‘natural’ photosystems far transcend as constructions the human artefacts with which these annual Volumes are largely concerned. It is becoming increasingly difficult to deny that a better photochemist has gone before. We may now turn to some of the significant developments in more conventional photochemistry. The carbonyl group continues to exert its perennial fascination. Thus Yates and Tam have provided useful evidence on the structural features which promote the ring-enlargement of cyclic ketones to oxacarbene intermediates. Rather controversially, Wagner and Thomas have invoked fluorine hyperconjugation to account for the photochemical behaviour of certain fluoroketones. Hydrogen-abstractions by carbonyl and azomethine systems have been compared by Alexander and Jackson. These systems can show a similar degree of reactivity towards primary hydrogen atoms, but the carbonyl systems are much the more chemically reactive towards secondary hydrogen atoms. In a study of photoenolization, Wagner and Chen have obtained evidence for the formation of two triplet species derived from the syn- and anti-conformers of o-methylacetophenone: the former has the shorter lifetime. Some photoenolization occurs from the S1state of the syn-conformer. hv Deshayes et al. have described examples of the useful process ROAc RH in the steroid series: the yields are high.
-
Introductory Review
vii
Upper triplet states continue to be proposed in carbonyl photochemistry. Thus Bellobono et al. have invoked a T2state in the photocyclization of a furanone to a coumarin. The formation of cyclic enones having trans C=C has previously been suggested, so it is interesting that examples have now been isolated and trapped as furan adducts (Hart and Suzuki). In view of the frequent use of t-butanol as a solvent for photoreactions, it should be noted that Stille and his co-workers have observed this solvent to be photochemically reactive, giving products suggestive of the formation of free methyl radicals, possibly via fragmentation of intermediate t-butoxyl radicals : Wubbels et al. have actually obtained a t-butanol/anthraquinone photoadduct in the presence of ammonia. Barton and his co-workers have continued to develop and exploit photochemical procedures in the natural products field, most recently for synthesis of the antibiotic bikaverin, and in an improved synthesis of aldosterone. Maier, Hartan, and Sayrac have reported some interesting studies on the photoproduction of cyclobutadiene, and have observed the possible formation of a cyclobutadiene-CO, complex. The short-lived tetrafluorocyclobutadiene has been obtained (Gerace, Lemal, and Ertl). Courtot and his co-workers have reported an interesting series of studies in the field of hexatriene photochemistry: the conformation of the ground-state species has emerged as an important factor. Some new studies by Bender and Brooks on the photoisomerization of barrelenes to cyclo-octatetraenes appear to provide results in conflict with previous proposals. Interesting developments continue to be reported in the field of aromatic photochemistry. Barltrop and Day have provided further applications of their procedure for analysing ring transposition reactions of aromatic molecules in terms of ring permutation patterns, and Chambers et al. have presented an interesting paper on the pyridazine-pyrazine conversion. The first examples of photochemical cine-substitution have been reported (Bryce-Smith, Gilbert, and Krestonosich). Two separate reports of the photochemical cleavage of a benzene ring have appeared (Saito, Takami, and Matsuura; Hasselmann and Laustrial). Katritzky and Wilde have shown that 3-oxido-1-phenylpyridinium undergoes photodimerization and photoisomerization by 2,s-bonding. The latter process is without precedent in pyridinium chemistry: the former is rare, but Nagano et nl. have provided a further example. Muszkat et al. have presented a notable analysis of mechanistic factors in stilbene photocyclizations. These and related reactions continue to find numerous useful applications in synthesis, e.g. of helicenes, apolignans, and berberine alkaloids. Anderson et al. have used a photochemical route to provide the first example of an isolated 1,Zdiazetidine : cyclization of a lH-2,3-benzodiazepine is involved, and the extrusion of N2 which might have been expected does not occur. There has been considerable interest in nitrile ylides during the year, for example the formation from azirines, and intra- and inter-molecular addition reactions to carbonyl and other groups. Barltrop, Day, and co-workers have reported that the photoisomerization of 2- to 3-cyanopyrroles appears to involve initial 2,s-bonding in the pyrrole ring, followed by ‘walk’ reactions of the NH group.
viii
Introductory Review
Scattered reports appear in the literature that various inorganic ions can act as photocatalysts of reactions involving organic species. An interesting example to appear during the year has been the observation that Cu2+salts strongly promote the photoisomerization of pyridine N-oxides to 2-formylpyrroles (Bellamy, Martz, and Streith). Hata has reported the use of eosin adsorbed on alumina as a photosensitizer for reactions of heterocyclic N-oxides using visible light. Sat0 et al. have described a remarkable new regioselective synthesis of mediumto-large ring azathiocyclols by irradiation of N-alkylphthalimides bearing -SR groups as remote substituents in the alkyl group, e.g. at C-5. Cyclization occurs on to a carbonyl group in the phthalimide ring via a-C to the S. The photoelimination of N2 from pyrazoles, triazolines, triazines, etc. continues to provide valuable procedures for synthesis of benzcyclopropenes, aziridines, azetes, and other strained systems. Likewise, the long known photoelimination of N2 from diazo-compounds to give carbenes continues to find new synthetic applications. The photolysis of 3-diazobenzofuranone provides an interesting new source of benzyne which involves two successive photochemical steps (Chapman et d).The first case of reversible photochromic valence isomerization between a diazo-compound and a diazirine has been described by Voight and Meier. Cadogan et al. have reported the first example of the addition of an a-oxocarbene to benzene. Oda et al. have generated the highly strained allenic 1,2,4,6-cyclo-octatetraeneby a chemical procedure involving the generation of an intermediate carbene, and have trapped it as an adduct with cyclopentadiene. Mykytka and Jones have used an intramolecular carbene-acetylene addition to generate the highly strained dibenzobicyclo[4,1 ,O]heptatriene (1) and have trapped this by cycloaddition of butadiene to the cyclopropene moiety. Few photochemists normally look for effects of light intensity on quantum yields, but the observation of such effects in the photoreduction of a cyclohexadienone by Schuster et nl. may stimulate investigations of this parameter in other photochemical systems, especially those involving free-radical intermediates.
H. Fischer and his co-workers have used a CIDNP procedure to show that enols having lifetimes of a few seconds at room temperature can be formed during the photoreduction of aliphatic alcohols and ketones, being derived from ketyl radicals. For example, acetone and isopropanol form the acetone enol MeC(OH)=CH, (together with pinacol, of course). At -70 "C, the lifetime of the enol is ca. 5000 s, and addition of this enol to acetone gives the oxetan (2) as the major product. Interest continues in the photodegradation of polychloroaromatics, doubtless under the stimulus of environmental concern, and several interesting studies have appeared, e.g. photoreduction by borohydride and by methanol. It is interesting that some of these reductions appear to be promoted by the presence of triethylamine (see Part 111, Chapter 5 ) .
Introductory Review
ix
Although no really striking developments in polymer photochemistry as such have appeared during the year, Schaaf and his co-workers have developed several polymer-bound dye and porphyrin sensitizers for the photochemical generation of singlet oxygen: these appear to have the advantage of functioning heterogeneously, and they show greater resistance to bleaching than do sensitizers not bound in this way. Boden has used ‘crowii ethers’ to render certain dyestuff salts for singlet oxygen generation soluble in organic solvents. Potassium perchromate (K,Cr08) is no longer recommended as a ‘clean’ source of singlet oxygen: according to Pitts et al., o-dibenzoylbenzene) as a diagnostic trap for singlet oxygen may not after all be wholly specific for this reagent, according to Howard and Mendenhall. The reaction may occur to some extent also by free radical-initiated oxidation. It is noteworthy that peracids give the product quantitatively without involvement of singlet oxygen (Boyer et a!.). In the field of solar energy conversion, there has been increasing interest in the photoproduction of hydrogen from water. Unfortunately, as already noted, a question mark now hangs over the striking report of the photodissociation of water in the presence of a monolayer-bound ruthenium complex. Further developments here will be awaited with the greatest interest. There are, however, a number of systems from which the photoproduction of molecular hydrogen has been observed, and attention is drawn to the important review by Stein. The three main approaches are (a) the use of transition metal compounds to provide photo-redox systems, sometimes in conjunction with an n-type semiconductor (see Creutz and Sutin), (b) photoelectrolysis of water using Ti02 or strontium- or potassium-titanate electrodes, and ( c ) the use of photosynthetic organisms containing the enzymes nitrogenase and hydrogenase. Among a number of interesting developments during the year, one may particularly note Lin and Sutin’s photogalvanic cell based on a R ~ ( b p y ) ~ + / F e ~ + system, and the ingenious 3-electrode electrical storage battery described by Hodes and co-workers which is chargeable by sunlight with a reported efficiency of up to 90%. Some important improvements have been made in the technology of manufacturing silicon suitable for photocells : these may substantially reduce the present high manufacturing costs (Chalmers and co-workers). Soukup and Shah’s ‘high voltage vertical multijunction solar cells’ may also provide practical photovoltaic devices of markedly improved efficiency. Some very efficient solar cells
X
Introductory Review
based on cadmium sulphide heterojunctions with other semiconductors have been further described, notably those employing CdS/InP junctions (Shay et al., Boer). James and Moon, among others, have described some promising solar cells incorporating gallium arsenide. The field solar energy conversion is undoubtedly attracting increased academic and industrial attention as the urgency of the need for renewable energy sources becomes more widely appreciated. Important practical developments in the years ahead are very much to be hoped for and, I think, expected. February 1977 D. BRYCE-SMITH.
Contents
lnfroduction and Review of the Year By D. Bryce-Smith
iii
Part I Physical Aspects of Photochemistry Chapter 1 Developments in Instrumentation and Techniques By M. A. West
3
1 Introduction
3
2 Plasma Sources
4
3 Laser Sources CW Lasers Pulsed Gas Lasers Dye Lasers Laser Dyes Solid-state Lasers Frequency Conversion Laser Measurements
5 5
6 9 10 12
4 Monochromators and Light Filters
15
5 Absorption Spectrometry U .v.-Visi ble Spectrometry 1.r. Spectrometry Two-phot on Absorption Techniques Photoacoustic Spectroscopy C.D. and M.C.D.
17 17 20 21 22 22
6 Preparative Techniques
23
7 Light Detection and Measurement Photodiodes Photomultipliers Other Photodetectors Radiometry and Photometry Miscellaneous
25 25 27 29 29 32
13
14
xii
Contents 8 Fluorescence and Phosphorescence Spectrometry U.v.-Visible Fluorescence Spectrometry Signal Processing Fluorescence Techniques Other Luminescence Equipment and Techniques Applications of Fluorescence Phosphorescence Spectrometry Raman Spectroscopy
33 33 34 35 37 38 41 43
9 Transient Absorption Spectroscopy Conventional Flash Photolysis Nanosecond Flash Photolysis Subnanosecond Photophysical Techniques Probe Technique Opt ical-gate Technique Streak Camera Technique Miscellaneous Applications
44 44 46 49 49 50 52 53
10 Transient Emission Spectroscopy Instruments and Methods Applications
53 53 57
11 Signal Processing
58
Chapter 2 Photophysical Processes in Condensed Phases By K. Salisbury 1 Introduction
60
60
2 Excited Singlet State Processes Radiative and Non-radiative Processes Ionic and Radical Phenomena Excimers Singlet Quenching by Energy Transfer Exciplexes and Electron Donor-Acceptor Complexes Heavy Atom Quenching
60 60 80 83 84 85 91
3 The Triplet State Radiative and Non-radiative Processes Triplet Quenching and Triplet Energy Transfer
92 92 96
4 Two-photon Processes
99
5 Photo-oxidation
100
6 Chemiluminescence
101
...
Contents
XI11
7 Photochromism
102
8 Some Low-temperature and Crystal Studies
103
Chapter 3 Gas-phase Photoprocesses By D. Phillips
105
1 Introduction
105
2 Alkanes, Alkenes, and Alkynes
105
3 Aromatic Molecules
108
4 Carbonyl and Oxygen-containing Compounds Free Radical Reactions
115 123
5 Nitrogen-containing Compounds
126
6 Sulphur-containing Molecules
131
7 Halogen Atoms and Halogenated Compounds
132
8 Metal Atom Reactions Mercury Cadmium, Zinc, and Magnesium Alkali Metals and Alkaline Earths Miscellaneous
139 139 141 142 144
9 Miscellaneous
144
10 Laser Isotope Separation
146
11 Atmospheric Photochemistry Extraterrestrial Phenomena Thermospheric and Stratospheric Reactions Tropospheric Reactions and Pollutants Detection and Estimation of Atmospheric Pollutants and Constituents Rare Gases Atomic and Molecular Hydrogen Atomic and Molecular Oxygen and Ozone HO, Reactions Atomic and Molecular Nitrogen, NO, Reactions CO, Reactions SO2 and H,S Reactions
148 148 148 150
151 153 154 155 159 161 163 164
Contents
xiv
Part /I Photochemistry of Inorganic and Organometallic Compounds By J , M. Kelly 1 Photochemistry of Transition-metal Complexes
Titanium Vanadium Chromium Molybdenum Manganese Rhenium Iron Ruthenium and Osmium Cobalt Rhodium and Iridium Nickel Platinum Copper, Silver, and Gold Mercury Lanthanides Actinides
167 171 171 172 177 177 177 177 180 184 189 191 191 191 192 192 194
2 Transition-metal Organometallics and Low-oxidation-state Compounds Titanium, Zirconium, and Hafnium Vanadium, Niobium, and Tantalum Chromium, Molybdenum, and Tungsten Manganese and Rhenium Iron and Ruthenium Cobalt, Rhodium, and Iridium Platinum Copper Gold Mercury
196 196 198 198 203 206 213 218 218 218 219
3 Metalloporphyrins and Related Compounds Haems and Cytochromes
22 1 225
4 Water, Hydrogen Peroxide, and Anions
226
5 Main-group Elements Magnesium Boron Aluminium and Thallium Silicon, Germanium, and Tin Lead Phosphorus, Arsenic, Antimony, and Bismuth Sulphur Selenium and Tellurium Halogens
228 228 228 229 229 23 1 231 232 232 233
xv
Contents
Part /I/ Organic Aspects of Photochemistry Chapter 1 Photolysis of Carbonyl Compounds
237
By W . M.Horspool 1 Introduction
237
2 Norrish Type I Reactions
238
3 Norrish Type I1 Reactions
244
4 Rearrangement Reactions
253
5 Oxetan Formation
255
6 Fragmentation Reactions
256
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreacti o ns of Cyclo hexad ienones and Quinones By W. M. Horspool
262
1 Cycloaddition Reactions Intramolecular Intermolecular Dimerization
262 262 266 27 1
2 Enone Rearrangements
273
3 Photoreactions of Thymines etc.
288
4 Photochemistry of Dienones Linearly Conjugated Dienones Cross-conjugated Dienones Miscellaneous Dienones
292 292 294 299
5 1,2-, 1,3-, and 1,4-Dienones
300
6 Quinones
310
Chapter 3 Photochemistry of Olefins, Acetylenes, and Related Compounds By W. M. Horspool 1 Reactions of Alkenes Addition Reactions Hydrogen Abstraction Reactions Halogeno-olefins Group Migration Reactions cis-trans-Isomerizat ion
314 314 314 315 317 318 32 1
xvi
Contents 2 Reactions involving Cyclopropane Rings
322
3 Isomerization of Dienes
335
4 Reactions of Trienes and Higher Polyenes
340
+
5 [2 21 Intramolecular Reactions
347
6 Dimerization and Intermolecular Cycloaddition Reactions
350
7 Reactions of Acetylenic Compounds
353
8 Miscellaneous Reactions
354
Chapter 4 Photochemistry of Aromatic Compounds By A. Gilbert
362
1 Introduction
362
2 Isomerization Reactions
362
3 Addition Reactions
367
4 Substitution Reactions
382
5 Intramolecular Cyclization Reactions
391
6 Dimerization Reactions
406
7 Lateral-nuclear Rearrangements
41 1
Chapter 5 Photo-reduction and -oxidation By H. A. J. Carless
413
1 Conversion of C=O into C-OH
413
2 Reduction of Nitrogen-containing Compounds
426
3 Miscellaneous Reductions
430
4 Singlet Oxygen
434
5 Oxidation of Aliphatic and Alicyclic Unsaturated Systems
436
6 Oxidation of Aromatic Compounds
443
7 Oxidation of Nitrogen-containing Compounds
447
8 Miscellaneous Oxidations
453
Contents
xvii
Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S. T. Reid
455
1 Nitrogen-containing Compounds Rearrangement Addition Miscellaneous Reactions
455 480 488
2 Sulphur-containing Compounds
492
3 Compounds containing other Heteroatoms
499
Chapter 7 Photoelimination By S. T. Reid
455
503
1 Photodecomposition of Azo-compounds
503
2 Elimination of Nitrogen from Diazo-compounds
509
3 Elimination of Nitrogen from Azides
515
4 Photodecomposition of other Compounds having N-N Bonds
521
5 Photoelimination of Carbon Dioxide
524
6 Fragmentation of Organosulphur Compounds
526
7 Miscellaneous Decomposition and Elimination Reactions
532
Part /I/ Polymer Photochemistry By D. Phillips 1 Introduction
541
2 Photopolymerization Photoinitiation of Addition Polymerization Photocondensation Polymerization and Photochemical Cross-linking Photograft ing
54 1 54 1
3 Optical Properties and Luminescence of Polymers
545
4 Photochemical Reactions in Polymers Photochemical Reactions in the Absence of 0, Photo-oxidation and Weathering PoIy(o1efins) (PE, PP) Poly(styrene) (PS)
549 549 55 1 55 1
544 545
551
xviii
Contents Poly(amides) and Poly(urethanes) Poly(viny1 chloride) (PVC) Elastomers Cellulose Wool Photodegradable Polymers U.V. Stabilization 5 Appendix: Review of Patent Literature Photopolymerizable Systems Table A1 : Prodegradants and U.V. Sensitizers Table A2: U.V. Absorbers and Stabilizers Table A 3 : Optical Brightening Agents
552 552 552 553 553 553 553 554 554 558 562 566
Part V Photochemical Aspects of Solar Energy Conversion By M. D.Archer 1 General Reviews
571
2 Photochemistry Valence Isomerizations Photochemical Decomposition of Water Electron Transfer Reactions
572 572 573 574
3 Photoelectrochemistry Photogalvanic Effects and Cells Semiconductor Electrodes Titanium Dioxide Electrodes Other Semiconducting Oxide Electrodes Cadmium Sulphide Electrodes Miscellaneous
575 575 577 578 579 580 582
4 Photochemistry in Vesicles, Micelles, and Artificial Membranes
582
5 Photosynthesis The Structure and Function of Photosynthetic Membranes Primary Photochemical Events in Photosynthesis Photosynthetic Hydrogen and Oxygen Evolution
583 583 585 585
6 Photovoltaic Cells Silicon Cadmium Sulphide Heterojunction Cells Gallium Arsenide Other Semiconductor Heterojunctions Shottky Barrier Solar Cells Theory Inorganic Materials Organic Materials
586 586 587 588 589 589 589 590 590
xix
Contents
Part V / Chemical Aspects of Photobiology By G.Beddard 1 Introduction
593
2 Photosynthesis Chlorophylls in viva and in vitro Photosystem I (PS I) Photosystem I1 (PS 11) Photosynthetic Bacteria Fast Fluorescence from the Light-harvesting Pigments
593 593 599 60 1 602
3 Vision
607 607 608
Retinals and Retinols Visual Pigments
Author I ndex
605
612
Part I PHYSICAL ASPECTS OF PHOTOCHEMISTRY
1 Developments in Instrumentation and Techniques BY M. A. WEST
1 Introduction Although the progressive trend is for more and more physics to enter into chemical applications, a state of affairs which has attracted comment by analytical chemists (Aiialyt. Chem., 1975,47, 2073), photochemists must surely welcome the application of lasers and electro-optic developments to aid their research. Fields such as absorption and emission spectroscopy, chemical kinetics, and more recently, preparative chemistry, have all benefited through higher spectral resolution, selectivity, sensitivity, etc. This two-year review (July 1974 to June 1976) discusses most of the obvious advances in instrumentation and techniques in photochemistry, photophysics, and related spectroscopy as well as referring to fringe and other developments which have potential for, or have yet to be applied to, studies on the interaction of light with matter. With such a wide subject content, it is not possible to be very critical of publications or to include all publications within the confined space of this chapter. Furthermore, although subjects have been arbitrarily separated into 10 sections, some areas could be equally well placed in several sections, for example, two-photon absorption in sections dealing with pulsed lasers, absorption, or even emission spectroscopy. Several key developments have taken place recently in a number of relatively new techniques. Photoacoustic spectroscopy, though discovered 95 years ago, has benefited considerably by recent research which shows its considerable potential for absorption spectrometry of solids and semi-solids. Preparative photochemistry using i.r. lasers is already proving itself as a powerful technique for isotopic separations and for producing specific products. The time resolution in transient absorption measurements has now been pushed back to femtoseconds, beyond which, chemistry, as we know it, does not exist because of the uncertainty principle. A list of recommended terms for spectroscopy was tabulated in a previous volume (Vol. 6, p. 62) and was reputably based on the S.I. system of units. Unfortunately, inconsistencies in these terms have been indicated by Mielenz,l who recommends use of more logical adjectives and nouns to describe quantities and terms which are based on the transport of energy according to the laws of geometrical optics. For example, by defining absorbance as the negative logarithm to base ten of internal transmittance, it should be clear that this refers to the transmittance of an absorbing material exclusive of losses at boundary surfaces K. D.Mielenz, Analyt. Chem., 1976,48, 1093.
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Photochemistry
and effects of interreflection between them. Any instrument used for the measurement of spectra should simply be called a spectrometer. The word spectrophotometer, though commonly used, is a misnomer since a photometer is an instrument that measures luminous flux. Since the adjective ‘luminous’ implies the integral effect of visual radiation as perceived by the human eye, the spectral analysis of luminous flux has no physical meaning. It is certainly more accurate and logical to use the term absorption spectrometer and in the same way the confusion over spectrofluorimeters and spectrofluorometers would be eliminated by the term fluorescence spectrometer. One suggestion unlikely to find acceptance by photochemists, however, is replacement of the firmly established quantum yield by radiant yield or photon yield.
2 Plasma Sources The low-pressure mercury lamp so commonly used for photochemistry has been and the intensity of the 253 nm line examined as a function studied recently of Hg pressure, tube radius, and operating current. The intensity rises to a peak at about 7 mTorr pressure and falls at higher Hg pressures and, at constant pressure, increases linearly with ~ u r r e n t . A ~ useful review emphasizing the chemical developments of inorganic phosphors discusses their applications in changing the output wavelength of an Hg lamp.* Instabilities in the output of an HPK mercury lamp have been overcome by operation from an optically stabilized supply resulting in a drift of 0.1%h-l over a 30 h period.6 The amount of obnoxious and hazardous ozone generated by xenon short arc lamps is reduced considerably by passing the normal cooling air through a baffled aluminium chamber containing iron oxide.* This ‘filter’ decomposes the ozone to oxygen with high efficiency, but only after a warm-up time of 3040 min. A comparison of Xe-Hg, D2arc, and H2 hollow-cathode lamps has been made in an evaluation of a suitable source for background correction in atomic absorption spectrometry.’ At shorter wavelengths, a new type of source generating the line radiation of the rare gas ions achieves an enhanced ion flux by incorporating a charged particle arrangement. Intense line spectra are obtained from the He, Ne, and Ar ions, affording a convenient windowless source of He(@ (30.4 and 25.6 nm) and Ne(I1) (46 nm) suitable for photoelectron spectroscopy.8 A microwave-discharge U.V. light source has been reported to yield significant photon fluxes at 26.9 and 40.81 eV.O Mention will be made in other sections of the use of light from a synchrotron, but it is worth noting here a collection of papers dealing with this intense plasma source and its applications.l* 2p
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T. J. Hammond and C. F. Gallo, Appl. Optics, 1976, 15, 64. T. J. Hammond and C . F. Gallo, Appl. Optics, 1976, 15, 308. A. L. N. Stevens, J. Luminescence, 1976, 12/13, 97. R. E. Pulfrey, Appl. Optics, 1976, 15, 308. W. C . Neely, A. D. West, and T. D. Hall, J. Phys. ( E ) , 1975, 8, 543. M. S. Epstein and T. C . Rains, Analyt. Chem., 1976,43, 528. F. Burger and J. P. Maier, J . Phys. ( E ) , 1975, 8,420. T. V. Vorburger, B. J. Waclawski, and D. R. Sandstrom, Rev. Sci. Instr., 1976, 47, 501. ‘Collection of Papers on Synchrotron Radiation and Applications in Vacuum U.V. Physics’, ed. E.-E. Koch, R. Haensel, and C. Kunz, Pergamon, 1974.
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3 Laser Sources Before reporting developments in laser sources, it is appropriate to comment on safety codes regarding eye protection. Although there is little doubt that nearly every laser system radiates a beam which is hazardous to the eye, current safety codes in this country and elsewhere need to be revised regularly in view of developments in laser sources. Minimum permissible exposures depend on laser wavelength, exposure time, and peak power and, for many lasers, are estimated and certainly not based on ophthalmic measurements of thresholds for retinal or corneal lesions. Although some current safety codes have been criticized for being confusing, too conservative, and unrealistic (Laser Report, 1976, 12, 6, 7), there has been a report that standards for the near-u.v. may be inadequate (Laser Focus, 1976, 12(1), 41) since the corneal-damage threshold for the Nz laser for 10 ns pulses is only 10 pJ cm-2. Even more disturbing is recent evidence showing that the eye is 800 times more susceptible to damage from blue light than from radiation in the near-i.r.ll Both laser users and developers must be aware of realistic safety requirements, particularly in view of present and planned legislation on safety. The following sections outline some of the numerous publications on lasers with a reporting bias towards high-energy U.V. and tunable sources of all wavelengths which are being, or can be, used in photochemistry and spectroscopy.
CW Lasers.-There are few U.V. lasers known with adequate CW output power, and frequency doubling of visible lasers is not normally very efficient. Intracavity SHG, with temperature-tuned KDP or ADP crystals in a folded argon ion laser cavity, produces an output power of 300 mW at 257.25 nm.12 The important design criteria for this 32% power conversion efficiency are: (i) temperature tuning of the SHG crystal to better than kO.02 "C; (ii) cutting the crystals at the Brewster angle; and (iii) producing a 50 pm beam waist in the crystal. A lower cost and potentially useful laser for photochemistry is a CW CuII laser obtained by exciting a neon discharge in a copper hollow ~ a t h 0 d e . l ~ Lines at 248.6, 250.6, 259.1, and 259.9 nm at a power output of between 7 and 210 mW have been reported. The He-Cd laser, which usually emits at 325 and 441 nm,14 can produce simultaneous emission on five wavelengths in the red, green, and blue which can be mixed to give a 'white light' 1aser.16 CW laser oscillations on 23 transitions of CU(II)between 450 and 799 nm were obtained by exciting He-Ar, He-Ne, or He-Xe discharge in a hollow copper cathode.16 A high output power (0.5 W) and a bandwidth of 0.004 nm have been reported for a rhodamine 6G (Rh6G) laser pumped by an argon ion 1aser.l' Removal of unwanted background fluorescence from this type of laser within 0.5 nm of the exciting Ar+ l1 l2 l3
W. T. Ham, H. A. Mueller, A. I. Goldman, B. E. Newman, L. M. Holland, and T. Kuwabara, Science, 1974, 185, 362. P. Huber, ODtics Comm.. 1975. 15, 196. J. R. McNei, G. J. Collins, K:B. Persson, and D. L. Franzen, Appl. Phys. Letters, 1976,28, 207.
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D. C. Fromm, G . M. Neumann, and E. M. Schmidt, Optics andLaser Technology, 1976,8, 68. J. Meckley, Laser Focus, 1975, 11(11), 44. J. R. McNeil, G. J. Collins, K. B. Persson, and D. L. Franzen, Appl. Phys. Letters, 1975, 27,
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E. E. Marinero, A. M. Angus, and M. J. Colles, Optics Comm., 1975, 14, 226.
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Photochemistry
line at 488 nm line has been achieved using an external diffraction grating.l* Two astigmatic and coma-free prism ring dye lasers have been described for the jet-stream CW system.lg CW laser action at 546 nm from an Hg laser 20* 21 has been obtained in one case 31 in a sealcd-off system, suggesting use as a low-power (3 mW) green laser. DOTC and hexacyanine-3 cyanine dyes pumped by a 1.5 W krypton laser produce laser emission covering the range 754-888 nm.22 A compact external cavity for use with Group 111 and IV compound semiconductor injection lasers incorporates a grating which allows tuning from 860 to 910 nm.23 Among i.r. lasers reported are those obtained by non-linear mixing of emission from Nd-YAG and Rh6G lasers in LiI03 (range 1.28-1.62 a spin-flip Raman laser for the range 1905-1850 cm-l which was calibrated by absorption spectroscopy of COS, NO, DBr, and H,O using acousto-optic detection,25and chemical lasers of HI; 27 and DF.20In one case, F atoms produced in a mixture of SF, and He by microwave-discharge apparatus produced a laser with a CW output power of 4 W between 2.5 and 2.9 pm.26 Laser gain profiles (at 10.8 pm) were measured in a low-pressure Na-catalysed N20-CO transverse flow chemical laser under a variety of flow conditions.28 261
Pulsed Gas Lasers.-The search for new U.V. lasers that are highly efficient has been particularly stimulated by requirements of isotope separation and laserinduced thermonuclear fusion. Electron-beam pumping of high-pressure noble gases is well known to be 30 and recent studies with xenon 31-33 and xenon-He-Ar mixtures 34 revealed a continuously tunable source over 5 nm at 172 34 Investigations of laser systems using collisional energy transfer to create population inversions between electronic states of acceptor molecules have concentrated on electron-beam pumping of gas mixtures, e.g. Xe-0,, Ar-N2.35 An intense band emission at 3 4 G 3 4 4 n m from Ar-I, mixtures has been attributed to emission from molecular iodine with an overall fluorescence yield of 13 k 4%. Since Velazco and Setser36 suggested that the diatomic 1 1 1 ~ 1 . ~ ~ 9
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K. Jain, W. T. Wozniak, and M. V. Klein, Appl. Optics, 1975, 14, 811. G. Marowsky, Appl. Phys. Letters, 1975, 26, 647. N. Djeu and R. Burnham, Appl. Phys. Letters, 1974, 25, 350. M. Artusy, N. Holmes, and A. E. Siegman, Appl. Phys. Letters, 1976, 28, 133. A. Donzel and C. Weisbuch, Optics Comm., 1976, 17, 153. H. Heckscher and J. A. Rossi, Appl. Optics, 1975, 14, 94. W. Lahmann, K. Tibulski, and H. Welling, Optics Comm., 1976, 17, 18. R. J. Butcher, R. B. Daniels, and S. D. Smith, Proc. Roy. Soc., 1975, A344, 541. D. Proch, H. Pummer, K. L. Kompa, and J. Wanner, Rev. Sci. Instr., 1975,46, 1101. J. M. Gagne, L. Bertrand, Y . Counturie, S. Q. Mah, and J. P. Monchalin, J . Opt. SOC.Amer., 1975, 65, 876. R. C. Benson, C. B. Bargeron, and R. E. Walker, Chem. Phys. Letters, 1975, 35, 161. D. J. Bradley, in ‘Lasers in Physical Chemistry and Biophysics’, ed. J. Joussot-Dubien, Elsevier, Amsterdam, 1975, pp. 7-23. J. P. Girardeau-Montaut, Onde Electr., 1974, 54, 456, 463. J. B. Gerard0 and A. W. Johnson, Appl. Phys. Letters, 1975,26, 582. D. J. Bradley, D. R. Hull, M. H. R. Hutchinson, and M. W. McGeoch, Optics Comm., 1975, 14, 1. S. C. Wallace and R. W. Dreyfus, Appl. Phys. Letters, 1974, 25, 498. J. K. Rice and A. W. Johnson, J. Chem. Phys., 1975, 63, 5235. M. V. McCusker, R. M. Hill, D . L. Huestis, D. C. Lorents, R. A. Gutcheck, and H. H. Nakano, Appl. Phys. Letters, 1975, 27, 363. J. E. Velazco and D. W. Setser, J. Chem. Phys., 1975,62, 1990.
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noble-gas halides were possible laser systems, the following have been observed to lase following electron-beam excitation: XeBr at 282 nm;38-40 KrF at 249 nm;37940-44 XeCl at 308 nm;40,43 XeF at 351 and 353 nm;40945-47 and ArF at 193 nm.37 These systems are of great interest as a new class of powerful tunable U.V. lasers. For example, using an axial electron-beam excitation scheme to excite a mixture of Ar, Kr, and F2, 108 J of laser energy corresponding to a peak power of 1.9 GW was obtained from KrF and 1.6 G W from ArF.37 Electron-beam pumping is not essential since transverse electrical excitation (similar to that used in the N, laser) of mixtures of He or Ne, Xe, and NF, at pressures between 300 and 1000 Torr produced strong laser emission at 351 and 353 nm (attributed to XeF) with an energy of 7 mJ (compared with 2 mJ from nitrogen under the same condition^).^^ Laser action on the U.V. bands of I2 at 342 nm49-51 and bromine at 292 nm 5 2 following electron-beam irradiation has been reported with an experimental arrangement similar to that used for the rare gas halide lasers.44 The nitrogen laser (at 337 nm) must be the most common laser used in photochemical laboratories. The literature on these devices up to 1974 has been reviewed,53 and a detailed analysis of their dynamic behaviour 54 and circuit theory and design55presented. In the last paper, the usual flat-plate design is modified to spiralled striplines rolled around the cavity. In this way, a reproducible power of 1.2 MW was obtained at a charging voltage of only 12 kV.55 Other models constructed include a double parallel-plate design similar to that of Basting and Steyer (Vol. 4, p. 88) with a third electrode in the cavity for preionization, giving an output power of > 3 MW and pulse energy of >20 mJ,56 a low divergence (0.2 x 0.3 mR) laser of maximum intensity 5 MW m~ad-,,~'a MW system from a simple 25 cm device,58and a low-threshold coaxial arrangement using a Nanolite pulser of maximum power 140 kW.5B A stabilization technique employing a corona-type discharge prior to pulsing a Blumlein circuit has been employed,60and calculations made on laser intensity 49s
J. M. Hoffman, A. K. Hays, and G . C. Tisone, Appl. Phys. Letters, 1976,28, 538. S. K. Searles and G. A. Hart, Appl. Phys. Letters, 1975, 27, 243. s9 S. K. Searles, Appl. Phys. Letters, 1976,28,602. 40 C . A. Brau and J. J. Ewing, J. Chem. Phys., 1975,63,4640. I1M. L. Bhaumik, R. S. Bradford, and E. R. Ault, Appl. Phys. Letters, 1976, 28, 23. Ia G. C. Tisone, A. K. Hays, and J. M. Hoffman, Optics Comm., 1975,15, 188. I 3 J. J. Ewing and C. A. Brau, Appl. Phys. Letters, 1975,27, 350. 44 J. A. Margano and J. H. Jacob, Appl. Phys. Letters, 1975, 27,495. 46 E. R. Ault, R. S. Bradford, and M. L. Bhaumik, Appl. Phys. Letters, 1975, 27, 413. 40 C. A. Brau and J. J. Ewing, Appl. Phys. Letters, 1975, 27, 435. I7 C. P. Wang, H. Mirels, D. G. Sutton, and S. N. Suchard, Appl. Phys. Letters, 1976,28, 326. 48 R. Burnham, N. W. Harris, and N. Djeu, Appl. Phys. Letters, 1976, 28, 86. p B J. J. Ewing, J. H. Jacob, J. A. Mangano, and H. A. Brown, Appl. Phys. Letters, 1976,28,656. J. J. Ewing and C. A. Brau, Appl. Phys. Letters, 1975, 27, 557. 61 R. S. Bradford, E. R. Ault, and M. L. Bhaumik, Appl. Phys. Letters, 1975, 27, 546. 6a J. R. Murray, J. C. Swingle, and C. E. Turner, Appl. Phys. Letters, 1976, 28, 530. 53 J. P. Girardeau-Montaut, Nouv. Rev. Opt., 1974, 5, 367. P. Richter, J. D. Kimel, and G. C. Moulton, Appl. Optics, 1976, 15, 756. 65 A. J. Schwab and F. W. Hollinger, I.E.E.E. J . Quantum Electron., 1976, QE-12, 183. 6 8 J. I. Levatter and S.-C. Lin, Appl. Phys. Letters, 1974, 25, 703. 67 B. Godard and M. Vannier, Optics Conzm., 1976, 16, 37. 6 8 H. M. von Bergmann, V. Hasson, and D. Preussler, Appl. Phys. Letters, 1975, 27, 553. H. Fischer, R. Girnus, and F. Ruhl, Appl. Optics, 1974, 13, 1759. O 0 V. Hasson, H. M. von Bergmann, and D. Preussler, Appl. Phys. Letters, 1976, 28, 17. s7
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Photochemistry
and linewidth.61 Construction of an N, laser operating at 1 atm pressure and producing 335 pJ in a 1 ns pulse6, and a segmented flat-plate Blumlein circuit generating 400 ps pulses at a peak power of 1 MW 63 have also been reported. Addition of SF6 to a nitrogen laser has been found to produce a considerable increase in output power 66 up to Electron-beam pumping of Ar-N, mixtures results in laser emission at 357.7 nm 67-69 with a much higher efficiency (0.08-0.4%) than nitrogen A compact Ar-N, excitation transfer laser emits 40 ns pulses at a repetition rate of 1 kHz with a peak power up to 300 kW.69 In this case, a 12-stage Marx bank generator drives the cathode directly with an input voltage of 540 kV. Travelling-wave excitation of high-pressure nitrogen can produce single pulses from the second positive band of N, with the duration decreasing from 300 ps at 1 atm to 50 ps at 6 Mixtures of argon and iodine-donor compounds (HI, CF31,or CH31) can be electron-beam pumped to produce lasing from iodine at 301 nm,71at average output powers up to 25 MW. Further investigations of the copper laser (reported in Vol. 6, p. 69) have shown that this could have potential as a high-energy visible Quasicontinuous pulsed laser output at 510.6 and 578.2 nm has been reported from 600 "C copper iodide discharges at repetition rates near 8 kHz 73 and up to 30 kHz with copper At slightly longer wavelengths, laser oscillations have been observed on the green bands of XeO and KrO excimers pumped by an electron beam at around 550 nm with peak powers up to 100 kW.75 A multiple wavelength laser could be obtained in a single laser tube by using metals known to lase individually. Copper and gold as laser materials, for example, produce a total power of 17 mW at repetition rates up to 1.7 kHz with simultaneous emission at 510.6, 578.2, and 627.8 nm.76 A discharge-heated lead vapour laser with emission at 406.2 and 405.7 nm has been r e p ~ r t e d . ~ ' High-power photochemical iodine lasers (emission at 1.31 5 pm) have the potential of providing the short and powerful pulses which are necessary for laser fusion. In order to reach maximum inversion quickly, it is necessary to pump CF31 or C3F,I molecules with a flash lamp or light from a laser-produced 649
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P. Richter, J. D. Kimel, and G. C. Moulton, Appl. Optics., 1976, 15, 1117. E. E. Bergmann, Appl. Phys. Letters, 1976, 28, 84. H. Salzmann and H. Strohwald, Optics Comm., 1974, 12, 370. C. S. Willett and D. M. Litynski, Appl. Phys. Letters, 1975, 26, 118. J. Itani, K. Kagawa, and Y . Kimura, Appl. Phys. Letters, 1975, 27, 503. 0. Judd, I.E.E.E. J. Quantum Electron., 1976, QE-12,78. S. K . Searles, Appl. Phys. Letters, 1974, 25, 735. E. R. Ault, M. L. Bhaumik, and N. T. Olson, Z.E.E.E. J . Quantum Electron., 1974, QE/10, 624; N. G . Basov, V. A. Danilychev, V. A. Dolgikh, 0. M. Kerimov, A. N. Labonov, and A. F. Suchlov, Pis'rna Zhur. Eksp. i Teor. Fiz., 1974,20, 124 (Chem. Abs., 1975, 81, 129 065). E. R. Auk, Appl. Phys. Letters, 1975, 26, 619. H. Strohwald and H. Salzmann, Appl. Phys. Letters, 1976, 28, 272. A. K. Hays, J. M. Hoffman, and G. C. Tisone, Chem. Phys. Letters, 1976, 39, 353. J. A. Piper, Optics Comm., 1975, 14, 296. I. Liberman, R. V. Babcock, C. S. Liu, T. V. George, and L. A. Weaver, Appl. Phys. Letters, 1974, 25, 334. C. J. Chen and G . R. Russell, Appl. Phys. Letters, 1975, 26, 504. H. T. Powell, J. R. Murray, and C. K. Rhodes, Appl. Phys. Letters, 1974, 25, 730. T. S. Fahlen, I.E.E.E. J. Quantum Electron., 1976, QE/12, 200. R. S. Anderson, B. G. Bricks, T. W. Karras, and L. W. Springer, I.E.E.E. J. Quantum Electron, 1976, QE-12, 313.
Developments in Instrumentation and Techniques
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plasma,78,79 or in one case, to increase the number density of the iodide by shock c o m p r e s ~ i o n . ~The ~ high-gain characteristics of these lasers may result in premature super-radiant emission along an amplifier chain unless the various amplifier stages are optically isolated from each other. This was accomplished by a single saturable absorber consisting of an electric discharge passed through a cell containing CF31 gas or iodine vapour.80 Q-switching and modelockinga2have been achieved with this laser, resulting in the latter case in 160 ps pulses. There have been numerous reports of carbon dioxide lasers, lasing at 10.6 pm, with details of high-power TEA lasers,83a chemical waveguide laser with energy from the exothermic chain reaction between D, and F2 initiated by flash p h o t o l y ~ i s ,and ~ ~ laser amplification by stimulated emission of CO, by transfer from products of the oxidation of alkaline-earth metal vapours in N20.85 Q-switching with aromatic halogenated hydrocarbons and rapid modulation by operating a thin film Pb,-,Sn,Te optical shutter have also been d e s ~ r i b e d . ~ ~ Conversion of a Coherent Radiation CO, laser to create a CO laser results in laser emission at 5.4-5.6 pm with a power of 1 W.88 Pumping CH3F gas with a 200 MW TEA CO, laser produced far4.r. laser pulses (at 496 pm) with powers > 1 MW.89 Laser action at 11.5 and 12.2 urn was observed in electron-beam stabilized electric discharges with He-Co-CS, and He-Ne2-CS, mixtures.g0 Dye Lasers.-The welcome development of dye lasers offering higher output powers, shorter pulse durations, and higher repetition rates have been accompanied by numerous studies of fluorescent dyes. There has been a growing interest in studies of both the photophysical and photochemical properties of these dyes in attempts to achieve conditions of high output power and minimum photochemical degradation. Average dye laser output powers of > 100 W have been demonstrated at repetition rates of 350 Hz in an arrangement in which the dye flow was transverse to the laser axis and pumped by a vortex-stabilized flashlamp in an elliptical r e f l e ~ t o r .A ~ ~similar transverse flow system has been used at repetition rates up ~ ,slab dye laser designed originally for photocoagulation produced to 1 ~ H Z . A 20 m J output (from Rh6G) with a 50 J flash.s3 A sound suggestion for increasing 78 79
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S. Ishii, B. Ahlborn, and F. L. Curzon, Appl. Phys. Letters, 1975, 27, 118. L. D. Pleasance and L. A. Weaver, Appl. Phys. Letters, 1975, 27, 407. S. Ishii, K. Fong, and B. Ahlborn, Rev. Sci. Instr., 1976, 47, 600. S. Ishii and B. Ahlborn, Rev. Sci. Instr., 1975, 46, 1287. E. D. Jones, M. A. Palmer, and F. R. Franklin, Opt. Quantum Electron, 1976, 8, 231. A. C . Walker and K. R. Rickwood, J. Phys. (E), 1976,7,432; P. F. Browne and P. M. Webber, Appl. Phys. Letters, 1976, 28, 662; F. Rheault, J.-L. Lachambre, P. Lavigne, H. Pepin, and H. A. Baldis, Reo. Sci. Instr., 1975, 46, 1244; J. Domey, ibid., p. 811; H.-S. Kwok and E. Yablonovitch, ibid., p. 814. T. 0. Poehler, R. E. Walker, and J. W. Leight, Appl. Phys. Letters, 1975, 26, 560. D. J. Benard, Chern. Phys. Letters, 1975, 35, 167. J. R. Izatt, G. F. Caudle, and B. L. Bean, Appl. Phys. Letters, 1974, 25, 446. A. V. Nurmikko and G. W. Pratt, Appl. Phys. Letters, 1975, 27, 83. J. A. Davis, Rev. Sci. Instr., 1975, 46, 323. D. E. Evans, L. E. Sharp, B. W. James, and W. A. Peebles, Appl. Phys. Letters, 1975,26, 630. L. Y . Nelson, C. H. Fisher, and S. R. Byron, Appl. Phys. Letters, 1974, 25, 517. W. W. Morey and W. H. Glenn, I.E.E.E. J. Quantum Electron., 1976, QE/12, 31 1. H. W. Friedman and R. G. Morton, Appl. Optics, 1976,15, 1494. P. Burlamacchi, R. Pratesi, and U. Vanni, Rev. Sci. Instr., 1975,46, 281.
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Photochemistry
the output power of a conventional flashlamp-pumped laser is to ignite a lowenergy prepulse discharge through the flashlanip just prior to firing the main flashlamp This prepulse or simmer mode of operation of flash lamps is well known to enhance the U.V. spectral output. Spectral characteristics of lamps have been given in a description of a coaxial dye cell pumped by a linear lamp, and considerable variations in peak power and energy were found at different wavelengths of a xenon d i s ~ h a r g e . ~ ~ High intensities in the 200-300 nm range needed to pump U.V. and blue dyes were obtained in a novel arrangement using a C0,-laser-produced Output powers of > 10 kW from p-terphenyl (lasing at 340 nm) were produced by a 10.6 pm input pulse energy of 5.6 J and more than 30 kW was obtained under the same conditions from Rh6G. By mixing two dyes in a single cell (e.g. Rh6G with coumarin 120) it is possible to obtain two or more lasing wavelengths on pumping with an N, laser.Q7A sixchannel flat-plate quartz dye cell has been used in a similar pumping arrangement to select specific dyes.ss This cell offers advantages over the rotating carousel because of a larger pumping area, more effective stirring, and a large dye reservoir (which reduces photodecomposition). Dye lasers of narrow bandwidth O Q s looare required for high-resolution spectral studies and selective photochemical excitation. In one example, peak powers of 50 kW in the visible at linewidths down to 6 x nm have been reported for a pressure-tuned, dye laser oscillator with an external interferometer and twostage amplifier pumped by a 1 MW N2laser.lo0 Theoretical analysis of dye-laser pumping has shown that only the singlet manifold needs to be considered in kinetic studieslO1 and that a rate equation, agreeing with experimental measurements, can be based on a study of the effects of pump radiation on singlet state absorption and fluorescence quantum yield.lo2 Laser Dyes. Many hundreds of fluorescent dyes have been examined for laser action, and only a few reports of these can be mentioned here. U.V. lasers, which are much needed for photochemical work, are still few and far between although a review, with useful references to Russian work, lists 30 flashlamppumped dyes lasing between 330 and 400nm.lo3 POPOP remains the most efficient dye in the vapour phase, producing radiation tunable from 380 to 410 nm.104-10s Unfortunately, the gain reaches a maximum well before the M. H. Ornstein and V. E. Derr, Appl. Optics., 1974, 13, 2100. M. Drake and R. I. Morse, Optics Comm., 1974, 12, 132. Q6 W. T. Silvfast and 0. R. Wood, Appl. Phys. Letters, 1974, 25, 275. 97 W. T. Silvfast and 0. R. Wood, Appl. Phys. Letters 1975, 26,447; R. K. Jain and A. Dienes, Spectroscopy Letters, 1974, 7, 491. P. E. Oettinger, Appl. Spectroscopy, 1976, 30, 362. J. E. Lawler, W. A. Fitzsimmons, and L. W. Anderson, Appl. Optics, 1976, 15, 1083; A. Yamagishi and H. Inaba, Optics Comm., 1976, 16,223. loo R. Wallenstein and T. W. Hansch, Optics Contm., 1976, 14, 353. lo1 S. Speiser, Chem. Phys., 1974, 6, 479. loa E. Saher, D. Treves, and I. Wieder, Optics Comnt., 1976, 16, 124. lo3 G. A. Abakumov, V. V. Fadeev, and R. V. Khokhlov, Spectroscopy Letters, 1975, 8, 651. lo' P. F. Liao, P. W. Smith, and P. J. Maloney, Optics Comm., 1976,17,219. lo6 G . Marowsky, F. P. Schaefer, J. Keto, and F. K. Tittel, Appl. Phys., 1976,9,143;P. W. Smith, P. F. Liao, C. V. Shank, T. K. Gustafson, C. Lin, and P. J. Maloney, Appl. Phys. Letters, 1974, 25, 144. lo6 M. Maeda and Y. Miyazoe, Jap. J. Appl. Phys., 1974,13,827. O4
!as J.
Developnzents in Instrumentation and Techniques
11
maximum of the pumping pulse and falls rapidly. This premature termination, attributed to photodeconiposition, is a severe limitation to high overall efficiency.lo4 Tables of lasing dyes with output wavelengths < 440 nm lo6and between 420 and 750 nni lo7have been published. Several new coumarin derivatives have been reported,lo8,logsome of which have superior thermo-optical properties with flashlamp pumping and lasing with high efficiency between 450 and 520 nm.lo8 A useful study of photodegradation of 7-diethylamino-4-methylcoumarinin ethanol showed that two chemically different reaction routes could be identified.l1° One of these routes leads to compounds which lase and the other to a substance which absorbs at the laser wavelength. Although oxygen is usually employed to quench the triplet state, in this case addition causes formation of the laser-inhibiting compound. Improved performance could be obtained by adding a non-oxidizable tripletstate quencher, replacing the CH, group on the coumarin by a less reactive CF3 group, and removing the inhibitor by appropriate chemical filtering. A comprehensive review on the lasing properties of 4-methylun~belliferone(lasing range 390-560 nm) includes other related coumarins.lll Laser action in trans-l,l,4,4-tetraphenylbutadiene in cumene shows that at low temperatures (- 80 "C) emission occurs at 435 nm, between -70 and - 80 "Cat both 435 and 498 nm, and at room temperature at 498 nm This behaviour has been attributed to a temperature-dependent photoisomer. Fluorol 7GA dye, if sufficiently purified, is a stable lasing material for the region 530-600 nrn.ll3 Chemical impurities in Rh6G [Cl-] decrease the quantum efficiency of fluorescence and shorten the usable lifetime of any lasing Other studies with this popular lasing dye include relaxation kinetics of dimerization 115and lasing properties in polyacrylonitrile polymers.l16 Studies of the lasing properties of Kiton red S and rhodamine B dyes with both long- and short-pulse excitation have shown how the performance is strongly influenced by substituents bonded to the C-9 of the main c h r o m ~ p h o r e . ~ ~ ~ Population inversion is difficult to achieve with cresyl violet, and optical filtering procedures designed to lower the laser threshold have been described.l18 Mixing the dye with RhGG produces efficient lasing between 646 and 700 nni because of transfer of excitation attributed to depletion by stimulated emission of population in an upper laser At longer wavelengths, efficient (211%) emission between 838 and 923 nm has been reported for various polymethine dyes pumped by a frequency-doubled Nd:YAG laser,119and a range of lo' J. B. Marling, J. G. Hawley, E. M. Liston, and W. B. Grant, Appl. Optics, 1974, 13, 2317. lo8 K.H.Drexhage, G. R. Erikson, G . H. Hawks, and G. A. Reynolds, Optics, Comm., 1975,15, 399.
E. J. Schimitschek, J. A. Trias, P. R. Hammond, R. A. Henry, and R. L. Atkins, Optics Comm., 1976,16,313. B. H. Winters, H. I. Mandelberg, and W. B. Mohr, Appl. Phys. Letters, 1974, 25, 723. ll1 S. C. Naydon, Spectroscopy Letters, 1975, 8, 815. lla C.Rulliere, J. P. Morand, and J. Joussot-Dubien, Optics Comm., 1975, 15, 263. 113 M.Lambropoulos, Optics Comm., 1975, 15, 35. 11* J. M. Drake and R. I. Morse, Optics Comm., 1975, 13, 109. 116 M. M. Wong and Z. A. Schelly, J . Phys. Chem., 1974, 78, 1891. ll6 S. Reich and G . Neumann, Appl. Phys. Letters, 1974,25, 119. 11' J. M. Drake, R. I. Morse, R. N . Steppel, and D. Young, Chem. Phys. Letters, 1975,35, 181. 11* D. E. Evans, J. Puric, and M. L. Yeoman, Appl. Phys. Letters, 1974, 25, 151. 119 C. D. Decker, Appl. Phys. Letters, 1975, 27, 607. loB
12
Photochemistry
dyes useful between 71&1080 nm was discovered with a Q-switched ruby laser as the pump source.120 A dye laser pumped by a pulse train from a mode-locked solid-state laser can emit short pulses coincidental in time with pumping pulses when the relative cavity lengths are mafched.l2l Recent work using Fabry-Perot tuning elements showed that transform-limited (12 ps) pulses covering a broad spectral tuning range (549-727 nm) could be generated with high efficiency in several laser dyes.121 By choosing a cavity of correct photon cavity decay time (e.g. 60 ps) and by controlling the level of pumping, it is possible to obtain high repetition rate, tunable, dye-laser pulses of subnanosecond duration from N,-laser-pumped dyes in the near U.V.and visible.122 Narrow-band ps pulses have been generated in an ultrashort (50 pm) cavity of rhodamine B pumped by a 530 nm modelocked Nd 1 a ~ e r . lDouble ~~ mode-locked operation occurs in mixtures of Rh6G and cresyl violet resulting in an ultrashort pulse at 574 nm followed by one at 644nm.124 A novel system for generating a single tunable high-power ps pulse involves passing a mode-locked dye-laser train through a dye amplifier which is pumped by a N2 1 a ~ e r . lAs ~ ~the amplifier gain is only available for a few ns, only one pulse in the train is amplified. Other reports of mode-locked pulses include the injection locking of a mode-locked high-power train to a (low-power) mode-locked CW laser to improve laser optical properties,126and the use of a mode-locked Arf laser to pump a dye and obtain < 500 ps (detector-limited) ~u1ses.l~~ Solid-state Lasers.-Ruby and neodymium lasers remain the most powerful and useful solid-state lasers and major developments have been directed towards more efficient harmonic conversion (see next section). CaLaS0AP:Nd [Ca2La7.,,Nd,.,,(Si0,),0,1 has been evaluated as a replacement for Nd:YAG in high repetition rate 1060 nm Q-switched laser systems; it was shown to operate at an average power of >1 J at 30Hz.12* Developments in electro-optic and other switches, particularly for single-pulse extraction from a mode-locked train, include descriptions of a multichannel laser-triggered spark gap which will switch up to 10 kV in five channels with a risetime of 5 M) NaOH undergoes photoionization (loss of H+) and the resulting emission at 375 nm is 78
’@
82
K. Rotkiewicz, Z . R. Grobowski, A. Krowerzynski, and W. Kuhnle, J. Luminescence, 1976, 12, 877.
H. Dodiuk and E. M. Kosower, Chem. Phys. Letters, 1975, 34, 253. (a) Y. H. Li, L.-M. Chan, L. Tyer, R . T. Moody, C . M. Himel, and D. M. Hercules, J. Amer. Chem. Soc., 1975, 97, 3118; ( b ) R . M . C. Henson and P. A. H. Wyatt, J.C.S. Furuduy IZ, 1975, 71, 669. I. Tatischeff and R . Klein, Phorochem. and Photobiol., 1975, 22, 221. S. Yamashita, M. Yoshida, and G. Jomita, Z . Nafurforsch., 1976, 31a, 361.
76
Photochemistry
Table 7 Fluorescence quantum yields of indole in aerated andlor deaerated solvent systems at excitation wavelength A. = 280 nm a Solvent
Water Acetonitrile Methanol n-Butanol Cyclohexane Purified cyclohexane n-Hexane n-But anol-w ater
(@i)air
0.274
0.008(7) 0.32 0.23-0.27-0.32 0.26 0.26, 2 0.00, (2) 0.16
99: 1
0.34 0.33-0.35 0.45 A 0.00,(4)
0.49 0.42
0.34
Ethanol-water 0.95 : 99.05 19 : 81 47.5 : 52.5 95 : 5 Ethanol-(water)-cyclohexane 9.5 : (0.5) : 90 0.95 : (0.05) : 99
0.24 0.33 0.35 0.28, i-0.00, (2)
0.39 0.40 0.45
Tryp (2 x M) in water is taken as standard of fluorescence quantum yield with air at A. = 280 nm. b Number in brackets indicates the number of measurements and stated accuracy corresponds to the mean measured deviation: without number, measurements are single ones. a
(@&fa
= 0.13
tentatively ascribed to the indole anion fluorescence. A series of somewhat contradictory reports on the role of electron ejection subsequent to excitation of tryptophan and its derivatives have appeared. Bryant et aLE3using a 265 nm laser-flash technique have concluded that in neutral aqueous solution three important primary products are formed, the neutral tryptophan radical (as a result of N-H bond fission), the triplet state, and the hydrated electron. It is suggested that after excitation the formation of a loose complex (Scheme 4)
Scheme 4
competes with relaxation to the fluorescent singlet state and that the complex may give back the neutral molecule or dissociate with deprotonation. Table 8 gives the quantum yields of e(aq) formation. The reason for the high values relative to earlier conventional flash results must be due to the rapid radicalcation electron recombination (within the complex). Bent and H a y ~ n , using * ~ a similar laser-flash system, but measuring the by the absorption method without a standard (the previous workers used a ferrocyanide reference), obtained much lower values (Table 8). Although it is not immediately obvious why the discrepancies exist, if the back reaction of the electron and radical cation is as fast and as important as claimed, measure@)e(,q)
83 84
F. D. Bryant, R. Santus, and L. I. Grossweiner, J. Phys. Chem., 1975, 79, 2711. D. V. Bent and E. Hayon, J. Amer. Chem. SOC.,1975, 97, 2612.
77
Photophysical Processes in Condensed Phases
Table 8 Photochemical electron quantum yields jor tryptophan and tyrosine Tryptophan 0.25 (neutral aqueous solution) Tryptophan 0.08 (pH = 6.0)
Tyrosine (neutral aqueous solution) Tyrosine
0.29 a 0.095
(pH = 7.5)
ments of e(aq) production may be expected to be both time and technique sensitive. Although in the latter two papers electron ejection was considered to be the result of a monophotonic reaction from the excited singlet state, other workers now dispute this. The effect of oxygen on the radical yield points to a triplet-state precursor.85 The initial decay rate of the isothermal fluorescence from tryptophan or phenolate as a result of radical-cation electron recombination increases with decreasing photoionization energy. This is probably the result of an increase in the electron-cation separation as the photoionization energy increases and thus imparts more kinetic energy to the electron.ss The fluorescence of tryptophancontaining peptides on paper or silica gel after different treatments may be used as a detection method in chr~matography.~~ A somewhat selective review of protein luminescence has appeared.88 The fluorescence of hydroxypyridines 89 has been discussed, and other workers have reported the fluorescence characteristics of substituted 2-methyl-l-isoquinolines,vo alkylated phenazinium ion-phenazyl radical v1 systems, some azaphenanthrenes,v2 and 4-pyridoxic acid and its 1act0ne.~~ 3,3’-Diethyloxadicarbocyanine iodide (DODCI) is important because of its use in the mode-locking rhodamine 6G dye-laser. However, some uncertainty exists as to the fluorescence lifetime of DODCI and its photoisomer (efficiently formed on photolysis). Using single and multiple picosecond pulse techniques, a value of 1.2 ns, in good agreement with some existing values, has been obtained for the lifetime of DODCI, while a value of 420 ps has been given to the lifetime of the excited singlet state of the p h o t o i ~ o m e r . ~ ~ Using sub-picosecond pulses from a mode-locked C.W. laser, and fitting the data to an equation having two exponentials, the ground-state recovery kinetics of malachite green have been measured, equation (17). In methanol a single exponential time constant of 2.1 ps is measured, while in more viscous solvents 86 87
8B
s2
93 gq
H. Templer and P. J. Thistlethwaite, Photochem. and Photobiol., 1976, 23, 79. K. K. Ho, J. Moan, and L. Kevan, Chem. Phys. Letters, 1976, 37,425. L. I. Larsson, F. Sundler, and R. Hakanson, J. Chromatography, 1976, 117, 355. R. E. Dale and L. Brand, Photochem. and Photobiol., 1975, 21,459. A. Weisstuch, P. Neidig, and A. C. Testa, J. Luminescence, 1975, 10, 137. R. A. Henry, C. A. Hiller, and D. W. Moore, J. Org. Chem., 1975,40, 1760. W. Rubaszewska and Z. R. Grabowski, J.C.S. Perkin ZI, 1975, 417. F. Masetti, U. Mazzucato, and J. B. Birks, Chem. Phys., 1975, 9, 301. N. P. Bazhulina, M. P. Kirpichnikov, Y. M. Morozov, F. A. Savin, R. M. Khomutov, and V. 0. Chekhov, Mol. Photochem., 1974, 6, 337. J. C. Mialocq, A. W. Boyd, J. Jaraudias, and J. Sutton, Chem. Phys. Letters, 1976, 37,236.
4
78
Photochemistry
+
R(r) = exp (- f / r , ) a exp ( - t / r &
(17)
a fast decay process converts S, to a high vibrational level of So, giving rise to partial recovery of the absorption. Subsequently, this hot ground state thermalizes at a slower rate for complete recovery of a b ~ o r p t i o n .The ~ ~ hexamethylindotricarbocyanine fluorescence spectrum and quantum yield are sensitive to
H20
Nd
NaTl
Figure 2 Schematic illustration of an AOT inverted micelle with an aqueous core @ = sulphosuccinate head group (Reproduced by permission from J. Amer. Chem. Soc., 1976,98, 2391)
solvent and t e m p e r a t ~ r e . ~The ~ fluorescence characteristics of aridine IIYe7 4-pyrones, 4-thiopyronesYand 4-pyridonesYe8some alkaloids and a d ~ e n a l i n e , ~ ~ and some aromatic thioketones (S, emission) loohave been examined. A study of the amphiphile, di-iso-octyl sodium sulphosuccinate (AOT), capable of forming inverted micelles (Figure 2) using the fluorescent probes anilinonaphthalenesulphonate (ANS), pyrene-sulphonic acid (PSA), and rhodamine B E. P. Innen, C. V. Shank, and A. Bergman, Chem. Phys. Letters, 1976, 38, 611. A. Eranian and 0. de Witte, Compt. rend., 1975, 281, 505. 97 J. 0. Williams, B. P. Clarke, and M. J. Shaw, Chem. Phys. Letters, 1976, 39, 142. N. Ishibe, H. Sugimota, and J. B. Gullivan, J.C.S. Faruduy IZ, 1975, 71, 1812. F. Nachtmann, H. Spitzy, and R. W. Frei, Analyt. Chim. Acta, 1975, 76, 57. l o o M. Mahaney and J. R. Huber, Chem. Physics., 1975,9, 371. 96
Photophysical Processes in Condensed Phases
79
has been carried out. The interest in this system stems from the fact that the inverted micelle is capable of containing water clusters in the central polar area of the micelle and it is of interest to determine the nature of these water clusters. ANS was found to be very sensitive to the size of the solubilized water clusters in that both its fluorescence yield and lifetime decreased with increasing radius of the aqueous micelle core. The microviscosity of the AOT micelles was examined using fluorescence polarization experiments and from the strength of rhodamine B fluorescence polarization in the absence of water it may be concluded that the micelle has a very rigid core. The microviscosity of the micelle core decreases with increase in the size of the water cluster. While the diffusion of ionic fluorescence quenchers is dependent on the water cluster size, oxygen may diffuse efficiently with or without the presence of water.lol The fluorescence behaviours of some fluorescent probes in aqueous solutions of cationic,lO*anionic,lo3 and non-ionic lo* surfactants have also been studied. Guanines undergo optical changes at low temperatures in diol-containing solvents by irradiation at A < 300 nm. Although the natures of the primary photoproducts can only be speculated upon, nevertheless, the observation of the well defined optical changes may be used as a semi-quantitative probe for the population of the excited states of guanines. The possibility of the involvement of these low temperature products in biological damage is not entirely excluded; but the special conditions of their formations and their thermal instability render this unlikely.lo5 Bilirubin has attracted a great deal of attention from photochemists because of its importance in the phototherapy of neonatal hyperbilirubinemia, and interest in the photophysical properties has also developed recently. A novel fluorescence emission (TP < 5 ns; , ,A 525 nm) obtained by cooling an EPA-dimethylformamide solution of bilirubin to 77 K has been obtained and has been assigned in a dangerously loose way to a second ‘species’of bilirubin which is in equilibrium and is favoured at low temperature. (This emission is not seen at room temperature.) lo6 Protoporphyrin IX dimethyl ester on pulse radiolysis produces an emitting singlet state (TF = 23 ns) and a non-emitting triplet state (TT > 240 ~ S ) . ~lo8 O ~The * fluorescence spectra and quantum yields have been obtained for a series of free base tetra-arylporphins and their Zn derivatives in which substituents were at the 2, 3, and 4 positions of the phenyl rings. Halogen substitution resulted in a decrease in as a result of induced intersystem crossing. Zn tetraphenylporphins exhibit an emission at 560 nm, which perhaps surprisingly has been assigned as hot-band fluorescence.log The recently isolated ‘large’
-
M. Wong, J. I
co + O W )
(144)
o(3q
(145)
+ CO,
+M
(146)
03+M
(147)
20,
(1 48)
0,
+H
(149) (1 50)
____+
HO,+M
(151)
I___,
OH
20H CO,
+ 0,
H,O,
(1 52) (1 53)
occurs.8o7These reactions account for the fact that 13C0, is formed in the presence of I3CO. Surface effects on the photodissociation of COzin relation to the Martian atmosphere have been discussed.808 SO, and H,S Reactions.-The quenching rate constant of H2S+ions by H2S has been reported to be 2.3 rt 0.3 x 10-Bcm3molecule-l s-1.809 Brief comment on the lifetimes of H2S and dimethyl sulphide in polluted (hours) and non-polluted (days) atmospheres has been made.s1o The flash photolysis of SO, produces a transient species whose lifetime is determined by the diffusion rate to the The species is formed by interaction between excited singlet SO, and its ground state, and has not been completely characterized, but may be a loosely bound dimer. Thus the photochemistry of this small but aeronomically important molecule, as Alice said, grows ‘curiouser and curiouser,’ given that the three triplet states are also implicated in atmospheric reactions. The phosphorescent ii3B, state has a zero-pressure decay rate of 3.8 +_ 0.6 x 10, s-l, with a rate constant for interaction with the ground state of 4.5 rt 0.1 x los 1mol-1 s-1.812 In a static system, the apparent quantum yield of photo-oxidation of SO, decreases owing to film formation in the vessel and a b a c k - r e a ~ t i o n .Recent ~ ~ ~ papers have reported oxidation of SO, in aqueous reaction of SO, with organic halogen compounds,816 and isomerization of cis-but-2-ene photosensitized by Finally, and wearily, it can be reported that i.r. laser studies on energy transfer in S80, have been carried I. Koyano, T. S. Wauchop, and K. H. Welge, J. Chem. Phys., 1975,63, 110. L. F. Loucks and R. C. Michaelson, J. Chem. Phys., 1975, 63,404. P. Papacosta and S. J. B. Corrigan, Chem. Phys. Letters, 1975, 36, 674. G. R. Mohlmann and F. J. de Heer, Chem. Phys. Letters, 1975, 36, 353. 810 R. D. Cadle, Atmos. Environment, 1976, 10, 417. 811 J. W. Bottenheim and J. G. Calvert, J. Phys. Chem., 1976, 80, 782. *la J. P. Briggs, R. B. Caton, and M. J. Smith, Canad. J. Chem., 1975, 53, 2133. P. A. Skotnicki, A. G. Hopkins, and C. W. Brown, J. Phys. Chem., 1975, 79, 2450. 814 S. Beilke, D. Lamb, and J. Muller, Arrnos. Environment, 1975, 9, 1083. 816 B. Gostisamihelcic and B. Kastening, 2. phys. Chem. (Frankfurt), 1975, 98, 443. 816 R. D. Penzhorn and W. G. Filby, J. Photochem., 1975,4, 91. 817 B. L. Earl, A. M. Ronn, and G. W. Flynn, Chem. Phys., 1975,9,307. 806
Part 11 PHOTOCHEMISTRY OF INORGANIC AND ORGANOMETALLIC COMPOUNDS By J. M. KELLY
1 Photochemistry of Transition-metal Complexes Recent publications of general interest include a monograph on inorganic photochemistry,l which nicely complements the earlier text of Balzani and Carassiti,2 a review of metal complex photochemistry covering the 1971-72 l i t e r a t ~ r e ,a~ summary of photochemical syntheses of inorganic compound^,^ and a discussion of spectroscopic investigations of transition-metal complex excited state^.^ The annual appearance of a report on luminescence properties of inorganic compounds in a sister Volume will be of interest to inorganic photochemists.6 The potential of transition metals for catalysis of the photodissociation of water has been recognized for many years. Balzani and co-workers have performed a useful service to inorganic photochemists by analysing the possible cyclic pathways for this process.’ (See also last year’s Report, p. 564.) This year has seen the realization of efficient production of molecular hydrogen and oxygen using visible light and monolayer-bound ruthenium@) bipyridyl complexes.* This remarkable discovery should provide an even greater stimulus to further research in this area. The quenching of electronically excited states of either organic or inorganic compounds by transition metal complexes is still an imperfectly understood process, despite the substantial number of studies carried out. Difficulties arise because of the variety of possible mechanisms (e.g. electron transfer, energy transfer, exciplex formation, catalysed inter-system crossing), and because of the sensitivity of the rate constant for quenching to such factors as solvent, transition metal involved, nature of the ligand, charge of the complex, and type of excited
a 3
4
a
‘Concepts of Inorganic Photochemistry’, ed. A. W. Adamson and P. D. Fleischauer, Wiley, New York and London, 1975. V. Balzani and V. Carassiti, ‘Photochemistry of Co-ordination Compounds’, Academic Press, London and New York, 1970. C. H. Langford and N. A. P. Kane-Maguire, in ‘M.T.P. International Review of Science’, Inorganic Chemistry Series Two, ed. M. L. Tobe, Butterworths, London, 1974, Vol. 9, p. 135. J. R. Wasson, in ‘Annual Reports in Inorganic and General Synthesis - 1974’, ed. K. Niedenzu and H. Zimmer, Academic Press, New York and London, 1975, Vol. 3. G. A. Crosby, Accounts Chem. Res., 1975,8, 231. A, J. Thomson, in ‘Electronic Structure and Magnetism of Inorganic Compounds’, ed. P. Day (Specialist Periodical Reports), The Chemical Society, London, 1976, Vol. 4, p. 149. V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta, and M. Gleria, Science, 1975, 189, 852. G. Sprintschnik, H. W. Sprintschnik, P. P. Kirsch, and D. G. Whitten, J. Amer. Chem. SOC., 1976,98, 2337.
167
168 Photochemistry state of the quencher available. A recent report considers the quenching of anthracene triplet by Cu2+,Ni2+,Co2+,and Mn2+,and of phenanthrene triplet by Ce3+,Pr3+,and Nd3+in methanol-water solutions over a wide temperature range (120-293 K).O At low temperatures, where the medium is viscous, the reaction rate is diffusion controlled but, in agreement with earlier work, the rate constants for quenching at higher temperatures are substantially below this limit. Under these latter conditions it is possible to calculate the intra-cage reaction rate constant k, [equation (1); T = aromatic molecule triplet state, Q = quenching kd +
k-d
~
(T-Q)
Products
(1)
metal ion]. The proposed mechanism for quenching is by energy transfer via exchange interaction. The low rate constants for quenching by the metal ions, despite favourable energy factors, are attributed to the relative inefficiency of transmission of the exchange interaction through the ligand solvent molecules. Energy transfer via exchange interaction is also the favoured mechanism for the quenching of the triplet states of organic sensitizers by tris(acety1acetonato)Fe"', -RulI1, - A P , and tris(dipiva1oylmethanato)Fe"' in benzene solution.l0V In this study the authors have recorded the efficiency of the quenching process as a function of sensitizer energy (Figure 1). This correlates well with the excited state energy of the metal complex acceptor determined spectroscopically. Thus it is apparent that energy transfer to metal-centred (ligand field) states of the Fell1 complex is substantially less efficient than transfer to its CT or intra-ligand states. These conclusions are confirmed by examination of the [Ru(acac),] complex where the ligand field states are at higher energy than the CT state, and for the [Al(acac),] species where only intra-ligand states are available. The considerable differences in rate constants for [Fe(acac),] and [Fe(dpm),] are attributed to the steric effect of the bulkier ligand on the efficiency of the exchange process. The influence of solvent environment on quenching rates has been examined for the interaction of anthracene triplet and Co2+ in mixed THF-water and t-butanol-water mixtures of various proportions.12 As was noted previously by the same authors for the naphthalene triplet, the value of the quenching rate constant passes through a minimum as the proportion of water in the organic phase is increased. This effect is ascribed to the changing nature of the solvation of the organic and ionic species. The general importance of electron transfer as a mechanism for the quenching of excited states of co-ordination compounds has been stressed by several reports this year. An illustration that this process may be responsible for the quenching of all classes of metal complex excited states has been given.13 Thus paraquat (1) deactivates the MLCT excited states of Ru"' complexes, the f-f* state of [Eu(phen),13+(phen = 1,lo-phenanthroline), and the intraligand excited state of Pd(octaethy1porphyrin). In all cases the mechanism proposed involves electron
* 10 l1 12
E. J. Marshall, N. A. Philipson, and M. J. Pilling, J.C.S. Faraday 11, 1976, 72,830. F. Wilkinson, Pure Appl. Chem., 1975,41, 661. F. Wilkinson and A. Farmilo, J.C.S. Faraday ZZ, 1976, 72, 604. V. A. Rogov, Y. I. Naberukhin, and Y. N. Molin, Zzvest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 1095.
R. C. Young, T. J. Meyer, and D. G. Whitten, J. Amer. Chem. Sac., 1976, 98, 286.
Photochemistry of Inorganic and Organornetallic Compounds
169
transfer. Although porphyrins and related compounds including chlorophyll are known to undergo both oxidative and reductive quenching, this behaviour has not previously been reported for co-ordination compounds. Balzani and co-workers l4 and Creutz and Sutin l6 have recently observed such behaviour for
Figure 1 Rate constants for quenching of Fe(acac),, Fe(dpm),, Ru(acac),, and Al(acac), as a function of ET, the energy of the trblet states being quenched (Reproduced from J.C.S. Furaday 11, 1976,72,604)
the excited state of [Ru(bipy),12+ (bipy = 2,2’-bipyridyl) when quenched by a variety of metallocyanide c o m ~ l e x e s and , ~ ~ also by reductants such as Eu”, S2042-, and [ R U ( N H ~ ) ~ ] ~Thus + . ’ ~ it could be shown that some metallocyanide complexes quench by energy transfer [equation (2)] {e.g. [Cr(CN),I3-}, some by oxidative electron transfer [equation (3)] {e.g. [Fe(CN)B]3-),and yet others by reductive electron transfer [equation (4)] {e.g. [MO(CN)~]~-) (Table 1). In l4
l6
A. Juris, M. T. Gandolfi, M. F. Manfrin, and V. Balzani, J. Amer. Chem. SOC.,1976,98,1047. C. Creutz and N. Sutin, Inorg. Chem., 1976,15,496.
170
*[Ru(bipy),12+ + Q *[Ru(bipy)J2+
+Q +
*[Ru(bipy)s12+ Q
-
Photochemistry
+ Q* [Ru(bipy),I3+ + Q [Ru(bipy),]+ + Q+ [Ru(bipy),12+
(2)
(3) (4)
Table 1 The rates oj'quenching of the luminescence of [Ru(bipy),12+by various metallocyanide complexes l4 E(*Q) "I E(Q+/Q)/ E(Q1Q-Y k, bl kJ mol-1 V V dm3mol-l s-l 234 148 28 3 28 1 31 1 561 275 27 1 230 = 204
kJ mol-l.
+-0.73
+0.36 > +0.75
-
+0.75 > +1.0 > +1.0
< -1.8 - 1.28 < -1.7 +0.36 -0.83 -
- 1.35
< -1.8
At 23 "C,ionic strength
3.4 7.5 3.3 6.5
x x x x
108 108
lo@ lo@ c 106 1.2 x lo@ 5.6 x loE < 106 < 106
= 0.50.
another paper it has been demonstrated that the MLCT excited state of [Ru(bipy),I2+ is deactivated by [Cr(bipy),13+,and that the metal-centred luminescent excited state of [Cr(bipy),13+is quenched by [ R ~ ( b i p y ) , ] ~ +In . ~both ~ cases the mechanism is one of electron transfer, even though in the first example, energy transfer is energetically favourable. The rate constant for the dynamic quenching of UOZ2+excited state by metal ions in aqueous solution correlates well with the ionization potential for the metal ion involved, and therefore a mechanism involving electron transfer seems most suitab1e.l' Electron transfer has also been shown to occur on the quenching of the triplet state of phenothiazine by Cu2+and Eu3+ ions in aqueous sodium lauryl sulphate micellar The influence of transition metal ions on the photochromism of a spiro-indolene-2,2'-benzopyran has been discussed.l@ Other examples of sensitization and quenching studies are discussed in the section dealing with the particular metal. As noted in last year's Report, picosecond laser-flash photolysis should prove to be a valuable technique for investigations of the properties of the excited states of co-ordination compounds, particularly under conditions similar to those used in photochemical studies. The results of such an investigation with Fe", Ru", and Cr"' compounds have been reported recently,20and are especially interesting in showing that non-radiative processes such as inter-system crossing are very fast indeed (7 < 5 ps). Burdett 21 has presented a theory for predicting the course of non-dissociative photochemical isomerizations of transition metal compounds. The method 16 17
18 20
21
F. Bolletta, M. Maestri, L. Moggi, and V. Balzani, J.C.S. Chem. Comm., 1975, 901. H. D. Burrows, S. J. Formosinho, M. da Graca Miguel, and F. Pinto Coelho, J.C.S. Furaday I, 1976, 72, 163. S. A. Alkaitis, G. Beck, and M. Graetzel, J. Amer. Chem. SOC.,1975, 97, 5723. D. Walther and E. G. Jager, 2. Chem., 1975, 15,236.
A. D. Kirk, P. E. Hoggard, G. B. Porter, M. G. Rockley, and M. W. Windsor, Chem. Phys. Letters, 1976, 37, 199. J. K. Burdett, Inorg. Chcm., 1976, 15, 212.
Photochemistry of Inorganic and Organornetallic Compounds 171 utilizes a simple molecular orbital approach based on consideration of metal d orbitals-ligand interactions. The site preference for the ligands in both excited and ground states is calculated from the particular d orbital configurations involved. For d s octahedral complexes it is presumed that the reaction proceeds by a thermal rearrangement of the excited state species to give the more stable excited state isomer. This then relaxes to the ground state. Good agreement is found with the experimental results for [Co(CN),(H,O),]- and Ru(PPh,),(C0)J2. For d8 square-planar compounds (e.g. Pt" complexes) the theory predicts that excitation of either the cis- or trans-isomer leads to the same distorted tetrahedral intermediate. This then decays back into the ground state, the proportion of cis- and trans-isomers depending on the position of intersection of the excited state and ground-state potential energy surfaces. As in previous Reports, the photochemistry of compounds of each transition metal will now be considered systematically. Transition metal organometallics, low oxidation-state compounds, and metalloporphyrins are the subjects of subsequent sections. Titanium.-A study of the photolysis of TiCI, in ethanol or methanol solution has been reported.22 Under these conditions the main species present in solution is solvated Ti(OR),Cl,. Irradiation at 300 nm leads to Ti"' and alkoxy radicals, which have been detected at low temperatures by a combination of e.s.r. and u.v.-visible spectroscopic techniques. The initial step in the reaction is presumed to be ( 5 ) (L = OR- or C1-). This is presumably also the first step in the lightTiIVL-
-
TiIx1L*
(5)
induced reaction of &unsaturated ketones in the presence of TiC14 in methanol An example in this report is the conversion of (2) into (3) in 65%
(2)
(3)
yield. It will be noticed that an extra (solvent-derived)carbon atom fragment has been incorporated into the product. It is reported that photolysis of an oxalato-TiIV complex in aqueous oxalic acid solution leads to reduction and hydrolysis of the species.24
Vanadium.-Methoxo-oxobis(8-quinoyloxo)vanadium(v), (VO)(OMe)Q,, has been used as a photo-initiator for the polymerization of methyImetha~rylate.~~~ 26 The polymerization is radical-initiated, and this suggests that the primary photochemical step is reaction (6). 21
83 84
2s 26
A. I. Kryukov, S. Y. Kuchmii, A. V. Korzhak, and 2. A. Tkachenko, Doklady Akad. Nauk S.S.S.R., 1975,222, 1134. T. Sato, G. Izumi, and T. Imamura, TetrahedronLetters, 1975, 2191. J. Shiokawa and A. Matsumoto, Asahi Garasu Kogyo Gijutsu Shoreikai Kenkyu Hokoku, 1974, 207 (Chem. Abs., 1976, 84,97 745). S. M. Aliwi and C. H. Bamford, J.C.S. Faraday I, 1975, 71, 1733. S. M. Aliwi, C. H. Bamford, and S. U. Mullik, J. Polymer Sci.,Part C, Polymer Symposia, 1975, 50, 33.
172 (VO)(OMe)Q2
-
Photochemistry (VIV0)Q2
+ *OMe
(6)
The phosphorescence of vanadium(v) species supported on silica gel has been m ~ n i t o r e d . It ~ ~ was further shown that this emitting species is reduced to vanadium(1v) by adsorbed ammonia or methanol. The photoreduction of vanadium(v) to vanadium(1v) in aqueous solution has been followed by polarography and by e.s.r.28 Chromium.-It is proposed that the photo-oxidation of pinacol [Me,C(OH)C(OH)Me,] by dichromate ion involves an initial two-electron transfer to give Crlv, as radicals could not be detected either by e.s.r. at low temperatures or by trapping with methylmetha~rylate.~~ In spite of the very considerable amount of work carried out on Cr"' complexes, the detailed mechanism of their photosubstitution reactions, and in particular photo-aquation, is still not fully understood. For most complexes it is clear that it is the lowest quartet state (4T2,in octahedral symmetry), and not the doublet (2Eu),which is photoactive, but this may not always be the case. Similarly, although stereochemical studies and experiments with macrocyclic ligands suggest that the substitution process proceeds via an associative rather than dissociative mechanism, conclusive proof of this is still lacking. These matters have been discussed in a short review,,O and in a detailed survey.31 Riccieri and Zinato 32 have published the results of a thorough investigation of the photoaquation reactions of tran~-[Cr(NH,),(H,0)Cl]~+ (4), trans[Cr(NH,)4(H,0)(NCS)]2+ (9,trans-[Cr(NH,),(H,0),l3+ (6), trans-[Cr(NH,),Cl,]+ (7), and trans-[Cr(NH,),(NCS)Cl]+ (8). In all cases, excitation in the ligand trans-[Cr(NH,),( H,O) C1I2+ trans-[Cr(NH,),Cl,]+
hV
hv
HIO
>
cis-[Cr(NH,),( H,O)C1I2+
'
cis-[Cr(NH3)JH,O)ClI2+
(7)
+ C1-
(8)
field bands of the complexes leads to aquation of the acido-groups, while NH3 aquation accounts for less than 10% of the reaction. Release of the axial ligand is accompanied by trans to cis isomerization. For example, the main products from (4) and (7) are those shown in equations (7) and (8). From a study of the wavelength dependence it could be shown that population of the lowest quartet state (4E)led almost exclusively to the labilization of the axial ligand, while the low yield of ammonia originated from the state, which lies at somewhat higher energy. (,E and 4B arise from the 4T,ustate under the non-octahedral symmetry of these complexes.) This is in accord with simple theory, which suggests that population of a a-antibonding orbital (the d , ~ orbital in the ,E state, or the d,+a orbital in the 4B state) should lead to axial or equatorial labilization respectively. As the authors point out, it is remarkable that cis-[Cr(NHJ427 Z8
A. M. Gritscov, V. A. Shvets, and V. B. Kazansky, Chem. Phys. Letters, 1975, 35, 511. M. Kitamura and H. Imai, Bull. Chem. SOC.Japan, 1975, 48, 1459. P. R. Bontchev, M. Mitewa, K. Kabassanov, and A. Malinovski, Inorg. Nuclear Chem. Letters, 1975, 11, 799. Kutal, J. Chem. Educ., 1975, 52, 502. E. Zinato, in ref. 1, p. 143. P. Riccieri and E. Zinato, J. Arner. Chem. SOC.,1975, 97,6071.
an C. 91 32
Photochemistry of Inorganic and Orgaltometallic Compounds 173 (H20)ClI2+,which is the common product of the ligand field photolysis of [Cr(NH,)6C1]2+,of (4) and of (7) is formed in all cases with a quantum yield close to 0.4, despite the differencesin the labilized ligands (NH,, H 2 0 , and C1-). This leads them to propose that the constancy of this quantum efficiency may be indicative of a common photophysical process. For compounds (6)-(8) U.V. irradiation leads to population of CT states. Bond fission resulting from such excitation was much less selective than from that caused by population of ligand-field states. [For example, for (4) at 254 nm Ocl- = 0.21 and O)NB,= 0.29.1 In this case homolytic rupture of the bonds is implicated. However, before the fragments so formed can escape from the solvent cage, the oxidized ligand recaptures an electron from the Cr", and the net products are therefore those of heterolytic cleavage. The extent of the stereoselectivity of the photo-aquation reactions of CrII' complexes is well exemplified in a study of the ligand-field band photolysis of trans-[Cr(en),X,]+ (X = Br, Cl) (en = 1,2-ethylenediamine).,, In both cases the principal product is [Cr(en),(H2O)XIa+. Using quantitative ion-exchange chromatography, it was possible to show that for X = C1 the product is >99.2% cis-complex, and that for X = Br it is >95% cis-complex. It is unlikely that such stereochemical preference can arise from thermodynamic or kinetic factors following dissociation of the excited state. The authors therefore propose that this stereospecificity is strong evidence for a mechanism in which entry of the solvent is concerted with halide loss. In agreement with the results obtained with related compounds, the small amount of NH, substitution (a678 = 0.003; OD,,, = 0.075; @4oe = 0.042) appears to arise from reaction of the 4Bstate. Ligand-field band excitation of 1 ,6-[Cr(en)(H,O),Cl,]+ (9) leads exclusively to chloride aquation (see Scheme l).,* Although photo-induced exchange of the aquo-ligand is also possible, arguments are presented to demonstrate that this is unlikely. Products (10) and (11) are formed in similar amounts. [For (lo), O,,, = 0.12, 0 4 0 0 = 0.22; and for (ll), @589 = 0.10, 0 4 0 0 = 0.15.1 Although it might appear that species (10) is formed by a stereo-retentive process, it is more probable that it too has arisen by a cis-trans isomerization step involving the H 2 0 ligands. The absorption spectrum of complex (9) exhibits essentially no splitting of the 4A2g-+4T2g band, implying that the 4E and 4B states are close in energy. This compound is therefore suitable for testing the relative importance of 0- and n-bonding in the excited state. As only axial labilization occurs, it appears that a-antibonding along the ClCrCl axis is the predominant effect, and that n-bonding factors are of minor importance. excited state of [Cr(bipy),],+ is remarkably long-lived in solution, and The its decay can be monitored by emission spectroscopy or by conventional flash photolysis. As discussed earlier (ref. 16) this excited state is deactivated by [R~(bipy),]~+, and in turn [Cr(bipy)J3+ may quench the luminescent state of [Ru(bipy),12+. For both reactions it has been proposed that electron-transfer mechanisms are operative, and this has been confirmed by flash photolysis.s6 33
36
W. J. Rosebush and A. D. Kirk, Canad. J. Chem., 1976,54,2335. R. T. Walters and R. G. Linck, Znorg. Chem., 1975, 14, 2098. R. Ballardini, G. Varani, V. Carassiti, and F. Scandola, '6th IUPAC Symposium of Photochemistry', Aix-en-Provence, July 1976, Abstract No. 4.
7
174
Photochemistry
OH2
C1
(9)
A OH2 (1 1) Scheme 1
More recently it has been found in quenching 36 and flash photolysis 37 experiments that this [Cr(bipy)J2+ doublet state reacts with water to form a [Cr(bipy),(H20)l3+transient species. The quenching of the photoracemization and phosphorescence of ( + )D[Cr(phen)$+ (phen = 1,lO-phenanthroline) by SCN- and O , have been investigated, and that by I- r e - e ~ a m i n e d .It~ ~ is shown that the efficiency for 4T2,-+ 2Eg intersystem crossing is greater than 95%, and that the photoracemization in acid solution proceeds via thermal population of the 4T2gstate from the 2Eg species. Hydroxide ion also partially quenches the phosphorescence, but causes an increase in the quantum yield for racemization. This is attributed to a reaction of the 2Egstate with OH-. Thus, from this observation and from those mentioned a b o ~ e it, appears ~ ~ ~ ~that ~ at least for these Cr"' chelate complexes, the 'rule' that only the quartet state is photoreactive is invalid. The importance of 4T2g+ 2Euintersystem crossing in Cr"' photochemistry has long been recognized. Reports this year indicate clearly that at room temperature in fluid solution this process is extremely rapid. Picosecond laser flash photolysis investigations with Cr(acac),, [Cr(NCS),I3-, and ~ ~ ~ ~ S - [ C ~ ( N H , ) , ( N Chave S)~]been carried out.20 In each case only the 2Eustate could be observed, indicating that the 4T2,species must undergo intersystem crossing in times less than the laser pulse width (5 ps). Independent evidence for the extreme rapidity of ISC in CrlI1complexes has been presented by Kane-Maguire and c o - ~ o r k e r s40. ~ The ~~ 36
37
33 39 40
M. Maestri, F. Bolletta, M. F. Manfrin, L. Moggi, and V. Balzani, see ref. 35, Abstract No. 65. M. S. Henry and M. Z . Hoffman, ref. 35, Abstract No. 43. N. A. P. Kane-Maguire and C. H. Langford, Inorg. Chem., 1976,15, 464. N. A. P. Kane-Maguire, J. E. Phifer, and C. G. Toney, Inorg. Chem., 1976, 15, 593. N. A. P. Kane-Maguire, D. E. Richardson, and C. G. Toney, J . Amer. Chem. SOC.,1976,98, 3996.
Photochemistry of Inorganic and Organometallic Compounds
175
+
photo-aquation of ( )=-[Cr(en)J3+ [equation (9)] has been followed by polarimetry in the presence of OH-. Hydroxide ion, selectively, and at the concentrations used, completely, deactivates the state. Variation of excitation wavelength had a marked influence on the percentage of reaction quenched, even though only the 4A2,-+ 4T2,band was excited (Figure 2). Further, the quantum
440
480
520
Figure 2 (A) Absorption spectrum of [Cr(en),Is+. (B) Variation of percentage reaction photoracemization as a function of excitation quenching by OH- of (+)~-[Cr(en),]~+ wavelength (Reproduced by permission from Inorg. Chem., 1976, 15, 593)
yield for phosphorescence in the absence of quencher shows a similar dependence; that upon excitation at 514 nm being only 63% of that recorded for 436 nm excitation. Both these observations indicate that crossing to the doublet state is more efficient at wavelengths shorter than 496nm. The authors suggest that this is evidence for effective competition of ISC processes with vibrational relaxation of the quartet state. An alternative possibility is that ISC might be competing with solvent-restricted relaxation of the quartet state to its thermally equilibrated state. However, this possibility may be ruled out as it has been demonstrated that the activation energies for phosphorescence after excitation at 460 or 514nm are identical. The explanation proposed for this variation in quantum yield is that excitation at wavelengths less than 490 nm corresponds to promotion to a point on the 4T2,surface above the intersection with the 2E, state, whereas at longer wavelengths the point reached is below the crossover, and the configuration of the 4T2,state is close to that of its vibrationally relaxed state. To rationalize the efficiency of crossing, it is also necessary to assume that at the crossover the 4T2,state is already substantially distorted from Oh
176
Photochemistry
Attempts to determine the lifetime of the 4T2,state in crystalline or rigid solvent matrices at low temperature have been performed using laser e ~ c i t a t i o n .42~ ~ , It was found that Cr(acac), and [Cr(CN),I3- in alcoholic glasses emit only phosphore~cence.~~ As no 'grow-in' of this emission can be observed, the lifetime of the initially formed 4T2, state must be less than 20 ns at 77 K, and less than 1 ps at 18 K. At 77 K [Cr(~rea)~],+ and [Cr(antipyrene),lS+show both fluorescence and phosphorescence, although in these cases the decays are none~ponential,~l1 4 2 and in the case of [Cr(antipyrene),13+ concentration dependent .41 The non-exponential nature of the [ C r ( ~ r e a ) ~ emission ]~+ decay has been found to be due to complexes in different environments. Most interestingly, it appears that the fluorescence is not prompt, but rather arises by thermal repopulation of the 4T2gstate from the 2E, state. This requires that the intersystem crossing from the 4T2,state must be very rapid indeed (k > log s - ~ ) . * ~However, Watson et al. state that because of the non-exponential nature of these emissiondecay processes, these compounds should be quoted as examples of delayed fluorescence 'only with due The c.d. spectra of a number of tris(fl-diketonato)Cr"' complexes have been determined following partial photo-induced resolution using circularly polarized light.43-45 The quantum yields for the photo-inversion are largest for tris(propanedialato)Cr"', and lowest for bulky-ligand-containing species such as tris(dipivaloy1methanaf~)Cr"'.~~ The influence of ligand deuteriation on the phosphorescence lifetimes of [Cr(NH3),]3+, [Cr(en),13+, and [Cr(NH,),(NCS),]- in rigid solution at 77 K has been in~estigated.~~ Effects are pronounced: e.g. for [Cr(NH,),],+ in methanolwater, T = 54 ps; for [Cr(ND3)J3+ in CD30D-D,O, r = 4350 ps. By comparison of these lifetimes with the frequencies for the ligand vibration overtone bands, it has been shown that a dipole-dipole mechanism is responsible for the non-radiative deactivation of these complexes. The phosphorescence lifetime of [Cr(CN),I3- has been studied in fluid organic solvents as a function of temperat ~ r e . ~The ' variation in lifetime for different solvents (e.g. at 25 "C, in D M F r = 6060 ps; in methanol, r = 19 ps) correlates with the solvent polarity parameter ET. This effect is ascribed to an enhancement of the radiationless decay of the complex produced by electrostatic perturbation of the solvent dipoles. Phosphorescence lifetimes in aqueous solution at room temperature have also been reported for [Cr(en),13+, [Cr(NH3)J3+, and [Cr(bipy),]3+.48 Other recent reports on luminescence from CrIII species include those on [Cr(en),I3+ and its deuteriated analogue,49 on [Cr(en),13++,[Cr(en),(ox)]+, and [ C ~ ( O X ) , ] ~on - , ~[Cr(NCS),(H20)6-,](3-n)+ ~ (n = 0_6),51 on CrS+ on various 41
p2 43
44
46 46
47 48
4B 6o
W. M. Watson, Y. Wang, J. T. Yardley, and G . D. Stucky, Znorg. Chem., 1975, 14, 2374. F. Castelli and L. S. Forster, J . Amer. Chem. Soc., 1975, 97, 6306. K. L. Stevenson and R. L. Baker, Znorg. Chem., 1976,15, 1086. H. Yoneda, U. Sakaguchi, and Y. Nakashima, Bull. Chem. Soc. Japan, 1975,48, 1200. B. Norden, Znorg. Nuclear Chem. Letters, 1975, 11, 387. I. B. Neporent, E. B. Sveshnikova, and A. P. Serov, Izvest. Akad. Nauk S.S.S.R., Ser. fiz., 1975,39, 1959. R. Dannoehl-Fickler, H. Kelm, and F. Wagestian, J. Luminescence, 1975, 10, 103. A. W. Adamson, C. Geosling, R. Pribush, and R. Wright, Znorg. Chim. Acta, 1976, 16, L5. C. D. Flint and A. P. Matthews, J.C.S. Faraday ZZ, 1976, 72, 579. P. E. Hoggard and H. H. Schmidkte, Spectrochim. Acta, 1975, 31, A, 1389.
Photochemistry of Inorganic and Organometallic Compounds
177
crystal lattices,62and on the magnetically induced circular emission of CrS+doped magnesium Molybdenum.-E.s.r. spectra have been recorded after the photolysis of several potassium diperoxomolybdates at low temperature^.^^ Manganese.-No excited state could be observed following picosecond laser photolysis of Mn0,- at room temperature, implying either that the excited state is very short-lived (T < 3 ps), or that its absorption is masked by that of the ground state.20 A report of reduction following irradiation of solid-state K[Mn(cydta)],3Hz0 (H,cydta = trans-1 ,ZcycIohexylenedinitrotetra-acet ic acid) has been noted .6Q
-
Rhenium.-As previously reported, photolysis of in acetonitrile leads to rupture of the quadruple Re-Re bond (equation 10). The reaction quantum [Re2C18]2-
+ 2MeCN
2[ReC14(MeCN),]-
(10)
yield is wavelength dependent, and irradiation in the longest wavelength band (Amx = 680 nm) causes no photocleavage. The results of laser flash-photolysis experiments with and [Re2Br8I2-in dichloromethane and acetonitrile have now been communicated.66 For [RezClsI2-,a transient (formed with 90% efficiency) having similar absorption spectrum and decay characteristics in either CH,Cl, or MeCN is observed, both following 337 and 615 nm excitation. This is assigned to a 0 ~ 7 7 ~ 6 ~ ( 6excited * ) ~ state, but it could be shown that this species is not that responsible for reaction (10). The authors speculate that the reaction pathway involves a halide-bridged intermediate formed from upper-excited states. Another report confirms that the long wavelength band of [Re,C1,I2- is indeed due to a 8-6* tran~ition.~' Iron.-Although the literature concerned with the photoredox chemistry of FeIrl complexes is very substantial, the precise nature of the primary photochemical processes has been, and still remains, a matter of controversy. This year, for example, the photoreduction of FeCl, or Fe(ClO,), in aqueous media is the subject of apparently contradictory reports.68,59 However, comparison of separate investigations is not easy as the course of the reaction is very dependent on the conditions used. Thus the quantum yield for Fe" production has been found to depend on the Fe"', Fell, and chloride ion concentrations, on pH, on excitation wavelength, on time of irradiation, and on the presence of radical scavengers (intended or accidental).68 Some of these problems arise because of 61
E. A. Solov'ev, G. P. Tikhonov, and E. A. Bozhevol'nov, Zhur. priklad. Spektroskopii, 1975, 23,434.
6a 63 64
s6
W. F. Coleman, J. Luminescence, 1975, 10, 72, 163. R. A. Shatwell and A. J. McCaffery, Mol. Phys., 1975, 30, 1489. G. L. Smorgonskaya, G. A. Bogdanov, G. L. Petrova, and M. V. Savina, Zhur. obshchei Khim., 1975,45, 2745. T. Takeuchi and A. Ouchi, Nippon Kagaku Kaishi, 1975, 7 , 1175 (Chem. Abs., 1975, 83, 106 150).
67 68 69
R. H. Fleming, G . L. Geoffroy, H. B. Gray, A. Gupta, G. S. Hammond, D. S. Kliger, and V. M. Miskowski, J. Amer. Chem. SOC.,1976, 98, 48. F. A. Cotton, B. A. Frenz, B. R. Stults, and T. R. Webb, J. Amer. Chem. SOC.,1976,98,2768. F. David and P. G. David, J. Phys. Chem., 1976, 80, 579. C. H. Langford and J. H. Carey, Canad. J. Chem., 1975, 53, 2430.
178
Photochemistry
the variety of Fe"' species which may be present in the solution, (e.g. [Fe(H20),l3+ (121, [WH20)60H12+(1 3), [Fe(Hzo)4(oH),Fe(H2o)4I4+ (14), and [Fe(H20),C1I2+(15)). Their relative concentrations depend on the pH, and on the concentrations of Fe"' and chloride ion. Further, the quantum yield for the initial process is difficult to estimate, because of the recombination reaction (11). For [Fe(H20),l2+
+ OH*+ H+
-
[Fe(H,0),I3+
+ H,O
(11)
FeCI, solutions at pH = 2.5, David and Davids8 have reported that the quantum yield for Fell production, following irradiation at 350 nm, decreases with chloride ion concentration. They therefore suggest that under these conditions the primary process is (12) and not (13). Langford and Carey60 have [Fe(H2O),OHl2+ [Fe(H,0),C1]2C
+
+ OH* [Fe(H20),l2+ + Cl.
[Fe(H20),12+
hv
(12) (1 3)
used t-butanol as a scavenger for hydroxyl radicals and chlorine atoms. (Other alcohols exhibit more complex behaviour-see below.) By this means and by selective excitation of the species concerned, they have determined the quantum yields for steps (13) and (14) to be 0.093 (at 350 nm) and 0.065 (at 254 nm) respectively. These values have been derived assuming that the mechanism for [Fe(H,O),],+ decomposition, under the conditions used, is as shown in steps (14)-(16), while that for [Fe(H20),Cl]a+ involves reactions (13) and (17)--(19). [Fe(H2o),I3+
+ MqCOH *CH,CMe,OH + Fe3+ C1* + Me,COH c1- + c1C1;- + [Fe(H20)s]2+ OH*
-
[Fe(H,0),l2+
+ OH* + H+
+ *CH,CMe,OH Fe2+ + H+ + HOCH,CMe,OH
H,O
HCI
+ -CH,CMe,OH
a,*[Fe(H2O)&1I2+
(14) (1 5 )
(16) (17)
(1 8)
+ C1- + H,O
(19)
The authors also remark that the apparent contradictions on the nature of the primary photochemical step in the literature arise partly because of neglect by other authors of steps (18) and (19). For certain scavengers (methanol, 2-propanol, formic acid, but not t-butanol) the yield of Fe" increases linearly with the concentration of the organic compound.eo This behaviour is ascribed to outersphere oxidation by the CT states of (12)-(14) of non-co-ordinated scavenger. Very interesting results, which may be relevant to the above discussion, have been reported by Plyusnin and Bazhin for the low temperature (77 K) photolysis of Fe'II in the presence of high concentrations of bromide ion.g1 Under these conditions, the species present are FeBr, and [FeBr4]-. The effects observed in frozen acidic aqueous solutions are quite different from those found in rigid alcohol glasses. In the first case Br2*-is formed, whereas in the ethanolic solutions this species is not produced, but [FeBr,12- and alcohol radicals may be 1o
J. H. Carey and C. H. Langford, Canad. J. Chem., 1975,53, 2436. V. F. Plyusnin and N. M. Bazhin, Khim. vysok. Energii, 1974, 8, 316.
Photochemistry of Inorganic and Organometallic Compounds
179
identified after irradiation. These results suggest that no bromine atom actually leaves the co-ordination sphere of the iron atom. It is proposed that after the initial photo-induced charge transfer, oxidation of species present in the second co-ordination sphere takes place, leading to the observed results. Photolysis (350-600 nm) of aqueous solutions of ferric bromide, under conditions where the main species present is [Fe(H20)6Br]2+,produces bromine with a quantum efficiency of 7.5 x lo-, at room temperature.62 Cox and Kemp have studied the e.s.r. spectra of radicals formed on photooxidation of alcohols, carboxylic acids, amides, and ketones by ferric chloride and ferric perchlorate at 77 K. The behaviour of these systems is in general similar to that observed for CeIV photo-oxidations. As identical results are obtained with ferric chloride and ferric perchlorate, it is presumed that the ferric chloride photo-oxidations in these experiments are not induced by chlorine atoms. Other authors have described the e.s.r. investigations of the photo, ~ ~of cellobiose 65 by ferric ions. oxidation of a l c ~ h o l sand Photolysis of hydrated ferric perchlorate in acetonitrile solution apparently yields FeIV compounds, as the products obtained when this reaction is carried out in the presence of cyclohexanol parallel those found when [Fe0I2+is generated by other means.66 Irradiation of (16) in the presence of anhydrous ferric perchlorate in acetonitrile gives (17)-(19).67 The ratio of (18) to (19) is very different
from that observed with other free radical initiators, suggesting that the stereochemistry of the products is controlled by co-ordination of the radical to the iron atom. Other reports on the photolysis of FeIII compounds in the presence of organic reagents describe the aerobic photodegradation of Fe"'(edta) 68 and Fe"'(nitri1otriacetate) 60 chelates, the photochemical reactions of Fe"'(citrate) cornplexe~,~~ the light-induced formation of radicals on irradiation of FeCI, in the presence of methacrylate esters 71 and he~-l-ene,'~ and the initiation of photopolymerization of methylmethacrylate by a Fe"l-triethy1enetetramine-carbon tetrachloride mixture.', 6a
6a 64 66
S.-N. Chen, N. N. Lichtin, and G. Stein, Science, 1975, 190, 879. A. Cox and T. J. Kemp, J.C.S. Faraday I, 1975, 71, 2490. 0. Hinojosa, J. A. Harris, and J. C. Arthur, Carbohydrate Res., 1975, 41, 31. E. Y. Davydov, G. B. Pariiskii, and D. Y. Toptygin, Zzuest. Akad. Nuuk S.S.S.R.,Ser. khim., 1974, 1747.
67
68
'O
71 7a
73
J. T. Groves and W. W. Swanson, Tetrahedron Letters, 1975, 1953. J. T. Groves, Tetrahedron Letters, 1975, 3113. H. B. Lockhart and R. V. Blakeley, Enoiron. Sci. Technol., 1975, 9, 1035. R. J. Stolzberg and D. N. Hume, Enuiron. Sci. Technol., 1975, 9, 654. N. A. Kostromina, N. V. Beloshitskii, and V. F. Romanov, Koord. Khim., 1975, 1, 1367. A. A. Nosonovich, S. V. Sogonova, S. Y. Kuchmii, L. E. Mazur, V. P. Sherstyuk, and A. I. Kryukov, Ukrain. khim. Zhur., 1975,41, 1330. A. I. Kryukov, S. Y. Kuchmii, A. V. Korzhak, and Z. A. Tkachenko, Doklady Akad. Nauk S.S.S.R., 1975, 222, 882. Y . Inaki, M. Takahashi, and K. Takemoto, J . Macromol. Sci., 1975, A9, 1133.
180
Photochemistry
Transient ground-state bleaching has been observed following picosecond laser photolysis of [ F e ( ~ h e n ) ~and ] ~ + [Fe(bi~y)~]~+.~O This has been assigned to either ligand field state. a 3(MLCT), lq,, or 3T1B Irradiation in the ligand-field bands of [Fe(CN),C0I3- causes reaction (20) to take place with a quantum yield of 0.90.74CT band excitation also induced CO [Fe(CN),C0I3-
+ H,O A
[Fe(CN),(H,0)]3-
+ CO
(20)
expulsion, although at present it is not clear whether this is due to non-radiative conversion to the reactive ligand-field excited state or to photoelectron ejection with consequent production of [Fe(CN),C0I2-, which is expected to release CO spontaneously. In another study it has been confirmed that on laser flash photolysis the ejection of an electron from [Fe(cN),l4- occurs by a single photon process.76 U.V. light accelerates the evolution of hydrogen from ferrous hydroxide gels, apparently by causing the disproportionation of the Fe(OH), to elemental iron.’, Ruthenium and Osmium.-Reports on the photochemistry of ruthenium complexes continue to be dominated by those dealing with [Ru(bipy),12+. This arises because of the remarkable properties of its lowest excited state, which luminesces in solution at room temperature and which is a very strong reducing agent. Although it is commonly classified as a triplet (d-n*) MLCT state, Crosby and co-workers have previously warned about the inaccuracy of this ‘spin label’, because of the dominant role of spin-orbit coupling. This research group has now published a series of detailed articles on the CT excited states of [Ru(bipy),12+ and related c ~ m p l e x e s . ~ ~From - ~ @ consideration of the lifetime and quantum yield for the emission at low temperatures, it may be deduced that the lowest excited state consists of a manifold of three energy levels. The second of these (the E level) lies about 10 cm-l above the first (the A l level), but decays approximately ten times faster, while the A 2 level, which is about 60 cm-1 above the Al, decays about 250-300 times more rapidly than it. The low values for the energy gaps between these levels is an indication of the large separation of the promoted electron from the metal, a feature which is apparent in the role of these states as strong reducing agents. The absorption of this lowest excited state of [Ru(bipy),12+has been monitored both in water and in acetonitrile solution following pulsed laser excitation at 265, 353, and 530nm.80 From a determination of its extinction coefficient, and by quenching with retinol, it has been shown that the quantum yield for formation of the excited state is 0.5 k 0.1 (excitation at either 265 or 353 nm). This contrasts with the previously accepted value of 1.O. The excited state of [Ru(bipy),I2+ could not be detected in a picosecond laser flash photolysis study,20presumably 76
76 77 78 78 *O
A. Vogler and H. Kunkely, 2. Naturforsch., 1975,30b, 355. U. Lachish, A. Shafferman, and G. Stein, J . Chem. Phys., 1976,64, 4205. G. N. Schrauzer and T. D. Guth, J . Amer. Chem. SOC.,1976,98, 3508. G. D. Hager and G. A. Crosby, J . Amer. Chem. SOC.,1975,97,7031. G.D.Hager, R. J. Watts, and G. A, Crosby, J. Amer. Chem. SOC.,1975, 97, 7037. K.W. Hipps and G. A. Crosby, J . Amer. Chem. SOC.,1975,97,7042. R. Bensasson, C. Salet, and V. Balzani, J. Amer. Chem. SOC.,1976,98,3722.
Photochemistry of Inorganic and Organometallic Compounds
181
because the excited state absorbs only weakly in the wavelength region (550650nm) available under the conditions of the experiment. However, the nonphosphorescent MLCT excited state of [Ru(bipy),(MeOH)J2+ was detected and its lifetime shown to be 620 ps. Interest in the photo-induced electron transfer reactions of ruthenium(I1) complexes continues this year. One of the most notable papers is that by Whitten
=O
Figure 3 Surfactant compounds used for the photochemical cleavage of water (Reproduced by permission from J. Amer. Chem. Soc., 1976, 98, 2337)
and co-workers on the properties of monolayers of the surfactant molecules shown in Figure 3.8 The absorption and emission properties of these monolayers on glass slides are very similar to those of [Ru(bipy),12+in solution. However, immersion of the slide in water completely quenches the luminescence of the complex, although addition of water has no effect on its emission in dioxan solutions. Further, irradiation of the monolayer assembly immersed in water with Pyrex-filtered light from a medium-pressure mercury lamp leads to the evolution of molecular hydrogen and oxygen (0E 0.1). The photosensitized decomposition of water appears to cause little permanent damage to the monolayers as over a thousand molecules of gas are evolved per molecule of complex. Recent work has demonstrated, however, that monolayers of highly purified substrates are inactive in inducing water photolysis. Although the mechanism for the process has not yet been elucidated, it is quite probable that the corresponding Ru"'
182
Photochemistry
-
complex is involved. Indeed, Creutz and Sutin 81 have shown that [Ru(bipy),],+ oxidizes OH-, resulting in the evolution of oxygen [equation (21)]. This reaction
+
[R~(bipy)~],+ OH-
[Ru(bipy),12+
+ 40, + Hf
(21)
exhibits a marked pH dependence, peaking in efficiency (80%) at pH9. Investigations using pulse radiolysis and stopped-flow methods suggest that species of the type (20) might be important.
(20)
[Ru(bipy),12+has been found to act as a photocatalyst for the oxidation of Fe2+to Fe3+by molecular oxygen.82 The quantum yield for the reaction depends upon the pressure of oxygen and upon the effective hydrogen ion concentration, but is insensitive to the concentration of Fe2+. The mechanism proposed for the reaction is that detailed in steps (22)-(28). While the electronic structure of (21) *[Ru(bipy),12+
+ 0, (21)
+ (21) + (21) [Ru(bipy),13+ + *02H [Ru(bipy),I3+ + Fe2+ 3H+ + *02H+ 3Fe2+ H+
H+
___+
*[Ru(bipy),.O2I2+ (21)
(22)
+ [Ru(bipy),P+ +
[Ru(bipy),I2+
+ [ R ~ ( b i p y ) ~ ]+ ~+ [Ru(bipy),12+ + 3Fe3+ + 2H,O [Ru(bipy)J2+
is not established, it is presumed to be a cage complex of [Ru(bipy),],+ and 0,-, although the extent of charge transfer need not necessarily be so great. As was mentioned in Section 1, electron transfer appears to be a quite general route for the intermolecular deactivation of the excited states of ruthenium(r1) complexes. Thus this mechanism has been shown to be operative in the quenching of [Ru(bipy),I2+, [Ru(phen),12+, [Ru(terpy)(bipy)(NH,)l2+ (terpy = 2,2',2"terpyridine), or [Ru(bipy),(CN)J by Fe3+ or paraquat.13 One of the most notable observations this year has been that [Ru(bipy),12+ excited state may undergo both reductive and oxidative quenching 149 l5 (see Table 1). Lin and Sutin 83 have communicated the results of experiments on the quenching of the MLCT excited states of [Ru(bipy),12+and [Os(bipy),12+ by oxygen, 8a
8s
C. Creutz and N. Sutin, Proc. Nat. Acad. Sci. U.S.A., 1975,72, 2858. J. S. Winterle, D. S. Kliger, and G. S. Hammond, J. Amer. Chem. SOC.,1976, 98, 3719. C.-T. Lin and N. Sutin, J. Phys. Chern., 1976, 80,97.
Photochemistry of Inorganic and Organornetallic Compounds
183
Fe3+, [Co(phen),13+, [ R u ( N H ~ ) ~ ]and ~ + , [Fe(CN)6]3-.83 It was found that the osmium complex is quenched 60-100% faster than its ruthenium analogue, probably because it is a better reducing agent. Steady-state photolysis of [Ru(bipy),12+in the presence of Fe3+ leads to the build-up of [Ru(bipy),],+ and Fez+. By determining this concentration as a function of irradiation intensity, it is possible to estimate k29/k30 (= 2.6 x lo3 at 25 "C).As the value of k30 is *[Ru(bipy),Ia+ [Ru(bipy)J3+
+ Fe3+ + Fe2+
-
___+
[Ru(bipy),13+
+ Fe2+ +
[ R ~ ( b i p y ) ~ ] ~ +Fe3+
(29) (30)
known from stopped-flow studies, it may be calculated that kze is 1.9 x log dm3 mol-ls-l. A photogalvanic cell has been constructed in which the cathode compartment containing a [Ru(bipy),12+-Fe3+ mixture is irradiated. This cell gives a current output comparable to that of the well-known iron-thionine system. Photo-induced electron-transfer reactions may be utilized in the study of highly reactive oxidized and reduced species such as organic radical-cations and anions.s4 For example, the recombination reaction (3 1) has been monitored following reduction of paraquat (P") (1) by flash-excited [Ru(bipy),12+, and subsequent oxidation of triphenylamine by the Ru"' complex [equations (32) and (33)]. NPh,'++ P'+ + NPha Pa+ (31)
+ P2+ + NPh,
*[Ru(bipy),12+ [Ru(bipy),13+
-
+
+ + NPh,'+
[ R u ( b i p ~ ) ~ ] ~ +P'+
(32)
[Ru(bipy),12+
(3 3)
Electron transfer from *[Ru(bipy),12+to the conduction band of SnO, gives rise to anodic photocurrents.8S The photocurrents may be quenched by Fe3+. No electron transfer from the valence band of semiconductors such as S i c to * [ R ~ ( b i p y ) ~could ] ~ + be detected. At least 95% of the quenching of *[Ru(bipy),lz+ by [Cr(bipy),13+proceeds via electron transfer, even though energy transfer would be energetically favourable.16 [R~(NH,)~(pyrazine)]~+ reacts with Cu2+ in solution to form compound (22).86 Flash-photolysis studies have indicated that this species undergoes
photo-induced electron-transfer forming (23), and thermal relaxation back to (22). The postulate that this process occurs by an inner-sphere mechanism is supported by the observation that [Ru(NH&,(~~Ac)]~+ (pyAc = 4-acetylpyridine) is not reversibly oxidized even in the presence of 0.35 mol dm-, Cu2+, although its spectroscopic properties are similar to those of [R~(NH~)~(pyrazine)]~+.
86
R. C. Young, T. J. Meyer, and D. G. Whitten, J . Amer. Chem. Soc., 1975, 97, 4781. M. Gleria and R. Memming, Z . phys. Chem. (Frankfurt), 1975, 98, 303. V. A. Durante and P. C. Ford, J. Amer. Chem. SOC.,1975,97, 6898.
184
Photochemistry
Irradiation of [R~(bipy),(N~)~]+ causes photoreduction of the complex [equation (34)].*' Although the lowest excited state has LMCT character, no free azide radical could be trapped by acrylamide. In a second light-induced
step, photosolvolysis of the Ru" complex occurs [equation (35)]. Photolysis of [Os(NZ)(NH3),]CI2has been reported to give [OS(NH,)~C~]~+ following initial photo-oxidation of 0s" to Os"', and subsequent photosubstitution of Nz by CI-.S8 Cobalt.-Photoredox reactions of Co"' complexes and theoretical models for these processes have been discussed in a recent review.8g Endicott and FerraudigOhave presented some new results for the photosubstitution reactions of Co"' and Rh"' complexes following ligand-field band excitation, and on the basis of these, they have critically discussed earlier theories 91 for the prediction of the products and quantum yields of these reactions. The experimental observations in this area are summarized by the authors as: (i) the quantum yields for pure LF band excitation of [Co(NH3),XI2+are in general small but very strongly wavelength-dependent, whereas those for [CO(CN)~X]~and for [Rh(NH3),XI2+are nearly wavelength-independent;(ii) complexes whose reactions are wavelength-independent have their lowest LF states at energies large compared to the activation energy for thermal ligand substitution; (iii) some correlations exist between non-radiative relaxation rates and photosubstitution quantum yields, but photoreactivity does not apparently increase with the lifetime of the excited state; (iv) the quantum yields are functions of medium conditions; and (v) the experimentally determined quantum yields do not correlate with ligand-field parameters such as the tetragonality parameter Dt. Therefore they propose an alternative model for these processes in which the reaction takes place from a vibrationally excited ground state. The role of the thermally equilibrated excited state is to provide a stereospecific distortion, which determines the configuration and momentum of the species when entering the ground state, and in this way it controls the nature of the products and the magnitude of their yields. Thus coupling between the excited state and the ground state may lead to reaction if the vibrational energy in the ground-state modes is greater than the activation energy for the thermal process and if a sufficient component of the momentum of the vibrationally excited system is along the critical reaction co-ordinate. Of course, for such a theory to be of predictive applicability much more detailed knowledge of both ground-state and excited-state potential energy surfaces will be required.
89
G. M. Brown, R. W. Callahan, and T. J. Meyer, Inorg. Chem., 1975, 14, 1915. A. P. Pivovarov, Y . V. Gak, G. I. Kozub, Y . M. Shul'ga, I. N. Ivleva, L. S. Volkova, and Y . G. Borod'ko, Koord. Khim., 1975,1, 1061. J. F. Endicott, in ref. 1, Chapter 3, p. 81. J. F. Endicott and G. J. Ferraudi, J. Phys. Chem., 1976, 80, 949. M. J. Incorvia and J. I. Zink, Inorg. Chem., 1974, 13, 2489.
Photochemistry of Inorganic and Organometallic Compounds
185
[Co(NH3),N3I2+exhibits unusual properties for cobalt(I1I)ammines in that LF band excitation leads to quite high quantum yields for ammonia aquation [equation (36)].g2 Irradiation at other wavelengths induces both substitution hv
[C0(NH3)5N3l2+
hv
[C0(NH3),N3l2+
[Co(NH3),(HzO)N3lZ+-t NH3 Co2+
(36)
+ 5NH3 + QNZ
(37)
[equation (36)] and redox decomposition [equation (37)]. The relative efficiency of these processes is markedly wavelength dependent (Figure 4). A comparison
0.6
0.5
0.4
0.3
0.2
0.1
I
I
I
300
400
X
n.-
1 50 0
1
m
Figure 4 Absorptioii spectrum of [CO(NH,),N,]~+,and quantum yields .for photoaquation [reaction (36)] (0) and photoredox [reaction (37)] (0) processes as a function of wavelength (Reproduced by permission from J. Amer. Chern. Soc., 1975,97, 6406)
of the photochemical properties of [Co(NH3),NCSI2+with those of [Co(NH,),N3I2+is instructive as both the ligand field strength and oxidation potential of NCS- and N3- are similar. However, the reactions observed for [Co(NH,),NCSI2+ are NCS- aquation and photoredox decomposition but not ammonia aquation. The quantum yields for Co" formation and those for the aquation reaction are greater for [ C O ( N H , ) ~ N ~ than ] ~ + for [Co(NH3),NCSlZ+,and this feature has been attributed to the sulphur atom increasing the rates of radiationless deactivation processes, possibly by a heavy-atom effect. The quantum efficiencies for Co" production are also markedly dependent on the excitation wavelength and the solvent (Figure 5). In particular, photolysis at h < 280 nm in glycerol-water solutions results in a pronounced increase in the quantum 82
G. J. Ferraudi, J. F. Endicott, and J. R. Barber, J. Amer. Chem. Soc., 1975, 97, 6406.
186
Photochemistry
yield for Co", and this effect appears to be quite general for compounds of the type [CO(NH,),X]~+,being found for X = N3, Br, C1, and NCS. Evidence has been presented to demonstrate that this phenomenon is due to photo-oxidation of the solvent. The primary process is presumed to be similar to that shown in
0.3
t
10
20
30 Excitation Energy I k K
40
50
Figure 5 Variations in the quantum yields for Co" production as a function of excitation energy for [Co(NH,),N3I2+ and [Co(NH3),NCSI2+in water ( 0 and A respectiuely) and in 50% water-glycerol (+ and 0 respectively) (Reproduced by permission from J. Amer. Chem. SOC.,1975, 97,6406)
[Co(NH,),NCSI2+ (24)
(24) H,O+
+ -OH + NCS*OH + R1R2CHOH R ~ R ~ ~+O H
H+
~0111
hv
___+
([Co(NH,),NCS]+, H,O+) (24)
(38)
[Co(NH,),NCSI2+
(39)
+ 5NH3 + NCS- + H,O+ *OH + H+ H,O + NCS. H 2 0 + R1R2eOH R~R~C=O + H+ + CO" Co2+
(40) (41) (42)
(43)
(44)
equation (38) for [CO(NH,)~NCS]~+, and secondary reactions such as those represented in equations (39)-(44) are predicted (R1R2CHOH = glycerol). Excitation of the CT bands (330 nm) of [Co(NH3),SCN12+ causes both photoredox (a = 0.48) and photo-isomerization reactions (@ = 0.24).93
8s
A. Vogler
and H. Kunkely, Inorg. Chim.Actu, 1975, 14, 247.
Photochemistry of Inorganic and Organometallic Compounds 187 Photolysis in the solid state leads exclusively to isomerization, and no Colt could be detected. By analogy with the case of [CO(NH,),NO,]~+,the isomerization is presumed to take place in the initially formed radical pair. The spectra of [CO(CN)~X]~and [CO(NH,),X]~+exhibit CT bands separated In~ contrast with this, the threshold by only between 2000 and 4 0 0 0 ~ m - l . ~ energy for photoredox activity in [Co(CN),N3I3- is 7000 cm-l higher than that in [ C O ( N H , ) , N ~ ] ~ This + . ~ ~apparent anomaly may be rationalized when account is taken of the spin state of the Co" fragment formed.e6 In the case of the cyanocomplex this will be the low-spin (doublet) state which correlates with the lCT state. However, for the Co" ammine complex the high-spin (quartet) species is the stable state, and as this correlates with the reactive ,CT state, a low-energy pathway for the reaction is available. This difference accounts for the experimental observations. Although medium effects have been considered for photoredox processes, much less work has been carried out with photosubstitution reactions. An interesting study has now been reported by Scandola et al., for the photoaquation of [Co(CN),I3- [equation (46)].Q6The equation is known to proceed via
dissociation of the TIg LF state. Examination of the variation in quantum yield for the reaction in a number of alcohol-water mixtures reveals a strong correlation with solvent viscosity but no apparent dependence on the percentage water or dielectric constant (Table 2). This viscosity effect is attributed to the solvent
Table 2 Solvent efects on [ c O ( c N ) 6 I 3 - photo-aquation Q6 DieIectric % Alcoholic Solvent
Water Methanol-wa ter G1ycerol-water Ethanol-water Glycerol-water 1 ,Zpropanediol-water Glycerol-water
soIvent 60 20 60 40 60 60
constant 78 54 73 50 67 57 60
Viscosity 1 .o 1.7 2.0 2.6 4.7 5.3 14.7
Photo-aquation
a)
0.31 0.27 0.24 0.25 0.17 0.1 4 0.10
preventing diffusive escape of the reaction pair formed after dissociation of the excited state. As the authors point out, this role of the solvent cage may cause the measured quantum yield to differ substantially from the primary value for bond cleavage. The photosolvation of [Co(CN),13- has also been studied in a variety of organic solvents (methanol, ethanol, acetonitrile, dimethylformamide and ~ y r i d i n e ) .In ~ ~all cases the quantum yield was in the range 0.28-0.32. The importance of the solvent cage has also been emphasized in a report on the photo-anation reactions of [CO(CN),(H,~)]~-with I-, N3-, HNs, and O4
O6 O7
V. M. Miskowski and H. B. Gray, Inorg. Chem., 1975, 14,401. J. F. Endicott and G. J. Ferraudi, Inorg. Chem., 1975, 14, 3133. F. Scandola, M. A. Scandola, and C. Bartocci, J. Amer. Chem. SOC.,1975, 97, 4757. K. Nakamura, K. Jin, A. Tazawa, and M. Kanno, Bull. Chem. SOC.Japan, 1975,48, 3486.
188
Photochemistry HCN.98 With the anions as reactants, it appears that five-co-ordinate [Co(CN)JZhas a definite existence, whereas with the uncharged compounds, an interchange mechanism between water and the HX species in the second co-ordination sphere is assumed. Photosubstitution of [Co(CN),IS- by HzO and OH- is a convenient synthetic route to the corresponding complex [CO~~'(CN),X,-,]~- (m = 3 or 4, X = OH or H20).9gPhotochemical isomerization of [Co(CN),(H,O),]- favours the formation of the thermally less stable trans-compound (e.g. at pH = 3, @ = 313 nm, @)cje*ans = 0.30, @)ttonlrcje = 0.04).100In contrast with the thermal trans-cis isomerization, the photochemical interconversion proceeds via a nondissociative twist mechanism. A theory to predict the course of this reaction has been developed by Burdett.21 Several publications on the photochemistry of [Co(phen),(ox)]+ and [Co(bi~y)~(ox)]+ have been p ~ b l i s h e d . ~ Langford ~ ~ - ~ ~ ~and co-workers lol have shown that LF band excitation of [Co(phen),(ox)]+ I- in acidic solution causes reaction (47) to take place. The presumed mechanism is represented in equations (48)and (50). Under the acidic conditions employed, the Co" products of steps 2[Co(phen),(ox)]+
-
* [Co(phen),(ox)]+ A [Co"(phen),(ox-)]+ [Co(phen),(ox)]+
+
OX*-
2C02+
+ 4phen + ox2- + 2C02
[Co1Yphen),(ox*)I+
(47)
(48)
[Co(phen),l2+
+ OX*-
(49)
[Co(phen),ox]
+ 2COZ
(50)
(49) and (50) decompose to give the overall stoicheiometry of reaction (47). While the reactive excited state cannot be unambiguously identified, the authors favour a ligand-field state (Tlg, 3T,,, or possibly even 6T2g).The quintet species might well be lowest in energy if the excited state is considerably distorted. More details on the outer-sphere redox reaction of [Co(phen),13+ and oxalate ion have been reported.lof Hennig et al. have also examined the redox decomposition of [Co(phen),(ox)]+ and [Co(bipy),(ox)]+ in solution lo2and in the solid state.lo3#lo4In neutral solution on irradiation at X < 400 nm (in bands variously assigned to CT, intra-ligand, or ligand field transitions) they observe that the quantum efficiency of the reaction is both wavelength and counter-ion dependent.lo2 In the solid state, the effect of the anion is such that the quantum yield diminishes in the order C3H,COz- > HC02- > F- > C1- > Br- > I- > Clod-. The reaction appears to proceed via steps (48) to (50), and as evidence for this the oxalate radical has been identified by e.s.r. In a study of the products formed on photolysis of various oxalato-Co"' complexes at low temperatures, a complex identified as a Co"' species has been observed.106 This L. Viaene, J. D'Olieslager, and S. De Jaegere, Bull. SOC.chim. belges., 1976, 85, 89. L. Viaene, J. D'Olieslager, and S. De Jaegere, Znorg. Nuclear Chem., 1975, 37, 2435. loo L. Viaene, J. D'Olieslager, and S. De Jaegere, Znorg. Chem., 1975, 14, 2736. lol C. P. J. Vuik, N. A. P. Kane-Maguire, and C. H. Langford, Canad. J . Chem., 1975,53,3121. lo* H. Hennig, K. Jurdeczka, and P. Thomas, 2. Chem., 1976, 16, 161. lo3 H. Hennig, K. Hempel, and P. Kertscher, 2. Chem., 1975, 15, 491. lo4 H. Hennig, K. Hempel, M. Ackermann, and P. Thomas, 2.anorg. Chem., 1976,422, 65. lo6 A. L. Poznyak, S. I. Arzhankov, and B. A. Budkevich, Doklady Akad. Nauk Belarus. S.S.R., 98
99
1975, 19, 905.
Photochemistry of Inorganic and Organometallic Compounds 189 complex decomposes to form CO" at temperatures above 200K. The triplet biacetyl- or triplet 9-carboxyanthracene-sensitizeddecomposition of [CO(OX),]~has been described.lo6 The process which is several orders of magnitude slower than the diffusion-controlled rate is presumed to proceed via energy transfer.lo6 Other reports deal with the e.s.r. and optical spectra of intermediate species formed on the low-temperature photolysis of cobaltammine carboxylate and chelate c o m p l e ~ e slo* , ~ ~the ~ ~reactions of aromatic compounds and C03'generated from [ C O ( N H ~ ) & O ~ ]the + , ~oxidation ~~ of Alizarin S by hydrogen peroxide catalysed by Co2+formed photochemically from [ C O ( N H ~ ) ~ N O ~ ] ~ + and the accelerating effect of U.V. light on the racemization of [Co(phen),l3+.ll1 Rhodium and Iridium.-The only photochemical reaction following LF band excitation of [Rh(NH3)6L]3+in aqueous solution at room temperature is the photo-aquation reaction (51).l12 The reactive excited state is the ,E species. At
+
CRh(NHa)5L]3+ H 2 0
A
[Rh(NH&(H20)l3+
+L
(51)
77 K in methanol-water glasses no photochemical reaction is observed, but detailed information on the energies and lifetimes of the thermally relaxed 3E states may be obtained. These data are presented in Table 3. It may be noticed
Table 3 Photo-aquationquantum yields and photoemission data for [Rh(NH,) &I3+ '12 Absorption Ligand L
La*lnm
Ammonia 4-Methylpyridine Pyridine
305 302 302 302 300 301 316
3-Chlorop yridine
Benzonitrile Acetonitrile Water
Free ligand PKB 9.3 6.0 5.3 2.8 - 10 - 10
-
Quantum yield at 25 "C 313 nm 0.075 0.091 0.14 0.34 0.35 0.47 0.43
Emission lifetime T at 77 K/(ps) 18.7 18.6 17.1 13.6 7.6 5.0 2.7
that the change in photo-aquation quantum yield parallels that for the ligand donor ability, although showing no correlation with parameters such as the absorption maxima in the absorption spectra. A relationship is also apparent between the photoreaction efficiency and the rate constant for non-radiative transition (/in). (As the emission is weak, T = l / k n . ) Of the possible mechanisms for the photosubstitution reactions the authors give serious consideration to the hot ground-state mechanism proposed by Endicott.DOHowever, no way of distinguishing between this and dissociative reaction of the excited state is obvious at present. S. Sakuraba, A. Kakuta, and R. Matsushima, Bull. Chem. SOC.Japan, 1975,48,2660.
lo6
lo7
A. L. Poznyak and V. V. Pansevich, Vestsi Akad. Navuk Belarus. S.S.R., Ser. khim. Navuk, 1975, 50.
A. L. Poznyak and S. A. Arzhankov, Doklady Akad. Nauk Belarus. S.S.R., 1975, 19, 439. log S.-N. Chen, M. Z. Hoffman, and G. H. Parsons, J. Phys. Chem., 1975,79, 1911. l10 S. D.Varfolomeev, S. V. Zaitsev, T. E. Vasil'eva, and I. V. Berezin, Doklady Akad. Nauk S.S.S.R., 1974, 219, 895. ll1M.Yamamoto and Y. Yamamoto, Inorg. Nuclear Chem. Letters, 1975, 11, 691. 112 J. D.Peterson, R. J. Watts, and P. C. Ford, J. Amer. Chem. SOC.,1976, 98, 3188. lo8
190
Photochemistry
The results of the photolysis of trans- and cis-[RhN,X,]+ [N4 = 2-en or cyclam (1,4,871l-tetra-azocyclotetradecane); X = C1, Br, or I] have been communi~ated.ll~ ?~~ ~ and compared with those of earlier reports on these syst e m ~11.6 ~With ~ ~the~ exception of cis-[Rh(en),Cl,]+, halide photo-aquation is the only process occurring. This reaction proceeds with retention of configuration. Quantum yields for the trans-complexes lie in sequence 0 1 - > @ ) B ~ - > @cI-, whereas the reverse order holds for cis-[Rh(cyclam),X,]+. In contrast with a previous report,l16 quantum yields are higher for CT than for LF band excitation. For cis-[Rh(en),Cl,]+ both ethylenediamine and chloride ion aquation were observed, although determination of the quantum yields was hampered by competing dark reactions. LF band photolysis of [RhC1613- yields [RhC1,(H2O)l2- with a quantum yield of 0.024.l'' This observation supports the hypothesis that for Rh"' complexes with weak-field ligands the photochemical reactions will proceed inefficiently. For photoredox reactions of Rh"' compounds it is important to understand more about the lability of the powerfully reducing Rh" species formed. This problem has been studied by following the decomposition of [Rh(NH3),C1]+, [Rh(NH3),(H20)I2+, and [Rh(NH3)4Br2]formed on electron capture by the corresponding Rh"' complex after pulse radiolysis.lls In all cases loss of two ligands occurs very rapidly to yield [Rh(NH3)J2f, which exchanges NH, molecules only on a millisecond time scale. Photochemical isomerization of L,RhHX, (L = tertiary phosphine or arsine ; X = Br or C1) has been described.ll@A mechanism involving dissociation of the arsine or phosphine ligand seems most probable. Emission from thermally non-equilibrated levels of [IrCl,(phen)(4,7-Me2phen)]Cl and [IrClz(phen)(5,6-Me,phen)]Cl has been analysed in 121 From these studies it is possible to derive selection rules for non-radiative transitions. These may be summarized as dn* -f dn*, or mm* + mm* but not dn* -+ mn*, and they are particularly strict when the energy gaps between the states are small, or when the states are localized on different parts of the molecule. With [IrCl,(phen),]Cl and [IrC1,(5,6-Me2phen)]C1at 77 K, emission is from the dn* (MLCT) and mn* states, respectively.lZ2However, at higher temperatures dd* emission predominates in both cases. This observation is ascribed to slow, thermally activated, radiationless transition between the CT- or ligand-centred states and the dd* (LF) states. Caution must therefore be exercised in assuming that the state emitting at low temperatures is the same as that involved in photochemistry under ambient conditions. J. Sellan and R. Rumfeldt, Canad. J . Chem., 1976, 54, 519. J. Sellan and R. Rumfeldt, Canad. J. Chem., 1976, 54, 1061. C. Kutal and A. W. Adamson, Inorg. Chem., 1973, 12, 1454. 116 M. M. Muir and W.-L. Huang, Inorg. Chem., 1973, 12, 1831. 11' N. A. P. Kane-Maguire and C. H. Langford, Inorg. Chim. Acra, 1976, 17, L29. n8 J. Lilie, M. G. Simic, and J. F. Endicott, Inorg. Chem., 1975, 14, 2129. llS C. E. Betts, R. N. Hazeldine, and R. V. Parish, J.C.S. Dalton, 1975, 2215. R. J. Watts, M. J. Brown, B. G. Griffith, and J. S. Harrington, J. Amer. Chem. SOC.,1975,97, 6029. 121 R. J. Watts, B. G. Griffith, and J. S. Harrington, J. Amer. Chem. SOC., 1976, 98, 674. 12a R. J. Watts, T. P. White, and B. G . Griffith, J. Amer. Chem. SOC., 1975, 97, 6914. llS
114 ll6
Photochemistry of Inorganic and Organometallic Compounds
191 Nickel.-The photochromism of [Ni(CS,N(CH,Ph),},]+ has been investigated using an n.m.r. detection method.123 The light-induced step (@ z 0.19) is a photoreduction of the NiIVto Ni" fragments [equation (52)]. 2[NiL3]+
+ 2Br-
NiL,
+ NiBr, + L,
(52)
Platinum.-On photolysis in acidic solution, [Pt(NH&I4+undergoes both aquation and reduction, whereas under alkaline conditions the primary products are reported to be [Pt(NH3)5NH2]3+and [Pt(NH3)4(NH2)2]2+.124 Other workers and of [Pt(en)have investigated the photolysis of C~S-[P~B~,(NO,)(NH,),],~~~ (py)(N02)2C1]+.126 Emission spectra of salts of the type K,PtC&-,Br, (n = 0-6) have been Photolysis of Pt" doped silver bromide crystals causes the formation of e.s.r. detectable centres, formulated as [PtBr6]5-.128 Recent studies on luminescent Pt" systems include those on [PtC1412- at 4 K,lagon the emission lifetime of MgPt(CN)4,7H20 as a function of temperature,130 and on the effects of pressure on the emission of various [Pt(CN),I2
salt^.^^^^ 13, Copper, Silver, and Gold.-A report on the photochemical decomposition of bis(diethyldithiocarbamato)copper(zI) has been Copper salts dramatically improve the yield of formylpyrroles produced on irradiation of pyridine 0 ~ i d e s . lTriplet ~~ quenching effects have been ruled out, and a probable explanation is that the copper ion interacts with one of the intermediates formed during the reaction. Further examples of CuI-amine complexes, which exhibit fluorescent thermochroism, have been r e p ~ r t e d . l ~ ~ - ~ ~ ~ Photolysis of silver salts of carboxylic acids provides a convenient route to alkyl ~adica1s.l~~ Silver ions enhance the rate of photopolymerization of N-vinylc a r b a z ~ l e .140 ~~~, A study of the photoreduction of [AuCI,]- by oxalate ion has been reported.141 123
D. P. Schwendiman and J. I. Zink, J. Amer. Chem. SOC.,1976, 98, 1248. R. M. Orisheva, S. P. Gorbunova, and G. A. Shagisultanova, Zhur. neorg. Khim., 1975, 20,
lZ4
1934.
126
128
R. I. Rudnyi, I. F. Golovaneva, 0. N. Evstaf'eva, A. V. Babaeva, and L. I. Solomentseva, Zhur. neorg. Khim., 1975,20, 422. R. I. Rudyi, I. F. Golovaneva, and 0. N. Evstaf'eva, Isuest. Akad. Nauk S.S.S.R.,Ser. khim., 1975, 1480.
V. Lipnitskii, N. M. Ksenofontova, A. B. Kovrikov, V. G. Popov, and D. S. Umreiko, Izvest. Akad. Nauk S.S.S.R., Ser. fiz., 1975, 39, 2241. lZ8 R. S. Eachus and R. E. Graves, J. Chem. Phys., 1975, 63, 83. lZQ H. H.Patterson, T. G. Harrison, and R. J. Belair, Inorg. Chem., 1976, 15, 1461. I3O G. Gliemann, H. Otto, and H. Yersin, Chem. Phys. Letters, 1975, 36, 86. lS1 Y. Hara, Chem. Letters, 1975, 1063. 132 M. Stock and H. Yersin, Chem. Phys. Letters, 1976, 40, 423. 133 K. K. M. Yusuff, P. M. Madhusudanan, and C. G. R. Nair, Current Sci., 1975, 44,221. lS4 F. Bellamy, P. Martz, and J. Streith, Heterocycles, 1975, 3, 395. ls6 H. D. Hardt and H. Gechnizdjani, Inorg. Chim. Acta, 1975, 15, 47. lSe H.D.Hardt and A. Pierre, Naturwiss., 1975, 62, 298. ls7 M. A. S. Goher, Naturwiss., 1975, 62, 237. 138 E. K. Fields and S. Meyerson, J. Org. Chem., 1976, 41, 916. 13s Y.Takeda, M. Asai, and S. Tazuke, Polymer J., 1975, 7 , 366. 140 M. Asai, Y.Takeda, S. Tazuke, and S. Okamura, Polymer J., 1975, 7 , 359. 141 B. S. Maritz, R. V. Eldik, and J. A. Van den Berg, J. S. African Chem. Inst., 1975, 28, 14. lZ7 1.
192
Photochemistry
Mercury.-The light-induced formation of dimethylmercury from inorganic mercury in aqueous acetic acid in the presence of HgO, HgS, or elemental sulphur has been Lanthanides.-The photo-oxidation of tryptophan and methionine in lysozymelanthanum(1n) complexes has been i n v e ~ t i g a t e d .Other ~ ~ ~ authors have utilized the luminescence of Eu"' in studies of its interaction with transfer RNA,144and with pyridoxylidene-amino-acid 146 Energy transfer from Tb3+to anthracene and 9-anthrylmethylketone has been studied in solvents of varying p01arity.l~~ It has been shown that in poor donor solvents, the rate of energy transfer to the 9-anthrylmethylketone is far greater than that to anthracene, because co-ordination to the Tb3+ion is possible with the former compound. Shakhverdov has investigated the quenching of fluorescent organic dyes by lanthanide ions in a number of organic solvents (alcohols, DMSO, and pyridine).148-150The quenching takes place in ion-pairs of the negatively charged dye molecule and the lanthanide ion. Dipole-dipole energy transfer is responsible for the quenching in many cases [e.g. with Nd"' and Hot''], but with some lanthanides the deactivation process probably involves either reversible photoreduction (e.g. with Eu"') or reversible photo-oxidation (e.g. with Ce"'). Energy transfer between the triplet state of coumarinium ion and Eu3+ in alkanesulphonic acid glasses at 77 K,151 between triplet phenanthrene and Ce3+,Pr3+,and Nd3+ in methanol-water solutions at temperatures between 120 and 293 K,g and between benzopyranopyridine derivatives and Eu3+ or Tb3+,152 have been reported. The technique of circularly polarized emission has been employed in the study of Tb"' and Ed1' complexes of optically active carboxylic acids in In the same publication it has been recorded that the efficiency of the quenching of Tb"' emission by Eu"' in solutions of L-malic acid is a function of pH. It is suggested that under certain pH conditions, the transfer of energy occurs between ions complexed to the same malic acid molecule. Intermolecular energy transfer between Tb(acac), and La(acac), or other tris(acety1acetonato)lanthanide complexes has been described.154,155 The rate of transfer is sensitive to the solvent used, being negligible for strongly co-ordinating compounds such as pyridine, but increasingly markedly for non-polar solvents such as benzene. This effect is attributed to the formation of mixed dimers in the non-polar solvent. Quenching of the excited state of U022+ by Eu3+proceeds only in part by energy H. Agaki, Y . Fujita, and E. Takabatake, Chem. Letters, 1976; 1, Nbpon Kagaku Kaishi, 1975, 1273. 14s G. Jori, M. Folin, G. Gennari, G. Galiazzo, and 0. BUSO, Photochem. and Photobiol., 1974, 19, 419. 144 J. M. Wolfson and D. R. Kearns, Biochemistry, 1975, 14, 1436. 145 V. F. Zolin, L. G. Koroneva, and V. I. Tsaryuk, Biofizika, 1975, 20, 194. 146 V. F. Zolin and L. G. Koreneva, Biofizika, 1975, 20, 198. 147 V. L. Ermolaev and V. S. Tachin, Optika i Spektroskopiya, 1975, 38, 1138. 14* T. A. Shakhverdov, Optika i Spektroskopiya, 1975, 38, 1228. us T. A. Shakhverdov, Optika i Spektroskopiya, 1975, 39, 786. lSo T. A. Shakhverdov and Z . N. Turaeva, Izvest. Akad. Nauk S.S.S.R.,Ser. fiz., 1975,39, 1952. lS1 P. G. Tarassoff and N . Filipescu, J.C.S. Chem. Comm., 1975, 208. lS2 A. Fujimoto, A. Sakurai, and E. Iwase, Bull. Chem. SOC.Japan, 1976, 49, 809. lSs C. K. Luk and F. S. Richardson, J. Amer. Chem. SOC.,1975, 97, 6666. lS4 G. D. R. Napier, J. D. Neilson, and T. M. Shepherd, J.C.S. Faraday IZ, 1975, 71, 1487. J. D. Neilson and T. M. Shepherd, J.C.S. Faraday ZZ, 1976, 72, 557.
142
Photochemistry of Inorganic and Organometallic Compounds 193 transfer; the rest of the deactivation occurs via some other unidentified pathway.lS6 The first report of fluorescence lifetime studies of a rare-earth chelate in the gas phase [Tb(ButCOCHCOBut),] indicates that the lifetime is much shorter than in Thus it decreases from solution and also markedly ternperat~re-dependent.~~~ approximately 1 ps at 235 "C to about 0.2 ps at 290 "C. The mechanism proposed requires intramolecular energy transfer from the rare earth ion to the chelate, followed by chelate relaxation. Tb3+Ions luminesce from the SD3as well as the 6D4excited state. (The states are separated by approximately 6000 cm-l.) By using a pulsed laser (265 nm) source for excitation, the 5D3to 5D4interconversion has been investigated both in borate glasses and D 2 0 solution.16* In D,O solution the rate constant for this process is rather low (1.5 x los s-l), and in concentrated solutions it is accompanied by an efficient Tb3+-catalysedinterconversion. In POC1,-SnCI, solution the decay kinetics of the 6D4luminescence of Tb3+ have been found to depend on the wavelength of e ~ c i t a t i 0 n . l This ~ ~ effect is also attributed to the slow 6D3--f 5D4radiationless transition. The rate of non-radiative crossing between excited states has been investigated for Eu"' and Tbl" p-diketone chelates using nanosecond laser excitation.160 For complexes such as Eu(PhCOCHCOPh), where the ligand triplet level lies above the 6D1state, initial excitation into a ligand-localized state is followed by population of both the and the lowest 5D0excited states. The subsequent internal conversion from the level to the state has been monitored.160 Several other reports on the photophysical properties of luminescent rare earth P-diketonates and related chelates have been published.1s1-16D It is well known that the luminescence of rare earth ions is markedly affected by the solvent. In particular, solvents with high-energy vibrations (e.g. O-H) cause rapid deactivation of the excited state. For Nd3+ in solutions of tributyl phosphate it has been demonstrated that small quantities of water quench the luminescence. Stern-Volmer kinetics are obeyed, and from this relationship the rate constant for quenching at 293 K (1.9 x lo6 dm3mol-1 s-l) has been deduced.170 The effect of water on the emission lifetime of Eu"' in acetone solution has been studied, and from the data so obtained it has been possible to 168
Is7 lS8 lS8 160 161
162 163
16*
B. D . Joshi, A. G . I. Dalvi, and T. R. Bangia, J. Luminescence, 1975, 10, 261. R. R. Jacobs, M. J. Weber, and R. K. Pearson, Chem. Phys. Letters, 1975, 34, 80. C. R. Goldschmidt, G. Stein, and E. Wuerzberg, Chem. Phys. Letters, 1975, 34, 408. P. Tokousbalides and J. Chrysochoos, J . Chem. Phys., 1976, 64, 1863. W. M. Watson, R. P. Zerger, J. T. Yardley, and G. D. Stucky, Inorg. Chem., 1975, 14,2675. N. S. Poluektov, I. I. Zheltvai, G. I. Gerasimenko, M. A. Tishchenko, and A. A. Kucher, Zhur. priklad. Spektroskopii, 1976, 24, 276. B. A. Knyazev, V. M. Moralev, and E. P. Fokin, Optika i Spektroskopiya, 1976, 40, 93. M. A. Tishchenko, N. S. Poluektov, and I. I. Zheltvai, Ukrain. khim. Zhur., 1975, 41, 197. T. Fukuzawa, N. Ebara, M. Katayama, and H. Koizumi, Bull. Chem. Sac. Japan, 1975, 48, 3460.
165 166 167
168 169
170
A. P. Aleksandrov, Optika i Spektroskopiya, 1975, 38, 561. G. E. Malashkevich and V. V. Kuznetsova, Zhur. priklad. Spektroskopii, 1975, 22, 230. E. T. Karaseva, A. P. Golovina, M. I. Gromova, and I. P. Efimov, Koord. Khim., 1975,1,260. N. S. PoIuetkov, V. N. Drobyazko, S. B. Meshkova, S. V. Bel'tyukova, and L. I. Kononenko, Doklady Akad. Nauk S.S.S.R.,1975, 224, 150. E. T. Karaseva and V. E. Karasev, Koord. Khim., 1975,1, 926. E. M. Zinina and A. V. Shablya, Optika i Spektroskopiya, 1975, 39, 686.
1 94
Photochemistry
derive the stability constants for the aquo-complexes of Eu"' under these cond i t i o n ~ The . ~ ~lifetimes ~ of various lanthanide tributyl phosphate complexes are lengthened by four to seven times on perdeuteriation of the ligand.17, Other authors have studied the photophysical properties of Eu"' in acetic acidchloroform or carbon tetrachloride solvent^,^^^ the effect of solvent on the spectra of Ed1' chelate and the influence of pulse intensity on the luminescence lifetimes and intensities of rare earth ions in solid matrices and in POC1,SnCl, Actinides.-Kemp and co-workers 176 have carried out a particularly detailed investigation of the interaction of photo-excited UOzz+with a wide variety of substituted carboxylic acids, and also with some esters, alkenes, and unsaturated ketones. A selection of these data is presented in Table 4. It has been observed Table 4 Stern- Volmer constants for the quenching of UOZ2+ emission by carboxylic acids (RC0,H) 176 R K&/dm3 mol-1 R Ksv/dm3mol -l H Me Et cyclohexy1 Ph EtO(CH,),
6.5 0.28 1.28 89 4150 203
CICH, BrCH, ICH, ICH2CHz MeCH=CH CH,=CHCH,
0.15 0.32 3 600 1960
8.74 1600
that the quenching of *UOZ2+ may proceed by a number of mechanisms depending on the substrate. For example, for 10 substituted benzoic acids, a Hammett plot yields a p value of -0.88. This is consistent with formation of an exciplex involving only a small amount of charge transfer. For olefinic compounds the authors also support the exciplex formation mechanism previously proposed by Matsushima for other systems. Iodine-containing compounds (e.g. ICH2CH2C0,H) quench very efficiently, probably due to charge transfer interaction of *U0,2+and the iodine atom of the substrate via a short-lived exciplex. Alkoxycarboxylic acids also cause effective deactivation of the excited UOz2+. However, in this case the quantum yield for UIV is found to be 1.89, suggesting that the principal route for quenching is by chemical reaction. Matsushima and co-workers have examined the UOZ2+-sensitized isomerization of stilbene in more The ratio of trans- to cis-stilbene is sensitive to the concentrations of stilbene and uranyl ion, varying from 0.96 to 3.0. While it has been shown that free radical processes are not important, and that triplettriplet energy transfer within an exciplex is the most probable mechanism, no thoroughly satisfactory explanation for the above phenomenon has been proposed. 171
J73 174
176 176
17'
V. P. Gruzdev and V. L. Ermolaev, Zhur. neorg. Khim.,1975,20, 2650. E. B. Sveshnikova, A. P. Serov, and V. P. Kondakova, Optika i Spektroskopiya, 1975, 39, 285. J. Chrysochoos, Spectroscopy Letters, 1975, 8, 771. L. I. Kononenko, S . V. Bel'tyukova, S. B. Meshkova, V. N. Drobyazko, and N. S . Poluektov, Dopovidi Akad. Nauk Ukrain. R.S.R., Ser. B, 1975, 816. A. V. Aristov, V. P. Kolobkov, P. I. Kudryashov, and V. S . Shevandin, Optika i Spektroskopiya, 1975, 39, 281. M. Ahmad, A. Cox, T. J. Kemp, and S. Quaisar, J.C.S. Perkin IZ, 1975, 1867. R. Matsushima, T. Kishimoto, and M. Suzuki, Bull. Chem. SOC.Japan, 1975, 48, 3028.
Photochemistry of Inorganic and Organometallic Compounds 195 The light-induced reaction of UOz2+and benzaldehyde in aqueous acetone solution gives Urv(0= 0.14) and benzil (0= 0.12), as well as small amounts of condensation products of benzaldehyde and a~et0ne.l'~This is consistent with the initial photochemical process (53). Other authors studying the photo-
+
*U022+ PhCHO
U02+
+ H+ + PhkO
(53)
reduction of uranyl ion by organic conipounds have described the detection of U02+following irradiation of solid uranyl f ~ r m a t e a, ~polarographic ~~ investigation of the photoreduction of U022+in ethanol,lEOand an attempt to induce photoreduction of U02+,prepared by laser photolysis of UOZ2+in ethanol.lE1 As the logarithm of the rate constant for quenching of *U022+by metal ions (Ag+ Ce3+, Co2+, Cu2+, Fe2+, Fe3+, Hg22+,Mn2+, Ni2+, and Pb2+) decreases linearly with increasing ionization potential of the metal ion, it is proposed that an electron-transfer process is operative.17 Possible mechanisms are illustrated in equations (54) and ( 5 5 ) . In the case of Mn2+as quencher, Mn3+was identified 9
after flash photolysis suggesting that at least in some reactions complete electron transfer ( 5 5 ) occurs. Self-quenching of U022+luminescence has been reported ( k , = 4 x lo6 dm3mol-1 s-l).lS2 In the same study it has been shown that the *UOZ2+lifetime is markedly temperature dependent (Eact = 41 kJmol-l), and it is proposed that the deactivation process may involve reversible photooxidation of water. It has been reported that the quenching of *UOZ2+by Ce3+ proceeds via energy transfer and another unidentified radiationless process.156 When mixed, solutions of U022+and Eu2+ he mi luminesce.^^^ The emitting species is formed by disproportionation of the UOz+ ions [reaction (57)]. Further investigation of
this system also revealed that U02+is an efficient quencher of U022+excited Photodimerization of (25) gives (26), whereas in the presence of UOz2+the product is (27).lS5 The stereospecificity is attributed to the bulk of the uranyl group to which one molecule of (25) co-ordinates. R. Matsushima, K. Mori, and M. Suzuki, Bull. Chem. SOC.Japan, 1976, 49, 38. B. Claudel, J. P. Puaux, and H. Sautereau, Compt. rend., 1975, 280, C, 169. 180 M. Feve, Compt. rend., 1974, 279, C, 721. 181 J. T. Bell and M. R. Billings, J . Inorg. Nuclear Chem., 1975, 37, 2529. lSa P. Benson, A. Cox, T. J. Kemp, and Q. Sultana, Chem. Phys. Letters, 1975, 35, 195. lS3 R. G. Bulgakov, V. P. Kazakov, G . S. Parshin, D. D. Afonichev, and G . L. Sharipov, Khim. uysok. Energii, 1975, 9, 92. la4 R. G. Bulgakov, V. P. Kazakov, and S. V. Lotnik, Khim. uysok. Energii, 1975, 9, 555. N. W. Alcock, N. Herron, T. J. Kemp, and C. W. Shoppee, J.C.S. Chem. Comm., 1975,785. 178
196
Photochemistry
PhCH=CHyO
COCH=CHPh
LIZPh
PhCH=CHkO
I
PhCH=CHCO
The experimental difficulties and the theory of laser-induced photochemistry for separation of 235Uand 238Uhave been discussed in a series of report^.^^^-^*^ In an interesting discussion of the quantum properties of the lowest excited state of U022+,it is mooted that the question whether the excited state is singlet or triplet is of little significance because of spin-orbit coupling. Arguments are presented to show that the state has i2 = 4 and is of even parity.lsO Other publications dealing with the spectroscopic properties of Uvl compounds discuss the influence of co-ordination geometry on the lifetime of U022+in crystalline environments,lsl SCF calculations of the electronic structure of UOa2+,ls2and ls4 the fluorescence of UF6.1g3The photochemical reduction of the plutonyl ion (PuO,~+)on irradiation in ethanol produces Pu4+ and Pu3+.lg5As in the case of U022+,the initial photochemical act appears to be one-electron transfer to give Pu02+. 2 Transition-metal Organometallics and Low-oxidation-state Compounds Wrighton lg6and Strohmeier lg7have summarized their groups' contributions to the photochemical generation of catalysts from organometallic compounds.
Titanium, Zirconium, and Hafnium.-Interest continues this year in the photochemistry of titanocene derivatives. Vitz and Brubaker have examined in more detail the photo-exchange reaction (58) reported earlier.198~ lg9 Quantum yields B. B. Snavely, Report UCRL-75725, 1974 (Chem. Abs., 1975, 83, 169 712). A. Hartford, Report UCRL-76601, 1975 (Chem. A h . , 1975, 83, 210 559). B. B. Snavely, R. W. Solarz, and S. A. Tuccio, Report UCRL-76923, 1975 (Chem. Abs., 1976, 84, 113 220). ISB B. B. Snavely, R. W. Solarz, and S. A. Tuccio, Lecture Notes in Physics, 1975, 43, 268. l 9 O C. K. Joergensen and R. Reisfeld, Chem. Phys. Letters, 1975, 35, 441. lgl G. C. Joshi, Indian J. Pure Appl. Phys., 1976, 14, 180. lg2 M. Boring, J. H. Wood, and J. W. Moskowitz, J. Chem. Phys., 1975, 63, 638. lQ3 A. Andreoni and H. Buecher, Chem. Phys. Letters, 1976, 40,237. 194 P. Benetti, R. Cubeddu, C. A. Sacchi, 0. Svelto, and F. Zaraga, Chem. P h p . Letters, 1976, 40, 240. lg6 J. T. Bell and H. A. Friedman, J. Znorg. Nuclear Chem., 1976, 38, 831. 196 M. S. Wrighton, D. S. Ginley, M. A. Schroeder, and D. L. Morse, Pure Appl. Chem., 1975, 41, 671. lg7 W. Strohmeier, J. Organometallic Chem., 1975, 94, 273. IBa E. Vitz and C. H. Brubaker, J. Organometallic Chem., 1976, 104, C33. lD9 E. Vitz, P. J. Wagner, and C. H. Brubaker, J. Organometallic Chem., 1976, 107, 301. lE6
lS7
Photochemistry of Inorganic and Organometallic Compoundr hv
Cp,TiCl, 4- (C6D6)2TiCl2
197
2Cp(C6D6)TiC&
(58)
are wavelength dependent (e.g. at 313 nm, 0 = 0.02; at 520 nm, @ = 0.007). In contrast with the observations of other workers,200no decomposition of Cp,TiCl, to give CpTiCl, could be detected when the solvent was carefully purified. This indicates that formation of cyclopentadienyl radicals is not a major reaction pathway for the excited state, and the mechanism for the exchange reaction is therefore still uncertain. Transfer of cyclopentadienyl ligands between (28) and Cp2TiClzto form the (CH,),-bridged dimer species proceeds only in low yield. Photo-induced interchange reactions similar to (58) have been observed for the Ti"' compound (29) and for Cp,VCl,.les Photolysis of the dialkyl complexes Cp,MR2 (M = Ti, Zr, and Hf) leads to the corresponding, highly reactive, co-ordinatively unsaturated metallocene MCp,. /
,TiCI,
(HzQ
CP
cp\
c1
cp
/ \ /
Ti
Ti
/ \ / \
cp
c1
cp
s-s /,s-s
,s/
Cp,Ti
Irradiation of the dialkyl compound in the presence of some suitable ligand L affords a most convenient route to the derivative Cp,TiL2. Recent examples are the preparation of Cp,Ti(CO), and its bis(7-indenyl) analogue by photolysis of the corresponding dimethyl compound in the presence of C0,,O1 the synthesis of (30) in 70% yield from Cp2TiMe, or Cp,Ti(CH,Ph), with elemental sulphur,202 and the formation of a polymeric bis(fluoreny1)zirconium compound from its dimethyl derivative.203The electronic structure of titanocene and derivatives is of considerable interest to theoreticians and is the subject of recent p ~ b l i c a t i o n s,05 .~~~~ The photopolymerization of styrene in the presence of TiC14, Ti(CH,Ph),, or CpTiClR (R = C1, Et or CPh3) has been monitored both by dilatometry and electrical conductivity measurements.206With Cp,TiClEt and Cp,TiC1CPh3 the polymerization is radical-initiated, confirming that the initial photoprocess is homolytic cleavage of the Ti-C(alky1) bond. For Cp2TiC1, and Ti(CH2Ph)4,no unambiguous mechanism could be proposed. However, for Ti(CH,Ph), the photochemical step is most probably a styrene-to-metal electron transfer within a styrene-Ti(CH,Ph), complex. TiC14, TiBr,, and VCl, are photo-initiators for the polymerization of isobutylene with visible light.207The propagating species are isobutylene radical cations produced on irradiation of the MX4-olefin complex [reaction (59)]. 200
201 2 oa 2 03
B04 206
2 06
2 07
R. W. Harrigan, G. S. Hammond, and H. B. Gray, J. Organometallic Chem., 1974, 81, 79. H. G . Alt and M. D. Rausch, 2.Naturforsch., 1975, 30b, 813. E. Samuel and C. Giannotti, J. Organometallic Chem., 1976, 113, C17. E. Samuel, H. G . Alt, D. C. Hrncir, and M. D. Rausch, J. Organometallic Chem., 1976,113, 331. J. W. Lauher and R. Hoffmann, J . Amer. Chem. SOC.,1976, 98, 1729. J. L. Petersen, D . L. Lichtenberger, R. F. Fenske, and L. F. Dahl, J. Amer. Chem. SOC., 1975, 97, 6433. T. S. Dzhabiev, F. S. D'yachkovskii, and L. I. Chernaya, Izvest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 1091. M. Marek, L. Toman, and J. Pilar, J. Polymer Sci., Part A-1, Polymer Chem., 1975,13, 1565.
198
Photochemistry [Me,C=CH,.MX,]
hv
[Me,C.CH,]’+
+ [MX.,]-
(59)
Vanadium, Niobium and Tantalum.-Emission from salts of [Ta(CO),]- and [Nb(CO),]- {but not [v(Co)&}has been recorded for samples both in the solid state and in rigid Interestingly this behaviour contrasts with that of the isoelectronic Mo(CO), and W(CO), from which no luminescence has been detected. The emission quantum yields and lifetimes are markedly temperature dependent (falling by approximately a hundred-fold between 22 and 100 K), as previously noted for other heavy-atom co-ordination compounds such as ruthenocene. Although the emission is assigned to that from the ‘triplet’ LF (3T1,) state, the apparent lack of distortion in the excited state suggests that it also has substantial (M-r* CO) CT character. Photo-substitution of [V(CO),]by acetonitrile or pyridine in solution proceeds with quantum yields of 0.5-0.6. Chromium, Molybdenum, and Tungsten.-Photosubstitution of carbon monoxide by other ligands continues to be widely used in the syntheses of substituted metal carbonyl compounds (some recent examples are collected in Table 5, p. 219), and for the generation of catalytically active species.1g6The prototype of this reaction has been that involving the Group VI metal hexacarbonyls. Since the pioneer work of Strohmeier, it has been assumed that the quantum yield for the photodecomposition reaction (60) is unity. However, Nasielski and Colas 209 have
now shown that at least in the case of Cr(CO),, the quantum yield is only 0.67. This was measured by allowing the co-ordinatively unsaturated Cr(CO), to be scavenged by pyridine, forming Cr(CO),py. Photolysis of Cr(CO), in low temperature matrices produces Cr(CO), having C,, 211 The visible spectrum of this species is very sensitive to the matrix material (Ne, Ar, Kr, Xe, CF4, or CH,), and this phenomenon has been attributed to a stereospecific interaction between the Cr(CO), and the matrix species. As the absorption bands of these ‘complexes’ are well separated [e.g. Amax Cr(CO),*- Ne, 628 nm; Cr(CO), **. Kr, 487 nm] selective excitation experiments are possible. In these cases, irradiation in mixed matrices (e.g. Xe-Ne) allow ‘ligand‘ exchange reactions such as (61) to be observed. The weak bond Cr(CO)5-.Ne
hv(619 nm) 7 Cr(CO)6--Xe hv(432 nm)
between Cr(CO), and methane in low-temperature matrices suggests that a similar interaction may be present at ambient temperatures for Cr(CO), in solvents such as cyclohexane which are normally considered to be non-coordinating. Cr(CO), may also be synthesized by the reaction of chromium atoms in carbon monoxide or mixed carbon monoxide-argon matrices. It had previously been suggested from i.r. evidence that the trigonal bipyramidal Dah 208
2oB 210
211
M. S. Wrighton, D. I. Handeli, and D. L. Morse, Znorg. Chem., 1976, 15, 434. J. Nasielski and A. Colas, J. Organometallic Chem., 1975, 101, 215. R. N. Perutz and J. J. Turner, J. Amer. Chem. Soc., 1975, 97, 4791. J. K. Burdett, M. A. Graham, R. N. Perutz, M. Poliakoff, A. J. Rest, J. J. Turner, and R. F. Turner, J . Amer. Chem. SOC.,1975,97,4805.
Photochemistry of Inorganic and Organometallic Compounds 199 isomer is also produced under the latter conditions.212 It has now been unambiguously demonstrated that the bands assigned to the species belong to Cr(CO), *- Ar, Cr(CO),-- CO, and Cr(CO)4.211 The structures of Mo(CO), and Mo(CO),, formed by irradiation of Mo(CO), in a methane matrix, have been found to be surprisingly It has long been felt that one of the restrictions in comparing photochemistry in matrices and in solution might be that the rigid environment of the matrix would prevent the escape of bulky ligands from the matrix cage. A recent report 214 on W(CO),L (L = pyridine or 3-bromopyridine) illustrates that matrix photochemistry may parallel that observed in Thus irradiation at long wavelengths (320 c h c 390nm) leads to ligand expulsion [equation (62)], whereas at shorter wavelengths (e.g. 254 nm) CO dissociation [equation (63)] is also observed. As reaction (62) is photoreversible it is probable that, as in other examples of matrix photochemistry, the expelled ligand remains in the second co-ordination sphere of the complex. W(CO),L W(CO),L
-
A hv
+L W(CO),L + co
W(CO),
(42) (43)
An interesting comparison of the photosubstitution chemistry of W(CO),(L-L) (L-L = bipy, phen, 5-MephenYor 5-Brphen) and W(CO),(py), has been published.21e Absorption and luminescence data establish that the lowest excited states of W(CO),(L-L) are of MLCT character, but are of the L F type for W(CO),(py),. It is to these distinguishing features that their differing photoreactivities are ascribed. Thus, irradiation of W(CO),(py), causes pyridine substitution (0z 0.23, essentially independent of A) and very inefficient (0z dissociation of CO. This is consistent with the predicted enhanced M-N bond lability in the LF excited state. On the other hand, the chelate complexes W(CO),(L-L) show only wavelength dependent CO-expulsion, and = 1.6 x 0 3 1 3 = 2.2 x no ligand substitution. [For W(CO),(phen) This suggests that CO-labilization is the reaction of upper excited LF states, and that the MLCT state is essentially unreactive. It is not clear, however, whether the observed lack of amine substitution is due to the nature of the reactive excited states or to chelate effects. The photosubstitution reactions of (arene)Cr(CO), complexes are of considerable interest, in part because the thermal (arene replacement) and photochemical (CO expulsion) processes are so different. An important example of the application of the photochemical substitution of CO in (arene)chromium carbonyls by phosphines is that which led to the first reported isolation of enantiomers of chiral CrO compounds Two reports on the quantum efficiency 21a a13 a14
216
216
a17
E. P. Kuendig and G. A. Ozin, J. Amer. Chem. SOC.,1974, 98, 3820. R. N. Perutz and J. J. Turner, J. Amer. Chem. SOC.,1975, 97, 4800. A. J. Rest and J. R. Sodeau, J.C.S. Chem. Comm., 1976, 696. M. S. Wrighton, Inorg. Chem., 1974, 13, 905; M. S. Wrighton, G. S. Hammond, and H. B. Gray, Mol. Photochem., 1973, 5, 179. M. S. Wrighton and D. L. Morse, J. Organometallic Chem., 1975, 97, 405. G. Jaouen, A. Meyer, and G. Simonneaux, Tetrahedron, 1975, 31, 1889.
200
Photochemistry for CO dissociation have been published 219 For the substitution in (benzene)Cr(CO), by pyridine, the quantum yield is 0.72 (at 313, 366, or 436 nm),218and that in (mesitylene)Cr(CO), by maleimide is 0.90.21QNo evidence was found in either study for exchange of the aromatic ligand. Apparently anomalous behaviour is exhibited by (toluene)Cr(CO), in the presence of cycloheptatriene.220 The reaction found was (64), which is particularly surprising as (toluene)Cr(CO),
+ CHT A
(CHT)Cr(CO),
+ toluene
(64)
its analogue CpMn(CO), reacts to give CpMn(CHT). Two groups of workers have described the photochemical conversion of compounds of type (31) 222 into (32).221* 2
p"" Et
- p"' Et
+
P""
Me
(65)
As noted in last year's Report, olefin metathesis reactions [e.g. (65)] may be catalysed by photolysis of W(CO), in carbon tetrachloride. Flash photolysis of W(CO), in CCl, leads to W(CO),Cl, which is a possible candidate for the active
species in the catalytic process.223 However, other workers favour W(CO)4C12 for this The above process suffers from the disadvantage of requiring high W(CO), concentrations. Warwel and Laarz have recently reported that mixtures of W(CO), and Bu'AICI, are very active photocatalysts at concentrations of 1 part catalyst to 11 300 parts of olefin.226 The mechanism for the photochemically induced 1,6hydrogenation of 1,3-dienes in the presence of Cr(CO), is still not completely elucidated. One of the species which may be involved is the corresponding (diene)Cr(CO)4. Complexes of this type have now been prepared by low-temperature photolysis of Cr(CO), and either 1,3-butadiene or trans-,trans-2,4-he~adiene.~~~ These species are not active as hydrogenation catalysts in the dark, but are readily activated by illumination. This suggests that (diene)Cr(CO), may be the active species. M. S. Wrighton and J. L. Haverty, Z . Naturforsch., 1975, 30b, 254. J. Nasielski and 0. Denisoff, J. Organometallic Chem., 1975, 102, 65. 2 2 0 P. L. Pauson and J. A. Segal, J.C.S. Dalton, 1975,2387. 221 A. N. Nesmeyanov, M. I. Rybinskaya, V. V. Krivykh, and V. S. Kaganovich, J. Organometallic Chem., 1975, 93, CS. 222 W. S. Trahanovsky and R. A. Hall, J. Organometallic Chem., 1975, 96, 71. 223 P. Krausz, F. Garnier, and J.-E. Dubois, J. Organometallic Chem., 1976, 108, 197. 224 A. Agapiou and E. McNelis, J. Organometallic Chem., 1975, 99, C47. 226 S. Warwel and W. Laarz, Chem. Ztg., 1975, 99, 502. 226 I. Fischler, M. Budzwait, and E. A. Koerner von Gustorf, J. OrganomefaZZic Chem., 1976, 105, 325.
218
21B
201
Photochemistry of Inorganic and Organometallic Compounds
Photolysis of CpMo(CO),(NCS) (or its iron analogue) leads to photochemically induced isomerization of the complexed thiocyanate ion [equation (66)].,,' Irradiation of CpMo(CO),(PPh,)X (X = Br or I) causes both cis- to hv
CpMo(CO),NCS
hv
CpMo(CO),(SCN)
(66)
trans-isomerization and disproportionation to give CpMo(CO),X and CpMo(CO)(PPh,),X. In this case the proposed mechanism requires either (or both) photo-induced CO- or PPh,-dissociation. Last year, photochemical syntheses of alkyldiazenido-complexes from Mo(N,),(dppe), [dppe = 1,2-bis(diphenylphosphino)ethane] and alkyl bromides were reported [reaction (67)].22* The scope of this reaction is clearer after Mo(N,),(dppe),
+ RBr A
MoBr(N,R)(dppe),
+ N,
(67)
(33)
investigations of the products formed from various alkylhalides (MeBr, MeI, MeCl, iodocyclohexane, and ~x-bromo-p-xylene).~~~ The iodo-compounds reacted to produce compounds analogous to (33). With the alkyl bromides this type of product was accompanied by MoBr(N,)(dppe), and for methylchloride, MoCl(N,)(dppe), was the sole product. Although the mechanism has not been discussed in detail, it seems probable that the initial photo-process is N2-dissociation. Photolysis of the tungsten analogue in the presence of dibromomethane yields the first diazomethane complex [reaction (68)].230 It was previously proposed
hv
W(N,),(dPP4, + CHzBr, [WBr(N,CH,)(dPPe),lBr + N2 (68) that irradiation of M(N2),(dppe), (M = Mo or W) in THF in the presence of methyl bromide gave (34) after acidification. X-Ray analysis has now shown that the structure of this product is [MBr(N-N=CH(CH2)30H}(dppe),]+Br-.231 P-b
(34)
Other reports mention that the conversion of cis-W(N,),(PMezPh), into NH, in methanol solution occurs on irradiation, as well as on r e f l u ~ i n g , ~and ~ , that photolysis of Mo(dppe),(C,H,) gives a carbon dioxide adduct in the presence of this gas.233 227 228
228 230
231
asa 233
D. G. Alway and K. W. Barnett, J. Organometallic Chem., 1975,99, C52. A. A. Diamantis, J. Chatt, G. J. Leigh, and G . A. Heath, J . Organometallic Chem., 1975, 84, C1 1. V. W. Day, T. A. George, and S. D. A. Iske, J. Amer. Chem. SOC.,1975, 97, 4127. R. Ben-Shoshan, J. Chatt, W. Hussain, and G. J. Leigh, J. Organometallic Chem., 1976,112, c9. P. C. Bevan, J. Chatt, R. A. Head, P. B. Hitchcock, and G. J. Leigh, J.C.S. Chem. Comm., 1976, 509. J. Chatt, A. J. Pearman, and R. L. Richards, Nature, 1976, 259, 204. T. Ito and A. Yamamoto, J.C.S. Dalton, 1975, 1398.
Photochemistry
202
A further account of the photosubstitution of the aryl isocyanide ligand in Cr(ArNC), by ~ l e f i n s and , ~ ~a~report on the electronic structure of M(CNPh)6 complexes 235 have been published. Irradiation of a mixture of CrCl, and Pr'MgBr yields CrPrip,236while in the presence of 1,3-cyclo-octadiene and 1,3,5-cyclo-octatriene,bis(cyc1o-octadieny1)chromium(r1) is formed.237 The photodecarbonylation reaction (69) is a con[W(CO),(COR)]-
[W(CO),R]-
+ CO
(69)
venient route to the novel pentacarbonyltungsten alkyl and aryl derivatives (R = Me, Ph, or CH2Ph).238 Three primary photoprocesses should be considered for the metal-metal bonded complex [CpMo(CO),],. These are (i) homolytic cleavage (70), (ii) heterolytic cleavage (71), and (iii) CO-substitution (72). Recent publications have described [CPMo(CO),Iz [CPMo(CO),Iz [CPMo(CO)31,
-
2CPMO(CO),
(70)
[CPMo(CO),I+ Cp,Mo2(CO),
+ [CPMo(CO),I-
+ co
(71)
(72)
the light-induced reactions of [CpMo(CO),], and [CpW(C0),l2 with halogeno240 the flash photolysis of [ C ~ M O ( C O ) , ] , ,and ~ ~ ~the photochemistry Photolysis of the dimer in carbon of [CpMo(CO),], in highly polar tetrachloride gives CpMo(CO),Cl, the quantum yield varying slightly with irradiation This result is consistent with homolytic cleavage of the Mo-Mo bond [reaction (70)], followed by abstraction of a chlorine atom from the solvent by the metal centre. Further evidence for the importance of reaction (70) as an initial photo-process has been obtained from flash photolysis experiments with mixtures of [CPM~(CO),]~ (M1 = Mo or W) and M22(CO)lo(M2 = Mn or Re). In these cases good yields of the mixed products are formed as shown in equation (73).239 The reaction presumably involves combination of the [CpMl(CO),],
+ M22(CO)lo A
2CpM1(CO),M2(C0),
(73)
CpM1(CO), and M2(CO), formed by the flash. Examination of the intermediates formed on flash photolysis of [CpMo(CO),], itself reveals, however, that two processes occur on photolysis in solvents such as acetonitrile, THF, or cycloh e ~ a n e . ~From ~ l the spectra and decay kinetics of the transient species, it is deduced that both reactions (70) and (72) take place. The lack of ionic strength 234
K. Iuchi, S. Asada, T. Kinugasa, K. Kanamori, and A. Sugimori, Bull. Chem. SOC.Japan,
a36
K. R. Mann, M. Cimolino, G. L. Geoffroy, G. S. Hammond, A. A. Orio, G. Albertin, and H. B. Gray, Inorg. Chim. A d a , 1976,16, 97. J. Mueller and W. Holzinger, Angew. Chem., 1975, 87, 781 ; Angew. Chem. Internat. Edn.,
1976, 49, 577. 236
1975, 14, 760. 237
as8 230
240 a41 248
J. Mueller, W. Holzinger, and F. H. Koehler, Chem. Ber., 1976, 109, 1222. C. P. Casey, S. W. Polichnowski, and R. L. Anderson, J. Amer. Chem. SOC.,1975,97,7375. M. S. Wrighton and D. S. Ginley, J. Amer. Chem. SOC.,1975, 97, 4246. C. Giannotti and G. Merle, J . Organometallic Chem., 1976, 105, 97. J. L. Hughey, C. R. Bock, and T. J. Meyer, J. Amer. Chem. Soc., 1975, 97,4440. D. M. Allen, A. Cox, T. J. Kemp, Q. Sultana, and R. B. Pitts, J.C.S. Dalton, 1976, 1189.
Photochemistry of Inorganic and Organometallic Compounds
203
effects proves that in these solvents of low polarity, heterolytic cleavage (71) is not a primary process. Both processes (70) and (72) are induced by either U.V. or visible light, indicating that initial population of either UU* or do* (Mo-Mo) states cause these reactions to take place. A quite different picture emerges for experiments in high-donicity solvents such as DMF, DMSO, and ~ y r i d i n e . , ~ ~ Under these conditions the anion [CpMo(CO),]- has been identified after photolysis of the dimer. The reaction is very efficient, the quantum yield being higher in the polar solvents than in non-polar (for pyridine, 300 nm) or acetonesensitized rearrangement of (158).92 The reaction is related to the lY3-sigmatropic shift reactions which are to be found in the singlet-state reactions of &-enones.
+
0
(154)
0 R-R=
0
R (157) 88
91 B2
M. A. Schexnayder and P. S. Engel, J. Amer. Chem. SOC.,1975,97,4825. P. S. Engel and M. A. Schexnayder, J. Amer. Chem. SOC.,1972,94,9252. P. S. Engel and M. A. Schexnayder, J. Amer. Chem. SOC.,1972,94,4357. H. Hart and S.-M. Chen, Tetrahedron Letters, 1975, 2363. B. Fuchs, M. Pasternak, and G. Scharf, J.C.S. Chem. Comm., 1976, 53.
I
284
Photochemistry
(159) a;
b;
R1
R2
R3 = H
= Ph, = R1 = R2 = R3 =
H
R1 = Ph, R2 = Me, R3 = H d; R1 = Ph, R2 = H, RS = Me C;
However, this photoreaction arises from the triplet state, and it is thought that a biradical intermediate is involved in the isomerization. It should also be noted that in recent years examples of sensitized lY3-migrationshave been reported.03 The enones (159) do not afford photoproducts by either direct or sensitized irradiati011.O~ The authors O 4 suggest that the absence of photoproduct in the case of (159a) is the result of an energy dissipation path involving a ‘free rotor’ effect.06 The feasibility of this path was demonstrated with (159c) and (159d) which undergo cis-trans-isomerization of the exocyclic methylene group when irradiated into the T -+ T* band.04 The dienone (160) gave three products (161), (162), and (163) when irradiated in dioxan-acetic acid.Qu A detailed account of the photochemical rearrangements of the hydroxysantonene (164) originally reported in note formg7has been published.Q8Rearrangement of the acetate (165a) takes place upon photolysis in the presence of a triplet quencher to afford the acetoxy-compounds (166a).O9
8@ /
0
(165) a; R = S-MeCO, b; R = a-MeCO,
0
(166) a; R = 01- Or P-MeCO, b;R=Me
c; R = Me
@’ O6 O7 O8
P. S. Engel and M. A. Schexnayder, J. Amer. Chem. SOC.,1975,97, 145. C. Lam and J. M. Mellor, J.C.S. Perkin 11, 1975, 519. H. E. Zimmerman and G. A. Epling, J. Amer. Chem. SOC.,1972,94, 8749. L. J. Dolby and M. Tuttle, J. Org. Chem., 1975,40, 3786. D. S. R. East, T. B. H. McMurry, and R. R. Talekar, J.C.S. Chem. Comm., 1974,450. D. S. R. East, T. B. H. McMurry, and R. R. Talekar, J.C.S.Perkin I, 1976, 433. T. B. H. McMurry and R. R. Talekar, J.C.S. Perkin I, 1976, 442.
285
Enone Cycloadditions and Rearrangements
(167)
(168) a; R
b; R c; R
= = =
a-MeCO, /I-MeCO, a-Me
R
=
01-
or p-Me
(169)
Direct irradiation in the absence of quencher leads to decarboxylated products (165c, 166b), and the loss of CO, is thought to involve a triplet process. Thus, the decarboxylation reaction presumably occurs by acetate 0-C bond fission, decarboxylation of the resultant acetoxy radical, and recombination of the methyl radical with the santonenyl radical. The decarboxylation also takes place with the epimer (165b), but in this instance the recombination takes place at C-11 (167). No evidence was obtained for recombination at C-6. Recombination at C-6 is however encountered in the photolysis of the dihydrosantonenes (168) which give rise to (169). Further study has suggested that the reaction in the dihydro-series is intermolecular in ~ h a r a c t e r . ~ ~ Irradiation (254nm) of the epoxide (170) in pentane solution affords the bicyclic compound (171).loo This reaction is thought to take place by fission of the C-C bond of the epoxide to afford the biradical(l72; perhaps this could be a carbonyl ylid). The biradical ring-closes to the furan derivative (173) which
qo Q
(172)
* = *or+,-
(173)
itself is photolabile and rearranges to the isolated product (171). The ketoepoxide (170) undergoes different reactions upon nn* excitation (C-0 fission) by irradiation at h 3 280 nm. The products from this process are the enones (174), (175), the keto-aldehyde (176), and the cyclobutanone (177), which is a photoproduct of (171).loo A study of the photochemistry of the two keto-olefins (178a) and (178b) has been published.lOl The U.V. spectra of the two compounds show no evidence for intramolecular interaction between the carbonyl and the olefinic chromophore. loo
lol
B. Frei and H. R. Wolf, Helu. Chim. A d a , 1976, 59, 82. H. Morrison, V. Tisdale, P. J. Wagner, and K. Liu, J. Amer. Chem. SOC.,1975, 97, 7189.
286
Photochemistry CHMe
p
h
v
R
3
Ph+H
0
R1
(179)
(178) a; R1 = H = R2,R3 = Me b; R1 = Et, R2 = R3 = H c ; R1 = H, R2 = Me, R3 = H
Ph (180) a; R1 = H, R2 = OH b; R1 = OH, R2 = H
However, the irradiation of (178a) brings about rapid isomerization to (178c) (the photostationary state contains 82% of the trans-isomer) by an intramolecular energy transfer process. Continued irradiation affords three products [(179), (180a, b)], which are thought to arise from the rearrangement of an intermediate bicyclo-oxetan (181). The other keto-olefin (178b) affords six products (Scheme 6) either by Norrish Type I1 processes or by rearrangement of an unstable bicyclic oxetan (see ref. 102 for an earlier account of the photochemistry of this ketone). This second keto-olefin also shows enhanced deactivation of the tripletexcited state of the carbonyl function in comparison with butyrophenone. The authors lol infer from the results that charge-transfer complexation is important in the photochemistry of the two ketones.
a)
(178b)
+
a)
= 0.001
= o.Ooo1
Ph HO Q, =
Et 0.0019
a
= 0.0042
Et 95% from preparative experiments: its use as a convenient route to semibullvalene, however, appears at present not to be viable.33 It has earlier been reported that the relative quanta1 efficiency for formation of 1,2- and 1,3-cycloadducts [i.e. (29) and (33), respectively] of ethylenes and benzene may be predicted from the ionization potential difference between the addends.25 In particular, where an ethylene has marked acceptor or donor properties with ionization potentials >9.6 or 68 kcalmol-1 are effective: the triplet of dichlorovinylene carbonate yields the 1,2- and 1,4-~ycloadductsin consecutive reactions with both benzene and naphthalene whereas S1naphthalene yields the products in parallel processes via a common intermediate. Both 1,Zadducts (38) and (39), and 1,4-adducts (40) and (41) are formed from naphthalene. Three isomeric 1 : 1 photoadducts of vinylfluoride and benzene have been detected, but their structures are as yet unknown.40
3B
K. E. Wilzbach and L. Kaplan, J. Amer. Chem. SOC.,1971,93,2073. R. B. Cundall and D. A. Robinson, J.C.S. Faraday ZZ, 1972,68, 1691. G. Subrahmanyam and R. Srinivasan, Tetrahedron, 1975,31, 1797. G. Hesse and P. Lechtken, Angew. Chem., 1971, 83, 143; Annalen, 1971,754, 1; H. D. Scharf and R. Klar, Tetrahedron Letters, 1971, 517; Chem. Ber., 1972, 105, 575. H. D. Scharf, H. Leismann, W. Erb, H. W. Gaidetzka, and J. Aretz, Pure Appl. Chem., 1975,
40
41 581. S. Tsunashima, H. E. Gunning, and 0. P. Strausz, J. Amer. Chem. SOC.,1976, 98, 1690.
86 97 88
371
Photochemistry of Aromatic Compounds
-.q (39)
(41)
(40)
The early observations by Koltzenberg and Kraft on photoadditions of 1,3dienes to benzene 41 have been followed up by several groups of Yang and co-workers have contributed much to the understanding of these reactions and now report the addition of 1,2-dihydrophthalic anhydride to benzene, naphthalene, and anthracene to yield adducts (42), (43), and (44) respecti~ely.~~ The formation of (42) is most interesting not only because cyclohexa-1,3-diene undergoes ring-opening in preference to benzene addition, but more importantly because this adduct is a very realistic precursor for the as yet unknown 1,41',4'-dimer (45) of benzene: cf. ref. 3. We look forward to reading soon of the
"
(45)
(46) 'l
4L
K. Kraft and G . Koltzenburg, Tetrahedron Letters, 1967, 4357, 4723. N . C. Yang, C. V. Neywick, and K. Srinivasachar, Tetrahedron Letters, 1975,4313.
372
Photochemistry
synthesis and properties of this ClzHlzisomer. Preliminary results suggest that sensitized irradiation of (42) yields (46). The original two independent reports of the photoaddition of furan to benzene appeared to be in conflict, particularly over the structures of the a d d u c t ~ .The ~~ workers have now collaborated and it transpires that most of the discrepancies arise from the thermal and photolabilities of the major adduct (47) and its Cope-rearranged isomer (48).44 Thus when high-intensity light sources are used and/or temperatures above ambient in the work-up procedure (i.e. preparative g . ~ . adducts ~ ~ ~ ) (47) and (48) are destroyed and the more photochemically (at
254 nm) and thermally stable minor adducts (49), (50), and (51) remain to be isolated. The question of the quantum efficiency of the process still remains in dispute. It is interesting to note here that not all 1,4-1',4'-diene-benzene adducts simply yield Cope isomers thermally: thus the adduct (52) from 1,2-dimethylenecyclohexane and benzene 45 undergoes a series of intramolecular and retroDiels-Alder reactions which result in the formation of buta-l,3-diene and tetralin.4s During a study of the S1and Tl energies of steroidal transoid dienes, Pusset and Beugelmans observed the formation of four unidentified adducts with both benzene and na~hthalene.~' Light-induced acyclic additions of amines to aromatic hydrocarbons have been known for some ten years,32but with some substituted arenes replacement of the substituent group also tends to occur. Thus primary and secondary aliphatic amines photoreact with fluorobenzenes to give adducts and substitution products, and evidence for an addition-elimination mechanism in the substitution reaction has been p r e ~ e n t e d .Irradiation ~~ (254 nm) of, for example, fluorobenzene with diethylamine gives NN-diethylaniline and the adducts (53)-(55). Adducts reflecting attack of the amine nitrogen at the 1-position and those which contain (a) J. C. Berridge, D. Bryce-Smith, and A. Gilbert, J.C.S. Chem. Comm., 1974, 964; (b) T. S. Cantrell, Tetrahedron Letters, 1974, 3959. 44 J. C. Berridge, D. Bryce-Smith, A. Gilbert, and T. S. Cantrell, J.C.S.Chem. Comm., 1975,611. 46 J. C. Berridge, D. Bryce-Smith, and A. Gilbert, Tetrahedron Letters, 1975, 2325. A. Gilbert and R. Walsh, J . Amer. Chem. SOC.,1976, 98, 1606. 47 J. Pusset and R. Beugelmans, Tetrahedron, 1976, 32, 791. IaD. Bryce-Smith, A. Gilbert, and S. Krestonosich, J.C.S. Chem. Comm., 1976,405. 43
373
Photoclzemistry of Aromatic Compounds
U N E t 2
or
FQtEt2
FDE
&HNEt2
'
(54)
(53)
H
(55)
a HCF group were not detected, and it is suspected that such adducts are unstable, and eliminate HF to give the aniline. Evidence that this may well be the case was provided from the reactions of difluorobenzenes with diethylamine which gave cine-substitution products together with the expected corresponding monofluoroaniline derivatives and various 1,2- or 1,4-acyclic adducts. Major contributions from an aryne intermediate in the formation of the cine-substitution products have been discounted, and an addition-elimination mechanism has been proposed which involves either zwitterionic Wheland-type intermediates [e.g. (56) and (57) from meta-difluorobenzene] and/or unstable chemical adducts [e.g. (58) and (59)]. These reactions provide the first known examples of photochemical cine-subst it ut ion. 48 Two reports within the year have described light-induced cleavage of a benzenoid ring. The irradiation of aromatic nitro-compounds which have either 3n7r* or 3 7 r ~ * lowest states in the presence of aromatic methoxy-compounds leads to selective addition of the nitro-group at the 1,2-positions of the latter a ~ e n e .The ~ ~ resulting adduct is very labile and undergoes fission to yield the diene (60). The second report claims that there is some evidence that the
6"' OMe
ArNO,*
'
A
Me0,C \
C
R2 O
R
'
aliphatic products from irradiation of phenylalanine at 254 nm result from a cleavage reaction of the aromatic ring.50 Although photoreduction reactions are reviewed in Chapter 5 of Part 111, it is worth noting here that aromatic hydrocarbons and phenols undergo photoreduction by aqueous sodium borohydride,61 and that the former are also photoreduced by 1,4-di~yanobenzene.~~ Indeed there have been several accounts in the literature this year in which cyanobenzenes have been incorporated into 4g
6o 61
I. Saito, M. Takami, and T. Matsuura, Bull. Chem. SOC. Japan, 1975,48, 2865. C . Hasselmann and G. Laustriat, Phorochem. and Photobiol., 1975, 21, 2, 133. D. Bradbury and J. Barltrop, J.C.S. Chem. Comm., 1975, 842. K. Mizuno, H. Okamoto, C. Pac, and H. Sakurai, J.C.S. Chem. Comm.,1975,839.
374
Photochemistry
reaction mixtures, sometimes with very significant changes in pathways. Thus although styrenes normally yield cyclobutane-type dimers, their irradiation in the presence of 1,2,4,5-tetracyanobenzenealso gives l-phenyI-l,2,3,4-tetrahydron a ~ h t h a l e n e .This ~ ~ type of dimer is suggested to arise via an ionic mechanism through photodissociation of the exciplex of the styrene derivative with the cyanobenzene. Photoaddition reactions of naphthalenes, and in particular naphthonitriles, continue to attract considerable interest. McCullough and co-workers have studied the reactions of 1- and 2-naphthonitriles with tetramethylethylene and reported that in benzene solution exciplexes are intermediates in the cycloaddition reaction, but that electron transfer dominates the chemistry in polar While both exciplex emission and adduct (61) formation still occur in acetonitrile, both have lower efficiencies than in benzene. Similarly in methanol the formation of adduct (62), previously described by C a n t ~ e l lfrom , ~ ~ 2-naphthonitrile is totally quenched and the photoreduction products (63) and (64) are
q:; H H
(61)
(62)
R1 = CN, R2 = H R1 = H, R2 = CN
Me Me
NC
RO
(67)
formed. Since the fluorescence of 2-naphthonitrile is quenched by the ethylene at a diffusion-controIled rate in methanol, it is suggested that both the photoaddition reaction and the quenching process in methanol and benzene have different mechanisms. The photoaddition of alkyl vinyl ethers to 2-naphthonitrile has been studied in great detail, and Pac and co-workers have published full details 66 of their earlier communication on this Using 313 nm radiation, only the single endo-[2 + 21 cycloadduct (65) is formed in 80-90% yield, whereas Pyrex-filtered irradiation (A > 280 nm) gives the cyclobutene (66) as the ultimate major product together with various 1 : 1 adducts (67)-(70) and the cyclobutane dimer of (67). These differences in selectivity of the vinyl ether addition are interpreted in terms of differences in the stability of the conE.* 65
56 67
J. J. McCullough, R. C. Miller, D. Fung, and W. s. Wu, J. Amer. Chem. SOC.,1975,97, 5942. T. S. Cantreli, J . Amer. Chem. SOC.,1972, 94, 5929. K. Mizuno, C. Pac, and H. Sakurai, J.C.S. Perkin I, 1975, 2221. K. Mizuno, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1973,219.
Photochemistry of Aromatic Compounds
NC
Q -
RO (68)
375
Ncq % ROSS ,-'
I
NC
(69)
OR
(70)
figurations of the intermediate exciplexes,68as deduced from the solvent dependence of quantum yields and fluorescence quenching. Adducts (67), (68), and (70) are not products of secondary photoreactions, but (69) is formed from (68) and (66) likewise arises from (67). The intermediate (71) is suggested for (66), (67), and (68). Readers should also be aware that the 2-naphthonitrile-methyl vinyl ether system has also been investigated by Chamberlain and McCullough when, together with the products described above, the 1 : 1 adduct (72) was isolated and suggested to arise from the fulvene derivative (73).59 The photochemical behaviour of 1-naphthonitrile has also been studied in the presence of phenylacetic acid derivatives, and from mechanistic studies the S1naphthaleneacid exciplexes, whose reactions are again found to be solvent-dependent (see Scheme 5), are suggested as intermediates.s0 The report describes the photoArCN*
+ RCH,CO,H
+ [ArCN .., RCH2C02H]* p H 6
MeC{
A~CN'
photoproducts
t--
RCH,CO~H
HArCN'
starting materials
+ RCH;I + CO2
Scheme 5
reduction and reductive alkylation of the naphthonitrile by rn- and p-methoxyphenylacetic acid and by phenoxyacetic acid. Thus p-methoxytoluene, 1,4-dihydro-l-naphthonitriie, (74), ( 7 9 , and (76) are formed from the naphthonitrile using the p-methoxy-acid derivative. Libman has also investigated this reaction with acridine as the electron-acceptor component, and the formation 63 b8 68
6o
T. Asanuma, M. Yamamoto, and Y . Nishijima, J.C.S. Chem. Comm., 1975, 609. C. Pac, T. Sugioka, K. Mizuno, and H. Sakurai, Bull. Chem. SOC.Japan, 1973,46, 238. T. R. Chamberlain and J. J. McCullough, Canad. J. Chem., 1973, 51, 2578. J. Libman, J. Amer. Chem. SOC.,1975,97,4139.
376
Photochemistry NC C H 2 e O M e
(74)
I H (77)
of (77) has been suggested to arise via simultaneous or consecutive electron- and proton-transfer from the carboxyl group of the acid to the acridine.61 Since the first report in 1965, there have appeared many enlightening accounts of the additions of diphenylacetylene derivatives to naphthalenes. A further account has now appeared which describes the formation of the adduct (78) from 1,4-diinethoxynaphthaleneand diphenylacetylene, and its thermal conversion into (79) and (80).62 U.V. irradiation of (79) yields (81), and (80) gives (81) more slowly than the corresponding conversions of (82), (83), or (84).
(78) R1 = R2 = OMe (81) R1 = H, R2 = OMe
(79) R1 = OMe, R2 = H (80) R1 = R2 = OMe (82) R' = H, R2 = OMe (83) R1 = K2 = Me (84) R' = R2 = H
This year has seen the publication of several important accounts of the interaction of 1,3-dienes with naphthalene and anthracene derivatives: the role of exciplexes in the photochemical processes has received particular attention. Libman has studied the details of the photochemical behaviour of octafluoronaphthalene towards conjugated dienes and reports that the reaction is markedly dependent on solvent polarity (as with so many other donor-acceptor Thus in cyclohexane solution the naphthalene and 2,4-dimethylpenta-l,3-diene give an 80% yield of a 1 : 1 mixture of the adducts (85) and (86) whereas with acetonitrile as solvent, although there is slow conversion of the naphthalene, the diene is consumed rapidly to yield the diene dimers (87) and (88) together with (85). As with other systems, although. the quenching efficiencies of the 61 62
63
J. Libman, J.C.S. Chem. Comm., 1976, 198. T. Teitei, D. Wells, and W. H. F. Sasse, Ausrral. J . Chem., 1975, 28, 571. J. Libman, J.C.S. Chem. Comm., 1976, 361.
377
Photochemistry of Aromatic Compounds
Ye
AY”
(87)
Me
(88)
naphthalene fluorescence by the diene are similar in the two solvents, the addition efficiencies vary greatly, and in acetonitrile the quantum yield decreases as the ionization potential of the diene decreases. It is suggested from the qualitative correlation between ionization potential of the diene and the quantum yield for the diene dimer formation in acetonitrile (at the expense of adduct formation) that the dimerization involves the intermediacy of solvated charge-transfer complexes or ion pairs. Formation of such intermediates is attributed to the occurrence of a solvent-induced crossing between the covalent and ionic potential energy surfaces of the naphthalene-diene system in acetonitrile. Sensitization of the diene with octafluoronaphthalene does not result in enhanced intersystem crossing, and it is concluded that Tl diene does not play an important role in the naphthalene-induced d i m e r i ~ a t i o n . ~ A~three-component exciplex 65 is suggested as an intermediate in the d i m e r i ~ a t i o n . ~ ~ With diene-arene systems, it is frequently postulated that the light-induced processes involve initial formation of an ‘encounter complex’ which can lead to the exciplex or chemical products. In many systems, the involvement of an exciplex has been conclusively proved and emission from such species is well documented; but as Yang and collaborators point out, the same cannot be said for encounter complexes. From a study of the fluorescence of the 9,lO-difluoroanthracene-2,5-dimethylhexa-2,4-diene(DMHD) system, however, these workers have now observed dual emission in a number of solvents.66 One of these emissions is only 440 cm-1 displaced from the anthracene, exhibits fine structure, and is not affected by change in solvent polarity. It is thus considered that this structured emission has all the characteristics of those expected for an encounter complex ‘and is so identified’. The other fluorescence is featureless and displaced well to the red. Further studies have been reported by the same group on the detection and characterization of exciplexes from anthracene and its halo- and cyano-derivatives with DMHD and their relationship to the photochemistry of 64 66
66
J. Libman, J.C.S. Chem. Comm., 1976, 363. J. Saltiel, D. E. Townsend, B. D. Watson, and P. Shannon, J. Amer. Chem. SOC.,1975,97,5688. N. C. Yang, D. M. Shold, J. K. McVey, and B. Kim, J. Chem. Phys., 1975,62,4559.
378 Photochemistry these The anomalously high quenching constant ( k , ~= 1500) for the fluorescence of 9,lO-dicyanoanthracene by D M H D is accounted for by a ground-state complex of the reactants, and end-absorption extending beyond 430 nm is observed. Two other groups have also commented upon arene-diene exciplexes. Saltiel and co-workers have reported details 66 of the emission properties of the 9,lO-dichloroanthracene-DMHD system in both methanol and acetonitrile.6s In the former solvent, the exciplex has T = 7.4 ns, emits with Am= ca. 485 nm, and is formed reversibly with an equilibrium constant at room temperature of 20 k 1 1 mol-l. In contrast, no exciplex emission was evident in acetonitrile at intermediate concentrations, but at higher diene concentrations triplex fluorescence (Arnx ca. 543 nm and T = 3.6 ns) was observed. These workers call for a thorough study of the photochemistry of these systems. Lewis and Hoyle have also emphasized the importance of reversible exciplex formation and noted that rate constants for fluorescence quenching generally decrease with increasing ionization potential of the diene.ss The involvement of exciplexes in addition processes has been further studied, and the relationship between the nature of the exciplex and the orientation of photocycloaddition in arene-1,3diene systems has been The reactions investigated in this context involved D M H D and cyclohexa-l,3-diene with 9,10-difluoro-, -dichloro-, -dibromo-, and -dicyano-anthracenes, acridine, and naphthalene under conditions such that 87.5-99% of the arene fluorescence was quenched by the diene: thus most of the S1arene was intercepted by the diene as an exciplex and hence photocycloaddition has to proceed mostly uia the exciplex. It was found that the majority of the arenes reacted with cyclohexa-1,3-diene in a 47ra 47ra mode to yield adducts of type (89), whereas only anthracene and naphthalene reacted with DMHD to give the respective47~g 4raadducts [e.g.(go)] : most of the anthracene
+
+
(91) R = F, CI, Br or CN O7
'O
(92)
N. C. Yang, D. M. Shold, and J. K. McVey, J. Amer. Chem. Soc., 1975,97, 5004. For a preliminary account see J. Saltiel and D. E. Townsend, J . Amer. Chem. SOC.,1973, 95, 6140. F. D. Lewis and C. E. Hoyle, J. Amer. Chem. SOC.,1975,97, 5950. N. C. Yang, K. Srinivasachar, B. Kim, and J. Libman, J. Amer. Chem. SOC.,1975, 97, 5006.
Photochemistry of Aromatic Compounds
379
derivatives gave the 47rs + 27r8adducts (91) with this diene. This greater reactivity of the cyclohexadiene over DMHD with excited arenes in 47r, + 47rBprocesses is explained by the more favourable overlap of the 7r-systems with the former diene. Also, adducts of type (90) and those from other transoid dienes have much greater strain than those of type (89), and this strain must have existed in the transition state thus making this process less favourable. It is interesting that acridine and anthracenes which have a halogen in the meso position react with cyclohexadiene in their terminal ring. On the other hand, (92) is formed from acridine and DMHD. The authors point out that there is an excellent correlation between the polarity of arene-l,3-diene exciplexes and the orientation observed in the cycloaddition processes. The photoaddition of cyclic dienes and cycloheptatriene to anthracene has been the subject of two recent reports. Common biradical precursors have been suggested for both 47r8 + 2r8and 4n8 + 47r8 adduct~,'~ and 4r8 47r8 and 47r8 67r8 adducts, re~pectively.~~ From work described in ref. 70, however, it is clear that the reaction pathways in these systems may be influenced by the nature of the exciplex, and further relevant information has been provided from a study of the 9-cyanoanthracene-cycloheptatrienesystem.73 Three adducts (93), (94), and (95) are formed from the irradiation of this system and the results
+
+
(93)
(94) R' (97) R'
= =
CN, R2 = H H, R2 = CN
+
suggest that the 47rs + 27~sadduct (93) and the 4r8 47r8 adduct (94) arise via different reaction pathways. Consistent with previous singlet excited states are involved in this reaction, and the formation of (93) (which is symmetryforbidden as a concerted process) is suggested to proceed via polar exciplexes and radical intermediates, the most stable of which is (96). Since collapse of (96)to a 4r8 + 4r8 adduct would, however, yield (97) and not the observed isomer (94), a 71
72
79
G. Kaupp, R. Dyllick-Benzinger, and I. Zimmermann, Angew. Chem. Internat. Edn., 1975,14, 491. T. Sasaki, K. Kanematsu, and K. Hayakawa, J. Amer. Chem. SOC.,1973,95, 5632. N. C. Yang and K. Srinivasachar, J.C.S. Chem. Comm.,1976,48.
380
Photochemistry
duality of mechanisms is suggested. The cyano and CH, groups of the addends are considered to orient in the same direction in the exciplex to give the most favourable dipole interaction (98). Such a species as (98) may either collapse to give (94) by a 47r, + 47r8 concerted reaction or follow a stepwise route to the biradical (96) which then gives (93). The light-induced acceleration of the Diels-Alder reaction of maleic anhydride and anthracene was reported 15 years and a similar study has now been made with acrylonitrile as the dienophile using 365 nm light.76 From sensitization experiments TI anthracene is discounted in the process, and a mechanism involving exciplexes is proposed: the reaction rate is increased by an increase in the acidity and polarity of the solvent. The involvement of exciplexes in the photoaddition of amines to arenes has previously been In particular, the reaction between anthracene and aniline has been suggested to involve such intermediate^,^^ and solvent effects have now been extensively s t ~ d i e d . ' ~The main finding is that fluorescence quenching is low in solvents in which the rate of adduct (99) formation is high: Ph
the latter process is again favoured in non-polar solvents. The overall mechanism is suggested to involve electron transfer followed by proton transfer, as in the case of the addition of tertiary amines to benzene.'* In polar solvents, electron transfer may almost be complete in the exciplex, which then rapidly yields a highly solvated ion-pair and this dissociates to solvated radical-anions and -cations. The solvent effects noted in the current work are rationalized by taking into account the effects on the ion-pair as well as on the radical pair formed after proton transfer. Hydroxylic solvents do not fit into the general scheme of solvent polarity effects, and this finding is attributed to the occurrence of proton transfer via two steps in such media (ix.from N to the oxygen of the alcohol and subsequently from the oxonium ion to the anthracene). Such a relay mechanism allows sufficient time for the molecules to separate and thus the addition efficiency is reduced. Over the past four years there has been renewed interest in the 9,lO-photoaddition of dienophiles to phenanthrene, a reaction first reported for maleic anhydride in 1961.79 The earlier controversy over the involvement of exciplexes 74
75 70
77 '1.3
78
J. P. Simons, Trans. Faraday Soc., 1960, 56, 391. N. Selvarajan and V. Ramakrishnan, Z . Phys. Chem. (Frankfurt), 1975, 96, 167. V. R. Rao, S. Vaidyanathan, U. K. Menon, and V. Ramakrishnan, Indian J . Chem., 1973,11, 231; S. Vaidyanathan and V. Ramakrishnan, Z . Phys. Chem. (Frankfurt), 1973, 85, 130. S. Vaidyanathan and V. Ramakrishnan, Indian J. Chem., 1975, 13, 257. D. Bryce-Smith, M. T. Clarke, A. Gilbert, G. Klunklin, and C. Manning, Chem. Comm., 1971, 916. D. Bryce-Smith and B. Vickery, Chem. and Ind., 1961, 429.
381
Photochemistry of Aromatic Compounds
or singlet biradicals in the addition of dimethyl fumarate to phenanthrene was settled last year in favour of the former by Caldwell and co-workers,80and this year further details concerning the intermediacy of both singlet and triplet exciplexes in the process have appeared.81 Reactions were performed in benzene solution using 347 nm radiation, and kinetic and quenching experiments were consistent with a diffusion-controlled reaction of the S1arene with So dimethyl fumarate to yield a weakly emitting singlet exciplex. This intermediate variously yields the oxetan (100) (2.4%), the cyclobutane adduct (101) (0.1%) stereospecifically, and undergoes intersystem crossing to the triplet exciplex (5.373, and decay to starting materials (92%). The triplet exciplex yields both (101)
PR: \
R3
C02Me, / R1 = R2 = H
C02Me (100)
(101) R4 = R3 = (102) R4 = R2 = H, R' = R3 = C02Me (103) R4 = R2 = H, R1-R3 = CO-0-CO
(3.2%) and its isomer (102) (1.7%), dissociates to Soolefin and Tl arene (67.2%), and decays to the Sostarting materials (27.9%). This latter process is presumed to arise via dissociation of the biradical which precedes the formation of (101) and (102). Hence the earlier evidence for the intermediacy of both singlet and triplet exciplexes in this addition process is greatly substantiated. The effect of alkyl substituents on the phenanthrene on the formation of the 1 : 1 arenemaleic anhydride adduct (103) has been investigated.82 The alkyl chain length does not seemingly affect the reactivity, but substituents on the 9,lO-positions increase the rate of reaction whereas those in other positions have the reverse effect. From time to time the photo-Diels-Alder reaction receives comment, but little has been reported on such processes with five-membered heterocycles :in particular, the additions to thiophen appear only to have been investigated with acetylenedicarboxylic The photocycloadditions of maleimide and maleic anhydride derivatives to five-membered heterocycles have now been reported.84 Both 2,3and 2,5-attack of the dienophile on the heterocyclic compound (104) occur and yield stereospecifically the endo and exo adducts (105) and (106), respectively. Photoreduction of the furan ring in benzo[b]furans by aliphatic amines to yield (107) has been reported by Lablache-Combier and co-workers.86 The chemical yields of such products increase with decrease in ionization potential
8a
83
*6
D. Creed and R. A. Caldwell, J . Amer. Chem. Soc., 1974, 96, 7369; R. A. Caldwell and L. Smith, J . Amer. Chem. SOC.,1974, 96, 2994. S. Farid, S. E. Hartman, J. C. Doty, and J. L. R. Williams, J . Amer. Chem. Soc., 1975,97,3697. E. G . Lekveishvili and E. G . Akhalkatsi, Soobschch. Akad. Nauk. Gruz, 1974, 76, 633. R. Helder and W. Wunberg, Tetrahedron Letters, 1972, 605; H. J. Kuhn and K. Gollnick, ibid., p. 1909; Chem. Ber., 1973, 106, 674. C. Rivas, C. Perez, and T. Makano, Rev. Latinoam. Quim., 1975, 6, 166. C. Parkanya, A. Lablache-Combier, I. Marko, and H. Ofenberg, J. Org. Chem., 1976,41, 151.
3 82
Photochemistry
+ (104)
R1,R2 = H or Me X=OorS
0
R3,R4 = H, Me or halogen Y
=
0, NH or NMe
of the amines, hence tertiary amines are the most effective. Exciplex intermediates are again suggested. 4 Substitution Reactions Two excellent reviews concerned with light-induced aromatic substitution reactions have been published this 1 3 ~ The authors are renowned for their very important contributions to this area of research and whether the reader is concerned with synthetic applications of these reactions or wishes to have a summary of present knowledge of photosubstitution reactions he will find either or both reviews invaluable. There are some basic ‘rules’ which describe the Thus as noted orientation of nucleophilic substitution in the excited many years ago, the nitro and methoxy groups are meta and ortho-para directing respectively, contrary to their effects in ground-state nucleophilic substitutions. With polynuclear aromatic compounds, certain positions (e.g. 1- in naphthalenes and azulenes, and 9- in phenanthrene) are more reactive than others, and merging (resonance) stabilization during product formation is an important consideration. Most of the substitution reactions involve the triplet m r * state of the arene, but others involve the singlet m*state, and there are indications that in some cases the aromatic molecule in its excited state undergoes dissociation to produce an ion which subsequently reacts with the nucleophile. There are few reported examples in which benzene undergoes light-induced substitution, hence a report concerned with the nitric oxide-benzene system is 86
E. Havinga and J. Cornelisse, Chem. Rev., 1975, 75, 353. J. Cornelisse, Pure AppE. Chem., 1975,41, 433.
Photochemistry of Aromatic Compounds
383
all the more interesting :** nitrobenzene, o-nitrophenol, p-nitrophenol, 2,4-dinitrophenol, and 2,6-dinitrophenol are all reported to be formed. The photosubstitution of halogen in aryl halides has in past years been studied in detail by Russian workers, who now report displacement reactions by sulphite, nitrite,89and cyanide ions.no Thus sulphite reacts with RC6H4X(R, X = p-Et2N, C1; p-Me2N, C1; p-EtNH, C1; p-NH2, C1; p-NH2, F; p-NH2, Br; p-NH2, I; p-PhNH, C1; 0-NH,, CI;and p-ONa, CI), and with 4-chloro-o-toluidine, 2-amino5-chloropyridine, and 1-amino-4-bromonaphthalene by substitution of the halogen: the reactions are deduced to arise from the triplet states of the halogen compounds. The reaction of NaNO, and Na,SO, with p-ClC6H4NR2(R = Me or Et) has been studied in the presence of other ions, and the quenching of the process was found to vary with the oxidation potential of the added ions, their redox reaction constants with nascent hydrogen, and their energy of chargetransfer by the ion to the solvent. Photosubstitution of halogen by the cyanide ion has been investigated in the naphthalene series where irradiation of aqueous t-butyl alcohol solutions of potassium cyanide and l-amino-4-chloro-, 1-amino4-bromo-, 2-amino-l-chloro-, or 1-amino-2-chloro-naphthalenes gives the corresponding aminocyanonaphthalenes.no The photocyanidation of aromatic hydrocarbons has been reported to be enhanced in the presence of both ‘crown ethers’ n1 and 1,4-dicyanoben~ene.~~ In the former account, naphthalene, phenanthrene, anthracene, and biphenyl all reacted readily with potassium cyanide dissolved in the cyclic polyether 18-crown-6 in anhydrous acetonitrile.nl Under such conditions, of course, the cyano nucleophile is unsolvated whereas when water is present the ion is strongly solvated. An extensive systematic study of the use of crown ethers in light-induced nucleophilic substitution reactions would be most informative. Similarly the photocyanidation of naphthalene and phenanthrene is observed to occur efficiently with sodium cyanide when 1,ddicyanoIn this case the reaction medium is dimethylformamidebenzene is water (3 : 1). The Pyrex-filtered irradiation of phenanthrene in this system gives 9-cyanophenanthrene (679, its photo-dimer, 9-cyano-9,lO-dihydrophenanthrene (4979, and dicyanated products. The authors stress the necessity for the presence of water and the dicyanobenzene otherwise the reaction rate is reduced to about 7% of that in their presence. The reaction is interpreted in turns of CN- attack on the cation-radicals of the arenes generated by electron-transfer from the excited hydrocarbon to the 1,6dicyanobenzene. No ground-state complexation was observed. With naphthalene the results were very similar, and l-cyanodihydronaphthalenes, 1-cyanonaphthalene, and dicyanotetrahydronaphthalene were formed together with other unspecified compounds. Both 0-and p-dicyanobenzenes are, however, photochemically labile in the presence of triethylamine, and the 254 nm irradiation of their acetonitrile solutions gives reasonable yields (56% from the p-isomer) of the corresponding ethylbenzonitriles together with 88
eo
K. Nojima, K. Fukaya, S. Fukui, and S. Kanno, Chemosphere, 1975, 4, 77. A. N . Frolov, E. V. Smirnov, N. I. Rtishchev, 0.V. Kulbitskaya, and A. V. Eltsov, Zhur. org. Khim., 1975, 11, 1464. A. N. Frolov, A. V. Eltsov, 0. V. Kulbitskaya, and V. V. Yunnikov, Zhur. org. Khim., 1975, 11,2623. R. Beugelmans, M. T. LeGoff, J. Pusset, and G. Royssi, J.C.S. Chem. Comm., 1976,377. K . Mizuno, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1975,553.
Dl e2
3 84
Photochemistry
smaller amounts of (108).93 rn-Dicyanobenzene is inert and oxygen also inhibits formation of the other ethylbenzonitriles. With methanol as solvent the starting materials are recovered from the irradiation, as with the tetracyanobenzenetoluene reaction,g4but in contrast with benzene and trieth~lamine,~~ and tetracyanoquinodimethane and toluene.95 The proposed mechanism for this cyano substitution involves initial addition of the amine to the arene ( c - refs. 48 and 78) to form, e.g., (109)zwhich loses HCN to yield the aromatized product (108).
6
€1 CN 6 / +
CN
E
t
3
N
%
a
N C ,CHMe Et,N (109)
-HCN\
CN /
Et
CHMe
Et,N' (108)
The step from (108) to p-ethylbenzonitrile is considered to be photochemical and-indeed irradiation of (108) in triethylamine does yield the observed product. Irradiation (254nm) of 1,2,4,5-tetracyanobenzene in tetrahydrofuran has been reported to yield tetrahydro-2-(2,4,5-tricyanophenyl)furan(1 lo), and as with the above system an addition-elimination mechanism is proposed (Scheme 6).98
(1 12)
Thus the primary step is considered to involve electron-transfer followed by radical coupling to yield the zwitterion (1 11). It is pointed out that the suggested rearrangement of (111) to the adduct (112) is analogous to that previously proposed for the intermediate (113) in the photoaddition of diethyl ether to benzene in the presence of trifluoroacetic acid to give (114).97 O3 O6 O6 O6
97
K. Tsujimoto, K. Miyake, and M. Ohashi, J.C.S. Chem. Comm., 1976, 386. A. Yoshino, K. Yamasaki, T. Yonezawa, and M. Ohashi, J.C.S. Perkin Z, 1975, 735. K. Yamasaki, T. Yonezawa, and M. Ohashi, J.C.S. Perkin Z, 1975, 735. M.Ohashi and K. Tsujimoto, Chem. Letters, 1975, 8, 829. D. Bryce-Smith and G. B. Cox, Chem. Comm., 1971,915.
Photochemistry of Aromatic Compounds
385
Each year a number of reports describe the photochemical dehalogenation of aryl halides, and in particular the dechlorination of chlorinated biphenyls, an environmental interest which has been highlighted by the recent accident at Seveso in Italy involving tetrachlorodibenzo-p-dioxin.The photodechlorination of 1,2,4-trichlorobenzene has been studied in both cyclohexane and propan-2-01 solutions and the primary products shown to be the 1,3- and 1,4-dichlorobenzenes.gs The product ratio is significantly different on direct and acetonesensitized irradiation, and in propan-2-01 solution under aerated conditions is fairly efficient (0= 0.44). Irradiation of cyclohexane solutions of tri- and tetra-chlorobiphenyls at 300 nm gives dechlorinated products, but quantum yields were reported to be only ca. 10-2.99 The reactivity of the biphenyls is dependent upon the position of the chlorine substituent ; o-chlorines cleave first and faster when p-chlorines are present on the same ring. The photoreduction of 3- and 4-chlorobiphenyl has been reported to occur in the presence of sodium borohydride or triethylamine, but with each isomer and reagent the mechanism of the reaction seemingly differs.loOThus, whereas the reaction of the 3-isomer with the borohydride is deduced from deuterium-labelling experiments to occur via a hydride-proton transfer mechanism, the 4-isomer reacts via a radical-chain mechanism of a type previously proposed for similar systems.101 With triethylamine as the reagent, the 4-isomer is efficiently' reduced via an electron-transfer process to yield biphenyl, but in the same system the 3-isomer yields 3-chloro-1,4-dihydrobiphenyl(0= 0.1) together with biphenyl (0= 0.3). Photolysis of both hexachloro- and hexabromo-biphenyls in methanol solution gives the corresponding tetra- and penta-halogenobiphenyls, but in aerated methanol the reaction rates are decreased by ca. 5O%.lo2 In the case of bromobiphenyls, evidence has been presented to show that the C-Br fission is preceded by electron transfer, and that the reaction is assisted by trieth~1amine.l~~ In this latter system, it is suggested that the triplet aryl bromide interacts with the amine to yield the amine radical-cation, the bromide anion, and an aryl radical which abstracts hydrogen from the solvent. The same workers have also investigated the photodegradation of polychloronaphthalenes in methanol s01ution.l~~ Dechlorination and formation of binaphthyl derivatives occur and, of the 20 B. Akermark, P. Baeckstrom, U. E. Weslin, R. Gothe, and C. A. Wachtmeister, Acta Chem. Scand. ( B ) , 1976,30,49. 98 L. 0. RUZO, S. Safe, and M. J. Zabik, J. Agric. Food Chem., 1975, 23, 594. l o o K. Tsujimoto, S. Tasaka, and M. Ohashi, J.C.S. Chem. Comm., 1975,758. Iol J. Barltrop and D. Bradbury, J . Amer. Chem. SOC., 1973, 95, 5085. Ioa L. 0. Ruzo and M. J. Zabik, Bull. Enuiron. Contam. Toxicof., 1975, 13, 181. l o 3 N. J. Bunce, S. Safe, and L. 0. Ruzo, J.C.S. Perkin I, 1975, 1607. lo4 L. 0. Ruzo, N. J. Bunce, S. Safe, and 0. Hutzinger, Buff.Enuiron. Contam. Toxicol., 1975,14, 341. 88
386
(ys& -1
Photochemistry
R1
0
S0,Me
(116) R' = R2 = C1, R3 = H (117) R1 = H, R2 = R3 = C1
(118) R1 = -
S
e
C
l
R2 = CI, R3 = H
R3 (119a) R1 = F, R2 = Me, R3 2 H (119b) R1 = C1, R2 = H, R3 = Me (120a)
R1 = -S o M e , R 2 = Me, R3 = H
(120b) R1 = - S G M e , R 2
R
H, R3 = Me
O
(121)
R
= C1 or F
put
(122)
___, \
But
OH But
OH But
OMe
Jiv
OMe
=
/
+
-
OH (123)
+
OMe
Me0
\ / But (124)
+
\ / OMe
Photochemistry of Aromatic Compoiinds
387
compounds examined, 1,8-dichloronaphthalene had the highest rate of reaction and the 1,2,3,4-tetrachloro-derivativethe lowest. In the light of current intensive efforts to dehalogenate aromatic compounds, it is interesting to note that for some workers at least, the photobromination of halogeno- and dihalogenobenzenes is still an absorbing and worthwhile area of research.lo5 Study of the irradiation of the thioester (115) has led to the observation of a somewhat exotic light-induced substitution of chIorine.lo8 Neither of the expected products (116) and (117) was detected, but (116) was implicated in the process because (118) (33%) was formed, together with 3,3’,4,4’-tetrachlorodiphenyl sulphide (15%) and 2-methylsulphonylbenzaldehyde (27%). Consistent with the intermediacy of (116) in the formation of (118), irradiation of (119a and b) in the presence of p-methylthiophenol gave (120a and b) in respective yields of 62 and 58%: irradiation of (121) also yielded the cyclized halogensubstituted product (122). Although photodealkylation of amines is well known,lo7N-aryl bond cleavage in aryl amines is unusual. This year, however, the photochemical deamination of phenylenediamines in acid solution has been described.lo8 For example, the irradiation of NN-dimethyl-p-phenylenediaminehydrogen sulphate or hydrochloride in methanol yields aniline as the major product via bond homolysis. It appears that the monoprotonated diamine is the reactive species. The formation of biphenyl derivatives from the photolysis of benzenoid compounds has again been noted with several systems. Thus irradiation of 2-t-butyl-4-methoxyphenol in benzene solution has been reported to yield the biphenyl derivatives (123)-(125), together with the intriguing adduct (126),lo9 and from methyl benzenesulphonate in methanol solution, biphenyl, anisole, and benzene are formed via radical reactions.l1° Irradiation of N-arylsulphonylSS-dimethylsulphoximides (127) in either benzene or toluene leads to biphenyl, again by aryl radical coupling.lll Photoisomerization of the 0- and m-methylbiphenyls yields mixtures of the three isomers in which the m-isomer predominates. Under the reaction conditions (254 nm radiation) the p-isomer is photostable: benzvalene intermediates are suggested.lll Lablache-Combier and his group have done much to help achieve an understanding of the light-induced substitution reactions of pyridine, quinoline, and isoquinoline, and their derivative^.^^, 112 They have now published full details concerning the mechanism of photosubstitution of such compounds by methanol in neutral and acidified (HCI) media, and report that pyridine, quinoline, 4-methylquinoline, isoquinoline, and 9-phenylacridine give initially the corresponding In neutral media hydrogen-abstraction semiquinone radicals in both lo6
lo8 lo’ lo*
loS 110 111
113
P. Gouverneur and J. P. Soumillion, Tetrahedron Letters, 1976, 133. G. Buchholz, J. Martens, and K. Praefcke, Tetrahedron Letters, 1975, 3213. See A. Schonberg, G. 0.Schenck, and 0.A. Neumiiller, ‘Preparative Organic Photochemistry’, Springer-Verlag, New York, 1968, p. 255. D.P. Specht, J. L. R. Williams, T. H. Chen, and S. Farid, J.C.S. Chem. Comm., 1975, 705. M. Mihara, T. Kondo, and H. Tanabe, Shokuhin Eiseigaku Zasshi, 1974, 15, 270. Y.Izawa and N. Kuromiya, Bull. Chem. SOC.Japan, 1975,48, 3197. R. A. Abramovitch and T. Takaya, J.C.S. Perkin I, 1975, 1806. A. Lablache-Combier, ‘616ments de Photochimie AvancCe’, ed. P. Courtot, Hermann Press, Paris, 1972, p. 293. A. Castellano, J. P. Catteau, and A. Lablache-Combier, Tetrahedron, 1975, 31, 2255.
388
Photochemistry
occurs from the nrr* excited state in a single-photon process, whereas in acidified methanol the photoreaction apparently involves two photons. In this latter case the reaction arises via electron-transfer from the methanol to a protonated upper excited triplet state of the aza-aromatic (see Scheme 7). The authors conclude Quinolinium ion
uinolinium] ion
hv
H++ &H,OH
.5'1 +
[Quinolinium] ion
m ., bH --.-.. +
.+
\__*-
7'1
uinolinium]
Tn
+ p p H 3 0 H ]
N
I H Scheme 7
that when n electrons are available, the nn* excited aza-aromatic compounds react with the hydrogenated solvents, but on protonation of the N-atom the photoreaction follows another path. Unsensitized photosubstitution of quinoline2- and -4-carbonitriles in alcohol solvents to yield the corresponding l-hydroxyethyl derivatives has been well researched.l14 The process has now been examined in ethanol with benzophenone sensitization, and differences from the unsensitized reaction have become 8 ~ p a r e n t . l ~Quinoline-2-carbonitrile ~ (128) yields no substitution product, but triazapentaphene (131) results. 2-(Hydroxy-
(128) R1 d = CN,RR2 = H 1 (129) R' = CN, R2 = Me (130) R' = CN, R2 = C1 ,Ph (132) R ' = -C,OH, R2 = Me
& \
Ph
N
(131)
NN
xN '
N'
CN
(1 33)
diphenylmethyl)-4-methylquinoline(132) and 4,4'-bi(2,2'-dicyano)quinoline (133) are formed from (129) and (130), respectively. It would appear that the conversion of (128) into (131) proceeds from the %n* state via energy transfer, whereas formation of (132) and (133) involves the primary formation of the ketyl radical of benzophenone and its subsequent reaction with ground-state (129) and (130). A further example of the light-induced Friedel-Crafts reaction has been reported, involving the benzoylation of anthracene.lls Benzoyl-, p-toluoyl-, N. Hata and I. Saito, Bull. Chem. SOC.Japan, 1974, 47,942, and references therein. nS N. Hata and R. Ohtsuka, Chem. Letters, 1975, 1107.
114
116
T. Tamaki, J.C.S. Chem. Comm., 1976, 335.
Photochemistry of Aromatic Compoiinds 389 and p-anisoyl chlorides were all observed to react photochemically with the arene to yield 2- and 9-aroylanthracenes. The anthracene fluorescence is quenched by the aroyl chloride, and the involvement of an exciplex between the S1aromatic compound and the ground-state acid chloride is suggested, but as with so many systems this could not be substantiated by exciplex emission. The formation of acetophenone by irradiation of acetyl chloride (but not acetic anhydride) in benzene has previously been reported.llsa Photochemical substitution in 9,lO-anthraquinone and its derivatives continues to attract the attention of several groups. Filipescu and co-workers have reported that photohydroxylation of the parent quinone occurs readily in concentrated sulphuric acid with near-u.v. or visible light to yield 2-hydro~yanthraquinone.~~~ Unlike the l-isomer, 2-hydroxyanthraquinone is normally tedious to prepare but the present one-step reaction can readily provide it in > 80% chemical yields. There appears to be some controversy over the mechanism of the previously reported photohydroxylation of sodium 9,10-anthraquinone-2-s~lphonate.~~~ A new mechanism which rules out the participation of kinetically free hydroxyl radicals has been proposed,llg but Stonehill and Clark have criticized the mechanistic conclusions as inconsistent with some of their results.120 Further it would appear that the mechanism outlined in ref. 119 would not operate in aerobic systems for which mechanisms proposed by the authors of ref. 120 were designed. The photosynthesis of aminoanthraquinone sulphonates has been reported from the irradiation with visible light of aminoanthraquinone and sodium sulphite in aqueous pyridine.121 A 92.6% yield of l-aminoanthraquinone2-sulphonate is obtained under air or nitrogen, but yields with an oxygen atmosphere are poor as the sulphite is oxidized to sulphate. A mechanism involving the triplet quinone (Scheme 8) is suggested. Two groups have commented on the photosubstitution of halogens in halogenoanthraquinones by 3ArH"
+
SO,T
SO,'-
+
H
__j
+
Ar'
SOa
H ArH +
&'\
so,-
+ 0,
+ ArS0,-
+
Hb2
./H + ArH
ArS0,'-
+
Ar\
Ar
\
H
-/
H
SO3-
Scheme 8 D. Bryce-Smith, G. B. Cox, and A. Gilbert, Chem. Comm., 1971, 914. G. G. Mihai, P. G. Tarassoff, and N. Filipescu, J.C.S. Perkin I, 1975, 1375. 11* A. D.Broadbent and R. P. Newton, Canad. J. Chem., 1972, 50, 381. lleJ. L. Charlton, R. G . Smerchanski, and C. E. Burchill, Canad. J . Chem., 1976,54, 512. H. 1. Stonehill and K. P. Clark, Canad. J . Chem., 1976, 54, 516. lZ1 J. 0.Morley, J.C.S. Chem. Comm., 1976, 88. 11'
390
Photochemistry
amines. Eltsov and co-workers have reported that the formation of 1,ddiaminoanthraquinone from irradiation of alcohol solutions of ammonia and l-amino4-halogenoanthraquinone occurs via the triplet state of the quinone.122 Inoue and co-workers, who have previously reported on similar have outlined the mechanism for the photoamination of sodium 1-amino-4-bromoanthraquinone-2-sulphonate (134),124carried out in aqueous aerated isopropyl alcohol solutions containing ammonia or alkylamines. It is suggested that an exciplex is formed between the anthraquinone and oxygen and that this undergoes nucleophilic attack of the amine resulting in peroxide formation and the substituted product (135).
(134) R = Br R = NR1R2,where R1,R2 = H, alkyl
(135)
The irradiation of substituted K-region arene oxides (136) has been previously reported to yield the ring-expanded products (137).126 Photolysis (254 or 350 nm) of the derivative (138), however, is now found to give the substituted phenanthrene (139) via a triplet excited intermediate, and this is suggested to confirm the singlet multiplicity for the rearrangement of (136) to (137).126 12a
lZ3 lZ4 lZ6 126
0. P. Stadzinskii, N. I. Rtishchev, and A. V. Eltsov, Zhut. org. Khim., 1975,11, 1133. H. Inoue and M. Hida, Chem. Letters, 1974, 255, and references therein. H. Inoue, K. Nakamura, S. Kato, and M. Hida, Bull. Chem. SOC. Japan, 1975,48,2872. N . E. Brightwell and G. W. Griffin, J.C.S. Chem. Comm., 1973, 37. G. W. Griffin, S. K. Satra, N. E. Brightwell, K. Ishikawa, and N. S. Bhacca, Tetrahedron Letters, 1976, 1239.
391
Photochemistry of Aromatic Compounds
5 Intramolecular Cyclization Reactions As in past years, examples of a wide diversity of light-induced cyclization reactions have been described. Stilbene-phenanthrene type processes still attract considerable interest, particularly for their use in the synthesis of helicenes. The concept that the sum of the free valence numbers of atoms involved in the cyclization step should exceed unity (i.e. CF* > 1.0) has been widely used to predict the positions of preferred cyclization. However, two groups have reported that Mulliken electronic overlap populations calculated from extended Hiickel (EH) wavefunctions are valuable indications of reactivity in photocyclizations and dimerization~.~~~, lZ8In a combined report,12gthis approach has been used to show that the reactivity of several pentahelicene derivatives can be directly related to the EH first excited state electronic overlap population of the pair of atoms involved in photocyclization, as well as to the difference in electronic overlap population of these atoms resulting from electronic excitation. This approach is likely to be more widely adopted now that its utility has been demonstrated. Many publications have discussed the dihydrophenanthrene intermediate (140) in the stilbene cyclization process from the viewpoint of its stability, stereochemistry (cis or trans), and nature of the reactive state in its formafion.lao Molecular orbital and energy strain studies have now been applied to such photocyclizations, as illustrated by a discussion of the formation of (140), 10b,10c-dihydrodibenzo[c,g]phenanthrene (141), and 14a,l4b-dihydrodibenzo[b,g]phenanthrene (142) and their ground- and excited-state reactions.131 The particular objective was to determine the factors responsible in the (143) --f (142)
(143)
(144)
v
130
W. H. Laarhoven, T. J. H. M. Cuppen, and R. J. F. Nivard, Rec. Trav. chim., 1968,87,687. K. A. Muszkat and S . Sharafi-Ozeri, Chem. Phys. Letters, 1973, 20, 397. A. H. A. Tinnemans, W. H. Laarhoven, S. Sharafi-Ozeri, and K. A. Muszkat, Rec. Trav. chim., 1975, 94, 239. See, for example, T. J. H. M. Cuppen, and W. H. Laarhoven, J. Amer. Chem. SOC.,1972,94,
131
K. A. Muszkat, S. Sharafi-Ozeri, G. Seger, and T. A. Pakkanen, J.C.S. Perkin I, 1975, 1515.
lZ7 12*
lZB
59 14.
392
Photochemistry
and (143) -+(141) conversions for a number of features, including the absence of the cyclized product (144), the large (ca. 42 kJmol-l) energy barrier in the latter process which is not present in the former, the low quantum efficiency of the (141) -+ (143) conversion and its marked temperature dependence, the relatively slow thermal reopening of (141), and the reason for the strong fluorescence of (141) (0= 0.7) when other dihydrophenanthrenes are nonfluorescent. Stilbene moieties held in a cis configuration by the molecular structure undergo facile light-induced cyclizations, although tetraphenylcyclopentadienone and phenyl-substituted furans are e ~ c e p t i 0 n s . l ~The ~ complex reversible photochemistry of the rigid cis-stilbene system, dixanthylidene (145), has now been studied in great detail, and temperature and external spin-orbit perturbation effects have been The investigation indicates the existence of three labile photoisomers all of which revert thermally to the starting isomer. One of the photoisomers is light-stable and thermally stable below - 140 "C. The quantum efficiency of the conversion of (145) into this isomer decreases with temperature, but is enhanced (up to 220-fold) by the spin-orbit coupling perturbers molecular oxygen, CS2, and ethyl iodide, and hence the conversion is considered to arise from the triplet state. This isomer has a lifetime of 0.05 ms at 0 "C and is the photochromic isomer previously described 134 and observed in practically all dianthrone and dixanthylidene derivatives : its structure involves torsional twist of about 50" about the central bond in (145). The other two isomers are photolabile, are formed from the singlet state of (145), and one is possibly the
precursor of the other. Both are cyclization products of the 4a,4b-dihydrophenanthrene type and one of them, which is photochemically converted back into (145), is deduced to have structure (146). The precursor of this isomer is suggested to be a conformer having a lower thermal stability than (146). Helixanthen (147) is obtained by thermal dehydrogenation of (146) by either molecular oxygen or iodine.133 Full details have been published of the cyclization of the rigid chromophore in substituted 2,3-biphenylbenzo[b]furans (148) in the absence and presence of 13a
133
134
W. M. Horspool, J. Chem. SOC.( C ) , 1971,400; D. T. Anderson and W. M. Horspool, Chem. Comm., 1971, 615; W. H. Laarhoven, T. J. H. M. Cuppen, and R. J. F. Nivard, Rec. Trau. chim., 1968, 87, 687. R. Korenstein, K. A. Muszkat, M. A. Slifkin, and E. Fischer, J.C.S. Perkin ZZ, 1976, 439. R. Korenstein, K. A. Muszkat, and E. Fischer, Mol. Photochem., 1972, 3, 379.
Photochemistry of Aromatic Compounds
393
aliphatic arnine~.l~~g 136 In the former case, the use of a variety of solvents gives only the fully aromatic compound (149; 52%), whereas in the presence of n-propylamine, the 1,4-dihydro derivative (1 50) (65%) results with only relatively minor amounts (12%) of (149). It is concluded from deuterium-labelling experiments that hydrogen atoms from the n-propyl chain of the amine are incorporated I
in the product: hindered amines give only the aromatic product (149). From the observation that acenaphthylene is reduced when incorporated into the irradiation of (148) in the presence of n-propylamine, it is proposed that the hydrogens are eliminated during cyclization not as atoms but in a ‘reductive form’. There have been several publications describing the formation of a phenanthrene by stilbene cyclization as a secondary photoreaction : three are mentioned here. Photolysis (254 nm) of benzpinacol carbonate in methanol yields C 0 2 , Ph2C0, diphenylrnethoxymethane, and tetraphenylethylene : the last compound subsequently yields 9,lO-diphenylphenanthrene in a separate photochemical Two groups have studied the photochemistry of 1,l-diphenylsubstituted vinyl halides in benzene 138 and ether 139 solutions. Both report the formation of tolan and its derivatives, but from the former system 9-phenylphenanthrene also results by reaction of the solvent with the intermediate biradical (151) and subsequent cyclization. The authors of ref. 139 report the formation of phenanthrene from experiments involving 1,2-disubstituted ethylenes which were designed to elucidate the mechanism of the formation of the acetylenes from the 1,l-diphenylethylenes. The use of stilbene cyclization in the synthesis of helicenes is very well known,32 and further examples have been reported. A systematic study of the reaction 136
la8
13? 13*
A. Couture, A. Lablache-Combier, and H. Ofenberg, Tetrahedron Letters, 1974, 2497. A. Couture, A. Lablache-Combier, and H. Ofenberg, Tetrahedron, 1975,31, 2023. G. W. Griffin, R. L. Smith, and A. Manmade, J . Org. Chem., 1976, 41, 338. T. Suzuki, T. Sonoda, S. Kobayashi, and H. Taniguchi, J.C.S. Chem. Comni., 1976, 180. B. Sket, M. Zupan, and A. Pollak, Tetrahedron Letters, 1976, 783.
394
Photochemistry
Photochemistry of Aromatic Compounds
395
with the compounds (152) and (153) using circularly polarized light has been described.140 The optical yields from the two series follow similar trends, and no asymmetric syntheses are observed in cases of higher benzologues of [lo]-helicene using the 290-370 nm photoband circularly polarized at 313 nm. The same group have also reported on the photosynthesis of [11]-, [12]-, and [14]-helicenes by use of double c y ~ l i z a t i o n s ,a~ ~procedure ~ previously employed for [131helicene.lP2 Two routes to each helicene were designed in order to compare yields and allow structural assignments. Such structural proofs are based on the fact that in each case only the desired helicene can be formed as a common isomer by each procedure. Thus [lll-helicene is formed from (154) and (155) in 54 and 84% yields, respectively, and similar approaches are described for the other two helicenes with yields between 10 and 45% depending on the precursor. All irradiations were in benzene solution in the presence of iodine using Pyrexfiltered radiation. Laarhoven and Kuin have examined the photocheinistry of 2-(2-benzo[c]phenanthrylethenyl)-l,6-methano[l01annulene (156).143 The interest here is in the fate of the bridge on the annulene ring on photocyclization of the two aromatic moieties. Irradiation (360 nm) of an ethanol solution of the cis-trans mixture of (156) in the presence of iodine gives an isolated yield of (157) of 60%. It is not known at present whether the methano-group is eliminated before or after cyclization, but the absence of methano[ l01annulene derivatives from the reaction mixture indicates that the decomposition is probably faster than the cyclization. The present type of cyclization process has also been examined for polymersupported 1,2-diarylethylene~.l~~ Irradiation of a suspension in benzene of l-(4-formylphenyl)-Z(Zbenzo[c]phenanthryl)ethylene (158) and 1-(4-formylphenyl)-2-(2-naphthyl)ethylene (159) attached to a styrene-divinyl benzene copolymer yields (160) and (161), respectively, after hydrolysis. The latter conversion was also noted when the polymer was irradiated in the absence of solvent.
(158)
R=
\
/
A. Moradpour, H. Kagan, M. Baes, G. Morren, and R. H. Martin, Tetrahedron, 1975,2139. R. H. Martin and M. Baes, Tetrahedron ,1976, 31, 2135. lP2 R. H. Martin, G. Morren, and J. J. Schurter, Tetrahedron Letters, 1969, 3683. 143 W. H. Laarhoven and N. P. J. Kuin, Rec. Trau. chim., 1975,94, 105. 144 J. M. Vanest, M. Gorsane, V. Libert, J. Pecher, and R. H. Martin, Chimia, 1975, 29, 343. 140
141
396
Photochemistry CHO
+
It is known that irradiation of o-divinylbenzene yields (162) by a (4 2) addition, plus traces of tetralin, dihydronaphthalene, and naphthalene: 2,3-divinylnaphthalene (but not the 1,2-isomer) also gives 5% of a (2 + 2) cyc10adduct.l~~Such reaction has now been investigated with 2-vinylstilbene where a stilbene-type cyclization is also very pr0bab1e.l~~Indeed, 15% of l-vinylphenanthrene was formed, but the major product (70% yield) was exo-5-phenylbenzobicyclo[2,l,l]hex-2-ene (163), and only 2% of the endo compound was
obtained. The differences in behaviour between o-divinylbenzene and 2-vinylstilbene are attributed, at least in part, to conformational differences. The authors also reported the formation of (164) and the product corresponding to (163) by photolysis of 2,2’-divinylstilbene. The conversion of azobenzene into benzo[c]cinnoline has been the subject of two reports. The essential presence of intramolecular hydrogen bonding for photochemical cyclization of azobenzene-o-carboxylic acids has been proved by and the process has been the lack of reaction of the esters in neutral observed to occur with some other azobenzene derivatives (165) in 95% concenProlonged trated sulphuric acid, and in CH,Cl, in the presence of Lewis irradiation of (165) in CH2C12alone does not yield products. Cyclizations of 1,4-diaryIbuta-1,3-dienesystems which are rigidly held in the cisoid-diene configuration have been well studied by Heller and co-workers for a number of years, and further reports have appeared. Many of these compounds display phot ochromism. The (22,3Z)-isomer of 2- benzylidene-2,3-dih ydro3-mesityl-3-phenylmethylenebenzofuran (166) yields the (22,3E)-isomer on irradiation; this in turn undergoes photocyclization via a conrotatory mode to 145
146
14’ 1 4
J. Meinwald and P. H. Mazzochi, J . Amer. Chem. SOC.,1967, 89, 696; M. Pomerantz and G. W. Gruber, J . Amer. Chem. SOC.,1971, 93, 6615; J. Meinwald, J. W. Young, E. J. Walsh, and A. Courtin, Pure Appl. Chem., 1970, 24, 509. M. Sindler-Kulyk and W. H. Laarhoven, J . Amer. Chem. SOC.,1976,98, 1052. C. P. Joshua, V. N. R. Pillai, and P. K. Ramdas, Indian J. Chem., 1975,13, 290. ~C. P. Joshua and V. N. R. Pillai, Indian J . Chem., 1975, 13, 1018.
397
Photochemistry of Aromatic Compounds Me
II
(165) R1,R2 = H, Me
(166)
H (167)
the trans-6,7a-dihydro intermediate (167),149which yields the trans-5a,6-dihydrobenzonaphthofuran (168) at 80°C and above by a 1,5-hydrogen shift, and is photo-oxidized to (169). Similar studies have been made with succinic anhydride and N-phenylimides (170) 150 and furanones (171).151 From the former class of compound (170), a thermally stable photochromic system has been developed and the potential of a multiphotochromic system investigated; but only one of the expected two cyclizations in fact occurred. The quantitative photorearrangement of derivatives of (171) to (172) provides a convenient synthesis of apolignan derivatives. Me
Me
H
Ph (169)
(170) X = 0 or NPh
I
H
(172)
(171)
This year many examples have been reported which involve cyclization of an ethylene onto an aryl group. The 1,l-diarylethylenes (173), (174), and (175) should all undergo facile cyclization at the positions indicated if (see ref. 129) is the most important feature which controls the p h o t o p r o c e ~ s .All ~ ~ ~these compounds are photoreactive but the products are dependent upon the reaction
zF*
149
150 151 152
J. S. Hastings, H. G. Heller, and K. Salisbury, J.C.S. Perkin I, 1975, 1995. R. J. Hart, H. G. Heller, R. M. Megit, and M. Szewczyk, J.C.S. Perkin I, 1975, 2227. H. G. Heller and P. J. Strydom, J.C.S. Chem. Comm., 1976, 50. R. Lapouyade, R. Koussini, and J. C. Rayex, J.C.S. Chem. Comm., 1975,11, 676.
14
398
Photochemistry
conditions. In degassed cyclohexane solution, irradiation (300nm) of (173) yields 85% of 9,1O-dihydro-9-phenylphananthrene,and (1 74) behaves ~imilar1y.l~~ The reaction is sensitized by xanthone and Michler’s ketone and quenched by oxygen without yielding 9-phenylphenanthrene, so triplet states are considered to be involved. Irradiation of (173) under the normal conditions for the stilbene -+ phenanthrene conversion does induce a slow reaction to yield the phenanthrene. The light-induced reaction of compound (176),which is formed by photolysis of cannabinol (177),yields the phenanthrene by dehydration and
(ys’,”b@
C6H4R1
+-
Ph (173)
+-
Ph (174)
R2
(175) R’, R2 = H, OMe
r i n g - c l o s ~ r e . In ~ ~contrast ~ with (173)and (174), (175)is not affected by direct or sensitized irradiation, but in the presence of oxygen, iodine, or CuBr,, acenaphthylenes (178)are formed, probably by singlet excited stafes.ls2 The mechanism of photocyclization of substituted o-allylphenols to benzofuran and benzopyran derivatives has been studied, and the role of intramolecular hydrogen-bonding between the hydroxy-group and the n-electrons of the allylic of 3-(2-hydroxybenzylidene)group has been d e m o n ~ t r a t e d . ~Irradiation ~~ 4,5-dihydrofuran-2(3H)-one (179) results in intramolecular acylation and formation of (180).166 lS3 lb4 166
lS8
See also P. H. G . Op Het Veld, J. C. Langendam, and W. H. Laarhoven, Tetrahedron Letters, 1975, 231, and references therein. A, Bowd, D. A. Swann, and J. H. Turnbull, J.C.S. Chem. Comm., 1975,797. S . Geresh, 0. Levy, Y. Markovits, and A. Shani, Tetrahedron, 1975,31,2803. I. R. Bellobono, L. Zanderighi, S. Omarini, and B. Marcandalli, J.C.S. Perkin 11, 1975, 1529.
Photochemistry of Aromatic Compounds 399 The stereochemistry of the known aryloxyenone photocyclization has been studied by reference to the compounds (181)-(183).15' In all cases a high-yield reaction occurred to give specifically the cis-fused decalone ring product [e.g. (184) from (18l)l. The relatively strain-free carbonyl ylide (185) is suggested as an intermediate.
p
0
Jo 0
Ph (181) R = Me (182) R = COZCHZCH,
% O
/
\
Ph
(183)
M&$33 0-
H'o / \
Comments on the use of the photocyclization of N-benzoylenamines in the synthesis of berberine alkaloids continue to appear, and the process has been studied for 11 derivatives of (186), when the berbin-8-ones (187) and (188) are formed.168 The same group has demonstrated the use of this reaction by the first total synthesis of ( rf: )-cavidine.lS9 Cyclization of simple a/3-unsaturated anilides is a known process,lSoand has now been reported for benzo[b]thiophen-2-carboxanilide (189).161 The unsubstituted compound (189) yields 40% of
(186) R s = H, OMe, -OCH,O-,
NOz, C0,Me
A. G. Schultz and W. Y . Fu, J. Org. Chem., 1976,41, 1483. I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1721. I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1791. See, for example, I. Ninomiya, S . Yamauchi, T. Kiguchi, A. Shinohara, and T. Naito, J.C.S. Perkin I, 1974, 1747. Y. Kanaoka, K . Itoh, Y.Hatanaka, J. L. Flippen, 1. L. Karle, and B. Witkop, J. Org. Chem.,
lK7
lS8 16s
160 lS1
(
1975, 3001.
400
Photochemistry
A
(190) and 15% of (191) via oxidative cyclization, but under non-oxidative conditions (191) is the major product and only traces of (190) are formed. Under similar anaerobic conditions, the N-methyl derivative yields the trans-fused isomer of (191); and whereas irradiation of (189) in D 2 0 yields (191) with deuterium in the 14-position, similar reaction of the N-methyl compound yields both the non-deuteriated trans-form of (191) and the corresponding N-methyl cis-16deuteriated (191) derivative. From these results it is deduced that the cis-14-hydrogen comes almost exclusively from the media whereas the trans14-hydrogen originates from an internal source. The occurrence of cyclization of 3-aroylchromones is found to be very dependent on substituents.ls2 Thus whereas 3-benzoyl-2-methylchromone (192) is seemingly stable under the reaction conditions, its isomer 3-(o-toluoyl)- chromone (193) readily forms benzoxanthenone (195) via the enol (194). A novel intramolecular cyclization of the thioketone group onto aryl moieties was reported four years ago by Lapouyade and de Mayo,ls3 and further details
0
162
lo4
OH
0
OH
P. G . Sammes and T. W. Wallace, J.C.S. Perkin I, 1975, 1845. R. Lapouyade and P. de Mayo, Canad. J. Chem., 1972,50,4068. A. Cox, D . R. Kemp, R. Lapouyade, P. de Mayo, J. Joussot-Dubien, and R. Bonneau, Canad. J. Chem., 1975,53,2386.
401
Photochemistry of Aromatic Compounds
R I
pfyR
\
/
S (196) R
\
II
=
-CPh
/
have now been p~b1ished.l~~ Polycyclic aromatic thiones (196) having free peri positions cyclize on nr* excitation to yield thiophen derivatives [e.g. (197)l. The formal 1,3-hydrogen migration was demonstrated to be intermolecular in one case by incorporation of deuterium from D 2 0 during irradiation. With the a-naphthyl derivative, the excited state responsible for the reaction was shown to be the nn* singlet. Each year many accounts appear describing the cyclization of aryl groups separated by heteroatoms, and comparable cyclizations which involve the loss of HHal. Details of the reaction and mechanism of the conversion of diphenylamine into carbazole have previously been noted,32 and the synthetic scope of the process with the substituted triphenylamines has now been reported.ls5 For many derivatives the reaction failed, and only with R1 = R2 = H, F, or OMe was the corresponding carbazole formed. With PhN(Me)-p-C6H4N(Me)Ph, however, a 10%yield of the indolo[3,2-b]carbazole (198) was obtained. Two groups have commented on the cyclization of diphenyl ethers leading to dibenzofurans, and one paper also described a process with Me
QJ-pJAyJI
M@oD OMe
OMe Me Me
Me (198)
(199)
thiodiphenyl ethers leading to dibenzothiaphens with yields in the range 4 0 60%.le6 The second group are concerned with the reactions of polyfunctional 2-methoxyphenyl phenyl ethers which were obtained by degradation of lichen lE5 188
W. Lamm, W. Jugelt, and F. Pragst, J. prakt. Chem., 1975,317,284. K. P. Zeller and H. Petersen, Synthesis, 1975,8, 532.
402
Photochemistry
depsidones.lB7 Such compounds as (199) and three of its more complex derivatives bearing 2-methoxy groups yield the corresponding dibenzofurans by the now well-known procedure (see ref. 32) involving loss of the elements of methanol. In contrast, other methoxyphenyl phenyl ethers photoisomerized to hydroxybiphenyls. The mechanism of these processes and the structural factors which favour the cyclization are discussed on the basic assumption that irradiation of the ethers results in initial formation of an uncleaved biradical species which undergoes various reactions leading to biphenyls, cleavage products, or dibenzofurans: the most favourable pathway is that which involves the most stable biradicals, and the direction of cleavage in asymmetrical ethers to give the hydroxybiphenyl is of course determined by the stability of the intermediate radicals. In a comparison of the photoreactions of Ph2CH-X-CHPh2 systems (X = NH, CH2, 0, and S), the transition state leading to products (e.g. biphenyl) is suggested to resemble the cyclized product Incorporation of a halogen atom in one of the rings naturally causes a considerable change in the photochemistry, and the irradiation of aqueous solutions of 2-iododibenzylamine hydrochlorides has provided a convenient synthesis of 6,7-dihydro-SH-dibenz[c,e]azepines (201).ls@Similarly, N-(2-halogenobenzyl)-~-phenethylaminehydrochlorides yield the corresponding 5,6,7,8-tetrahydrodibenz[c,e]azocines (202).
(201) Iz = 1 (202) n = 2
Me0
OH (203)
OH (205)
A further example in natural product synthesis of the use of photocoupling of two aryl rings by loss of HX has been reported, and involves cyclization of the ( f)-bromodiphenyl (203) to the (+)-spirodienone (204) as a key step in the first synthesis of the aporphine alkaloid ( + )-boldine (2O5).l7O 16' 168
170
J. A. Elix and D. P. Murphy, Austral. J. Chem., 1975,28, 1559. R. W. Binkley, S. C. Chen, and D. G . Hehemann, J. Org. Chem., 1975,40,2406. P. W. Jeffs, J. F. Hansen, and G . A. Brine, J . Org. Chem., 1975,40,2883. S . M. Kupchan, C. K. Kim, and K. Miyano, J.C.S. Chem. Comm., 1976,91.
Photochemistry of Aromatic Compounds
403
Irradiation of the meta-bridged bromo-compound (206) yields the three transannular products (2O7)-(209).l7l Light-induced cyclization of 2,6-dichlorocinnamates (210) and loss of hydrogen chloride yields 5-chlorocoumarin by
(206) n = 2 or 3
(207) ?TI = 3, IZ = 2 (208) nt = IZ = 3 (209) nt = 4,IZ = 2
reaction from the singlet excited state of the cis cinnamate isomer and formation of an unstable ortho-quinomethylketen (211) as the product precursor.172Consistent with this proposal, irradiation of the cinnamate at - 190 "C yields a new red species with structured absorption out to 640 nm, and on warming to - 170 "C this spectrum is replaced by that of the coumarin: the i.r. spectrum of the red species is also consistent with structure (211). Among the products from the photolysis of the dienone (212) is the dehydrobrominated cyclized product (213),173and irradiation of 9-a-bromopropionylanthracene has been reported to yield 2-methyl-1-aceanthrenone(214) and 9-vinylanthryl ketone.17* N-Phenylpyrrole undergoes simple substitution by loss of hydrogen bromide with dibromomaleic anhydride, and the product (215) is ideally constructed for a further intramolecular photoprocess and indeed yields (216) as the final Both dehydrobrominations are considered to arise from triplet states. 171
178
173 17*
175
S. Hirano, H. Hara, T. Hiyama, S. Fujita, and H. Nozaki, Tetrahedron, 1975,31,2219. R. Arad-Yellin, B. S. Green, and K. A. Muszkat, J.C.S. Chem. Comm., 1976,14. C. W. Shoppee and Y. S. Wang, J.C.S. Perkin I, 1976, 695. T. Matsumoto, M. Sato, and S. Hirayama, Bull. Chem. SOC.Japan, 1975,48, 1659. T. Matsuo and S. Mihara, Bull. Chem. SOC.Japan, 1975,48,3660.
404
Photochemistry Br
Ph
Br
Ph
//
0
Me
Further details of the earlier reported 176 photocyclization of N-chloroacetyl2,5-dimethoxyphenethylamine have been but the products and suggested intermediates are unchanged and were reviewed two years The common feature in all these examples of this type of cyclization is intramolecular electron-transfer from the S1aromatic chromophore to the chloroacetyl moiety.179 The exciplex so formed undergoes C-Cl homolysis and the resulting radical couples with the aryl radical cation to yield the cyclized products. Such photocyclizations of the seven isomeric N-chloroacetylindolylethylamines (217) have been examined in attempts to correlate the reactivities of the positions with frontier electron densities calculated by unrestricted Hartree-Fock molecular orbitals.lso A variety of azepinoindoles and azocinoindoles are formed by cyclization at the ortho- and peri-positions. With the 3-, 4-, and 6-isomers of CH,Cl
CHzCHR
o=c'
"HCOCH,CI
R (217)
(218)
(217) high reactivity is observed, whereas the other positional isomers are less reactive and no cyclization at position 1 is detected. The mechanism suggested involves radical intermediates for the unsubstituted N-compounds and radicalcation species with N-alkyl derivatives. This reaction has also been examined by other workers with 2-(N-chloroacetylpiperidylalkyl)indoles, e.g. (21 8).lE1 In general, cyclization occurs at the indole 3-position, i.e. to yield (219) from (218), when the indolylalkyl group is attached to the 2- or 3-position of the piperidine 176 177 178 l7@
lS1
Y. Okuno and M. Kawamori, Tetrahedron Letters, 1973, 3009. Y. Okuno, M. Kawamori, K. Hirao, and 0. Yonemitsu, Chem. and Pharm. Bull. (Japan), 1975,23, 2584. See Vol. 6, p. 487. Y. Okuno and 0. Yonemitsu, Tetrahedron Letters, 1974, 1169. S. Naruto and 0. Yonemitsu, Tetrahedron Letters, 1975, 3399. R. J. Sundberg and F. X. Smith, J . Org. Chem., 1975,40,2613.
405
Photochemistry of Aromatic Compounds
ring : methanol is preferable to benzene as a solvent. Intramolecular cyclization onto the indole ring system has been noted in other cases. The irradiation of (220) in ethanol and in the presence of iodine yields both (221) and (222) in respective yields of 20 and 5O%.ls2 I n the absence of iodine, but the presence of air, only (222) is formed. With the isomeric derivative (223) of (220), only one product (224) is formed in oxidizing media, but the three products (225)-(227) result under non-oxidizing conditions.ls3 The indole (228) likewise yields (229) and (230) under oxidizing and non-oxidizing conditions, respectively. CN
(220) R
(219)
=
(223) R =
(221) X = N, Y = Z = CH (222) Y = N, X = Z = CH (224) X = Y = CH, Z = N
(228) R =
(229)
(230)
A novel route to N-bridgehead compounds by cyclization of l-styrylimidazoles has been described.ls4 For example, irradiation of the imidazoles (231) in methanol leads to cyclization at the 2-position of the imidazole ring and the formation of imidazo[2,l-a]isoquinolines(232). The reverse mode of cyclization involving 2-styrylbenzimidazoles in C-N bond formation has also been demonstrated,ls4and sterically hindered N-vinyliminopyridiniumylides (233) have been reported to yield a variety of products on photolysis, including the cyclized ylide (234).lS6 C . Dieng, C . Thal, H. P. Husson, and P. Potier, J . Heterocycl. Chem., 1975, 12, 455. C. Riche and A. Chiaroni, Tetrahedron Letters, 1975, 4567. la4 G. Cooper and W. J. Irwin, J.C.S. Perkin I, 1976, 75. Ia6 A. Kakehi, S. Ito, T. Funahashi, and Y . Ota, J . Org. Chern., 1976,41, 1570. 182
lS3
406
Photochemistry
(231) R1, R2, R3 = H, CO,Me, Me, Ph
(234)
(232)
(233) R
=
Me or Et
(235)
The cyclization arising from light-induced loss of sulphur from compounds (235) 186 and (236) is revieved in Part 111, Chapter 6. 6 Dimerization Reactions Work prior to 1975 on the light-induced dimerization of anthracenes has been very well reviewed by Cowan and Drisko.l** It has previously been suggested that the photodimerization observed with certain compounds in the crystalline state is the result of exciton trapping at dislocation sites in the Such exciton trapping properties of an idealized plane defect core have been studied theoretically, as has the macroscopically strained region around the core, and the theory has been applied to the case of crystalline anfhracene.lQo It is suggested that the core trapping initiates the dimerization, but that the compressive strains which are set up in the dimerization region following the formation of some dimer are responsible for the subsequent trapping to produce further dimer in these regions. The photochemistry and photophysics of a number of 9-substituted anthracene sandwich pairs have been studied in the corresponding photo-dimer crystal matrices, and in methylcyclohexane matrices at 6 K.lQ1The photodimerization of 9-methyl-, 9-chloro-, and 9-cyano-derivatives of anthracene in the dimer matrix at 6 K occurs with unit quantum yield, but the presence of excimer fluorescence from sandwich pairs indicates that the topochemical orientation is not perfect. Activation processes which lead to reaction involve molecular reorientation from more stable groundstate configurations, and these are achieved within the constraints imposed by the T. L. Gilchrist, C. J. Moody, and C. W. Rees, J.C.S. Perkin I, 1975, 1964. K. Praefcke and C. Weichsel, Tetrahedron Letters, 1976, 1787. lR8 D. 0.Cowan and R. Drisko, 'Elements of Organic Photochemistry', Plenum Press, New York, 1975, Chapter 2. lRe For reviews of the subject see: M. D. Cohen and B. S. Green, Chem. in Britain, 1973,9,490; J. M. Thomas and J. 0. Williams, Prog. Solid State Chem., 1971,6, 121. ln0 P. E. Schipper and S. H. Walmsley, Proc. Roy. SOC.,1976,348,203. lB1 J. Ferguson and S. E. H. Miller, Chem. Phys. Letters, 1975,36, 635. lR6 lR7
Photochemistry of Aromatic Compounds
407
solvent or crystalline cage. Other workers have also reported on the luminescence of the sandwich dimer of anthracene produced by photocleavage of dianthracene in methylcyclohexane at 77 K.lQaTopochemical dimerization has been used as a new method for enantiomeric purification.lQ3 The basic principle is to attach chemically a photodimerizable molecule to an enantiomerically enriched sample of an alcohol, amine, etc. The photodimerizable molecules containing the chiral group may crystallize in either the photoactive a-form or in the light-stable y-form, as the short distance for /3-packing is precluded by the bulky chiral group. Irradiation of the mixture then yields the meso photodimer from the a-form whereas the y-form is unaffected and easily separated from the reaction mixture. This
Me
I
oYo-z-Ar approach has been applied to the enantiomeric separation of three 1-arylethanols which were condensed with 9-anthroic acid to yield the corresponding anthroates (237). The crystalline esters were exposed to U.V. light, and the unaffected monomer was extracted simply from the sparingly soluble dianthracenes (238) which reverted to the monomers at their melting points. The chemical yield is quoted as >80% and the enantiomeric purities of the recovered unreacted monomer > 90%. It is to be hoped that this novel method of optical purification will be extended to other systems. The quantum yields of photodimerization and fluorescence have been measured for 9-anthroamide, and methyl, ethyl, n-butyl, t-butyl, and cyclohexyl9-anthroates, as a function of c o n ~ e n t r a t i o n .From ~ ~ ~ these efficiencies and data of fluorescence lifetimes, rate ratios and individual rate constants have been evaluated for several mechanistic schemes. Concentration quenching of the monomer fluorescence, formation of excimers, and photodimerization studies have been reported for anthracene derivatives which have substituents in the side rings of the anthracene nucleus.1Q5Such investigations have shown, not surprisingly, that steric constraints have profound effects on the formation of the excimers and photodimers. Somewhat similar studies have been made by Castellan, who has also examined Isa
lgs
Io4 lS5
P. C. Subudhi, N. Kanamaru, and E. C. Limy Chem. Phys. Lett., 1975,32,503. M. Lahav, F. Laub, E. Gati, L. Leiserowitz, and Z. Ludmer, J. Amer. Chem. Suc., 1976, 98, 1620. R. S. L. Shon, D. 0. Cowan, and W. W. Schmiegel, J. Phys. Chem., 1975,79,2987. I . E. Obyknovennaya, T. M. Vember, T. V. Veselova, and A. S. Cherkasov, Optika i Spektroskopiya, 1975,38,1127.
408
Photochemistry
the effect of solvent on the efficiency of the The quantum yields for dimerization of anthracene and some 2,4-substituted derivatives have been recorded, and it has been reported that disymmetry of charge on the meso positions, the presence of halogen and groups capable of inducing nn* transitions, and again steric constraints all hinder the process; but the effects of solvents (benzene, diethyl ether, acetonitrile, and ethanol) are only weak. The structures of the photodimers of tetracene produced from irradiation of M solutions in benzene have been determined by X-ray diffraction.lg7 2 x Two dimers are formed: structure (239) is assigned to the one which is soluble in organic media, and (240) to the insoluble isomer.
(240)
Three accounts have described various examples of the well-known 9,lO9’, 10’-intramolecular photodimerization of bianthryl derivatives. Applequist and Swart reported a new improved synthesis of 9,9’-dianthrylmethane derivatives and have re-examined their light-induced reactions as the previous studies 19* were apparently made ‘on the wrong Whereas (241a and b) gave (242a and b), (241c and d) were inert, an observation consistznt with the fact that no dianthracene with vicinal bridgehead halogens has yet been reported. The intramolecular dimerization of the dianthracene (243) to yield (244) has been studied in some The quantum yield of the reaction is wavelengthdependent and at 450-470 nm is reported to be 0.70 & 0.06, whereas at wavelengths shorter than 420 nm, it is lower at 0.45 & 0.04. The results are interpreted lB6
lB7
A. Castellan, Compt. rend., 1975, 281, C , 221. J. Gaultier, C. Hauw, J. P. Desvergne, and R. Lapouyade, Cryst. Structure Comm., 1975, 4, 497.
D. E. Applequist, M. A, Lintner, and R. Seale, J . Org. Chem., 1968,33,254. D. E. Applequist and D. J. Swart, J . Org. Chem., 1975,40, 1800. H. Shizuka, Y. Ishii, M. Hoshino, and T. Morita, J . Phys. Chem., 1976,80, 30.
IB8
lB8
aoo
409
Photochemistry of Aromatic Compounds
to mean that direct excitation to the transannular excited state at the longer wavelength is much more fruitful of reaction than excitation to the locally excited state [lLa or lBb] of an anthracene moiety. X
Y (241) a; X = Y = H b; X = H , Y = Br c; X = Y = B r
(242) a; X b; X
= =
Y =H H , Y = Br
d;X=Y=Cl
(243)
(244)
All previously reported intramolecular dimerizations of such systems have involved the anthracene 9,1@9’,1O’-positions : this year, however, an exception to this has been described. Thus Bouas-Laurent and his co-workers, who are well known for their studies on the intermolecular process, have observed that irradiation of bis-(g-anthryl)-l, 1,3,3-tetrarnethyldisiloxane (245) leads to unsymmetrical dimerization of the anthracene moieties with the formation of (246).201This is the first example of photodimerization involving the 1,4-positions and is rationalized by the steric hindrance of the bulky Me,Si groups preventing closure between the 9,lO- and 9’,1O’-positions. The dimers from phenanthrenes had been previously deduced to have head-totail structures (247) and cis configurations about the cyclobutane ring.202X-Ray structure analysis has now shown this assignment to be correct for the photodimer of 9-cyano-10-methoxyphenanthrene and, as with anthracene, an exciplex intermediate is 201
aoa a03
G. Felix, R. Lapouyade, H. Bouas-Laurent, and B. Clin, Tetrahedron Letters, 1976, 2277. R. Galante, R. Lapouyade, A. Castellan, J. P. Morand, and H. Bouas-Laurent, Compt. rend., 1973,277, C, 837. C. Courseille, A. Castellan, B. Busetta, and M. Hospital, Cryst. Structure Comm., 1975,4, 1.
410
Photochemistry
SiMe,
I I
0
(245)
(247)
Photodimerization in the naphthalene series is currently restricted to the p-cyano- and p-alkoxy-derivatives : thermal reactions of the trans-photodimer of 2-methoxynaphthalene have been reported this year.2o4 Although many examples of photodimerization of polynuclear aromatic compounds are known, uncondensed benzenoid aromatic rings do not yield such products and until recently the only example of the reaction with a monocyclic heteroaromatic compound involved the sunlight dimerization of 2-aminopyridine
(yoSJ.
bq cr 0
+
N
Ph
I
0
Ph
N
Ph (249)
R
1
R
=
H, 7-Me, and %Me
(251) 204
T. Teitei, D. Wells, P. J. Collin, G. Sugowdz,and W. F. H. Sasse, Austral. J. Chem., 1975,28, 2005.
41 1
Photochemistry of Aromatic Compounds
h y d r o ~ h l o r i d e .Katritzky ~~~ and Wilde have now reported that 3-oxido-l-phenylpyridinium (248) undergoes both light-induced valence bond tautomerism and dimerization.206 Thus photolysis (350 nm) of (248) in ethyl acetate leads to formation of (249), for which there is no precedent in pyridine chemistry, and (250) as primary products: the exo- and endo-isomers of (251) are formed thermally by addition of (249) to (248). Reversible photodimerization has also been noted with 2-methyl-sym-triazolo[1 ,5-alpyridines (252).207 7 Lateral-nuclear Rearrangements The mechanism of the photo-Fries reaction of phenyl acetate has been established in both the vapour and solution phases, and the involvement of radical intermediates has been demonstrated.208 The rearrangement has now been studied in the presence of p-cyclodextrin when the reaction showed some selectivity with the formation of the o- and p-hydroxyacetophenones in a 1 : 6.2 ratio; phenol production was decreased. Methyl-a-glucopyranoside was reported to have little effect.20gThe rearrangement has also been studied with a number of aryl esters and amides (253) in which the acyl part is derived from a-amino-acids.210 Although some variations were reported, approximately equal yields of the 1,2,3- and 1,2,4-isomers (254) were generally obtained. R
(253) R = H, Me, CHMe,, CH2Ph, 3-indolylmethyl, or (CH,),NHCO,CH,Ph X=OorNH
(255) R = H (256) R = Me
(257) (258)
O
(254)
R1 = NHCOPh, R2 = H R1 = H, K' = NHCOPh
Acetanilides have been known for many years to undergo the photo-Fries rearrangement, and work in this area is generally related to Shizuka and Tanaka's fundamental studies reported in 196tL211 The reaction has now been studied with the four fully aromatic amides (255)-(258) in ethanol solution with 254 nm radiation, and (255) has been subjected to a detailed investigation at various 206 208 207
2os 210
*11
E. C. Taylor and R. 0. Kan, J. Amer. Chem. SOC.,1963,85,776. A. R. Katritzky and H. Wilde, J.C.S. Chem. Comm., 1975,770. T. Nagano, M. Hirobe, M. Itoh, and T. Okamoto, Tetrahedron Letters, 1975, 3815. J. W. Meyer and G. S. Hammond, J. Amer. Chem. SOC.,1972,94,2219; C . E. Kalmus and D. M. Hercules, ibid., 1974,96,449. M. Ohara and K. Watanabe, Angew. Chem., 1975,87,880. H. Keroulas, C. Ouannes, and R. Beugelmans, Bull. SOC.chim. France, 1975, 793. H. Shizuka and I. Tanaka, Bull. Chem. SOC.Japan, 1968,41,2343.
412 Photochemistry wavelengths in a variety of solvents, and in the presence and absence of oxygen.212 It was reported that quantum yields for formation of 2- and 4-aminobenzophenones and products from free-radical precursors decreased as the solvent polarity was increased, and also with increase in the wavelength of the exciting radiation. The presence of oxygen had apparently no effect on the rearrangement, but benzoic acid was formed, it is suggested, from free radicals which escaped from the solvent cage. The authors tentatively interpret their results in terms of an energy-dependent radiationless transition to a reactive singlet state of the carbonyl group. Cleavage of the N-C bond follows to yield free radicals, as outlined in ref. 211. A type of photo-Fries reaction has been observed on photolysis of the aromatic enol-esters (259) to give the p-diketones (260).213 The well-known light-induced rearrangement of diphenyl ethers to hydroxy214 The work described in ref. 167 biphenyls 32 is the subject of two reports.le7~ followed from a study of the cyclization of such molecules to dibenzofurans, and the other report outlines the selective rearrangement of p-phenoxyphenol to phenylhy droquinone.
(259) R = Me, Ph, 2-naphthyl, or 2-anthryl
(260). 0-
Three years ago, a report on the photochemical rearrangement of azoxybenzene to hydroxyazobenzene apparently substantiated the proposal that the reaction proceeded by way of the cyclic intermediate (261),216but it has now been reported that the two isomers (262) and (263) photorearrange to the same o-hydroxyazo-compound (264).21a The mechanistic problem is effectively resolved by the further observation that (263) photoisomerizes to (262), so there is no need to disturb the earlier conclusion. 212 213
214
215
D. J. Carlsson, L. H. Gan, and D. M. Wiles, Canad. J. Chem., 1975,53,2337. D. Veierov, T. Bercovici, E. Fischer, Y. Mazur, and A. Yogev, Helv. Chim. Acta, 1975, 58, 1240. A. Ehrl, Atomkernenergie, 1975, 25, 293. D. J. W. Goon, N. G. Murray, J. P. Schoch, and N. J. Bunce, Canad.J. Chem., 1973,51,3827. N. J. Bunce, Canad. J. Chem., 1975, 53, 3477.
5 Photo-reduction and -oxidation ~
~~
BY H. A. J. CARLESS
1 Conversion of C=O into C-OH This year has seen a growing realization of the difficulties, which have sometimes been overlooked, when making quantitative measurements of the photoreduction of carbonyl compounds. Steel and co-workers1 have investigated the well known role of transient light-absorbing photoproducts [e.g. (2) and (3) in Scheme 11 believed to be Ph2C0 3Ph2C0
'Ph2C0
+ RH
2Ph2kOH
2Ph$OH
+ 'Ph2C0 Ph2kOH
+ R.
Ph,C(OH)C (OH)Ph2
-
(1)
Ph OH
P h 2 6 0 H -t R(3) Scheme 1
formed during the photoreduction of benzophenone in the presence of hydrogen donors. Both U.V. absorption spectra and benzophenone triplet lifetime measurements show the presence of transients which are sensitive to oxygen. The decay of two species with half-lives of 1.7 and 27 h in iso-octane is noted. The major photoproduct, benzpinacol (l), has a low quenching constant (4 x lo6 1 mol-1 s-l) for triplet benzophenone, but the unstable compounds appear to be diffusioncontrolled quenchers. This complicates any measurements of the quantum yield of photoreduction in hydrogen donors, because the transients can affect J. Chilton, L. Giering, and C. Steel, J. Amer. Chem. Soc., 1976, 98, 1865.
413
Photochemistry the measured values both by competing light absorption and by their tripletquenching effect. Steel et a1.l have used Fourier-transform n.m.r. as a useful method for the investigation of these light-absorbing transients. The advantages of this method are that solutions of relatively low concentrations (ca. moll-l) can be quickly analysed in sealed tubes, free from the complication of exposure to oxygen. The n.m.r. spectra show that benzpinacol is formed immediately as the major photoproduct from benzophenone in propan-2-01 or cyclohexane; it is not formed by a slow dark reaction of the light-absorbing transients. In fact, transients such as (2) and (3) can amount to only a small fraction of the total product (perhaps 2%). Schuster and his co-workers have raised some interesting general points concerning photoreduction in their study of light-intensity effects in the photochemistry of cyclohexadienone (4). The main products from irradiation of (4) in propan-2-01 are the reduction product p-cresol (3,the cyclopentenone ether (6), chloroform, and acetone (Scheme 2). The most important observation is a 414
0 6 kfMe2 0
OH
0
+ CHCl3 + Me,CO
-I-
Me CC1,
Me
Me
(4)
marked dependence of the quantum yield of p-cresol ( 5 ) formation on the light intensity, whereas formation of ( 6 ) is negligibly affected. Relevant values for formation of (5) in propan-2-01 are shown in Table 1, and similar trends are
Table 1 Eflect of 366 nm light intensity on the quantum yield of p-cresol(5) from dienone (4) Solution deoxygenated No No No No No a Yes
Yes
Light intensity / 10lephotons cm-2 min-l 6.89 12.4 27.7 106.5 104.3 6.9 108.3
Total photons absorbed1 1018 6.08 5.58 6.29 6.23 6.88 6.13 6.48
(5)
0.02 0.05 0.06 0.12 0.01 1.91 1.54
LSolutionstirred during irradiation.
observed in diethyl ether or cyclohexane. The differences between aerated and deoxygenated solutions are obvious, and are attributed to the interception of radical intermediates such as (7) by oxygen, thereby inhibiting the formation of ( 5 ) . Stirring lowers the quantum yield, indicating that the diffusion of oxygen in a
D. I. Schuster, G. C. Barile, and K. Liu, J . Amer. Chem. SOC.,1975, 97,4441.
415
Photo-reduction and -oxidation
the solution is important. At high light intensities, oxygen in the solution is rapidly depleted, and reaction proceeds by the mechanism outlined in reactions (1)-(5). In the absence of oxygen, the quantum yield for formation of ( 5 ) (4)
- hu
3(4)*
1(4)*
+ Me,CHOH
(7)
eCls
+ Me,CHOH + (4)
___+
Me,eOH
3(4)*
+ Me,eOH ( 5 ) + CCI, CHCl, + M&OH (7) + Me,CO (7)
(1) (2)
(3) (4) (5)
actually decreases slightly with increasing light intensity. This may be because the steady-state concentration of free radicals increases with light intensity, and radical-radical reactions such as reaction ( 6 ) (discussed later in this section) serve to terminate the free-radical chain of reactions (3)-(5). The triplet (4) 2Me,dlOH
Me,CHOH
+ MeC(OH)=CH,
(6)
is the precursor of the zwitterion (8), which leads to (6), and this pathway is not noticeably affected by oxygen. Formation of ( 5 ) is quenched by cyclohexa1,3-diene and by di-t-butyl nitroxide, although these quenchers cannot be intercepting the triplet state of (4) because the formation of (6) is little quenched. Consequently, it is proposed that the quenchers can act not only by tripletenergy quenching, but also by scavenging intermediate radicals, as previously suggested by other workers.a, Certainly the Stern-Volmer plot of ( 5 ) quenching against cyclohexadiene concentration is non-linear, with an initially steep portion at low diene concentrations. OH
0-
Schuster considers that more attention should be paid to light-intensity effects, especially in reactions with radical intermediates. He points out that some of the observed changes in the course of photochemical reactions on varying the wavelength of the incident light could be due to light-intensity effects in such systems. A further publication has reinforced the view 6, that decafluorobenzophenone (9) is unsuitable for use as an actinometer. Not only do fast dark reactions occur when irradiating low concentrations of (9) in propan-2-01, but also the reaction becomes more complex with increasing concentration of (9). a
ti
P. J. Wagner, J. M. McGrath, and R. G. Zepp, J. Amer. Chem. SOC.,1972, 94, 6883. D. R. Charney, J. C. Dalton, R. H. Hautala, J. J. Snyder, and N. J. Turro, J. Amer. Chem. SOC.,1974, 96, 1407. G. Gauglitz and U. Kolle, J. Photochem., 1975,4, 309. J. Dedinas, J. Amer. Chem. SOC.,1973, 95, 7172. P. Margaretha, J. Gloor, and K. Schaffner, J.C.S. Chem. Comm., 1974, 565.
41 6 Photochemistry Murai and Obi have investigated the photochemistry of benzophenone, acetophenone, and benzaldehyde in alcoholic solvents at 77 K under high light intensities (although the actual fluxes used are not mentioned). Whereas the lowest triplet excited state of benzophenone does not produce the ketyl radical (Scheme 3; X = Ph) below 100 K and at normal light intensitie~,~ reactions do N
OH
0
II
(Ph-C-X)"
+ RH
I
Ph-C-X
+
Rm
Scheme 3
appear to take place at high light intensities via a biphotonic process involving a higher excited triplet state of the carbonyl compound.8 This hydrogen abstraction from solvent produces ketyl radicals and solvent-derived radicals (Scheme 3 ; X = Ph, Me, or H), both of which were detected by e.s.r. In the case of benzaldehyde, competing a-cleavage to produce benzoyl radicals is also important. Laser photolysis of benzophenone in benzene and cyclohexane leads to the ketyl radical (Ph&OH),1° which could be made to fluoresce by simultaneous irradiation with a beam of electrons.ll An e.s.r. study has been made of the ketyl radical produced on irradiation of benzophenone labelled at the carbonyl by 13C.12 Nanosecond flash photolysis measurements suggest that transient hydrogen abstraction by triplet benzophenone provides a pathway in its deactivation route, even in the absence of photoprodu~ts.~~ In the presence of the hydrogen donors NN-dimethyltoluidine or 3-methylindole, xanthone reacts to give the xanthone ketyl radi~a1.l~Quenching of this ketyl radical by oxygen is very rapid ( k = 2-3 x lo91 mol-l s-l). Using a specifically deuteriated steroid, Breslow and co-workers l5 have confirmed the earlier proposal l6for the mechanism of the stereospecific functionalization of the steroid nucleus by means of the photochemical hydrogen abstraction of attached benzophenone esters. The photoreduction of the bridgehead phenyl ketones (10)-(12) has been rep0rted.l' Both pinacol and carbinol are formed in the yields shown in Scheme 4,the remainder of the products arising from a-cleavage and subsequent reactions. A re-investigation of the photochemistry of methyl 2-naphthyl ketone shows that dilute solutions in propan-2-01 do appear to undergo slow reduction on irradiation at 350 nm, although the reaction products could not be isolated.18 In more concentrated solutions, triplet self-quenching leads to a reduction in product formation. H. Murai and K. Obi, J. Phys. Chem., 1975,79,2446. T. S. Godfrey, J. W. Hilpern, and G. Porter, Chem. Phys. Letters, 1967, 1, 490. l o 0. Brede, W. Helmstreit, and R. Mehnert, 2.phys. Chem. (Leipzig), 1975, 256, 505. l1 B. W.Hodgson, J. P. Keene, E. J. Land, and A. J. Swallow, J . Chem. Phys., 1975,63, 3671. la H. Murai, M. Jinguji, and K. Obi, J. Phys. Chem., 1976, 80, 429. l3 M. R. Topp, Chem. Phys. Letters, 1975, 32, 144. l4 A. Garner and F. Wilkinson, J.C.S. Faraduy 11, 1976, 72, 1010. l 5 R. L. Wife, D. Prezant, and R. Breslow, Tetrahedron Letters, 1976, 517. l6 R. Breslow, S. Baldwin, T. Flechtner, P. Kalicky, S. Liu, and W. Washburn, J. Ainer. Chem. SOC.,1973, 95, 3251. l 7 H.-G. Heine, W. Hartmann, F. D. Lewis, and R. T. Lauterbach, J. Org. Chem., 1976, 41, 1907. l 8 D.I. Schuster and M. D. Goldstein, Mol. Photochem., 1976, 7 , 209.
417
Photo-reduction and -oxidation
b4b
Ph
OH OH
0
II
Ph-c-R
I
Mc,CHOHp
1
OH
Ph-C-C-Ph I I
+ Ph-C-RII
36% 26% 74%
9% 11% 17%
I1v
R R
H
Scheme 4
Photoreduction of the benzanthrone (13) occurs on irradiation in hexane or alcohol ~ o l u t i o n s .Presumably, ~~ the 7 ~ nature 7 ~ ~of the lowest excited states accounts for the low quantum yield (ca. at 365 nm) of reduction. Irradiation
of 3-acetylcoumarin (14) in propan-2-01 produces a C-4-linked dihydro-dimer in high yield.20 Hydrogen abstraction by the acetyl group, followed by dimerization of the resulting coumarinyl radicals, is thought to be responsible for this reaction. Thus, a similar irradiation of 3-methoxycarbonylcoumarin in propan-2-01 does not lead to reduction. An interesting paper has appeared concerning the detection by CIDNP spectroscopy of enols formed during the photoreduction of aliphatic aldehydes and ketones.21 The enols (with lifetimes of 1@ 20s) are formed by disproportionation reactions of ketyl radicals bearing a hydrogen atom on the carbon adjacent to the radical centre (e.g. as shown for acetone in Scheme 5). These enols seem to be a general feature of aliphatic aldehyde and ketone photoreduction. Consequently, the enolization reaction presents an important route to carbonyl deactivation, because slow thermal ketonization of the enol regenerates starting materials. A study of [2H6]acetone photoreduction by propan-2-01 enables the ratio of rate constants to be found for the reactions in I @ P. Bentley, J. F. McKellar, and G. 0. Phillips, J.C.S. Perkin ZZ, 1975, 1259. 2o
21
K.-H. Pfoertner, Helv. Chim. Acta, 1976, 59, 834. B. Blank, A. Henne, G . P. Laroff, and H. Fischer, Pure Appl. Chem., 1975,41,475.
418
Photochemistry Me,CO
+ Me,CHOH
k
hv
-4 Me,C(OH)C(OH)Me,
2Me$OH kb
I Me,CHOH Scheme 5
+ MeC(OH)=CH,
Scheme 5. Disproportionation of the ketyl radicals (kd) predominates over combination (kc),kd/kc = 3.4, and over the back-reaction which involves OH hydrogen abstraction (kb), kb/kd = 0.3, in acetonitrile at 26 "C. Irradiation of acetone-propan-2-01 mixtures at room temperature produces pinacol as the sole isolated product. However, irradiation at low temperature ( - 70 "C) increases the lifetime of the enol so much ( 2 5000 s; cf. 15 s at room temperature) that the major product becomes an oxetan (15), formed by photocycloaddition of acetone to its eno1.22 MeC(OH)=CH,
+ Me,CO
'lv
H O Q , M e Me Me
CIDNP studies on benzaldehyde photolysis in hexane solution have shown that hydrogen abstraction by triplet benzaldehyde occurs from another molecule of ground-state benzaldehyde [reaction (7)] rather than from hexane, giving a ketyl and a benzoyl radical.2s Further CIDNP studies now reveal a rate constant 3PhCH0
+ PhCHO
-
PhCHOH
+ PhkO
(7)
of 1 x lo61mol-1 s-1 for any subsequent exchange reaction between the ketyl radical and ground-state benzaldehyde [reaction (8)].24 PhCHOH
+ PhCHO
PhCHO
+ PhCHOH
(8)
A full account of the CIDNP spectra arising from irradiation of aliphatic aldehydes in a variety of solvents has been p~blished.,~The relative importance of hydrogen abstraction us. a-cleavage in different solvents varied only for propionaldehyde; acetaldehyde gave self-abstraction, whereas isobutyraldehyde and pivalaldehyde appeared to give only a-cleavage in all the solvents studied. A CIDNP study of photoreactions of formaldehyde in solution showed that the primary process was hydrogen abstraction by triplet formaldehyde from another ground-state formaldehyde molecule.2s A similar self-abstraction step can be the initiating process in aldehyde photo-oxidation in the liquid phase for but-2-ena1, heptanal, and ben~aldehyde.~' 22
2s 24 26
as
A. Henne and H. Fischer, Helv. Chim. Acta, 1975, 58, 1598. P. W. Atkins, J. M. Frimston, P. G . Frith, R. C. Gurd, and K. A. McLauchlan, J.C.S. Faraday ZZ, 1973, 69, 1542. P. G. Frith and K. A. McLauchlan, J.C.S. Faraday ZI, 1975, 71, 1984. H. E. Chen, M. Cocivera, and S. P. Vaish, Canad. J. Chem., 1975,53,2548. J. A. Den Hollander and J. P. M. Van der Ploeg, Chem. Phys. Letters, 1976,37, 149. J. C. Andre, M. Bouchy, and M. Niclause, J. Photochem., 1976, 5, 1.
Photo-reduction and -oxidation
419
Funke and Cerfontain2* have examined in detail the photoreduction of cyclopropanecarbaldehyde (16) and cyclobutanecarbaldehyde. For example, (16) irradiated in propan-2-01 led to the nine products shown in Scheme 6. The
Me,CO
Me,C(OH)C(OH)Me,
Me(CH,),CHO
0 OH
0 0
II
+
II
0 OH0 II I II
A
H OHOH
+
[t-CH,OH
+
PC-C I -M / e
I
I
0
+
pC-(C II H , ) , C H O
H Me Scheme 6
first step would be expected to be formation of the cyclopropylhydroxymethyl radical (17): the structures of all the observed products are understandable in terms of three competing reactions of (17), viz. radical combinations, hydrogen abstraction from solvent and from aldehyde, and rearrangement to 4-oxobutyl radicals [(lS) in Scheme 71. Relatedly, Davies and Muggleton 29 report that the
(19)
Scheme 7
same radical (17) ring-opens and also rearranges via enolic hydrogen abstraction to give (19), so that some other reaction products might possibly have been expected in Funke and Cerfontain's work. Photoreduction of cyclopropyl methyl ketone in the presence of 1-cyclopropylethanol leads to a similar ringopening and rearrangement of the 1-cyclopropyl-1-hydroxyethyl 28
29
C. W. Funke and H. Cerfontain, J.C.S. Perkin II, 1976, 669. A. G. Davies and B. Muggleton, J.C.S. Perkin II, 1976, 502.
Photochemistry
420
whereas photoreduction of cyclobutanecarbaldehyde gives no evidence for ring-opening of the cyclobutylhydroxymethyl Irradiation of the ?&unsaturated carbonyls (20) in hydrogen-donating solvents such as pentane, propan-2-01, or toluene led to photoreduction of the carbonyl group of (20) as one of the observed reactions.30 0
(20)
R
=
Me or H
McKelvey has begun a series of experiments aimed at understanding photochemical hydrogen abstraction from carbohydrates, using ketone sensitizers and Abstraction occurs model compounds such as 2-metho~ytetrahydropyran.~~ from C-2, and the methoxytetrahydropyranyl radical then undergoes the further reactions of methyl loss or ring-breaking. A further paper 32 extends this work to the 2-methoxy-4-methyltetrahydropyrans(21) and (22), and there is an interesting conformational effect on abstraction. Both isomers (21) and (22) give the products shown in Scheme 8, as might have been expected from the earlier
M&OMe,
0
Do +
Ph &CO,Me
However, (21) reacts eight times faster than (22), showing a preference for axial hydrogen abstraction. These results can be explained by an anomeric effect: if oxygen non-bonding orbitals antiperiplanar to the C-H bond being broken stabilize the transition state for abstraction, (21) (two interactions) would be more reactive than (22) (one interaction). Irradiation of 1,4-dioxan leads, by hydrogen abstraction, to two pairs of diastereoisomers, (23) and (24).33 It is postulated that a dioxyl radical (25) gives ring-breaking similar to that mentioned above,31 leading to ethoxyacetaldehyde, then (photochemically) to acetaldehyde, and the photoreduction of this in dioxan gives the diastereoisomeric alcohols (24). It is not obvious whether absorption by impurities or direct absorption by dioxan provides the original source of dioxyl radicals (25). M. P. Zink, H. R. Wolf, E. P. Miiller, W. B. Schweizer, and 0. Jeger, Helv. Chim. Acta, 1976, 59, 32. a1 3a
33
R. D. McKelvey, Carbohydrate Res., 1975, 42, 187. K. Hayday and R. D. McKelvey, J. Org. Chem., 1976,41,2222. P. H. Mazzocchi and M. W. Bowen, J. Org. Chem., 1975,40, 2689.
42 1
Photo-reduction and -oxidation H
Ledwith 34 has reviewed the photoinitiation of polymerization by aromatic carbonyl compounds, quoting examples which involve photochemical hyd,rogen abstraction as the radical-generating step. The diphenylketyl radical (Ph,COH), generated photochemically by irradiation of benzophenone in propan-2-01, gives a novel radical substitution reaction on the 4-cyanopyridinium ion, probably as a result of an electron-transfer reaction from ketyl to 4-cyanopyridinium Previtali and Scaiano 38 have continued their theoretical study of the photoreduction of carbonyl triplets, applying it this time to the rates of hydrogen abstraction from bonds other than C-H. The furan-2,3-dione (26) reacts photochemically at the C-3 carbonyl group with cyclohexene or 2-methylbut-2-ene, to give mixtures of oxetans and hydrogen-abstraction Choo and Wan 38 have made a comparative study of CIDNP and CIDEP (electron polarization) spectra in the photoreduction of pyruvic acid in hydrogen-donating solvents. Ammonia has been found to give an unusual electron-transfer catalysis in the photochemical hydrogen abstraction of anthraquinone (AQ) (27) from the 0
0
relatively poor hydrogen donor t-butyl Irradiation of (27) in the presence of t-butyl alcohol gives the adduct (28)) but the quantum yield of reaction is greatly enhanced by ammonia (@ = 0.0058 becomes @ = 0.10 in the a4
36 3*
37 38 9B
A. Ledwith, J. Oil Colour Chemists' ASSOC.,1976, 59, 157. B. M. Vittimberga, F. Minisci, and S. Morrocchi, J . Amer. Chem. SOC., 1975, 97, 4397. C. M. Previtali and J. C. Scaiano, J.C.S. Perkin ZI, 1975, 934. W. Friedrichsen, Annalen, 1975, 1545. K. Y.Choo and J. K. S. Wan, J. Amer. Chem. SOC.,1975,97,7127. G. G. Wubbels, W. J. Monaco, D. E. Johnson, and R. S. Meredith, J. Amer. Chem. SOC., 1976,98, 1036.
422
Photochemistry
presence of 0.6M NH3). Quenching experiments suggest that the reaction goes through a triplet intermediate of (27), which interacts more rapidly with ammonia than with t-butyl alcohol. A mechanism is proposed [and outlined in reactions (9)-(13)] which involves exciplex formation between triplet AQ as acceptor and ammonia as donor [reaction (lo)]. Hydrogen-atom abstraction by the exciplex, 3AQ
[AQ'-NH,'+] AQ'-
+ Me,COH 3AQ + NH3
''
>
'lo
k
[AQ'-NH,'+] ---+ Me,COH
+
+ *CH,C(OH)Me,
(H+) _I___,
AdH
+ *CH,C(OH)Me,
[AQ'-NH;+]
+
(10)
AQ NH3 NH4+ AQ*-
+
(9)
(1 1)
+ *CH,C(OH)Me, (12)
(28)
(13)
or possibly by free ammonia radical ion (NH3'+), leads eventually to (28). A kinetic analysis shows that k , = 7.1 x lo4 lmol-ls-l, whereas klo = 2 x lo7Imol-ls-l, thus showing the important role of ammonia in complex formation. McLauchlan and Sealy 40 have questioned the assumption that triplet quinones always react with alcohols (e.g. R,CHOH) to produce hydroxyalkyl radicals (e.g. R&OH). E.s.r. spin-trapping experiments have led to the detection of alkoxy-radicals (e.g. R,CHO*) which may have been formed directly or else by initial electron transfer [reactions (14) and (15); Q = quinone]. Although these workers were unable to obtain a precise estimate of the quantum yield of
+ RzCHOH RZCHOH" + RZCHOH 3Q
___+
+ R,CHOH*+ R,CHO* + RzCHOHz+
Q*-
(14)
(15)
alkoxy-radical production, they do provide indications that it is a significant process in the photochemistry of quinone-alcohol systems. Combined CIDNP and CIDEP studies have been made of the photoreduction , ~ ~CIDNP studies of the photoof tetrafluoro-p-benzoquinone in d i ~ x a n and reduction of a series of 1,4-benzoquinones in propan-2-01 have been r e p ~ r t e d . " ~ The much-researched photoreduction of duroquinone (D) (29) has received further attention.43 The triplet state of duroquinone is formed with unit efficiency following excitation in cyclohexane, ethanol, or water. Quantum 0
0 (29) 40 41 42
4a
OH (30)
K. A. McLauchlan and R. C. Sealy, J.C.S. Chem. Comm., 1976, 115. H. M. Vyas and J. K. S. Wan, Canad. J. Chem., 1976, 54, 979. D. A. Hutchinson, H. M. Vyas, S. K. Wong, and J. K. S. Wan, Mol. Phys., 1975,29, 1767. E. Amouyal and R. Bensasson, J.C.S. Faraday I, 1976,12, 1274.
423
Plioto-reduction and -oxidation
yields of photoreduction to the semiquinone radical (*DIP) are 0.4 5 0.1 in ethanol, 0.09 k 0.03 in cyclohexane, and 0.00 in water. Triplet-triplet annihilation [reaction (16)] is the only pathway for photoreduction of (29) in water, and obviously becomes appreciable at high 3D concentrations, producing 3D + 3D
-
Do+
+ Do-
(1 6 )
the radical anion (D*-) which leads on to reduction product. Formation of the duroquinone methide (30) is not a major pathway for deactivation of the 3D state in The quantum yield of photoreduction of 1,4-naphthoquinone is equal at two wavelengths (0ca. 0.90 at 334 nm and 436 nm), corresponding to transitions into the m* and nr* states, which implies unit efficiency for the m* -+ nrr* c o n v e r ~ i o n . ~ ~ Electron transfer from hydroxide ion produces the quinone radical-anion on photoreduction of p-benzoquinone or 1,4-naphthoquinone in water,46 or of anthraquinone in ethanolic potassium hydro~ide.~' The photoreduction efficiency of quinone sulphonates in water is affected by the presence of cationic surf act ant^.^^ Cohen and his co-workers have continued their extensive studies of the photoreduction of ketones by amines, devoting their attention this time to the effect of amine concentration and solvent on the photoreducti~n.~~ The generally accepted pathway (Scheme 9) for reduction involves rapid formation of a chargetransfer complex (31) between triplet ketone and amine. Then, either hydrogen
c 0
II
C,
0
+
>N'
I
CH
' \
Scheme 9
transfer to produce radicals (a) or quenching (6) occurs. Such a mechanism would predict a linear plot of (quantum yield)-l us. (amine concentration)-l. However, plots for photoreduction of benzophenone by cyclohexylamine in either benzene or t-butyl alcohol are curved. Higher quantum yields are obtained at higher amine concentrations (> 0.02 moll-l) than would be expected 4p 46 46
47 4a
49
D. Creed, J.C.S. Chem. Comm., 1976, 121. J. Rennert and P. Ginsburg, J. Photochem., 1975, 4, 171. S. Hashimoto, H. Takashima, and M. Onohara, Nippon Kagaku Kaishi, 1975, 1019. V. Ya. Oginets, Khim. vysok. Energii, 1975, 9, 190. K. Kano, Y. Takada, and T. Matsuo, Bull. Chem. SOC.Japan, 1975,48, 3215. A. H. Parola, A. W. Rose, and S. G. Cohen, J. Amer. Chem. SOC., 1975, 97, 6202.
Photochemistry from extrapolation of the results at low amine concentrations. The effect is less marked in aqueous pyridine (at pH 12) for the reduction of 4-benzoylbenzoate ion by triethylamine, and is not evident in the reduction by 2-butylamine under these conditions. Explanations for these results are based on two related arguments: (i) the amine catalyses the transfer of a proton from radical-cation to radical-anion (a) in the compIex (31), (ii) a ground-state complex of ketone and amine is formed which, after excitation, interacts with amine in solution to produce (31). It seems that much more work is required before these proposals could be proven. Arimitsu and co-workers 50 have extended their earlier laser photolysis studies 51 of the quenching of triplet benzophenone by various amines. Tertiary aromatic amines (e.g. NN-diethylaniline and NN-dimethyl-p-toluidine) give rise to electron transfer in acetonitrile as solvent, producing benzophenone radical-anion and amine radical-cation. However, hydrogen abstraction is the observed process in benzene, producing the benzophenone ketyl radical. Using a range of solvents, it has been shown that the two processes of electron transfer and hydrogen abstraction compete according to the polarity of the solvent. The sum of the quantum yields for ionic dissociation and photoreduction is unity for both the benzophenone-diethylaniline and the benzophenonedimethyltoluidine systems. Primary and secondary aromatic amines or aliphatic amines produce the benzophenone ketyl radical in all the solvents used. Roth and Manion have been able to distinguish the spectra of the neutral aminoalkyl radicals (32) and the aminium radical-cations (33) by means of their CIDNP hyperfine coupling p,p’-Disubstituted benzophenones (X = Cl, Me, or MeO) irradiated in acetonitrile in the presence of triethylamine 424
(33)
(34)
gave CIDNP signals assigned to diethylvinylamine which implied the intermediacy of neutral radicals (32). On the other hand, irradiation of decafluorobenzophenone (9) in acetonitrile in the presence of NN-diethyl-p-toluidine suggested the radical-cation (33) as an intermediate. The amine NN-diethylp-toluidine reacted with several other triplet aromatic ketones in acetone to give evidence for both (33) and (32), i.e. an electron-transfer process and a net hydrogen abstraction. These results are certainly in agreement with those of Arimitsu et aLY5Oand illustrate the importance of solvent in determining the species formed in such systems. Unfortunately, Roth and Manion52could not tell whether species (33) and (32) were formed consecutively or independently. A full account has appeared of the a-diketone (triplet) sensitized decomposition of the acetoin derivatives (34) where R is a nitrogen-containing substituent such as pyrrol-Zyl or ind01-3-yl.~~Electron transfer from nitrogen to photoexcited 60
61 s2 63
S. Arimitsu, H. Masuhara, N. Mataga, and H. Tsubomura, J. Phys. Chem., 1975,79,1255. S. Arimitsu and H. Masuhara, Chem. Phys. Letters, 1973, 22, 543. H. D. Roth and M. L. Manion, J. Amer. Chem. SOC.,1975,97, 6886. H.-S. Ryang and H. Sakurai, J.C.S. Perkin I, 1975, 1590.
Photo-reduction and -oxidation
425
a-diketone leads to a further fragmentation through the N-containing radicalcation (see Vol. 6, p. 521). A study of the photoreduction of 4-benzoylbenzoate ion by methionine (35) and related compounds (36)-(43) shows the relative importance of chargetransfer interaction of the excited ketone with either sulphur or nitrogen as the electron-donor atom.54 For example, triplet ketone interacts with the methionine anion to produce the triplet exciplexes shown in Scheme 10. Later steps lead I\
3(
,C = 0) -k MeSCH,CH2CH(NT3,)CO2-
Scheme 10
on to decarboxylation and hydrogen abstraction which yields ketyl radicals. Quantum yields of ketyl radical production (@)ketyl) and rate constants for complexation (kh) are shown in Table 2. For the S-containing compounds Table 2 Photoreduction of 0.003 mol 1-1 4-benzoylbenzoate ion by 0.04 mol 1-1 methionine and related compounds pH 12 7
Reducing agent MeSCH,CH,CH(NH,)CO,MeSCH,CH,CH(NHCOMe)C02MeSCH2CH2CH2NH, MeSCH,CH,CH,CO,MeSCH,CH,CH,NHCOMe MeCH(NH,)CO2MeCH(NHCOMe)CO,MeCH(NHC0Me)Me MeOCH,CH,NH,
PH7
7 7
kirl
kir/
@)k&pl
0.90 0.43
0.28 0.09 0.09 0.93 0.03 0.07 0.67
1 mol-l s-l 1.6 x log 1.5 x lo9 2.0 x 109 1.3 x 109 1.3 x lo9 1.6 x lo8 2.1 x lo6 1.0 x lo6 1.8 x lo8
@ketsrl
0.55 0.12 0.11 0.13 0.07 0.26 0.05 0.05 0.21
1 mol-l s-l 1.2 x 109 1.5 x 109 1.7 x 109 1.1 x 109 1.1 x 109 -2 x 105 -2 x 105 -3 x 105 1.1 x 106
(35)-(39), values of ki, are high (1-2 x lo9 1 mol-1 s-l) and independent of pH or other functional groups. The amino-compounds (40) and (43) show values of kh about an order of magnitude smaller, at pH 12. In contrast, these amines are largely protonated at pH 7, and the values for ki, are small. The b4
S . G . Cohen and S . Ojanpera, J. Amer. Chern. SOC.,1975,97, 5633.
426 Photochemistry amides (41) and (42), as expected, show quite low values of kir. Consequently, it can be deduced that the initial charge-transfer interactions for (35) and (37) are ca. 90% at sulphur and 10% at nitrogen (because kir = lOki,,), and for (36) >99.9% at sulphur. The quantum yield for reduction arising from interaction at sulphur may be quite low (< 0.09), so that the larger values observed for (35) and (37) may be taken as evidence for electron transfer (kxin Scheme 10) within the triplet exciplexes. A related mechanism is responsible for the synthesis of medium- and large-sized rings which occurs by cyclization on irradiation of sulphur-containing phthalimides (44).66 Charge-transfer interaction of excited 0
0 (44) IZ = 5, 6, 8, 9, 10, or 12
carbonyl with thioether may lead to easy removal of protons from C-H bonds adjacent to the sulphur atom. The resulting biradicals subsequently undergo ring closure, and reaction is therefore regioselective. A variety of 3-aminopropiophenone derivatives gave photopinacolization products (2-25% yield) on U.V. irradiation.66 Other p-amino-ketones such as N-methyl-4-piperidone7 tropinone, or 1-diethylaminobutan-3-one apparently underwent photoreduction to the corresponding /I-amino-alcohols on irradiation at 238 or 313 nm in hexane, although no reaction products were Interaction of singlet excited alkanones with diethylamine and triethylamine probably generates an exciplex.68 Singlet reaction rates, as measured by alkanone fluorescence quenching, show a strong dependence on the steric accessibility of the alkanone carbonyl group. 2 Reduction of Nitrogen-containing Compounds Dopp has published an interesting review of the triplet-state reactions of aromatic nitro-compounds, covering hydrogen abstractions and photoreductions of nitrobenzenes, nitronaphthalenes, and nitropyridine~.~~ Cu and Testa 6o have reported the photoreduction of 5-nitroquinoline in 50% aqueous propan-2-01 in the presence of hydrochloric acid. These authors (see Vol. 7, p. 401) have outlined the reaction mechanism as electron transfer from chloride ion to triplet nitrocompound, leading to 5-amino-6,8-dichloroquinoline.61 Photoreduction of 1-,2-, 3-, and 4-nitro-9-acridones to the corresponding amino-9-acridones occurs in high yield on irradiation in alcoholic m 66
b7 68
6Q 6o 6a
Y. Sato, H. Nakai, T. Mizoguchi, Y. Hatanaka, and Y. Kanaoka, J , Amer. Chem. SOC.,1976, 98, 2349. H. J. Roth, A. Abdul-Baki, and T. Schrauth, Arch. Pharm., 1976, 309, 2. A. M. Halpern and A. L. Lyons, J. Amer. Chem. Soc., 1976,98,3242. J. C. Dalton and J. J. Snyder, J . Amer. Chem. SOC., 1975,97, 5192. D. 0. Dopp, Topics Current Chem., 1975, 55, 49. A. Cu and A. C. Testa, Mol. Photochem., 1974, 6, 473. A. Cu and A. C. Testa, J. Phys. Chem., 1975,79,644. V. Zanker and E. Cmiel, Annalen, 1975, 1576.
427
Photo-reduction and -oxidation
Several further examples of the photoreduction of nitrogen-containing heterocyclic compounds have been reported during the year. The pyridine (45) upon irradiation in aqueous acetonitrile in the presence of diethylamine gives the reduction products (46) and (47), rather than an adduct of amine with heterocycle.63 It is possible that (47) is a secondary photolysis product, formed from (46).
RaR RaR hv, EtpNH
R
$R
+
H
R
H
(45) R = C0,Et
H
(47)
(46)
CIDNP Spectroscopy has again been applied to the photoreduction of acridines in the presence of hydrogen donors (see Vol. 7, p. 402). Libmans4 has examined the role of a singlet radical pair in the hydrogen abstraction by excited acridine (48) from carboxylic acids. The generalized reaction products and the intermediate radical pair are shown in Scheme 11. French workers 65
+
RC0,H
(48)
H
H Scheme 11
have likewise studied the singlet radical pair formed by abstraction of benzo[a]and benzo[c]-acridines from dioxan or tetrahydrofuran. Six-membered monoaza-aromatics such as pyridine, quinoline, 4-methylquinoline, isoquinoline, and 9-phenylacridine are able to produce radicals analogous to (49) on irradiation in methanol.66 The reaction is believed to be 83 84 66
E8
K. Kano and T. Matsuo, Tetrahedron Letters, 1975, 1389. J. Libman, J.C.S. Chem. Comm., 1976, 198. G. Vermeersch, N. Febvay-Garot, S. Caplain, and A. Lablache-Combier, Tetrahedron, 1976, 32,935. .A. Castellano, J. P. Catteau, and A. Lablache-Combier, Tetrahedron, 1975, 31, 2255.
428 Photochemistry one of hydrogen abstraction by the heterocycle nn* excited state. Although these same radicals are produced on irradiation in HC1-acidified methanol, a different biphotonic process pertains. In this latter case, it is proposed that electron transfer occurs from alcohol to an upper excited triplet state of the protonated heterocycle. An e.s.r. investigation of the U.V. irradiation of acridine and quinoline in 2-methyltetrahydrofuran at low temperatures has been published,67 and the effect of [Cr(CN)J3- ion on the photoreduction of acridine in ethanol noted.gs The photoreduction of benzo[c]cinnoline in aqueous acidic alcohols involves the protonated species (50) and yields the protonated 5,6-dihydro-derivative (51).69 Further reduction of (51) on irradiation in ethanol at wavelengths greater than 400 nm produces 2,2'-diaminobiphenyl, possibly through the participation of MeeHOH radicals. The curious formation of carbazole from (50) on ,~~ irradiation at wavelengths less than 380 nm has been reported p r e v i o ~ s l y and is thought to represent an alternative pathway for (51) reaction, although the exact route remains unknown. The photoreduction of porphyrins to chlorins by tertiary amines has been studied by e.s.r. and flash phot~lysis.~l
(52) R = Me or Ph
A full account has appeared of the photoreductive ring cleavage of 3,5-disubstituted isoxazoles (52) to amino-enones (53), from which it now seems that reaction can be catalysed by Cu" The photoreduction of imines still attracts attention. The reduction of some N-aroylimines [e.g. (54)to (55)] occurs cleanly on irradiation in propan-2-01.7~ Ph2C=NCOPh (54)
v Me,C11HOHr Ph,CHNHCOPh
(55)
87
A. Castellano, J. P. Catteau, and A. Lablache-Combier, Photochem. and Photobiol., 1976, 23,
6*
K. Nakamaru and H. Murakami, Sci. Reports Hirosaki Univ., 1975,22,31 (Chem. Abs., 1976,
135. 84, 42 868).
72
H. Inoue, T. Sakurai, and F. Tanaka, Bull. Chem. Soc. Japan, 1975,48,924. H. Inoue and Y. Matsuka, Chem. Letters, 1972, 713. Y.Harel, J. Manassen, and H. Levanon, Photochem. and Photobiol., 1976, 23, 337. T. Sato, K. Yamamoto, K. Fukui, K. Saito, K. Hayakawa, and S. Yoshiie, J.C.S. Perkin I,
ps
A. Padwa and W. P. Koehn, J. Org. Chem., 1975,40,1896.
89 70
71
1976, 783.
429 However, there are puzzling differences in the mechanisms of reaction, according to the substitution pattern. Thus, the compound (54), like alkylimines, apparently reacts by the ‘chemical sensitization’ route, in which traces of sensitizer (e.g. benzophenone) generate the ketyl radicals which are the effective reducing agent. On the other hand, the N-aroylimines (56)-(58) are photoreduced to (59)-(61)
Photo-reduction and -oxidation
Me Ph I I Me(CH,),C-C=NCOAr
Me Ph
I I &Me (CH,),?-CHNHCOAr
I
Me (56) n (57) n (58) n
= = =
Me
1, Ar = Ph 2, Ar = Ph 1, Ar = p-MeOC,H,
respectively, with reaction shown to occur from the nn* triplet state of the imine. The failure of (56)-(58) to give a Norrish Type I1 reaction suggests that the initially occurring hydrogen abstraction is by the carbonyl oxygen atom rather than by the imine nitrogen. Triplet quenching experiments show a low rate of reduction for (56)-(58) (k N 1 x lo31 mol-l s-l in propan-2-01), and a high rate of triplet decay, which account for the low quantum yields of reduction (a ca. lo+). Irradiation of the N-acetylimine (62) in toluene leads to the products of solvent addition (63) and reduction (64).74175 Again, evidence is presented for a mechanism of hydrogen abstraction by the excited imine.
Ph,C=NCOMe
PhMe
+
Ph,b-NHCOMe
+ Ph,CHNHCOMe
Irradiation of the iminolactone (65) in propan-2-01 gives reduction to a mixture of the dl- and meso-dimers of the radical (66).7s This free radical is exceptionally stable, and hence the dimers possess an unusually weak C-C bond.?? A study of the photochemistry of 4-acylpyrimidines has enabled Alexander and Jackson78to estimate the relative reactivities of the C=O and C=N groups in these pyrimidines towards intramolecular hydrogen abstraction. Both C=O and C=N triplets have about equal reactivity towards primary C-H hydrogen abstraction ( k N 7 x lo7s-l). Curiously, C=N triplets are not much more S. Asao, N. Toshima, and H. Hirai, Bull. Chem. SOC.Japan, 1975,48, 2068. N. Toshima, S. Asao, and H. Hirai, Chem. Letters, 1975, 451. 7* T. H. Koch, J. A. Oleson, and J. DeNiro, J. Org. Chem., 1975, 40,14. ‘’ T. H. Koch, J. A. Oleson, and J. DeNiro, J. Amer. Chem. SOC.,1975, 97, 7285. E. C. Alexander and R. J. Jackson, J. Amer. Chem. SOC.,1976,98,1609. 15 74
7L
Photochemistry
430
reactive towards secondary hydrogen abstraction (k = lo8 s-l), whereas C=O triplets are so (k = 1.2 x log s-l). 3 Miscellaneous Reductions The role of transition-metal complexes in photo-assisted hydrogenation continues to be investigated. Schroeder and Wrighton 7g have examined photocatalysed alkene isomerization and hydrogenation using [Fe(CO),]. Hydrogenation occurs on irradiation (300-380 nm) of mixtures of [Fe(CO),] and hydrogen with alkene, the key photochemical step being proposed as generation of [H,Fe(CO),(alkene)]. For simple alkenes, hydrogenation and isomerization occur at comparable rates. Butynediol is photohydrogenated to butenediol by use of the catalyst [IrCI(CO)(PPh,),].80 KroppS1 has continued his work on the photochemistry of alkenes. Unsymmetrically tetrasubstituted alkenes on irradiation in methanol give the corresponding alkanes, besides the products of addition of methanol. Similarly, 2-isopropylidenenorbornane leads to 2-endo-isopropylnorbornaneas a major component of the complex mixture of products. Kropp has now produced evidence in such systems for the ejection of free electrons, formed from the alkene following electron promotion to a Rydberg excited state. The only volatile products from sensitized irradiation of the exo- and endo-isomers of 5-chloronorbornene, 5-hydroxynorbornene, and 5-acetoxynorbornene are the corresponding norbornanes.82 Unexpectedly, no rearrangements or exo-endo isomerizations are noted during these photoreductions. The bicyclic enone (67) 0
is reduced on irradiation in the presence of cyclopentene or cyclohexene as hydrogen donors, yielding (68) amongst the observed products.8s The photoreduction of phenols by sodium borohydride to the corresponding cyclohexenols and cyclohexanols occurs in a surprisingly specific manner.84 By means of n.m.r. shift reagent studies, Barltrop and Bradbury have been able to ascertain the positions of deuterium incorporation on irradiation of p-cresol in aqueous sodium hydroxide solutions in the presence of NaBD,. The 4-methylcyclohexenol product (69) has deuterium incorporated only at the positions shown in Scheme 12. As also shown in this Scheme, a reaction mechanism is proposed to explain these results which involves phenoxide dissociation into phenoxyl radicals and solvated electrons. Subsequent attack by [BDJ- at C-1 of the phenoxyl radical is followed by intramolecular deuterium transfer by a cyclic 79
84
M. A. Schroeder and M. S. Wrighton, J. Amer. Chem. SOC.,1976, 98, 551. W. Strohmeier and K. Gruenter, J. Organometallic Chem., 1975, 90, 0 4 8 . H. G. Fravel and P. J. Kropp, J. Org. Chem., 1975,40,2434. S. J. Cristol, R. P. Micheli, G. A. Lee, and J. E. Rodgers, J. Org. Chem., 1975,40, 2179. A. Kunai, T. Omori, K. Kimura, and Y . Odaira, Bull. Chem. SOC.Japan, 1975,48, 731. D. Bradbury and J. Barltrop, J.C.S. Chem. Comm., 1975, 842.
43 1
Photo-reduction and -oxidation
0-
D
(69)
R
= H or a boron derivative Scheme 12
route. Further protonation and reduction steps lead to (69). Only this kind of pathway can account for both the observed positions and stereospecificity of deuterium incorporation. The 4-methylcyclohexanol also produced may be a secondary product, as it can be formed from 4-methylcyclohexenol on irradiation in the presence of phenol and NaBH,. Attack of hydride ion on excited aromatic compounds can lead to r e d ~ c t i o n . ~ ~ However, a different mechanism for the photoreduction of aromatic hydrocarbons by NaBH, is followed in the presence of an equivalent of an electron acceptor like 1,4-dicyanoben~ene.~~ Photo-Birch reduction to the dihydroderivatives occurs for phenanthrene, anthracene, or naphthalene, on irradiation in aqueous acetonitrile in the presence of NaBH, and 1,ddicyanobenzene. It seems likely that an exciplex of aromatic electron donor (D) with 1,4-dicyanobenzene acceptor (A) dissociates to give aromatic radical-cations, which are then attacked by borohydride ion (Scheme 13). Alternatively, borohydride may D
+ A ---%D'+
+A"
+
HZD A Scheme 13
BH.,-
H,O
>
HD'+X-
HD- + A
attack the exciplex directly. A similar mechanism to that of Scheme 13 no doubt also applies to the photoreaction of cyanide ion with phenanthrene and naphthalene in the presence of 1,4-di~yanobenzene.~~ Formation of an exciplex in which the aromatic molecule is an electron acceptor rather than an electron donor can also lead to photoreduction. Libman 86
J. A. Barltrop, Pure Appl. Chem., 1973, 33, 179. K. Mizuno, H. Okamoto, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1975, 839. K. Mizuno, C. Pac, and H. Sakurai, J.C.S. Chem. Comm., 1975, 553. J. Libman, J. Amer. Chem. SOC.,1975, 97, 4139.
Photochemistry has described examples of this type in the reduction and reductive alkylation of 1-cyanonaphthalene (70) on irradiation in acetonitrile in the presence of methoxyphenylacetic acids (71a) and (71b) or phenoxyacetic acid (71c). The products are shown in Scheme 14. There is good evidence for exciplex formation in such 432
+
RCH,CO,H
(70) a; R = p-MeOC,H, b; R = rn-MeOC,H, .~ C ; R = PhO
'W
>
(71) -t
+ (RCHJ,
RMe
-t CO,
Scheme 14
systems [reaction (17)], and electron transfer from (71) to (70) in the exciplex [reaction (1 S)] would lead to cyanonaphthalene radical-anions. These may then take up a proton from the (71) radical-cation [reaction (19)], leading eventually to the observed products. The reaction of 0- and p-dicyanobenzenes in the ArCN*
+ RCH,CO,H
[ArCN *** RCH,CO,H]* ArCN'-
+ '+RCH,CO,H
-
[ArCN
RCH,CO,H]*
(17)
ArCN'-
+ '+RCH,C02H
(1 8)
+ RkH, + CO,
(19)
HArCN=
presence of triethylamine 8g probably occurs by a similar route. Certainly, the interception of exciplexes by chemical reactions is an increasingly important field in photochemistry. Photoreduction of substituted benzo[b]furans by aliphatic amines [e.g. (72) to (73)] probably involves electron transfer from amine to benzofuran, which
R
(72) a; R = H b;R=Me
(73)
produces the aromatic radical-anion. The reactivities of various substituted benzofurans have been correlated with calculated spin densities on these radicalanions; only when spin density is highest at the 2-position (as opposed to the 4-position) is a stable photoreduction product observed.go Lablache-Combier also reports the reductive photocyclization of some 2,3-diphenylbenzo[b]furans in n-pr~pylamine.~~ Interest in the fate of polychlorinated aromatic compounds such as polychlorinated biphenyls (PCB) and tetrachlorodibenz-p-dioxinin the environment K. Tsujimoto, K. Miyake, and M. Ohashi, J.C.S. Chem. Comm., 1976, 386. C. PBrkAnyi, A. Lablache-Combier,I. Marko, and H. Ofenberg, J. Org. Chem., 1976,41, 151. A. Couture, A. Lablache-Combier, and H. Ofenberg, Tetrahedron, 1975, 31, 2023.
Photo-reduction and -oxidation
433
has led to several studies of the relevant photochemistry. Japanese workers O2 have described the photoreduction of 3- and 4-chlorobiphenyl by sodium borohydride in aqueous acetonitrile to yield biphenyl. The authors favour a mechanism of hydride attack on excited chlorobiphenyl, although it is not clear on what grounds they have rejected the previously proposed radical chain mechanism for reductions of this type.g3 A Swedish groupg4has studied the photochemical dechlorination of 1,2,4-trichIorobenzeneas a model compound relevant to PCB. The primary products on irradiation in cyclohexane or propan-2-01 are 1,3- and 1,4-dichlorobenzene, the product ratio of 1,3- : 1,44sorners being significantly different on direct irradiation (0.15) from that on acetone sensitization (4.8). These facts suggest that both singlet and triplet excited states give rise to dechlorination. Reductive dechlorination is the major reaction on irradiation of hexachlorobiphenyls in of unsymmetrically substituted tri- and tetra-chlorobiphenyls in c y c l o h e ~ a n e , and ~ ~ of polychloronaphthalenes in methanol The tranquillizer chlorpromazine gives a similar reductive dechlorination on irradiation in propan-2-01.~~ The photochemistry of polybromobiphenyls, which are used as plasticizers and flame retardants, has also been in~estigated.~~,Reductive debromination is the main reaction of bromobiphenyls on irradiation at ca. 300 nm in cyclohexane ~ 0 1 u t i o n .As ~ ~for PCB,loO2-substituted biphenyls show enhanced quantum yields of reaction over the 3- or 4-substituted isomers. The presence of triethylamine assists such reductions, and an electron transfer from triethylamine to excited bromo-compound may be responsible. The photoreduction of esters has aroused some interest. PBte lol has explored the reaction mechanism of the previously reported photoreduction of esters to alkanes in wet hexamethylphosphoramide (HMPA) [reaction (2O)].lo2 It appears R1C02R2
hv
RICO,H
+ R2H
that HMPA is the source of hydrogen, and that reaction goes through a radical intermediate. Moreover, absorption of light by either HMPA or ester can initiate reaction. One possibility is that an exciplex is formed in which an electron can be transferred from HMPA to ester. Subsequent reaction of the ester radical anion with water leads to reduction. Photoreductive removal of the toluene-p-sulphonyl group from tosylate esters of steroids occurs on irradiation in the presence of sodium borohydride, yielding alcohols.103 B2 93 84
g8 g7
K. Tsujimoto, S. Tasaka, and M. Ohashi, J.C.S. Chem. Comm., 1975, 758. J. A. Barltrop and D. Bradbury, J. Amer. Chem. SOC.,1973,95,5085. B. Akermark, P. Baeckstrom, U. E. Westlin, R. Gothe, and C. A. Wachtmeister. Acta Chem. Scand., 1976, B30, 49. L. 0. Ruzo and M. J. Zabik, Bull. Enuiron. Contam. Toxicol., 1975, 13, 181. L. 0. Ruzo, S. Safe, and M. J. Zabik, J. Agric. Food Chem., 1975, 23, 594. L. 0.RUZO,N. J. Bunce, S. Safe, and 0. Hutzinger, Bull. Enuiron. Contam. Toxicol., 1975,14, 341.
A. K. Davies, S. Navaratnam, and G. 0. Phillips, J.C.S. Perkin II, 1976,25.
N. J. Bunce, S. Safe, and L. 0. RUZO,J.C.S. Perkin I, 1975, 1607. L. 0. Ruzo, M. J. Zabik, and R. D. Schuetz, J. Amer. Chem. SOC.,1974,96, 3809. H.Deshayes, J. P. Pete, and C. Portella, Tetrahedron Letters, 1976, 2019. ln2 H. Deshayes, J. P. Pete, C. Portella, and D. Scholler, J.C.S. Chem. Comm., 1975,439. lo3 Y . Kondo, K. Hosoyama, and T. Takemoto, Chem. andPharm. Bull. (Japan), 1975,23,2167. 89
loo Io1
434 Photochemistry de Mayo lo4has summarized the hydrogen abstraction reactions given by aliphatic and aromatic thiones. Irradiation of organic disulphides in aldehyde solvents results in reductive fission of the S-S linkage, producing equimolar amounts of the corresponding thiol and acylated thiol.lo6 Lastly, attempts have been made to reach a better understanding of the photoreduction of thiazine dyes in aqueous solution.lo6 4 Singlet Oxygen The purpose of this section is to mention some of the organic aspects of the chemistry of singlet molecular oxygen (lo2, lAg). Ohloff lo7has reviewed the use of singlet oxygen as a reagent in organic synthesis, with emphasis on the preparation of important flavours and fragrances. New or modified sources of continue to be developed. Most dyes used as sensitizers of lo2production are anionic compounds and thus insoluble in aprotic solvents; consequently, this problem must be circumvented for work in such solvents. Schaap and co-workers lo*have published a full account of their work on the use of polymer-based dye sensitizers for generation.loB Such sensitizers have the advantages of being able to function heterogeneously, being more stable towards bleaching than unbound dye, and being easily removed after reaction. For example, polymer-bound Rose Bengal functions quite efficiently, giving a quantum yield of lo2of 0.43 in dichloromethane. It is also possible to prepare polymer-bound sensitizers such as eosin-Y, ffuorescein, chlorophyllin, and haematoporphyrin (the last two possibly being of importance in biological oxidation studies). As an alternative to heterogeneous sensitization, Boden 110 has described the use of homogeneous photosensitization. The dyes Rose Bengal and eosin-Y can be made soluble in carbon disulphide or dichloromethane by the use of crown ethers (e.g. 18-crown-6) or quaternary ammonium salts (e.g. tricaprylmethylammonium chloride), and can then function as sensitizers of lo2production. Other workers 111 have examined the generation in aqueous micellar systems. Singlet oxygen, generated by irradiation of of oxygenated aqueous solutions of methylene blue, can diffuse into sodium dodecyl sulphate micelles, where it may react with solubilized organic substrates (1,3-diphenylisobenzofuran,in this case). This mechanism of transfer may be important in relation to the known effects of lo2in causing damage to biological systems. Peroxides are providing several new sources of lo2.Phthaloyl peroxide (74) in benzene appears to generate lo2at room temperature or on gentle heafing.ll2 P. de Mayo, Accounts Chem. Res., 1976, 9, 52. M. Takagi, S. Goto, and T. Matsuda, J.C.S. Chem. Comm., 1976,92. lo6 R. Bonneau, J. Pereyre, and J. Joussot-Dubien, Mol. Photochem., 1974, 6, 245. lo' G. Ohloff, Pure Appl. Chem., 1975, 43, 481. lo8 A. P. Schaap, A. L. Thayer, E. C. Blossey, and D. C. Neckers, J. Amer. Chem. Soc., 1975,97, lo'
lob
lop ll0 ll1
3741. E. C. Blossey, D. C. Neckers, A. L. Thayer, and A. P. Schaap, J. Amer. Chem. Soc., 1973,95,
5820. R. M. Boden, Synthesis, 1975, 783. A. A. Gorman, G. Lovering, and M. A. J. Rodgers, Photochem. and Photobiol., 1976, 23, 399.
K.-D. Gundermann and M. Steinfatt, Angew. Chem. Znternat. Edn., 1975,14, 560.
435
Photo-reduction and -oxidation
PP
0
II
\
c/o II
0
Ozonization of benzaldehyde, 2-methyltetrahydrofuran, and isopropyl methyl ether produces hydrotrioxide intermediates [e.g. (75) from benzaldehyde], which decompose at or below room temperature to generate 102.113 From a study of the products of cholesterol oxidation, Smith and Kulig 114 have found evidence that the base-catalysed disproportionation of hydrogen peroxide [reactions (21) and (22)] does produce lo2in addition to ground-state oxygen (302). A ratio of
+ HO- 7 HOO- + HzO HzOz + HOOHZO + HO- + H,O,
_I__,
(21) 0 2
(22)
: production of at least 1 : 3 is suggested, although this estimate is based only on the product yields. There is spectroscopic evidence for generation of (both lAg and lC,+) in the self-reaction of secondary peroxy-radicals derived from linoleic acid.lls Such a process could be involved in microsomal lipid peroxidation. Pitts and his group had previously recommended the aqueous decomposition of potassium perchromate (K3Cr0,) as a ‘clean’ source of 102,116 but have now gone on to show the occurrence of other oxidative pathways during its deThus, the relative reactivity of purine and pyrimidine bases to perchromate is not as expected for lo, attack, and other unidentified products also arise from the oxidation of certain cycloalkenes. The upper limit for the yield of lo2production is estimated to be ca. 6%. Osmium(I1) and iridium(n1) complexes have been found to sensitize photo-oxidation by a lo2route.l18 The variation of the quantum yield of lo2production as a function of pH has been measured for the dye toluidine blue as sensitizer.lls The quenching of lo2by organic molecules continues to receive attention. The rate constants for physical quenching of lo, by bilirubin lZo and biliverdin lZob are large and similar (2.3-2.5 x los and 3.3 x los 1 mol-1 s-l, respectively). However, the rate constant for chemical reaction with lo, is much greater for 118 114 116
F. E. Stary, D. E. Emge, and R. W. Murray, J. Amer. Chem. Soc., 1976, 98, 1880. L. L. Smith and M. J. Kulig, J. Amer. Chem. Sac., 1976, 98, 1027. M. Nakano, K. Takayama, Y.Shimizu, Y.Tsuji, H. Inaba, and T. Migita, J. Amer. Chem. SOC.,1976, 98, 1974.
117
J. W. Peters, J. N. Pitts, I. Rosenthal, and H. Fuhr, J. Amer. Chem. SOC.,1972,94,4348. J. W. Peters, P. J. Bekowies, A. M. Winer, and J. N. Pitts, J. Amer. Chem. SOC.,1975, 97,
11.9
J. N. Demas, E. W. Harris, C. M. Flynn, and D. Diemente, J. Amer. Chem. SOC.,1975,97,
116
3299. 3838. 119
120
R. Pottier, R. Bonneau, and J. Joussot-Dubien, Photochem. and Photobiol., 1975,22, 59. (a) B. Stevens and R. D. Small, Photochem. and Photobiol., 1976,23, 33; (b) C. S. Foote and T.-Y. Ching, J. Amer. Chem. Soc., 1975, 97, 6209.
436 Photochemistry bilirubin (1.7-4 x lo8, cf. < 3 x los 1 mol-ls-l for biliverdin). The azodioxide (76) appears to exist in equilibrium with its dinitroso-isomer (77).121 Although azo-dioxides such as (76) are efficient triplet quenchers, they do not quench lo2significantly. On the other hand, nitroso-compounds such as (77)
n
c1
C1 'NO
do quench at approaching diffusion-controlled rates (e.g. 9.3 x lo9 1 mol-1 s-l for 2-methyl-2-nitrosopropane). There has been dispute as to whether the dismutation of the superoxide radical anion (027produces or ground-state oxygen [reaction (23)]. 202'-
+ 2H+
----+
H202
+ 02(lAg or 3Xs-)
(23)
Guiraud and Foote122have now established that lo, is rapidly quenched by 0;-, with a rate constant of 1.6 x lo91 mol-1 s-l in DMSO, which may help to clarify the situation. The bimolecular emission of lo2in aqueous solutions [reaction (24)] is enhanced by cyclic tertiary diamines such as 1,4-diazabicycloO,pA,)
+ O2pA,)
-
+ hv
202(3Zu-)
(24)
[2,2,2]octane and NN'-dimethylpiperazine, which is surprising because tertiary amines are generally observed to be quenchers of 102.123
5 Oxidation of Aliphatic and Alicyclic Unsaturated Systems The controversy continues as to whether the reaction of singlet oxygen with olefins to give allylic hydroperoxides involves an ene-reaction or a perepoxide intermediate. Two theoretical studies of the reaction of singlet oxygen (l&) with ethylene have lent some support to the feasibility of a perepoxide intermediate [(78), Scheme 151 as a shallow minimum on the potential surface.124,126 Dewar and Thie1125have calculated that the transition state for reaction with lo2is reactant-like, and that the perepoxide (78) can rearrange with a higher activation energy to 1,2-dioxetan (79); a concerted route to dioxetan is not favourable. The same method applied to the reaction of propene with lo2 describes the most favoured mechanism for allylic hydroperoxide formation as a two-step process via a perepoxide intermediate. The perepoxide readily rearranges to the product of an ene reaction, For electron-donating substituted olefins, such as vinylamine or 2,3-dihydropyranYformation of a zwitterion [e.g. (SO)] is the preferred addition pathway (Scheme 16). Ring closure of zwitterion to 121 123
la4 126
P. Singh and E. F. Ullman, J. Amer. Chem. SOC.,1976,98,3018. H. J. Guiraud and C. S. Foote, J. Amer. Chem. SOC.,1976, 98, 1984. C. F. Deneke and N. I. Krinsky, J. Amer. Chem. SOC.,1976,98, 3041. S. Inagaki and K. Fukui, J. Amer. Chem. SOC.,1975,97, 7480. M. J. S. Dewar and W. Thiel, J. Amer. Chem. SOC.,1975, 97, 3978.
437
Photo-reduction and -oxidation
0
0-0 I I CH2-CH,
+
,0.-P
J
CH,-kHz
(79)
Scheme 15
dioxetan, or to perepoxide, occurs easily, If the perepoxide can in turn rearrange to ene product (Sl), there should be competition between the ene reaction and formation of a dioxetan. Dewar and co-workers126have also sought to explain the formation of the epoxides which have sometimes been observed in the reaction of olefins with loz.Calculations of the three possible reactions (25)-(27) suggest that for
Lo+-0co+-o[=O+-O-
+ +
-O,('h,,
CH2=CH2
Lo
4
+
0 3
2 LO
sterically hindered olefins, reaction (27) is unfavourable. Reactions (25) and (26) would then compete to produce a mixture of epoxide and dioxetan (as is indeed observed in some cases). However, for sterically unhindered olefins, reaction of the perepoxide with olefin [reaction (27)] would be the dominant pathway, leading to epoxide. Hence, ethylene should react with lo2to yield oxiran. Since this prediction appeared, it has been demonstrated that la6
M. J. S.Dewar, A. C.Griffin, W. Thiel, and I. J. Turchi, J. Amer. Chem. SOC.,1975,97,4439.
Photochemistry formaldehyde chemiluminescence can be detected from the gas-phase reaction of singlet oxygen (lAg) with eth~1ene.l~'Formaldehyde chemiluminescence would arise from fragmentation of a vibrationally excited dioxetan, and is therefore evidence for the formation of 1,Zdioxetan in this system. Arrhenius parameters have been determined for reactions of lo, with some substituted butenes, cyclopentenes, and cyclohexenes in the gas phase.12* Differing reactivities are mainly due to varying activation energies, rather than preexponential factors. An investigation of the kinetic aspects of dye-sensitized photo-oxygenation of olefins in a gas-liquid reactor has concentrated on masstransfer pro blems.12a The conformationally fixed cyclohexylidenecyclohexanes (82) and (83) react with lo, to give, in each case, a mixture of the two stereoisomeric allylic hydroperoxides (84) and (85) (Scheme 17).130 The ratio of (84) : (85) from (82) was 438
OOH
OOH
Scheme 17
60 :40 and from (83) was 33 : 67. Such a result cannot be explained on the basis of a concerted ene mechanism, but supports the formation of an intermediate, which could possibly be a perepoxide, in the reaction. Oxidation of several unconjugated cyclic dienes by lo2gave a normal ene reaction to yield allylic hydroperoxides, and there was no evidence for transannular reaction.lsl cis,cisCyclodeca-l,6-diene appeared to be inert towards loz. Dye-sensitized photo-oxidation of terpenes can sometimes produce complex mixtures. Thus, the Rose Bengal-sensitized oxidation of linalool in methanol allowed the isolation and identification of eight whilst a-terpineol gave six compounds on photosensitized Magnesium phthalocyanine has been used as a photosensitizer in the oxidation of a- and P - ~ i n e n e .Photo~~~ la@ lSo lS1
Isa lSa
D. J. Bogan, R. s. Sheinson, and F. W. Williams, J. Amer. Chem. SOC.,1976,98, 1034. R. D. Ashford and E. A. Ogryzlo, J. Amer. Chem. SOC.,1975,97,3604. D.Brkic, P. Forzatti, I. Pasquon, and F. Trifiro, J. Photochem., 1976,5, 23. R. M. Kellogg and J. K. Kaiser, J. Org. Chem., 1975, 40, 2575. A. Horinaka, R.Nakashima, M. Yoshikawa, and T. Matsuura, Bull. Cham. SOC.Japan, 1975, 48, 2095. K. h a , Nippon Shokuhin Kogyo Gakkai-Shi, 1973, 20,43 (Chem. Abs., 1975,83, 162 382). Y. S. Cheng, M. D. Tsai, J. M. Fang, and S. S. Hsu, Hua Hsueh, 1975, 8 (Chem. Abs., 1976, 84, 105 816).
H. Kropf and B. Kasper, Annalen, 1975, 2232.
Photo-reduction and -oxidation
439
sensitized oxidation of the sesquiterpene lactone lipiferolide (86) produces peroxyferolide (87), which has also been isolated from a plant It is unusual to find an allylic hydroperoxide from a naturally occurring source, and (87) may well arise in the leaves of the plant by chlorophyll-mediated lo2 addition to (86). Singlet oxygen reacts with germacratriene (88) to yield allylic
HOQ
hydroperoxides at the isopropylidene double bond ca. nine times more rapidly than at the endocyclic double bonds, in contrast to the relative reactivity of these double bonds on epoxidation of (88).136 Germacrone (89) reacts with lo2to give a complex mixture, from which only one allylic hydroperoxide (arising by C-5 attack of oxygen) could be isolated in 2.5% yield.131 There have been further reports of the reactivity of cholesterol l l 413' ~ and some fatty acids 13' towards lo,. The text of an interesting lecture by Bartlett has been published, in which the formation of dioxetans from '0, and alkenes is reviewed.13* Excited states can be generated photochemically, or by intermolecular or intramolecular energy transfer. Zimmerman has published another route to photochemical rearrangement without light, which involves a dioxetan in the intramolecular generation of an excited As shown in Scheme 18, the dioxetans (91) prepared by low-temperature photosensitized oxidation of the methylenecyclohexadienes (90) decompose on heating to produce cyclohexadienone (92) and its known photorearrangement product (93). Curiously, the rearranged product (93) is still observed even when the other carbonyl fragment produced is methyl 2-naphthyl ketone, despite the fact that this latter ketone has a lower triplet energy (ET 59 kcal mol-l) than the cyclohexadienone (92) (ET 68.5 kcal mol-l). This observation may support theoretical predictions of the preferential production of nr* excited triplets in such dioxetan fragmentations, rather than the rn* triplets of methyl 2-naphthyl ketone. Further examples of the reaction of alkylthio-substituted alkenes with lo2 have been reported. For the ethylthiocycloalkenes (94) 140 and (95),141 the dioxetans formed decompose by C-C or C-S bond cleavage. The ratio of the two pathways depends on the ring size of the substrate 140 and the conformation ls6
R. W. Doskotch, F. S. El-Feraly, E. H. Fairchild, and C.-T. Huang, J.C.S. Chem. Comm.,
lS6
T. W. Sam and J. K. Sutherland, J.C.S. Perkin I, 1975, 2336. F. H. Doleiden, S. R. Fahrenholtz, A. A. Lamola, and A. M. Trozzolo, Photochem. and Photabiol., 1974,20, 519. P. D. Bartlett, Chem. Sac. Rev., 1976, 5, 149. H. E. Zimmerman and G. E. Keck, J. Amer. Chem. SOC.,1975,97,3527. W. Ando, K. Watanabe, and T. Migita, Tetrahedron Letters, 1975, 4127. W. Ando, K. Watanabe, and T. Migita, J.C.S. Chem. Comm., 1975,961.
1976,402. 13'
lS8 lS8 140
141
4 40
Photochemistry
(90) a; R = Ph b; R = m-MeOC,H,
c; R
=
(91)
2-naphthyl
kG8O
OC
i-
Ph
&Ph
i- MeCOR
Ph
Ph
(92) 8 3 4 8 %
(93) 17-12%
Scheme 18
SEt
(94) n
=
5, 6, 7, 8, 10 or 12
n
(95) a; R1 = R2 = H b; R1 = Me, R2 = H c; R1 = Pri, R2 = Me d; R1 = Ph, R2 = H e; R1 = H, R2 = But
of the dioxetan. 141 Photochemical oxidation of 1-ethoxy-2-ethylthioethylene has been ~ e p 0 r t e d . l ~ ~ It has been suggested that the formation of glyoxal in the mercury-photosensitized reaction of oxygen with acetylene may proceed via attack of lo2 (lAg) on the a~ety1ene.l~~ The charge-transfer complex formed between ethylene and oxygen at low temperatures has been irradiated at 206 nm.144 Cyclic conjugated dienes generally react with lo2in a [4 21 addition reaction to yield 1,4-endo-peroxides. For example, dye-sensitized photo-oxygenation of the cyclopentadiene (96) produces a mixture of 1,4-endo-peroxide (97) together with products arising from endo-peroxide ~earrangernent.~~~ Mention must be
+
m (96)
Ira
143
144 146
(97)
R. I. Shekhtman, V. A. Krongauz, V. Yu. Borovkov, and E. N. Prilezhaeva, Izuest. Akad. Nauk S.S.S.R., Ser. khim., 1975, 1139. S. L. N. G. Krishnamachari and T. V. Venkitachalam, Mol. Photuchem., 1976, 7 , 75. H. W. Buschmann, Ber. Bunsengesellschaftphys. Chem., 1974, 78, 1344. W. Skorianetz and G . Ohloff,Helv. Chim. Acta, 1976, 59, 1.
Photo-reduction and -oxidation
441
made here of the fact that Barton and co-workers148have used the triphenylmethyl cation and other electrophiles as catalysts in the photo-oxygenation of ergosteryl acetate to the peroxide. These catalysts allow the ‘spin-forbidden’ addition of triplet (ground-state) oxygen to cisoid conjugated dienes, yielding endo-peroxides. A reaction route involving excitation of a diene-catalyst complex to a triplet state and spin-allowed reaction with triplet oxygen could give ground-state peroxide. Ergosterol is converted into the peroxide in fungi by simultaneous photo-oxidative and enzymic The photo-oxidation process is probably sensitized by known pigments in the fungi. Cyclohexa-l,3-dienes generally react with lo2to form endo-peroxides which may rearrange thermally to lY3-diepoxides.Such reactions have been used in the synthesis of crotepoxide, an anti-tumour agent, from sensitized photooxygenation of the cyclohexadiene (98).14* The diacetate (99), however, was inert towards lo2. Singlet oxygen reacts with the oxepin-benzene oxide (100) CH20CH,Ph
$:: (98) R = H
(99) R = COMe
(102) R = H (103) R = Me
system and the arene oxide (101) to form a l,.l-endo-peroxide in each case.149 The presence of an angular methyl group causes differences in the observed lo, reaction with dienes (102) and (1O3).l6O Thus, the diene (102) yields mainly (> 80%) endo-peroxides, whereas (103) produces a mixture of the two allylic hydroperoxides (104) (10-15%) and (105) (85-90%) resulting from an ene reaction. Kondo and Matsumoto lS1 have examined the relative reactivity of acyclic ene and diene systems towards lo2.p-Myrcene (106), which has both isolated and conjugated double bonds, gives reaction at the isolated double bond more readily, yielding allylic hydroperoxides (107) and (108). These products may react more slowly with lo2at the diene system to form lY4-endo-peroxides(109) and (110) respectively. From a study of (106) and other acyclic monoterpenes, D. H. R. Barton, R. K. Haynes, G. Leclerc, P. D. Magnus, and I. D. Menzies, J.C.S. Perkin I, 1975, 2055. 14’ M. L. Bates, W. W. Reid, and J. D. White, J.C.S. Chem. Comm., 1976, 44. 148 M. R. Demuth, P. E. Garrett, and J. D. White, J. Amer. Chem. SOC.,1976, 98, 634. l*@C. H. Foster and G. A. Berchtold, J. Org. Chem., 1975,40, 3743. lS0 I. Sasson and J. Labovitz, J. Org. Chem., 1975, 40, 3670. M. Matsumoto and K. Kondo, J. Org. Chem., 1975,40,2259. lP6
442
Photochemistry 0OH
OOH
p. (110)
(109)
the double-bond reactivity towards lo2appears to be trisubstituted alkene > 2-substituted lY3-diene> 1,l-disubstituted or lY2-disubstituted alkene. The same authorslS2have gone on to make use of this order of reactivity in the synthesis of some furanoterpenes by photosensitized oxidation of p-myrcene. For example, reduction of the endu-peroxide ring of (109) and dehydration produces a synthesis of the furan ring, with subsequent steps leading to the frrranoterpene perillene (1 11). Furans can similarly be produced following the
(1 11)
lo2photo-oxygenation of polyaryl-substituted cyclopentadienols.lSS Sensitized photo-oxidation of the s-trans-diene grouping of the steroid oestra-4,g-dien17/%01-3-onegives an allylic hydroperoxide,16*and direct photo-oxidation of the s-trans-diene grouping in the diterpene abietic acid has been noted.lss Dye-sensitized photo-oxidation of the furan ring of the furanolactone (112) to give the products shown in Scheme 19 is a key step in the synthesis of the alkaloid camptothecin.ls6 Photosensitized oxidation of the furanoid ring of petasalbin has been reported.lS7 Et OC0,Me
g YI 031 OH
10,
+
0
lSa IS3
K. Kondo and M. Matsumoto, Tetrahedron Letters, 1976, 391. J. J. Basselier, J. P. Le ROUX,F. Caumartin, and J. C. Cherton, Bull. SOC.chim. France, 1974, 2950.
M. Maumy and J. Rigaudy, Bull. SOC.chim. France, 1975, 1879. us A. Enoki and K. Kitao, Mokuzai Gakkaishi, 1975, 21, 101. lS6 E. J. Corey, D. N. Crouse, and J. E. Anderson, J. Org. Chem., 1975, 40, 2140. lS7 K. Naya, R. Kanazawa, and M. Sawada, Bull. Chem. SOC.Japan, 1975,48,3220. 16'
Photo-reduction and -oxidation
443
Addition of lo2at -70 "C to the s-cis-diene system of the polyarylfulvenes (1 13) gives reasonably stable 1,4-endo-peroxides [except (1 14g)l which undergo an interesting rearrangement at room temperature in the presence of methanol or ethanol to form 1,Z-dioxetans (115) (Scheme ZO).168
R1
R1
Ph
Ph
Ph
Ph H
R3
R2 = R3 = Ph (114) R1 = R2 = Ph, R3 = Me C; R1 = R2 = Ph, R3 = p-ClC,H, d; R1 = R2 = Ph, R3 = p-NO&H, e; R1 = R2 = Ph, R3 = p-MeOC,H, f ; R1 = R3 = Ph, R2 = H g; R1 = R2 = H, R3 = Ph
(113) a; b;
R1 =
Scheme 20
o-Alkyl-substituted aromatic carbonyl compounds undergo intramolecular hydrogen abstraction on irradiation to form an enol, which can react with oxygen to produce an endo-peroxide. This process allows a regioselective hydroxylation at the C-8 methyl group of (116).159 Oxidation of the C=C AcO
0
Me
Me (116)
functional group occurs in several other photoreactions. Photo-oxidation of 2methyl-ly4-naphthoquinone (vitamin K3) in ethanol yields the 2,3-epoxide.ls0 The dye-sensitized photo-oxidation of 2',4',6'-trihydroxychalcone to the corresponding flavonol has been reported,l6l and photolysis of bis-(3-hydroxyflavenylidene)and bis-(3-epoxyflavenylidene) in air leads to oxidative cleavage of the 2,3-double bond of one half of the bisflavenylidene molecule.lsa 6 Oxidation of Aromatic Compounds This year has seen several extensive reports of the 1,4-cycloaddition of singlet oxygen to compounds having a double bond in conjugation with an aromatic system. Thus, the photosensitized oxidation of l-vinylnaphthalenes (117)in carbon tetrachloride results in 1,4-attack by lo2to produce the endo-peroxides (1 18),le3 Ib8
lLD
160
la%
J. P. Le Roux and C. Goasdoue, Tetrahedron, 1975, 31, 2761. W. A. Ayer and D. R. Taylor, Canad. J. Chem., 1976,54, 1703. J. M. L. Mee, C. C. Brooks, and K. H. Yanagihara, Biochem. Biophys. Res. Comm., 1975, 65, 228. H. M. Chawla and S. S. Chibber, Tetrahedron Letters, 1976, 2171. R. J. Molyneux, H. Aft, and P. Loveland, Chem. and Ind., 1976, 68. M. Matsumoto and K. Kondo, Tetrahedron Letters, 1975, 3935.
444
Photochemistry
(117) a; R1 = Ph,R2 = H b; R' = R2 = H c; R1 = R2 = Me
d; R1 = Me,R2 = H R1 = H,R2 = Me
e;
The reaction is stereospecific, as shown by the exclusive formation of (118d) from trans-l-propenylnaphthalene(117d) and only (1 18e) from the cis-isomer (1 17e). a-Substituents on the side-chain inhibit the 1,4-cycloaddition, and no endo-peroxide could be isolated. Instead, allylic hydroperoxides were found, no doubt arising from a normal ene-reaction of lo2 on the side-chain of the aromatic compound. For such (a-substituted-vinyl)naphthalenes, it seems possible that steric interactions with the naphthalene peri-hydrogen hinder a conformation in which the side-chain is coplanar with the naphthalene ring, and hence 1,4-attack by lo2 is prevented. Kondo and co-workers 164 have also examined the behaviour of 2-vinylthiophens (119) on photosensitized oxidation.
(119) a; R1 = b; R' =
c; d; e; f; g;
R1 = R1 = R1 = R1 = R1 =
R2 = Me,R3 = H R3 = H Me,R2 = R3 = H Ph,R2 = R3 = H R2 = H,R3 = Ph R2 = H,R3 = Me R2 = R3 = Me R2 =
(120)
Again, endo-peroxides (120) are formed as a result of 1,4-attack of lo2on the aromatic and vinyl double bonds, However, in the case of the (a-substitutedviny1)thiophens (119f) and (119g), endu-peroxide formation does compete with production of allylic hydroperoxide. This observation, when compared with those described above for l-vinylnaphthalenes, becomes understandable in terms of a much reduced steric interaction between the substituted side-chain and the thiophen ring. Foote et alls6 have published a full account, with further examples, of their earlier work lB6on the formation of diepoxy-endo-peroxides (122) in the photooxidation of indene and substituted indenes (121) in acetone at -78 "C. The probable mode of formation of the adducts (122) involves initial lY4-additionof lo4
M. Matsumoto, S. Dobashi, and K. Kondo, Tetrahedron Letters, 1975,4471. P. A. Burns, C. S. Foote, and S. Mazur, J. Org. Chem., 1976,41, 899. C. S. Foote, S. Mazur, P. A. Burns, and D. Lerdal, J . Amer. Chem. Sac., 1973,95,586.
lE6
lo6
Photo-reduction and -oxidation
445
mR3 R2
R2
lo*
Me,CO, - 78 'CO
lo2to the indene, followed by rearrangement of this endo-peroxide to a diepoxide, and attack of a second molecule of lo2. A similar reaction of 1,Zdihydronaphthalenes (123) with lo2occurs at -78 "C in acetone, although generally in these examples both 1,4-~ycloaddition and ene-reaction compete to yield mixtures of diepoxy-endo-peroxides and allylic hydroperoxides.le7 The reaction pathway is quite sensitive to the substitution pattern of (123), since (123a) gives
(123) a; R1 = H,R2 = Ph b; R1 = Ph, R2 = H C; R' = R2 = Ph d; R1 = Me,R2 = Ph e ; R1 = Me, R2 = H f; R1 = R2 = H
only allylic hydroperoxide whereas (123b) gives only diepoxy-endo-peroxide. The products from lo2attack on l-phenylcycloalkenes depend upon the cycloalkene ring size.le8 l-Phenylcyclopentene yields only an allylic hydroperoxide, l-phenylcyclohexene gives a 3 : 1 mixture of allylic hydroperoxide and a double endo-peroxide, and 1-phenylcyclobutene gives a more complex mixture (Scheme 21), the composition of which is solvent-dependent. The authors claim that their results can be rationalized on the basis of the formation of three intermediates, perepoxide, dioxetan, and endo-peroxide.ls8 0
0
II
a
Ph
'02
+ cC-H QooH Ph C-Ph I1
0
II
+
C-H
V" \=/
Scheme 21 lo'
lb8
P. A. Burns and C. S. Foote, J. Org. Chem., 1976,41, 908. C. W. Jefford and C. G . Rimbault, Tetrahedron Letters, 1976, 2479.
446
Photochemistry
Naphthalene does not react with lo, to form a 1,4-endo-peroxide. Nevertheless, this endo-peroxide (126) is reasonably stable and has been produced indirectly by dye-sensitized photo-oxidation of the [l01annulene (124).ls0 Treatment of the resultant amine (125) with nitrosyl chloride leads to elimination of
N20 at low temperatures, and consequent production of (126). Gentle warming of the peroxide (126) quantitatively produces naphthalene and loa,rather than forming a 1,3-diepoxide. However, the intermediate peroxide (125) can rearrange to 1,3-diepoxide and therefore allows the synthesis of syn-naphthalene 1,2:3,4d i e p ~ x i d e . ~1,4,5,8,9-Pentamethylanthracene '~ is readily photo-oxidized to the expected 9,10-end~-peroxide,l~~ and the photo-oxidation of anthracene on an alumina catalyst has been r e p 0 ~ t e d . l ~The ~ effects of solvent and reactant concentrations on the self-sensitized photo-oxidation of rubrene have been investigated in 1,3-Diphenylisobenzofuran(127) has proved a popular substrate for photooxidation studies, being oxidized by lo2to o-dibenzoylbenzene. It has now been shown that (127) is also very susceptible to autoxidation by free-radical initiators at 30 "C and that this process does also yield some o-dibenz~ylbenzene.~~~ Caution is therefore required when deducing lo2as an intermediary from the conversion of (127) into o-dibenzoylbenzene. Photo-oxidation of (127) in aromatic solvents leads to the known endo-peroxide, which is fairly stable at 20 "C,but photo-oxidation of (127) in carbon tetrachloride gives o-dibenzoylbenzene quantitatively. The peroxy-acid oxidation of (127) also yields o-dibenzoylbenzene, but evidence has been produced against the involvement of '0% in this ~eacti0n.l~~ The photolysis of pyridine N-oxide in benzene is well known to lead to phenol in competition with the formation of intramolecular rearrangement products of the N-oxide. The former conversion has been studied as one which mimics the biological hydroxylation of aromatic rings. It has now been found that the internal oxygen rearrangements can be blocked by protecting the N-oxide as the boron trifluoride complex, when much increased yields of phenol are Stein and co-workers have reported further on the photochemical oxidation of benzene in aerated aqueous solutions, but they have not been able to provide any more evidence on the structure of the unisolated p h o t o p r o d u ~ t . ~ ~ ~ M. Schafer-Ridder,U. Brocker, and E. Vogel, Angew. Chem. Znternat. Edn., 1976, 15,228. E. Vogel, H.-H. Klug, and M. Schiifer-Ridder, Angew. Chem. Znternat. Edn., 1976,15,229. 171 H. Hart, J. B.-C. Jiang, and R. K. Gupta, Tetrahedron Letters, 1975, 4639. li2 S. Nakanishi and K. Ito, Nippon Kagaku Kaishi, 1975, 687. 173 H. D. Brauer and H. Wagener, Ber. Bunsengesellschaftphys. Chem., 1975, 79, 597. 17* J. A. Howard and G . D. Mendenhall, Canad. J. Chem., 1975,53,2199. 17s R. F. Boyer, C. G. Lindstrom, B. Darby, and M. Hylarides, Tetrahedron Letters, 1975,4111. lie G. Serra-Errante and P. G. Sammes, J.C.S. Chem. Comm., 1975, 573. 17' Y . Ilan, M. Luria, and G. Stein, J. Phys. Chem., 1976, 80, 584.
170
447
Photo-reduction and -oxidation Ph
(128) a; n = 1, X = COCO2H b; n = l , X = COzH c; n = 2 , X = COzH d ; n = 2 , X = OH
Ph (127)
e; n
=
2,X
=
Me0 (129) a; X = Me
b; X C;
NHAc
X
= =
CH,OMe CHO
A full paper has appeared describing the dye-sensitized photo-oxidation of g-hydroxyphenylpyruvic acid (128a) reported last year (see Vol. 7, p. 416), and this work has now been extended to the oxidation of other p-substituted phenols (128b--e).178 Interest in the yellowing of wood pulp has led to studies of the products from the participation of lo2in the photo-oxidation of lignin model compounds containing phenolic 17g or styryl functional groups such as (129).lS0 Photoexcited aromatic nitro-compounds are able to give oxidative cleavage of the aromatic ring of aromatic methoxy-compounds (e.g. see Vol. 6, p. 543).181 7 Oxidation of Nitrogen-containing Compounds Photo-oxidation of amines has again been the subject of several reports this year. Davidson and Tretheweylea have shown from kinetic data that, in the photooxidation of triethylamine sensitized by Rose Bengal in aqueous methanol, both singlet oxygen [reactions (28) and (29)] and radical intermediate pathways [reaction (30)] are involved. The relative importance of each pathway depends Dye (TI)
Amine Dye (TI)
-
+ 30a
+ lo2
+ Amine
___+
____+
Dye (So)
-
+ lo,
[Complex]
[Complex]
Products
(28) (29)
Products
(30)
upon amine concentration: at low concentration of triethylamine (< 0.1 moll-1) the singlet-oxygen route contributes significantly, whilst at higher concentrations (0.4 mol I-l) a radical mechanism dominates. In continuation of their studies on the dye-photosensitized oxidation of tertiary amines, French workers have reported the oxidation of alkaloids which contain an N-methyl heterocyclic ring, such as An iminium ion is apparently a general intermediate in the formation of products, and the transformation of iminium ion to enamine has been used in the synthesis of some indole alkaloids.184 Secondary and tertiary amines such as (130) undergo dyesensitized photo-oxidation with the production of nitroxyl radicals [e.g. (131)].lS6 I. Saito, Y.Chujo, H. Shimazu, M. Yamane, T. Matsuura, and H. J. Cahnmann, J. Amer. Chem. Soc., 1975, 97, 5272. 17@ G. Brunow and M. Sivonen, Paperi Puu, 1975, 57, 215, 219. lB0 G. Gellerstedt and E. L. Pettersson, Acta Chem. Scand., 1975, B29, 1005. 181 I. Saito, M. Takami, and T. Matsuura, Bull. Chem. SOC. Japan, 1975, 48, 2865. ls2 R. S. Davidson and K. R. Trethewey, J.C.S. Chem. Comm., 1975, 674. Y.Hubert-Brierre, D. Herlem, and F. Khuong-Huu, Tetrahedron, 1975, 31, 3049. R. Beugelmans, D. Herlem, H.-P. Husson, F. Khuong-Huu, and M.-T. Le Goff, Tetrahedron Letters, 1976, 435. V. B. Ivanov, V. Y.Shlyapintokh, 0.M. Khvostach, A. B. Shapiro, and E. G. Rozantsev, J. Photochem., 1975, 4, 313.
178
448
Photochemistry OH
OH
Singlet oxygen is implicated in this process, which occurs in low quantum yield (10-2-10-4) but high overall chemical yield. The direct photo-oxygenation of the aromatic amines p-phenylenediamine and NN-dimethylaniline has been noted.lsB The photo-oxygenation of p-phenylenediamine in cyclohexane leads to p-benzoquinonediimine (Q = 0.01) and 4,4'diaminoazobenzene (Q = 0.003), perhaps via a charge-transfer complex of singlet excited amine with oxygen. Indian workers have again investigated the ketone-sensitized photo-oxidation of diphenylamine which yields diphenyl nitroxide,ls7,lS8whilst a similar photo-oxidation sensitized by methylene blue instead produces N-phenyl-p-benzoquinonirnine.les Enamines generally behave as electron-donating substituted olefins and are attacked by lo2 (see Section 5 ) to yield 1,Zdioxetans or their thermal decomposition products. Sensitized photo-oxidation of the enamines (1 32) in pyridine at room temperature is believed to proceed through a 1,Zdioxetan intermediate.1go According to substitution pattern, the dioxetan decomposes
R'
\
R3
I
c=c, A
R4
N
X
eo
W 0 (132) a; R1 = H, R2 = Me,R3 = Ph, X = 0 b; R1 = R2 = Me,R3 = Ph, X = 0 c; R1 = R2 = Me, R3 = Ph, X = CH, d ; R1 = H, R2 = Me, R3 = Et, X = 0 e ; R1 = H,R2 =: Me,R3 = Et,X = CH, f; R1 = R2 = Me, R3 = Pri, x 0
H
(133)
either by ring fission to two carbonyl fragments (C-C fission) or by C-N bond cleavage. There is thus a resemblance to the competitive C-C or C-S bond cleavages noted for the dioxetans from lo2attack on vinyl s ~ l p h i d e s141 .~~~~ The products of direct photo-oxidation of the enamide (133) include the dioxetan from lo2addition to the olefinic bond, and compounds arising by rearrangement of the dioxetan.lQ1 Photo-oxidation of an enamine has been reported in the synthesis of the aporphine alkaloid, cepharadione B.lS2 Rose Bengal-sensitized photo-oxidation of steroidal etiojervane derivatives which are ap- or K. Maeda, A. Nakane, and H. Tsubomura, Bull. Chem. Soc. Japan, 1975,48,2448. W. R. Bansal, S. Puri, and K. S. Sidhu, J. Indian Chem Soc., 1975, 52, 308. lS8 N. R. K. Raju, M. Santhanam, B. Sethuram, and T. N. Rao, Indian J. Chem., 1975,13,493. lS9 W. R. Bansal, N. Ram, and K. S. Sidhu, Zndian J. Chem., 1975, 13, 987. ln0 W. Ando, T. Saiki, and T. Migita, J, Amer. Chem. Soc., 1975, 97, 5028. F. Abellb, J. Boix, J. Gbmez, J. Morell, and J.-J. Bonet, W e b . Chim. Acta, 1975, 58, 2549. lea J. M. Sah, M. J. Mitchell, and M. P. Cava, Tetrahedron Letters, 1976, 601. lS6
Photo-reduction and -oxidation
449
jly-unsaturated enamines produces the corresponding a/3- or jly-unsaturated ketones.lg3 In both reports,lg2,lg3there is a critical dependence on solvent polarity: a polar solvent such as methanol assists the photo-oxidation of enamine to ketone at the expense of the formation of other products. The photo-oxidation of 2-methyl-2-nitrosopropane in the gas phase proceeds by a dissociative mechanism, as outlined in reactions (31)-(33).lg4 The identified BdNO
+ O2 + NO
But*
ButO,*
hv
ButNO*
But.
+ NO
But02* ButO*
__I_,
+ NOz
(31) (32)
(33)
reaction products are t-butyl nitrate (62%), acetone (18%), 2-methyl-2-nitropropane (14%), t-butyl nitrite (2%),and isobutene (2%). Most of these products are derived by combination and fragmentation of the t-butoxyl radicals formed in reaction (33). Photo-oxidation of [2H,]azomethane produces deuteriated methanol, methyl hydroperoxide, and dimethyl peroxide.lgs The methylene blue-sensitized photo-oxidation of a nitrone (134) has been reported by Ching and F 0 0 f e . l ~ ~Quantitative conversion of (134) into the
hydroperoxide (135) is observed on oxidation at - 63 "C in deuteriochloroform, and it seems most likely that a normal ene-reaction of lo2(see Section 5 ) is involved in this conversion, rather than a 1,3-dipolar cycloaddition. Singlet oxygen, generated by dye-photosensitization, is able to cleave the C=N bond in benzophenone oxime (136a), its anion, or the O-methyl ether (136b), producing benzophenone and nitrite (Scheme 22).lg7 An unstable dioxazetan, from lo2 Ph2C=N- OR
lo'
> Ph.,C=O
+
O=N-OR
(136) a; R = H b; R = Me Scheme 22
addition to the C=N bond, would be a plausible intermediate in this cleavage react ion. A new method has been reported for the conversion of lactams (137) into imides (138) by irradiation in the presence of benzophenone and oxygen.1g8 A. Murai, C. Sato, H. Sasamori, and T. Masamune, Bull. Chem. SOC.Japan, 1976, 49,499. J. Pfab, J.C.S. Chem. Comm., 1976, 297. lP6 J. Weaver, R. Shortridge, J. Meagher, and J. Heicklen, J. Photochem., 1975, 4, 109. lP6 T.-Y. Ching and C. S. Foote, Tetrahedron Letters, 1975, 3771. In7 C. C. Wamser and J. W. Herring, J. Org. Chem., 1976, 41, 1476. 19* J.-C. Gramain, R. Remuson, and Y. Troin, J.C.S. Chem. Comm., 1976, 194. lD3
ln4
450
Photochemistry
(137) a; R = Me,n = 1 b; R = H, n = 1 c; R = Me,n = 2
The lactams are inert towards lo2,and evidence is put forward to support a hydrogen abstraction mechanism in which triplet benzophenone abstracts hydrogen specifically from the methylene group adjacent to the nitrogen atom. The radical produced then reacts with oxygen, eventually yielding (138). Lightner et al. have continued their series of publications on the dyesensitized photo-oxidation of pyrroles with studies of N-phenylpyrr~le,~~~ 2,3,5-trimethylpyrr0le,~~~ and t-butylpyrroles.201In each case it is believed that the initial steps are lo2attack via 1,4-addition to form an endo-peroxide and possibly 1,2-addition to form a dioxetan. The suggestion is made that the endo-peroxide may in part rearrange to dioxetan below room temperature (a rearrangement also observed for polyarylfulvenes 168). The thermally unstable endo-peroxides can in fact be observed by n.m.r. at ca. -80 "C in [2H6]acetone or Freon 11, and are precursors to the isolated photoproducts.201s202 There is continuing interest in the photochemistry of bilirubin, which is the pigment responsible for neonatal jaundice. Rates of reaction of bilirubin with lo2have been calculated (see Section 4).120 The self- or dye-sensitized photo-oxygenation of some oxopyrromethenes (139) and other monopyrroles related to bilirubin has been investigated.203 R3
R2
(139) a; R1 = R3 = R4 = Et, R2 = Me b; R1 = R2 = H, R3 = R4 = Et c; R1 = Et, R2 = Me, R3 = R4 = H
Dye-sensitized photo-oxidation at room temperature of N-substituted indoles leads to cleavage of the 2,3-double bond [e.g.:(140) produces (141)], perhaps via a d i o ~ e t a n .However, ~~~ when the irradiation of (140) is carried out at -70 "C a peroxidic intermediate, possibly (142), can be intercepted by functional groups of the side-chain to give a 3-hydroperoxyindoline (143) in high yield.206 Analogously, other Japanese workers have provided full details of their earlier report 206 on the involvement of a 3-hydroperoxyindoline in the photo-oxidation Ips 2oo 201 202
204 205
208
D. A. Lightner, D. I. Kirk, and R. D. Norris, J. Heterocyclic Chem., 1974, 11, 1097. D. A. Lightner and L. K. Low, J . Heterocyclic Chem., 1975, 12, 793. D. A. Lightner and C . 4 . Pak, J . Org. Chem., 1975,40,2724. D. A. Lightner, G. S. Bisacchi, and R. D. Norris, J. Amer. Chem. Soc., 1976, 98, 802. D. A. Lightner and Y.-T. Park, Tetrahedron Letters, 1976, 2209. I. Saito, M. Imuta, S. Matsugo, H. Yamamoto, and T. Matsuura, Synthesis, 1976, 255. I. Saito, M. Imuta, S. Matsugo, and T. Matsuura, J. Amer. Chem. SOC.,1975, 97, 7191. M. Nakagawa, T. Kaneko, K. Yoshikawa, and T. Hino, J. Amer. Chem, SOC.,1974,96,624.
451
Photo-reduction and -oxidation
rn 00-
'02
- 70°C
H2CH20
Me
I
1 0 ,
20°C
COCH2CH20H N-CHO Me
OOH
OLD Me
(141)
of the N-methyltryptamine (144a), and now describe the isolation of this species (145a).207Such compounds as (145) are also isolable from the photo-oxidation of the tryptamine and tryptophan derivatives (144b) and (144~).~O*
(144) a; R1 = H, R2 = Me b; R1 = H, R2 = C0,Me
(145)
c; R1 = R2 = C02Me
Other reported examples of the photo-oxidation of nitrogen-containing heterocycles include the reaction of substituted pyrazines such as (146) and pyrimidines with lo2to form endo-peroxides (Scheme 23),200and the photooxidation of the reduced lumiflavin cation.210 Dioxetans are produced in some
(146) a; R
= PhCH2 b;R=Me Scheme 23
heterocyclic oxidations. Thus, a stable dioxetan is formed from attack of lo2 at - 50 "C on the 4,5-double bond of the 1,2'-dimer of 2,4,5-triphenylimidazole (lophine).211 Oxidations of 3-ben~ylidenepiperazine-2~5-diones such as (147) ao7
aoD
alo
M. Nakagawa, K. Yoshikawa, and T. Hino, J. Amer. Chem. SOC.,1975, 97, 6496. M. Nakagawa, H. Okajima, and T. Hino, J. Amer. Chem. SOC.,1976, 98, 635. J. L. Markham and P. G. Sammes, J.C.S. Chem. Comm., 1976, 417. N . Lasser, H. Levanon, and J. Feitelson, Photochem. and Photobiol., 1975, 22, 7 . G. Rio and B. Serkiz, J.C.S. Chem. Comm., 1975, 849.
452
Photochemistry
(147) a; b;
R1 = H, R2 = Ph R1 = Ph, R2 = H
(148)
with lo2at 25 "C give cleavage of the benzylidene group, via formation of fairly stable dioxetans.212 Photo-oxidation of either isomer (147a) or (147b) leads to the same dioxetan (148), which suggests that a non-concerted formation of dioxetan may be involved. A zwitterionic intermediate would be in accord with calculations for lo2attack on electron-donating substituted 01efins.l~~ There have been several reports of the photo-oxidation of amino-acids. Riboflavin-sensitized photo-oxidation of methionine, which leads to several products, has been noted at various pH values.213 The photo-oxidation of methionine by 4-benzoylbenzoate ion 64 has already been discussed (see Section 1 and Scheme 10). The oxidation product, methional (MeSCH,CH,CHO), is formed in 100% yield at pH 7. Irradiation of flavin mononucleotide in the presence of sulphur-containing amino-acids causes deamination by a closely related mechanism.214 Electron abstraction by flavin mononucleotide from sulphur is followed by intramolecular electron transfer from carboxylate anion to the sulphur radical centre, which leads on to decarboxylation and deamination. Photo-oxidation of tryptophan produces N'-formylkynurenine, which may then act as a sensitizer for further photodynamic action, resulting in the degradation of proteins. The pathway by which N'-formylkynurenine acts as sensitizer of tryptophan degradation has been shown to involve partly lo2production and partly triplet-state hydrogen abstraction with consequent production of oxygen radical-anion (02'-)from ground-state oxygen.215 Adenine accelerates the lumiflavin-sensitizedphoto-oxidation of tryptophan, histidine, met hionine, and guanine, without altering the reaction products.216 The photosensitized oxidation of tyrosine derivatives in the presence of sodium alginate has been investigated.217,218 The photo-inactivation of enzymes is often a result of the specific photooxidation of certain amino-acid residues which they contain. Papain, for example, on methylene blue-sensitized photo-oxidation, loses one histidine residue and 220 Destruction of histidine residues is also reported to becomes be involved in the Rose Bengal-sensitized photo-oxidation of a-glucan phosphorylases,221rabbit haemopexin,222and the visible-light-induced photo212
213 21r 21s 216
217 218
21D 220
a21
2a2
P. J. Machin and P. G. Sammes, J.C.S. Perkin I, 1976, 628. H. Nakamura, Koshien Daigaku Kiyo, 1975,4, 13 (Chem. Abs., 1975, 83,59 239). J. R. Bowen and S. F. Yang, Photochem. andPhotobiol., 1975, 21, 201. P. Walrant, R. Santus, and L. I. Grossweiner, Photochem. and Phorobiol., 1975, 22, 63. A. Yoshimura and S. Kato, Bull. Chem. SOC.Japan, 1976, 49, 813. G. R. Seely and R. L. Hart, Phorochem. and Photobiol., 1976, 23, 1. G. R. Seely and R. L. Hart, Photochem. and Photobiol., 1976, 23,7. K. Okumura and T. Murachi, J. Biochem. (Japan), 1975, 77, 913. A. Ohara, S. Fujimoto, and H. Kanazawa, Chem. and Pharm. Bull. (Japan), 1975,23,967. A. Kamogawa and T. Fukui, Biochim. Biophys. Acta, 1975, 403, 326. V. L. Seery, W. T. Morgan, and U. Muller-Eberhard, J. Biol. Chem., 1975, 250, 6439.
Photo-reduction and -oxidation
453
oxidation of dinitrophenylhistidine-200 human carbonic anhydrase B.223 Brief reviews have appeared which cover various aspects of the photosensitized oxidation of amino-acids 224 and proteins.224,225 Applied to proteins, such techniques can give valuable information on their three-dimensional (tertiary) structure.22s In the photo-oxidation of lanthanide ion-lysozyme complexes, the lanthanide ion can convey some protection from photo-oxidative attack to nearby tryptophan and methionine residues, which again allows useful 'mapping' Photodynamic sensitization of the of the tertiary structure of inactivation of lysozyme by 8-methoxypsoralen is a result of lo2production.22s 8 Miscellaneous Oxidations Interest in the fate of chlorinated hydrocarbons in the environment continues. The photolysis of chlorofluoromethanes in the presence of oxygen or ozone 229 and the photo-oxidation of vinyl chloride in air 230 have been studied. Heicklen and co-workers have reported the mercury-photosensitized oxidation of 1,l-dichloroethene 231 and t r i c h l o r ~ e t h e n e . ~ ~ ~ Several examples of the photo-oxidation of organic molecules catalysed by metallic ions have been published. Copper@) salts have been shown to play an essential role in the photo-oxidation in methanol of @-unsaturated ketones such as dypnones ( 1 4 9 a - - ~ )and ~ ~ ~mesityl oxide (149d). It now appears234that endo-peroxides may be intermediates in this reaction, because (150) has been R
Me
H. NMe,
hCOR
Me
(149) a; R = Ph b; R = p-MeC,H, c; R = p-BrC,H, d;R=Me
( 1 50)
isolated, and this could arise by oxygen addition to the s-cis-dienol form of (149d). A method for the formation of remote double bonds by cupric acetate-catalysed photolysis of alkyl nitrites depends upon the ability of Cu" salts to oxidize an intermediate alkyl radical to an a1kene.235Irradiation of alkyl nitrites in benzene in the presence of Cu" ions leads to 23-36% of the corresponding &-unsaturated 223 224 226
22e 227
M. Kandel, A. G. Gornall, L. K. Lam, and S. I. Kandel, Canud. J. Biochem., 1975,53,599. G. Jori, Photochem. and Photobiol., 1975, 21, 463. G. Laustriat and C. Hasselmann, Photochem. and Photobiof., 1975, 22, 295. G. Jori, Anais Acad. brasil. Cienc., 1975, 45, Suppl., 33 (Chem. Abs., 1975, 83, 127 573). G. Jori, M. Folin, G. Gennari, G. Galiazzo, and 0 . Buso, Photochem. and Photobiol., 1974, 19, 419.
228 229
230 231 232
233 a34
236
W. Poppe and L. I. Grossweiner, Photochem. and Photobiol., 1975,22,217. R. K. M. Jayanty, R. Simonaitis, and J. Heicklen, J . Photochem., 1975, 4, 381. T. Kagiya, K. Takemoto, and Y . Uyama, Nippon Kagaku Kaishi, 1975, 1922. E. Sanhueza and J. Heicklen, J. Photochem., 1975, 4, 17. E. Sanhueza and J. Heicklen, J. Photochem., 1975, 4, 161. T. Sato, K. Tamura, K. Maruyama, and 0. Ogawa, Tetrahedron Letters, 1973, 4221. T. Sato, K. Tamura, K. Maruyama, 0. Ogawa, and T. Imamura, J.C.S. Perkin I, 1976, 779. Z. Cekovid and T. Srnik, Tetrahedron Letters, 1976, 561.
Photochemistry alcohols. Iron(1n) is an effective photo-oxidant for a wide range of organic 454
Hydrogen abstraction by an alkoxy-radical, generated from the photolysis of a-peroxynitriles, forms the basis of a method for the introduction of a functional (cyano) group into unactivated C-H The unusual cleavage of a cyclopropane ring by lo2has been reported for (151), which gives mainly ring-opened products that can be accounted for by the formation and subsequent breakdown of a 1,Zdioxolan intermediate.238 The reaction of a-keto-carboxylic acids with lo2appears to generate peroxy-carboxylic acid by oxidative decarboxylation [reaction (34)].239"However, this peroxy-acid reacts immediately with more a-keto-carboxylic acid [reaction (35)], so that the overall reaction product observed is a carboxylic acid. RCOC02H RC03H
+ lo2
+ RCOCOZH
___I*
+ C02 2RCOzH + COa RC03H
(34)
(3 5 )
A study has been made of the mechanism and rates of attack of lo2on cyanine and it has been noted that lo2is able to oxidize selenides to selenoxides in good yield.241Photo-oxidation of the drug phenothiazine has been studied by e.s.r. and photo-oxidation of p-ketoalkylpyridinium iodides leads to or-diketone~.~~~ The photo-oxidation of derivatives of the triterpene lupan-29-01 has been Irradiation of solutions of ozone in saturated hydrocarbons at -78 "C with visible light gives alcohols and ketones derived from ozone-hydrocarbon complexes by oxygen insertion into C-H Irradiation with U.V. light (254 nm), however, leads to competing attack by excited oxygen atoms [O(lD)] on the hydrocarbon. Ground-state atomic oxygen [O(3P)]is often generated by photolytic methods, and its reactions with organic compounds have been reviewed.24s Another report of the products of photo-oxygenation of diethyl ether has appeared,lss and is broadly in agreement with that reported last year.247 2a6
238
240
242 243
244 246 246
ar7
A. Cox and T. J. Kemp, J.C.S. Faraday I, 1975, 71,2490. D. S. Watt, J. Amer. Chem. SOC.,1976, 98, 271. R. H. Rynbrandt and F. E. Dutton, J. Org. Chem., 1975, 40, 3079. C. W. Jefford, A. F. Boschung, T. A. B. M. Bolsman, R. M. Moriarty, and B. Melnick, J. Amer. Chem. SOC.,1976, 98, 1017. G. W. Byers, S. Gross, and P. M. Henrichs, Photochem. and Photobiol., 1976, 23, 37. L. Hevesi and A. Krief, Angew. Chem. Internat. Edn., 1976, 15, 381. I. Rosenthal and R. Poupko, Tetrahedron, 1975,31,2103. T. Mukaiyama, K. Atsumi, and T. Takeda, Chem. Letters, 1975, 1033. A. Vystrcil, V. Krecek, and M. Budesinsky, Coll. Czech. Chem. Comm., 1975, 40, 1593. T. H. Varkony, S. Pass, and Y. Mazur, J.C.S. Chem. Comm., 1975, 709. R. E. Huie and J. T. Herron, Progr. Reaction Kinetics, 1975, 8, 1. C. von Sonntag, K. Neuwald, H.-P. Schuchmann, F. Weeke, and E. Janssen, J.C.S. Perkin ZZ, 1975, 171.
6 Photoreactions of Corn pounds containing Heteroatoms other than Oxygen BY S. T. REID
1 Nitrogen-containing Compounds Rearrangement.-The mechanism of photoisomerization about the C-N double bond has been the subject of a number of separate investigations. Two different pathways have been proposed for the direct and triplet-sensitized syn-antiphotoisomerizations of the 4-nitrophenylhydrazones of benzaldehyde and certain of its p-substituted derivatives.l A twisted triplet intermediate is proposed for the sensitized process, whereas in the direct isomerization the reaction is said to occur either concurrently with or subsequent to internal conversion of electronic into vibrational energy. Evidence for two different pathways in an analogous study of the 4-nitrophenylhydrazone of pyridine-Zaldehyde has also been published.2 The syn-isomer undergoes complete conversion into the antiisomer on direct excitation, but a photoequilibrium between the two isomers is rapidly established in the triplet-sensitized process. Thermally reversible synanti-isomerization has been reported in the 2-phenylhydrazones of 1,2,3-triketones and related 1,2-diketone~,~ and an investigation of the mechanism of tripletstate photochemical isomerization of benzoylacetanilide and pyrazolone azomethine dyes has been de~cribed.~In the latter case, two pathways were distinguished, the first involving torsion about the azomethine bond and the second involving inversion at the nitrogen atom. Electron-donating substituents facilitate the torsion mechanism in contrast with electron-withdrawing substituents, which favour the inversion mechanism. cis-Azoalkanes, unlike the corresponding aryl azo-compounds, undergo photodecomposition with loss of nitrogen at room temperature. In an attempt to obtain stable cis-azoalkanes, the cis-isomers of the azo-compounds (1) derived from adamantane and norbornane were prepared by irradiation of the corresponding trans-isomers (2) in toluene at 0 oC.6 The cis-isomers reverted to the trans-isomers on heating without substantial competing loss of nitrogen. Equilibrium isomer ratios at 320 and 430nm for the photochemical cis-transisomerization of a series of l-aryl-3-(3-methylbenzothiazolin-2-ylidene)triazenes l
a
G. Condorelli, L. L. Costanzo, S. Giuffrida, and S. Pistara, 2.phys. Chem. (Frankfurt) 1975, 96, 97. G. Condorelli, L. L. Costanzo, L. Alicata, and A. Giuffrida, Chem. Letters, 1975, 227. P. Courtot, R. Pichon, and L. Le Saint, Tetrahedron Letters, 1976, 1181. W . G. Herkstroeter, J. Amer. Chem. SOC.,1976, 98, 330. P. S. Engel, R. A. Melaugh, M. A. Page, S. Szilagyi, and J. W . Timberlake, J. Amer. Chem. Soc., 1976, 98, 1971.
455
456
Photochemistry
(3) have been determined;s the reaction proceeds via a pathway similar to that found for azobenzene. Further studies of the photochromism of salicylaldehyde 2-quinolylhydrazone (4) have been described. The coloured form ( 5 ) is remarkably stable in both
H
hv, 250-400 n m L
"b
H-*'
w
A
HI ._....I
N\
N
protic and aprotic solvents at room temperature. Structural requirements for this isomerization have been deduced from an examination of a series of substituted salicylaldehyde 2-q~inolylhydrazones,~ and details of the kinetics and mechanism of the thermal decay of the coloured form have been published.s A photostationary equilibrium is rapidly established on irradiation of either of the isomeric 2-diethylamino-l,3-diphenylprop-2-en-l-ones (6) and (7) in diethyl ether.g The major products of prolonged irradiation are the dihydroisoquinoline (8) and a mixture of cis- and trans-chalcone (9); their formation can best be accounted for in terms of a Norrish Type I1 process involving the biradical species (10). Analogous photoreactions have been reported for other 2-dialkylamino-l,3-diphenylprop-2-en-l-ones, but deconjugation of the apunsaturated ketone appears to be preferred in cases where this is possible, as in a
E. Faughaenel, R. Haensel, W. Ortmann, and J. Hohlfeld, J. prakt. Chem., 1975,317, 631. M. F. Zady, F. N. Bruscato, and J. L. Wong, J.C.S. Perkin I, 1976,2036. J. L. Wong and M. F. Zady, J. Org. Chem., 1975,40,2512. J. C . Arnould and J. P. Pete, Tetrahedron Letters, 1975, 2459.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
ACH=CHPh
Ph
457
*
t
(9)
+ [MeCH=NEt]
Me
-
he (10)
J01
OH
the derivative (11) which affords the isomer (12) in 50% yield on irradiation in diethyl ether or methanol. An alternative cyclization has been observed in dialkylaminocyclohexenones.l0 On irradiation, 2-piperidinocyclohex-2-en-1-one (13) is converted into the azetidine (14) by a process which presumably must involve y-hydrogen abstraction by the carbonyl group to form the biradical(l5); a singlet excited state appears to be involved. In a number of derivatives, and especially those with N-tosyl groups, competing cyclization to the corresponding
0
lo
J. C.Arnould and J. P. Pete, Tetrahedron Letters, 1975, 2463.
458
Photochemistry azetidinol (16) occurs. Surprisingly, no evidence for cyclization has been observed in the photochemistry of the related 2-alkoxycyclohexenones. Photochemically induced electrocyclic processes continue to attract much attention. Quenching studies indicate that the photoisomerization of l-ethoxycarbonyl-lH-azepine (17) occurs via a singlet excited state;ll a triplet state is available but unreactive. Arylated 1,3-dihydr0-2H-azepin-2-ones(18) are
C02Et
(18) R1 = R2 = But R3 = Me, Prn, or C,HI1
converted by an analogous disrotatory process into the isomers (19) in high yield on irradiation in benzene.lz Cyclization is also observed on irradiation of diazepines, as, for example, in the conversion of lH-1,2-diazepines into the corresponding 2,3-diazabicycl0[3,2,O]hepta-3,6-dienes.~~In contrast with their carbocyclic analogues the benzocycloheptatrienes, which react principally by [1,7] hydrogen shift and ring contraction, lH-2,3-benzodiazepines (20) undergo a
'qR3 R
R'
hv, Pyrex
s
'
,
R1'
-qR2
R2 (20)
:
(21)
R1
R2
R3
H OMe
H Me H
H H
H H H H H
Me Me Ph H PhCH2
H Ph H Ph
Ph
facile valence isomerization to give the 2a,7-dihydro[l,2]diazeto[4,1-a]isoindoles (21).l* The reaction, which provides the first example of an isolable 1,2-diazetine, is virtually quantitative, and there is no nitrogen extrusion as might be expected by analogy with the photochemical behaviour of 1,2-benzodiazepines. A more recent report describes the related conversion of 3H-1,2-diazepines into the corresponding 1,2-diazet0[4,1-a]pyr~oles.~~ l1 la
lS
'l lS
G. Jones and L. J. Turbini, J. Photochem., 1976, 5, 61. H.-D. Becker and K. Gustafsson, Tetrahedron Letters, 1976, 1705. J. P. Luttringer, N. PCrol, and J. Streith, Tetrahedron, 1975,31,2435. A. A. Reid, H. R. Sood, and J. T. Sharp, J.C.S.Perkin I, 1976, 362. C. D. Anderson, J. T. Sharp, E. Stefaniuk, and R. S. Strathdee, Tetrahedron Letters, 1976,305.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
459
Reversible cyclization of the benzoazahexatriene intermediate (22) has been proposed to account for the formation of N-acetylbenzoazetines (23) from 1-acetyl-2-cyano-l ,2-dihydroquinolines (24) on Pyrex-filtered irradiation in diethyl ether or ethanol.16 A competing irreversible cyclization of the triene (22; R = Me) to the dihydroindole (25) predominates on further irradiation.
AC
NC
Ac
N I CO,Et
(z8)
p.
0 N I
An aza[l3]annulene (26) of unknown configuration has been obtained by lowtemperature irradiation of the heterocycles (27), (28), and (29).17 The azaannulene (30) is also formed on irradiation of the aziridine (29). The 2-azabicyclo[2,2,l]hexane ring system has been synthesized by irradiation of N-substituted 3-allylamino-5,5-dimethylcyclohex-2-en-l-ones (31) in cyclohexane.18 For the lo
M. Ikeda, S. Matsugashita, F. Tabusa, H. Ishibashi, and Y. Tamura, J.C.S. Chem. Comm.,
l7
1975, 575. G. Frank and G. Schroder, Chem. Ber., 1975, 108, 3736.
Y. Tamura, H. Ishibashi, M. Hirai, Y. Kita, and M. Ikeda, J. Org. Chem., 1975,40,2702.
Photochemistry
460
N-methyl, N-allyl, and N-phenyl derivatives, intramolecular cycloaddition takes place to give exclusively or predominantly the less stable stereoisomer (32). The N-acetyl derivative (31; R = Ac), however, is converted into a 1 : 1-mixture of isomers (32; R = Ac) and (33); this difference in stereospecificity is tentatively
____, cyclo hv, pyrex hexane I
&-Jk HT
Me
Me (34)
(35)
accounted for in terms of different excited states. It is not clear why the corresponding dimethyl derivative (34) takes an unusual course, giving 7-azabicyclo[4,3,0]nonan-2-one (35) as the sole product.l@ Valence tautomerization to the 1,4-diazocine (36) is proposed to account for the photoreaction of the azetidino[3,2-b]pyridine (37).20 The final products of irradiation in diethyl ether at - 78 “C
c Et
I
C-N=C=C, C H’;‘ Et !
Me
l* *O
HI
/
Ph H
(39)
Y. Tamura, H. Ishibashi, and M. Ikeda, J . Org. Chem., 1976,41, 1277. J. W. Lawn, M. H. Akhtar, and W. M. Dadson, J . Org. Chem., 1975,40, 3363.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
461
are the l-(S)-pyrrole (38) and the N-(A)-ketenimine (39). The optical purity of the products corresponds to about 80% retention of configuration for the proposed [1,3] sigmatropic rearrangement. The stereochemistry of the reaction is therefore in agreement with orbital-symmetry predictions. The photochemistry of 1,3-oxazin-6-one (40) appears to be similar to that previously described for a-pyrone. Irradiation in an argon matrix gave two unstable species to which structures (41) and (42) have been tentatively assigned on the basis of spectral evidence.21 Further irradiation led to decomposition and the formation of HCN, C 0 2 ,and acetylene.
(44)
(43)
The study of heterocyclic analogues of the stilbene -+ phenanthrene cyclization continues to attract some attention. Novel pyridocarbazoles have been obtained in the absence by the photocyclization of 1-/3-indolyl-2-pyridylacrylonitrile~;~~ of oxygen, pathways competing with dehydrogenation are sometimes observed.23 4-Methylsulphonylbenzo[c]cinnolines were prepared by photochemical cyclodehydrogenation of the corresponding 2-methylsulphonylazobenzenes in 98% sulphuric A novel route to the imidazo[2,1-a]isoquinoline ring system has been accomplished by irradiation of cis-1-styrylimidazole (43) in methanol in the presence of iodine.25 Unlike the stilbene --f phenanthrene cyclization, an intermediate of the dihydrophenanthrene type is not possible in this case, and a dipolar species (44) has been proposed. Analogous cyclizations occur with substituted l-styrylimidazoles and with l-styrylbenzimidazole. The cyclization of 1-dimethylamino-2,2-bis-(9-fluorenylidenemethyl)ethyleneis followed by elimination of dimethylamineto give benzo[e]fl~oreno[9,1-kZ]acephenanthrylene.~~ Further synthetic applications of the photocyclization of enamides have been reported. Details of the preparation of alkylated benzophenanthridinones (45) in 50% yield by irradiation of the N-cyclohexenyl-l-naphthamides(46) have been published,27and similar cyclizations have been employed in the synthesis 21 22
23
24 25 26
27
A. Kranz and B. Hoppe, J. Amer. Chem. SOC.,1975, 97, 6590. C. Dieng, C. Thal, H. P. Husson, and P. Potier, J. Heterocyclic Chem., 1975, 12, 455. C. Riche, A. Chiaroni, H. Doucerain, R. Besselievre, and C. Thal, Tetrahedron Letters, 1975, 4567. C. P. Joshua and V. N. R. Pillai, Indian J. Chem., 1975, 13, 1018. G. Cooper and W. J. Irwin, J.C.S. Perkin I , 1976, 75. C. Jutz and H.-G. Lobering, Angew. Chem. Internat. Edn., 1975, 14, 418. I. Ninomiya, T. Naito, and A. Shinohara, Japan Kokai, 74 134 679 (Chem. Abs., 1975, 83, 28 125).
16
462
Photochemistry
of protoberberine alkaloids 28 and of ( & ) - c a ~ i d i n e . ~ A~ novel stereoselective synthesis of lycorine-type alkaloids using the key intermediate (47), obtained by photocyclization of the enamido-ketone (48), has also been described.80 Two different mechanisms are believed to be involved in the non-oxidative cyclization of benzo[b]thiophen-2-carboxanilide (49 ; R = H) and its N-methyl derivative (49; R = Me).31 The former affords the cis-fused product (50), whereas the latter is converted into the trans-isomer (51); the formation of the trans-isomer is
(49)
28
2B
31
\
[l,5] shift
I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1720. I. Ninomiya, T. Naito, and H. Takasugi, J.C.S. Perkin I, 1975, 1791. H. Iida, S. Aoyagi, and C. Kibayashi, J.C.S. Perkin I, 1975, 2502. Y. Kanaoka, K. Itoh, Y . Hatanaka, J. L. Flippen, I. L. Karle, and B. Witcop, J. Org. Chem., 1975,40, 3001.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
463
presumably the result of a suprafacial 1$5-hydrogen shift in the trans-dihydrointermediate (52). Oxidative cyclization of a different type is observed on irradiation of the fully methylated 6-(benzylidenehydrazino)uracils (53) and this provides a new synthetic route in up to 90% yield to pyrazolo[3,4-d]pyrimidines (54).3a Carbazoles and indolo[3,2-b]carbazoles have been synthesized by photocyclization of substituted triphen~1arnine.s.~~
Me
hie
Me
(53)
(54)
The photochemical generation of nitrile ylides from 2H-azirines has been reviewed.34 These species are now widely used in synthetic chemistry and new applications continue to be reported. The low-temperature irradiation (- 196 "C) of 2,3-diphenyl-2H-azirine (55) has been reinvestigated and leads almost quantitatively to the formation of the dipolar species, benzonitrile-benzylide (56), with = 0.78.35 Irradiation ( A = 345 nm) of the benzylide (56) resulted in almost Ph H
b H Ph
N
-
+
hv
complete reconversion into the 2H-azirine (0= 0.15). The benzylide (56) also underwent a quantitative conversion into 2,5-diazahexa-lY3,5-triene(57) on heating to - 160 "C, thus providing evidence that the triene is formed not only via an indirect route involving the bicycle (58) as previously demonstrated, but also by direct dimerization of the benzylide. Evidence for the intermediacy of dipolar species (59) in the photochemical conversion of ( 5 8 ) into (57) has been presented. Rearrangement via the ylide to give the oxazoline (60) is observed on 32 33 34
3b
F. Yoneda and T. Nagamatsu, Bull. Chem. SOC.Japan, 1975, 48, 1484. W. Lamm, W. Jugelt, and F. Pragst, J. prakt. Chem., 1975, 317, 284. A. Padwa, Angew. Chem. Internat. Edn., 1976, 15, 123. A. Orahovats, H. Heimgartner, H. Schmid, and W. Heinzelmann, Helv. Chim. Acta, 1975, 58, 2662.
464
Photochemistry PhwcHzR
+ Ph-C-N-CH
/,v _ I ,
I
CH,R
N
irradiation of 2-hydroxymethyl-3-phenyl-2H-azirine(61 ; R = OH) in benzene, whereas in the 2-chloro- or 2-bromo-analogues (61; R = C1 or Br) an unexpected rearrangement involving a novel 1,4-halogen shift occurs to give the N-vinylimines (62).3s The ylides derived from spiro-azirines (63) are readily trapped by methanol, giving the imines (64).37 In contrast to these results, however, irradiation of 2-phenyl-l-azaspiro[2,2]pent-l-ene (65) resulted in an unusual photochemical cycloelimination to give products such as the azirine (66) derived from the novel carbene 2-phenylazirinylidene.
\\ f
’
- ..
N CH2’
(63) n = 1, 2, or 3 (65) n = 0
p,, OMe
Intermolecular and intramolecular addition reactions of nitrile ylides have been reported. Irradiation (280-350 nm) of a benzene solution of 3-phenyl2H-azirines in the presence of carboxylate esters leads to the formation of 5-alkoxy-3-oxazolines by regiospecific addition of benzonitrile-methylide to the ester carb0ny1.~~ Intramolecular addition of ylide to a carbonyl is observed on irradiation of 2-formyl-3-phenyl-2H-azirine (67) to give 2-phenyloxazole (68) in 70% yield.39 Analogous transformations were found in 2-vinyl-substituted 2H-a~irines,~~, *O but the major product of irradiation of (2)-3-phenyl-2-styryL 87 88
40
A. Padwa, J. K. Rasmussen, and A. Tremper, J.C.S. Chem. Comm., 1976, 10. A. Padwa and J. K. Rasmussen, J. Amer. Chem. SOC.,1975,97, 5912. P. Gilgen, H.-J. Hansen, H. Heimgartner, W. Sieber, P. Uebelhart, H. Schmid, P. Schonholzer, and W. E. Oberhlnsli, Helv. Chim. Acta, 1975, 58, 1739. A. Padwa, J. Smolanoff, and A. Tremper, J . Amer. Chem. SOC.,1975, 97, 4682. A. Padwa, J. Smolanoff, and A. Tremper, J. Org. Chem., 1976, 41, 543.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
465
2H-azirine (69) is the benzazepine (70). The preference for cyclization to a sevenmembered ring is a consequence of the proposed linear geometry of the dipolar intermediate. Sensitization and quenching experiments indicate that the primary photoreaction of the 2-vinyl-substituted 2H-azirine system occurs from the singlet state.
CHO (67)
phpd" Ph
/
The ylide derived from 2-phenyl-3-methyl-3-allylazirine(71) has a different fate; the major product of irradiation is the 2-azabicyclo[3,1,O]hex-Zene (72), and the reaction is viewed as an intramolecular 1,l-cycloaddition of the carbene (73) to the alkene.41 A more recent study provides support for a stepwise pathway for this addition,42 and similar additions have been reported in related sys As part of a continuing study of phototransposition in aromatic and heteroaromatic systems, the irradiation of 3-, 4-, and 5-methyl-2-cyanopyrroles in acetonitrile has been On the basis of product analysis the pathway outlined in Scheme 1, involving 2,5-bonding followed by a 1,3-sigrnatropic shift 41
43
44
A. Padwa and P. H. J. Carlsen, J. Amer. Chem. SOC.,1975, 97, 3862. A. Padwa and P. H. J. Carlsen, J. Amer. Chem. SOC.,1976, 98, 2006. A. Padwa, A. Ku, A. Mazzu, and S. I. Wetmore, J. Amer. Chem. SOC.,1976,98, 1048. J. Barltrop, A. C. Day, P. D. Moxon, and R. R. Ward, J.C.S. Chem. Comm., 1975, 786.
466
Photochemistry
major product
minor product Scheme 1
of the nitrogen atom, has been proposed for the photorearrangement of 2-cyanopyrroles. A second 1,3-sigmatropicshift is necessary to account for the formation of the minor product. The suggestion that 2,5-bonding in excited pyrrole initiates the transposition is supported by a correlation diagram for the T,T* state of pyrrole. An analogous intermediate is believed to be implicated in the photoisomerization of the azine monoxide (74),46 and reaction of the mesoionic 1,2,4-triazo1-3-ones (75) to give the corresponding benzimidazoles, azobenzenes,
&
h
&
N=N
, &H2Me2+
N=O
NO
+
CH=NMe,
0
CH=NOH
fashion, whereas the cis-isomer underwent rapid ring cleavage to give products derived from the 1,3-bisformylcyclopentane derivative (207). The effects of temperature on the reactivity of the adducts of N-nitrosopiperidine with camphene and pinene have been investigated,142and in the presence of oxygen, addition of N-nitrosoamines to norbornene takes place to give cis,exo- and trans-2-nitrato-3-aminon~rbornanes.~~~ Oxygen is believed to intercept photochemically generated NO to form a nitrogen trioxide radical which in turn is scavenged by a carbon-radical intermediate. Cleavage of the benzene ring of aromatic methoxy-compounds on irradiation in the presence of aromatic nitro-compounds is thought to involve addition of the nitro-group as illustrated for 1,4-dimethoxynaphthalene in Scheme 7.144 Cleavage occurs selectively at the 1,%bond with respect to the methoxy-group, and aromatic nitro-compounds having lowest n , ~ *triplet are more effective f
Ar
OMe
..
___, OMe
OMe
+ ArN: Me0
CHO
i
ArNHz + ArN=NAr Scheme 7 H. H. Quon and Y. L. Chow, Tetrahedron, 1975, 31,2349. K. S. Pillay, K. Hanaya, and Y. L. Chow, Canad. J . Chem., 1975,53, 3022. lP4 I. Saito, M. Takami, and T. Matsuura, Bull. Chem. SOC.Japan, 1975, 48, 2865. lP2
488
Photochemistry
than those having lowest n,n* triplet. Precedence for this addition is to be found in the reaction of nitrobenzene with cyclohexene. The reported addition of water and benzene to 17/3-acetoxy-4-aza-androst5-en-3-one on irradiation in benzene is not easily e~p1ained.l~~ Miscellaneous Reactions.-The well known synthesis of aldosterone 21-acetate by photolysis of the 1lp-nitrite of corticosterone acetate suffers from the disadvantage that radical attack at c-19 competes with the desired attack at c-18. This problem has now been overcome and an improved route to aldosterone devised by the introduction of extended conjugation into the nitrite, resulting in an increase in the separation between C-19 and the llp-oxygen atom;146thus, on irradiation, the 11-nitrite of 1lfl-hydroxypregna-l,4-dien-3-one(208) affords 0H.
(208)
(209)
the C-18 oxime (209) in 55% yield. The best solvents for this reaction were found to be THF and acetonitrile. Nitrite photolysis has also been employed in the synthesis of ( k )-tetrahydroanhydroaucubigenone from 2-(2-nitrosoxyethyl)7-oxabicyclo[3,3,0]octan-3-one.147 The syntheses of 11-deoxy-18-hydroxycorticosterone and 18-hydroxycorticosterone21-acetates have been accomplished by irradiation of the appropriate steroidal 20-nitrites in the presence of oxygen, leading to functionalization at C-18 and the formation of C-18 nitrates.14* Interception of 8-alkyl radicals, generated by nitrite photolysis, has been achieved with cupric acetate;140 both unsaturated alcohols and cyclic ethers are obtained in this way, as shown, for example, for 2-hexyl nitrite in Scheme 8.
i
Cu(OAc),
Scheme 8 145
146
147
lQ8 149
J. Boix, J. Gbmez, and J.-J. Bonet, Helv. Chim. Acta, 1975, 58, 2545. D. H. R. Barton, N. K. Basu, M. J. Day, R. H. Hesse, M. M. Pechet, and A. N. Starratt, J.C.S. Perkin I., 1975, 2243. H. Obara, H. Kimura, J. Onodera, and M. Suzuki, Chem. Letters, 1975, 221. D. H. R. Barton, M. J. Day, R. H. Hesse, and M. M. Pechet, J.C.S. Perkin I, 1975,2252. Z. CekoviC and T. SrniC, Tetrahedron Letters, 1976, 561.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
489
A process and a reactor for the photo-oximation of cycloalkanes using a gaseous nitrosating agent such as nitrosyl chloride have been described in the patent 1 i t e r a t ~ r e . lThe ~ ~ photochemical nitrosation of chlorocyclododecane with nitrosyl chloride affords syn- and anti-isomers of a-chlorocyclododecanone oxime together with dichlorocyclododecane, chloronitrocyclododecane, chlorocyclododecyl nitrate, and chlorocyclododecanone.lsl Abstracts reporting the analogous nitrosations of cyclododecane 152 and cyclo-octane 153 have been published, but details are not available. New reports provide additional evidence that carbon-nitrogen bond cleavage is the major pathway in the photochemistry of nitrosoalkanes. Substituted hydroxylamines are the major products of irradiation of 2-nitrosoisobutyronitrile and 1-nitrosocyclohexanecarbonitrile with red light (570-700 nm) and appear An identical homolysis to be derived exclusively by C-N bond hom01ysis.~~~ is involved in the preparation of gem-chlorobromo-compounds by photolysis of the corresponding gem-chloronitroso-derivatives in the presence of excess bromine,166and contrary to earlier reports the photo-oxidation of nitrosoalkanes is now claimed to involve the same primary homolytic process.166 The NO group also appears to be removed photochemically from N-methyl-N'-nitro-N-nitrosoguanidine without oxygen parti~ipati0n.l~~ Nitrogen-nitrogen bond homolysis has been proposed to account for the formation of piperidine and N-formylpiperidine from N-nitropiperidine on irradiation in methanol.ls8 The details of this process are by no means clear, and indeed other workers have failed to observe homolysis of this type. In the presence of HCl and cyclohexene, however, addition of N-nitropiperidine occurs via the piperidinylium radical to give 2-nitro- and 2-methoxy-1-piperidinocyclohexanes. A complex series of reactions has been described for the irradiation of nitroethane in c y c l o h e ~ a n e and , ~ ~ ~attempts to rationalize the sequence have been complicated by further reaction of the photoproducts. trans-Azocyclohexane di-N-oxide (210) is the major product of irradiation (254 nm) of nitroethane in cyclohexane; it is further converted into hydroxyiminocyclohexane (21 1) and N-cyclohexylcaprolactam (212) by irradiation ( A > 290nm and X > 250nm respectively). The formation of nitrosocyclohexane appears to preclude the possibility of a direct photochemically induced deoxygenation of the nitroalkane and requires that, even in solution, all major products of the irradiation of nitroalkanes arise by an initial C-N homolytic bond cleavage. The primary processes 160 lS1
lS2
lK4
lK6 lSB 16'
lKB lKB
G. Lucas, U.S.P. 2 853 729 (Chem. Abs., 1975, 83, 18 926). Y. A. Gromoglasov, A. V. Iogansen, L. A. Levashova, V. V. Karchikhina, M. N. Enikeeva, G. A. Kurkchi, 0. V. Levina, V. A. Valovoi, V. P. Baeva, and A. A. Samoilenko, Neftekhimiya, 1974, 14, 770. M. P. Lazareva, V. V. Karchikhina, I. A. Levashova, 0. V. Levina, L. G. Zelenskaya, Y. A. Gromoglasov, and M. N. Enikeeva, Chem. Abs., 1976, 84,4125. K. E. Kuznetsova, L. E. Levashova, A. A. Streltsova, A. D. Proshenkova, and Y . A. Gromoglasov, Chem. Abs., 1975, 83, 95 979. B. G. Gowenlock and J. Pfab, Annalen, 1975, 1903. J. Pfab, Tetrahedron Letters, 1976, 943. J. Pfab, J.C.S. Chem. Comm., 1976, 297. Y . Ioki, A. Imamura, C. Nagata, and M. Nakadate, Photochem. andPhotobiol., 1975,21,387. R. W. Lockhart, R. W. Snyder, and Y . L. Chow, J.C.S. Chem. Comm., 1976, 52. S. T.Reid and E. J. Wilcox, J.C.S. Chem. Comm., 1975, 647.
490
Photochemistry
IIV, 254 nm
EtNoz
cyclohexane
’
hv, X > 290 nm
observed in the gas-phase photolysis of 1-nitropropane are analogous to those previously described for nitroethane and 2 - n i t r 0 p r o p a n e ~whereas ~ ~ ~ the photochemical decomposition of tetranitromethane in a variety of solvents is thought to take place predominantly by an ionic mechanisrn.ls1 Attempts to prepare imidazoles by photocyclization of 3-amino-Znitrobenzo[blthiophen derivatives (213) were unsuccessful; a product mixture was obtained from which the nitro-compound (214) and the oxime (215) were separated.lsa
Unsaturated oximes are themselves photochemically reactive. 3-0xo-17/3acetoxyandrosta-ly5-dieneoxime (216), on irradiation in methanol, is converted into the lactam (217) together with the parent ketone and four photoproducts derived The triplet states of 0-acyl aromatic ketoximes have excitation energies close to those of the parent ketones and have considerable A. R. Khan, Chem. Letters, 1975, 879. V. I. Slovetskii and V. P. Balykin, Izvest. Akad. Nauk. S.S.S.R.,Ser. khim., 1975,2186. le2 P. N. Preston and S . K. Sood, J.C.S. Perkin I, 1976, 80. 162 P. N. Preston and S. K. Sood, J.C.S. Perkin I, 1976, 80. le3 J. Repoil6s, F. Servera, and J.-J. Bonet, Helv. Chim. Acta, 1974, 57, 2454. l80
lE1
Photoreactions of Compounds containing Heteroatoms other than Oxygen
491
T,T*character.le4
They readily undergo homolytic cleavage of the N-0 bond to iminyl radicals and acyloxyl radicals. The diphenylmethaniminyl radical (218), generated in this way from benzoate (219), undergoes aromatic substitution in benzene as well as dimerization to the azine (220).lS6 The mechanisms of such 0
II ,0-C-Ph
Ph
0 Irv
Ph
)=No
Ph
Ph)=N
+
II
*O-C--Ph
Ph Ph
Ph
Ph
substitutions in benzene and in toluene have been studied.lsa Radical phthalimidation of aromatic compounds can also be effected photochemically by decomposition of N-tosyloxyphthalimide in the presence of electron-rich aromatic substrates.lS7 The recently reported photocyclization of 3-dialkylaminoacrylophenoneshas now been extended to 3,3-bis(dialkylamino)acrylophenones (221) which, on
(221) R
=
H, Et, or Prn
irradiation in benzene, are converted into the pyrroles (222).lSS The mechanism of these transformations is still not clear. The benzophenone-sensitized photoreactions of 2-quinolinecarbonitrile derivatives in ethanol are dependent on the nature of the 4 - s ~ b s t i t u e n t . ~Nucleophilic ~~ photosubstitutions of halogen in aminochloropyridines and in aminohalogenopyrimides have been described.170 Articles reviewing the photochemistry of pyrazolone derivatives used as the photosensitized reactions of amino-acids and and the M. Yoshida, H. Sakuragi, T. Nishimura, S. Ishikawa, and K. Tokumaru, Chem. Letters, 1975, 1125. las H. Ohta and K. Tokumaru, Bull. Chem. SOC.Japan, 1975, 48, 2393. lSa S. Ishikawa, H. Sakuragi, M. Yoshida, N . Inamoto, and K. Tokumaru, Chem. Letters, 1975, 8 19. la7 J. I. G. Cadogan and A. G. Rowley, J.C.S. Perkin I, 1975, 1069. la8 H. Aoyama, T. Hasegawa, T. Nishio, and Y. Omote, Bull. Chem. SOC.Japan, 1975,48, 1671. laS N. Hata and R. Ohtsuka, Chem. Letters, 1975, 1107. 170 A. N. Frolow, A. V. El'tsov, and 0. V. Kul'bitskaya, Khim. geterotsikl. Soedinenii, 1974,12, 1645. 171 J. Reisch, Gyogyszereszet, 1975, 19, 81. G. Jori, Photochem. and Photobiol., 1975, 21, 463. lS4
492 Photochemistry photochemistry of the hydrazo-, azo-, and azoxy-groups 173 have appeared during the course of the year.
2 Sulphur-containing Compounds Certain aspects of the photochemistry of organic sulphur compounds have been reviewed.174 Interest in the study of photorearrangement reactions in sulphurcontaining compounds has been maintained. The stable Z-form of a-(thiopyran2-ylidene) ketone (223) is transformed into the E-isomer (224) on irradiation;175
the photoproduct reverts to starting material by a dark process which obeys first-order kinetics. Z-Methyl thiobenzohydroximates are photochemically converted into their thermally stable E - i ~ o m e r s and , ~ ~ ~an equilibrium mixture of stereoisomers (225) and (226) is obtained on irradiation of the sulphone (225).17' Irradiation of 3-phenyl-2H-thiopyran 1, l-dioxide (227) in methanol yields a mixture of adducts (228) and (229).17* Isolation of the latter provides convincing evidence that the cyclic sulphone does not require the incorporation of an
atom bearing a free electron pair for ring-opening to occur, and therefore argues in favour of a mechanism involving cycloreversion to the sulphene (230). Full details of the 'Dewar' thiophen structure (231) of the photoproduct of 2,3,4,537s
174 176
17' 178
R. J. Drewer, in 'Chemistry of the Hydrazo, Azo, and Azoxy Groups', Vol. 2, ed. S. Patai, Wiley, 1975, p. 935. J. D. Coyle, Chem. SOC.Rev., 1975, 4, 523. C. T. Pedersen, C. Lohse, N. Lozach, and J.-P. Sauv6, J.C.S. Perkin I, 1976, 166. W. Walter, C. 0. Meese, and B. Schroder, Annalen, 1975, 1455. H. A. Selling, Tetrahedron, 1975, 31, 2387. J. F. King, E. G. Lewars, D. R. K. Harding, and R. M. Enanoza, Canad. J. Chern., 1975,53, 3657.
Photoreactions of Compounds containing Heteroatorns other than Oxygen
493
tetrakis(trifluoromethy1)thiophen (232) have been p~b1ished.l~~ The role, if any, of this species in the photorearrangement of substituted thiophens remains uncertain. The incorporation of deuterium, observed on irradiation of 5-phenylisothiazole in D,O-diethyl ether solution, supported the formation of a tricyclic sulphonium cation intermediate.lsO Various intermediates have been proposed to account for the conversion of 1,2,3-thiadiazole 2-oxides (233) into the isomeric
F3kfF3 hv
F3C
CF3
~
’ A
0-
0-
(235)
3-oxides (234);ls1the isolation of a low yield of the 1,2,5-thiadiazole (235) from the diphenyl derivative (233; R1 = R2 = Ph) is somewhat surprisingly taken as evidence for an intramolecular cycloaddition pathway. As a continuation of the study of the photochemistry of mesoionic systems, the photoreactions of the 1,3,4-thiadiazoles (236) have been examined.la2 Spectral evidence for initial rearrangement to the acyclic species (237) has been presented,
(236) R1 Ph Me Ph Ph
R2 Me Ph Ph p-MeOC,H,
Y . Kobayashi, I. Kumadaki, A. Ohsawa, Y. Sekine, and H. Mochizuki, Chem. and Pharm. Bull. (Japan), 1975, 23, 2773. lBo M. Maeda, A. Kawahara, M. Kai, and M. Kojima, Heterocycles, 1975, 3, 389. lS1 H. P. Braun, K.-P. Zeller, and H. Meier,AnnaZen, 1975, 1257. R. Mukherjee and R. M. Moriarty, Tetrahedron, 1976, 32, 661.
17
494
Photochemistry
and the products are presumed to be derived by a further photochemical N-N bond homolysis. A re-examination of this decomposition provided further evidence for a ring-opening process and also led to the detection of COS as a A revised and simplified mechanism has been proposed for these transformations. Competition between reversible ring-opening and elimination of C02 in the mesoionic 4-phenyl-l,3,2-oxathiazolylio-5-oxide(Scheme 9) has 0
Scheme 9
been shown to be dependent on the molecular environment of the reactant.ls4 1,2-Dithiolyl radicals and dithioketonate anions are the initial products of irradiation of a series of 3,5-disubstituted 1,2-dithiolylium salts in ethanol,lss and irradiation of diary1 sulphides in cyclohexane in the presence of iodine affords dibenzothiophens.166 The synthesis of medium- to large-ring azathiocyclols has been achieved by an unusually regioselective remote photocyclization of sulphide-containing phthalimides.ls7 For example, the phthalimides (238) are converted on irradiation in acetone into a mixture of nine-membered (239) and seven-membered (240) ring compounds. This reaction can surprisingly be extended to even larger rings, and a special mechanism must therefore be implicated in which the sulphur atom facilitates the formation of the macrocyclic transition state; a tentative explanation involving enhanced proton transfer from the methylmercapto-group in a chargetransfer complex has been advanced. The corresponding O-methyl derivatives fail to undergo the same cyclization. One direct application of this cyclization has been the construction of a cyclic peptide Polycyclic aromatic thiones (241) having a freeperi-position cyclize, on excitation in the n -+ T band, to give thiophen derivatives (242).189 At least in one case, the formal 1,3-hydrogen lE3 18(
lS6
A. Holm, N. H. Toubro, and N. Harrit, Tetrahedron Letters, 1976, 1909. I. R. Dunkin, M. Poliakoff, J. J. Turner, N. Harrit, and A. Holm, Tetrahedron Letters, 1976, 873. C. T. Pedersen and C. Lohse, Acta Chem. Scand., 1975, 29B, 831. K. P. Zeller and H. Peterson, Synthesis, 1975, 532.
lR6
Y.Sato, H. Nakai, T. Mizoguchi, Y . Hatanaka, and Y. Kanaoka, J. Amer. Chem. SOC.,1976, 98, 2349.
lE8 189
Y . Sato, H. Nakai, T. Mizoguchi, and Y . Kanaoka, Tetrahedron Letters, 1976, 1889. A. Cox,D. R. Kemp, R. Lapouyade, P. de Mayo, J. Joussot-Dubien, and R. Bonneau, Canad. J. Chem., 1975, 53,2386.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
495
0
0 (238) R1 = Me, Et, .But, or PhCH,
P R 2
(239) R2 = H, Me, or Ph Ph
a H
YS
\
/
p h i r 2.L (243) Ar = Ph or /3-naphthyl
migration has been shown to be intermolecular. In contrast, arylalkyl thiones (243) having an activated p-position undergo cyclization to cyclopropanethiols (244) on n + ~ T T * excitation.lao The reason for the difference in behaviour between thiones and the corresponding ketones which do not undergo cyclization is not clear; a simple rationalization is that the greater size of the n 3p orbital makes hydrogen abstraction easier. p-Hydrogen abstraction is also observed in substituted thiochromanone sulphoxides, but the major reaction pathway involves rearrangement to the cyclic sulphenate.lQ1 This rearrangement is formally analogous to the formation of oxacarbenes from ketones, but the photochemistry is made more complex by further cleavage of the weak 0 - S bond. The disulphide (245) is the only product of irradiation of the sulphoxide (246) and is believed to be formed via the sulphenate (247) as shown in Scheme 10. Homolytic S-S bond cleavage followed by radical recombination rather than a concerted 1,3-sigmatropic rearrangement is proposed to account for the novel A. Couture, M. Hoshino, and P. de Mayo, J.C.S. Chem. Comm., 1976, 131. 1. W.J. Still, P. C. Arora, M. S. Chauhan, M. H. Kwan, and M. T. Thomas, Canad.J. Chem., 1976,54,455.
lBo 191
Photochemistry
496
(245) Scheme 10
as& hNHc NHCOMe
hv, MeCN Pyrex
R
’
R
2
(248) R = H or OMe
R yH2Ph
COCH,Ph
/
HN +S
M
O
Me 0,C
v
11
EtOH
’ N
X
C0,Me
E;IH PhCH,CON H - p - - > N / 0
CO, Me
(253)
-
I1v 0 MeOH
NH
Photoreactions of Compounds containing Heteroatoms other than Oxygen
497
rearrangement of bis-(o-acetylaminophenyl) disulphides (248) to 2-methylbenzothiazoles (249).ID2 The rearrangement of the isothiazolone (250) to the thiazole (251) is assumed to involve initial homolytic S-N bond cleavage, but details of the formation of this and other products are not completely clear.le3 A thiazole (252) is also obtained as the major product of irradiation of the cephalosporin (253) in methan01.l~~ The photoreactions of simple aromatic and aliphatic thiones have been reviewed.lB6 The most distinctive characteristic is the frequent, if not general, ability of the excited thione function to give products derived from a higher singlet state. Adamantanethione, however, on irradiation (500 nm) gives the n,n* triplet efficiently; this species undergoes addition to alkenes to give thietans in a regiospecific manner and to adamantanethione to give a dimer, the 1,3-dithietan.lDs The regiospecificity in thietan formation is that expected from the most stable biradical; prior formation of a triplet exciplex may be involved. Photoaddition of aryl thiones (254) to bis(methy1thio)ethyne (255) has been
Rf R3
1q-S
I
*C=C -SMe
MeS'
MeS
SMe
reported to give the @-unsaturated dithioesters (256);lS7the unstable dithiet (257) is presumably an intermediate in this addition. The photochemically induced addition of a primary amine, l-aminobutane, to lY3-dimethyl-4-thiouracil (258) leads to the formation of two diastereoisomeric adducts (259).lD8 In the presence of a tertiary amine, however, 1,3-dirnethyl-4-thiouracilis converted into a mixture of tetrahydrodipyrimidine~.~~~ Photochemically generated thiyl radical additions have also been widely reported. 3-Phenylthiacyclohexane(260) and 2-methyl-4-phenylthiacyclopentane (261) have been synthesized by competing intramolecular thiyl addition in the unsaturated thiol (262).200The dithiole (263) and a 1,4-dithian were obtained by lea lea
lS6 lge 19'
lg0
aoo
Y. Maki and M. Sako, Tetrahedron Letters, 1976, 851. Y. Maki and M. Sako, Tetrahedron Letters, 1976, 375. Y. Maki and M. Sako, J, Amer. Chem. SOC.,1975, 97, 7168. P. de Mayo, Accounts Chem. Res., 1976, 9, 52. A. H. Lawrence, C. C. Liao, P. de Mayo, and V. Ramamurthy, J. Amer. Chem. SOC.,1976, 98, 2219. A. C. Rrouwer and H. J. T. Bos, Tetrahedron Letters, 1976, 209. J.-L. Fourrey, Tetrahedron Letters, 1976, 297. J.-L. Fourrey and J. Moron, Tetrahedron Letters, 1976, 301. V. P. Krivonogov, V. 1. Dronov, and N. K. Pokoneschikova, Khim. geterotsikl. Soedinenii, 1975, 9, 1204.
498
Photochemistry
S
S
irradiation of the thiol (264) and its S-substituted derivatives;201the formation of the dithiole via the isomeric disulphide (265) may involve a 1,3-sigmatropic process.
0
0
Irradiation of cyclic disulphides in the presence of aldehydes results in S-S bond cleavage and the formation of mono S-acylated dithiols.202The addition of photochemically generated thiyl radicals to diphenylvinylphosphine has been described; the formation of different products can be accounted for in terms of competing thiyl radical attack on the alkene or at Sulphur dioxide has again found use in the trapping of photochemically generated 1,4-biradi~als,~~* and O-ethyl-l-thionaphthoateis recommended for use as a photosensitizer for excitation by visible light of up to 500 nm wavelength because of its high triplet yield and low photochemical reactivity.206 aol SOa
$0‘ SO6
L. Dalgaard and S. 0. Lawesson, Acra Chem. Scand., 1974,28B, 1077. M.Takagi, S. Goto, and T. Matsuda, J.C.S. Chem. Comm., 1976,92. D. H. Brown, R. J. Cross, and D. Millington, J.C.S. Dalton, 1976,334. N.K. Hamer, J.C.S. Chem. Comm., 1975, 551. M. Gisin and J. Wirz, Helv. Chim. Acta, 1975,58,1768.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
499
3 Compounds containing other Heteroatoms The photochemistry of silicon-containing compounds continues to attract increased attention and a wide variety of reactions have been reported. Renewal of interest in silacyclobutenes prompted a study of the photochemistry of l,l-dimethyl-2-phenyI-l-silacyclobut-2-ene (266).206Irradiation in acetone gave the adduct (267) in 83% yield. The reaction is viewed as arising via cycloreversion to the siladiene (268) followed by 1,4-addition to acetone. The diene
Ei-Me Ph A e
hv.
ph
d
(266)
bk,CO
Si-Me I Me
n : e Ph ,sic Me Me
(268)
(267)
(27 1)
(268) would be expected to have some 1,2- and 1,4-zwitterionic character by analogy with the known chemistry of the C=Si double bond. Irradiation of the novel 9-silabicyclo[4,2,1]nona-2,4,7-triene(269), with or without sensitizers, led to the formation of the silabarbaralane (270) and the product (271) of intra,2] c y c l i ~ a t i o n . ~ ~ ~ molecular [,2 The first unsymmetrical photodimerization of the anthracene ring system has been claimed.208 Irradiation of bis-(9-anthryl)-l,l,3,3-tetramethyldisiloxane (272) in diethyl ether affords the [,4 ,4] intramolecular adduct (273). Steric
+
+
Me - Si-Me
I
0
d Av
I
Me-Si-Me
206
ao7 208
P. B. Valkovich and W. P. Weber, Tetrahedron Letters, 1975, 2153. T. J. Barton and M. Juvet, Tetrahedron Letters, 1975, 2561. A. G. Schultz, J. Org. Chem., 1975, 40, 3467.
500 Photochemistry effects are said to be responsible for the unsymmetrical dimerization of the anthracene nucleus, but no previous examples of this type of behaviour have been published. Sensitized photocycloaddition is observed between the silyl enol ethers of a-tetralone and a-indanone and electron-deficient a l k e n e ~ . ~ * ~ Two types of reaction are known for acylsilanes, namely reversible siloxycarbene formation and a less common radical cleavage. Irradiation of 1,l-diphenyl-l-silacyclohexan-2-one(274) in cyclohexane affords dimers (275) and Ph P h
(274)
Ph Ph
P h Ph
\/
\/
+ Ph’
I
Ph
Ph’
Ph
(275)
(276), arising presumably via the siloxycarbene (277) which is trapped in methanol as the cyclic acetal (278).210 1,l-Diphenyl-l-silacyclopentanewas also obtained. The photochemical reactions of benzoyltrimethylsilane with substituted phenylsilanes can readily be accounted for in terms of the insertion of siloxycarbene into the Si-H bond of the phenylsilane.211 A novel addition is observed between phenylpentamethyldisilane (279) and certain alkenes (280) on irradiation in benzene.212 The major products are the o-disubstituted benzene derivatives (281), which are believed to arise by addition
OSi
Me, Si Me
(279)
209
*lo
212
(281)
R1 = H, R2 = SiMe,
R1 = RZ = Me R1 = Me, R2 = CMe=CH,
G. Felix, R. Lapouyade, H. Bouas-Laurent, and B. Clin, TetrahedronLetters, 1976, 2277. K. Mizuno, H. Okamato, C. Pac, H. Sakurai, S. Murai, and N. Sonoda, Chem. Letters, 1975, 237.
zll
hv +
A. G. Brook, J. B. Pierce, and J. M. Duff, Canad. J. Chem., 1975,53,2874. H. Watanabe, N . Ohsawa, M. Sawai, Y. Fukasawa, H. Matsumoto, and Y. Nagai, J. Organometallic Chem., 1975, 93, 173.
Photoreactions of Compounds containing Heteroatoms other than Oxygen
501
of alkene to the unstable intermediate (282). Ph2Si=CH2 or its biradical equivalent has previously been described in the photolysis of pentaphenylmethyldisilane. When vinylsilanes such as dimethylvinyl-, trimethylvinyl-, and ethyldimethylvinyl-silane are used as the alkene in this reaction, silepin derivatives are always formed in low yield in addition to the normal adducts.21s The mercury-sensitized photodecompositions of trichlorosilane 214 and of halogenomethyldimethylsilanes 21s have been described. The aryl selenide (283) undergoes photocyclization in benzene in the presence of toluenesulphonic acid to give the benzoselenophen (284) in 60% yield.21s The selenocarbonyl ylide (285) is presumably involved. OH
Irradiation ( A > 280 nm) of benzyl diselenide (286) in acetonitrile results in extrusion of selenium and the formation of dibenzyl selenide (287).217Homolytic Se-Se bond cleavage is not involved, and a radical Se-C bond cleavage is proposed to account for product formation and kinetic observations. The preparation of carbazoles by the photochemical extrusion of dimethyl phenylphosphonate from oxazaphosphoranes has been reported.21s The isolation of isomerically pure 3-methoxy-7-methylcarbazole(288 ; R1 = MeO, PhCH,SeSeCH,Ph (286)
818 214
als 216
217
hv
PhCH,SeCH,Ph
+
Se
(287)
M. Ishikawa, T. Fuchikami, T. Sugaya, and M. Kumada, J. Amer. Chem. SOC.,1975, 97, 5923. M. Ishikawa, T. Fuchikami, and M. Kumada, Tetrahedron Letters, 1976, 1299. K. G. Sharp, P. A. Sutor, T. C. Farrar, and K. Ishibitsu, J. Amer. Chem. Soc., 1975,97,5612. I. N. Jung and W. P. Weber, J. Org. Chem., 1976, 41, 946. J. Y. C. Chu, D. G. Marsh, and W. H. H. Gunther, J. Amer. Chem. SOC.,1975,97,4905. J. I. G. Cadogan, B. S. Tait, and N. J. Tweddle, J.C.S. Chem. Comm., 1975, 847.
Photochemistry R2 = Me) from the phospholine (289; R1 = MeO, R2 = Me) can be satisfactorily accounted for by participation of a species such as (290). A preliminary account of the successful preparation of the phosphindole system by photocyclization of an o,o’-bis(phenylethyny1)triphenylphosphine has been published.21e Details of the photoaddition of tetrafluorodiphosphine to fluorinated ethylenes have been described,220and a new preparation of monoalkyl phosphates by irradiation of simple dianisyl alkyl phosphates has been reported.221 Mercurysensitized irradiation ( A = 254 nm) of boranes or carbaboranes has been found to be a convenient method for synthesizing the corresponding boron-boroncoupled boranes or carbaboranes.222 502
220 a21 24a
N. Winter, Tetrahedron Letters, 1975, 3913. W. K. Glanville, K. W. Morse, and J. G. Morse, J. Fluorine Chem., 1976, 7 , 153. R. A. Finnigan and J. A. Matson, J.C.S. Chem. Comm., 1975, 928. J. S. Plotkin and L. G. Sneddon, J.C.S. Chem. Comm., 1976, 95.
7 P hotoel imination BY S. T. REID
This chapter is principally concerned with the photochemically induced fragmentation of organic molecules accompanied by the formation of small molecules such as nitrogen, carbon dioxide, and sulphur dioxide. Photodecompositions resulting in the formation of two or more sizeable fragments are reviewed in the final section. Fragmentations arising by Norrish Type I and Type I1 reactions of carbonyl-containing compounds are considered in Part 111, Chapter 1. 1 Photodecomposition of Azo-compounds The photolysis of azoalkanes provides a convenient route for the generation and subsequent study of alkyl radicals; the most important competing process is trans-cis-isomerization. A low-field CIDNP study of ethane formed by photolysis of azomethane in carbon tetrachloride solution indicates that the photodecomposition occurs predominantly from the singlet state.l Examination of the Stern-Volmer plot of the nitrogen quantum yield for the photolysis (366 nm) of azoisopropane in the gas phase over an extended pressure and temperature range led to the conclusion that decomposition occurred via vibrationaIly excited upper singlet and triplet states with the onset of dissociation of the vibrationally equilibrated triplet state as the temperature is increased.a The phenyldiazenyl radical PhN2*has been established as an intermediate in the decomposition of arylazoalkanes. A recent study of the photolysis of the labelled azo-compound (1) clearly demonstrates that phenyl migration in the /
Me
N=N15
\ /
C
Ph’
‘Me
Ph Me
I 2 Ph-C* I hv
*N=Nl5-Ph
Me
diazenyl radical (2) does not readily O C C U ~ . Various ~ a,a’-dichloro-, a,a’-dialkoyloxy-, and a&-dibenzoxy-azoalkanes are reported to undergo mesu-dl photointerconversion on direct irradiation through pyrex,* and the photoelimination a
J. A. den Hollander, J.C.S. Chem. Comm., 1976, 403. G. 0. Pritchard and F. M. Servedio, Internat. J. Chem. Kinetics, 1975, 7 , 99. N. A. Porter and J. G. Green, Tetrahedron Letters, 1975, 2667. N. Levi and D. S . Malament, Israel J. Chem., 1975, 12, 925.
503
504 Photochemistry of nitrogen from methyldi-imide, MeN=NH, proceeds via a radical chain mechani~m.~ The study of the photoelimination of nitrogen from 1-pyrazolines continues to offer a unique opportunity to examine the mechanism of these photodecompositions, and also provides a useful synthetic route to cyclopropanes. Irradiation of the 1-pyrazoline(3) in diethyl ether affords ( - )-cyclocopacamphane (4) in 92% yield.s The pyrazoline diester ( 5 ) is similarly converted into the
Y
Y
,CO, Me
/
Me0,C
C0,Me (5)
Me0,C
C0,Me
C0,Me (7)
(6)
cyclopropane (6) which on further irradiation is reversibly transformed into the isomer (7).' 1,3-Biradicals are usually proposed as intermediates in photodecompositions of this type. In support of this, e.s.r. spectral evidence for the formation of the cyclopenta-l,3-diyl radical on irradiation of 2,3-diazabicyclo[2,2,l]hept-2-ene in a cyclohexane matrix at 5.5 K has now been described.8 Over 95% retention of configuration is observed in the photoelimination of nitrogen from the chiral 1-pyrazoline (8).g This is in agreement with the intervention of a short-lived singlet biradical with little zwitterionic character.
6
-0-menthyl
p h ~ ~ ~ - O - m e n t h y l hv -N,
Ph
+
Ph Ph (8)
Stereospecificity is not observed, however, in the photodecomposition of cis- and trans-3,5-diphenyl-l-pyrazoline, where rotational isomerization appears to compete with cyclization in the biradical.1° The cis-diphenylpyrazolineyields the cisand trans-diphenylcyclopropanesin a ratio of 51.5 : 48.5, whereas the trans-isomer
lo
S. K. Vidyarthi, C. Willis, and R. A. Back, J . Phys. Chem., 1976, 80, 559. E. Piers, M. B. Geraghty, R. D. Smillie, and M. Soucy, Canad. J. Chem., 1975, 53, 2849. T. Toda, K. Nakano, A. Yamae, and T. Mukai, Tetrahedron, 1975,31, 1597. S . L. Buchwalter and G. L. Closs, J. Amer. Chem. SOC., 1975,97, 3857. R. L. Dreibelbis, H. N. Khatri, and H. M. Walborsky, J. Org. Chem., 1975, 40, 2075. M. Schneider and H. Strohacker, Tetrahedron, 1976, 32, 619.
505 affords the same products in the ratio 14.5 : 85.5. A minor competing pathway in the photodecomposition of 3,5-diphenyl-l-pyrazolineleading to the formation of phenylcarbene has been detected by other workers;ll two possibilities for the generation of this carbene exist and are shown in Scheme 1, namely (a) a retro1,3-dipolar addition followed by photolysis of phenyldiazomethane or (b) a
Photoelimination
Ph
Pv Ph-
+
N2
+
PheH Scheme 1
PhcH
+
N,
direct fragmentation with elimination of nitrogen. At present, it is not possible to distinguish between these pathways. Both cis- and trans-3,5-divinyl-l-pyrazolines(9) are known to undergo photoelimination of nitrogen to give trans-l,2-divinylcyclopropane(10) and cyclohepta1,4-diene (11) via the same allylic biradical. A study at - 50 "C now confirms
(1 1)
beyond reasonable doubt that the cycloheptadiene is formed by thermal Cope rearrangement of the unstable cis-l,2-divinylcyclopropane(12).12 The view that the di-n-methane rearrangement of barrelene proceeds by way of a triplet cyciopropyldicarbinyl biradical is widely supported. The cyclopropyldicarbinyl radical (13) has now been generated independently by irradiation
(14)
(13)
(1 5 )
n S. L. Buchwalter and G . L. Closs, J. Org. Chem., 1975, 40, 2549. l2
M. Schneider, Angew. Chem. Internat. Edn., 1975, 14, 707.
(16)
506 Photochemistry of the azo-compound (14), leading to the formation of barrelene (15) and semibullvalene (16).13 Sensitized irradiation affords predominantly the semibullvalene, thus supporting the proposed intermediacy of triplet biradicals in the di-n-methane rearrangement. The greater tendency for barrelene formation in the direct irradiation indicates that intersystem crossing in the azo-compound is inefficient.
(19) R = Ph, 2,5-(MeO)2CaH3,Prn or Pri
(20)
A series of 4-substituted 3-nitro-3-methyl-1-pyrazolines (17) have been converted by irradiation in benzene into the corresponding nitrocyclopropanes (18).14 The reaction is complicated by competing but poorly understood photoreactions of the nitro-group giving the pyrazoles (19) and (20). Singlet biradical intermediates are proposed to account for the retention of configuration observed in
hv benzene
(21)
'
R1 = Ra = Me R1 = Me, R2 = But 0
R2
Ph
0
-
Ph
(23) l8
l4
H. E. Zimmerman, R. J. Boettcher, N. E. Buchler, and G. E. Keck, J. Amer. Chem. SOC., 1975, W,5635. L. Valades, M. Jimenez, and L. Rodriguez-Hahn, Rev. Latinoamer. Quim., 1975, 6, 152.
Photoelimination
507
the direct photodecomposition of four new em-double pyrazolines prepared from norbornadiene and dimethyl phenyldiazornethylphosph~nate.~~ Benzophenonesensitized photodecomposition leads to an intractable mixture of isomeric biscyclopropanes. The photodecomposition of diazobicyclohexane (21), however, takes a different course and results in the formation of the diazo-compound (22), characterized spectroscopically.l6 Further irradiation of this intermediate affords the bicyclo[l,l,O]butane (23), presumably via the carbene. Pyrazoles readily eliminate nitrogen on irradiation to yield the corresponding cyclopropenes. In this way, the first example of a [ 2 ~ , 6 nspirene ] (24)has been
prepared by irradiation of the pyrazole (25).17 The first spirocyclopropabenzenes (26) have also been prepared by photodecomposition of the pyrazoles (27) at - 20 OC.18 These products can be expected to be even less stable than previously reported cyclopropabenzenes and do in fact readily rearrange on heating to the cycloheptenes (28). Photodecomposition of pyrazole (29) yields small amounts of the indenes (30; R1 = Me, R2 = C0,Me; R1 = CO,Me, R2 = Me) together with the cyclopropene (31).le Photoelimination of nitrogen from triazolines has again been used in the synthesis of aziridines. Thus, bicyclic aziridines (32) have been prepared in good yield from the corresponding triazolines (3 3),20 and aziridines have similarly A similar approach to the been obtained from 4-cyan0-5-aminotriazolines.~~ l5
l6 l7
2o
21
H. Cohen and C. Benezra, Canad. J. Chem., 1976, 54,44. W. Welter and M. Regitz, Tetrahedron Letters, 1976, 1473. H. Durr and B. Weiss, Angew. Chem. Internat. Edn., 1975, 14, 646. H. Durr and H. Schmitz, Angew. Chem. Internat. Edn., 1975,14,647. V. V. Razin, Zhur. org. Khim., 1975, 11, 1457. M. H. Akhtar, A. Begleiter, D. Johnson, J. W. Lown, L. McLaughlin, and S.-K. Sim, Canad. J. Chem., 1975, 53, 2891. F. Texier and J. Bourgois, J. Heterocyclic Chem., 1975, 12, 505.
508
Photochemistry
H
0
hv __f
- Nz
I
R
(32)
N-N
/
Me
\
Me
synthesis of several heteromethylenecyclopropanes has been employed. The first synthesis of 1,2-dimethyldiaziridinone (34) has been accomplished by irradiation of the 2-tetrazoline ( 3 3 , the product being characterized spectroscopically.22 Analogous ring contractions are reported in imino- and methylenetetrazolines, leading to diaziridinimines and cyclic carbodi-imides respectively. The photoelimination of nitrogen from cyclic azo-compounds is not, of course, are obtained on limited to five-membered rings. Three products, (36)-(38),
(36)
(37)
(38)
direct irradiation of the azo-compound (39) and their formation is taken as evidence for the intermediacy of the 2,2’-bis-(l, l-dimethylallyl) biradical (40).23 Although full details of the nitrogen elimination process have not been established, aa
H. Quast and L. Bieber, Angew. Chem. Internat. Edn., 1975, 14, 428. T. J. Levek and E. F. Kiefer, J. Amer. Chem. SOC.,1976, 98, 1875.
509
Photoelimination
the formation of the cyclobutane (37), involving as it does rotation around the 2,2'-bond, precludes a concerted ring closure. Thermally unstable 2-arylbenzazetes (41) are produced by photolysis of 4-arylbenzotriazines (42) at - 80 "C and can be trapped as cycloadducts with suitable dimersn2*At room temperature, the benzazetes readily give dimers (43).
Rey
hv, 300nm
N+N
\
(42)
R
=
-80 O C / - N
H, Me, or C1
(41)
Ar (43)
R
Photoelimination of nitrogen is also reported in 3-chloro- and 3-bromo3-methyldia~irines,~~ whereas photochemically induced cis-trans-isomerization is observed in preference to elimination in a series of configurationally isomeric 3 ,S-diphenyl-1,2-diazacyclo-oct-1-enes .26
2 Elimination of Nitrogen from Diazo-compounds Photoelimination of nitrogen from diazo-compounds provides a simple and versatile route for the generation of carbenes. The formation of triplet carbenes by photolysis and thermolysis of diazo-compounds has been ~eviewed.~' The gas-phase photolysis of diazo-n-butane has been studied at various pressures and with added gases.28 Vibrationally excited but-1-ene and methylcyclopropane are formed via singlet carbene. The photodecomposition of diazoanthrone leads to two species of anthranylidene having different spin states;29these two states are directly interconvertible. Irradiations carried out in benzene, toluene, cyclohexene, cyclohexane, and hexafluorobenzene in the presence and absence of triphenylphosphine demonstrate that the triplet state has the ability to abstract hydrogen atoms selectively whereas the singlet state interacts with nucleophilic centres such as the terminal nitrogen atom of the diazo-group. Triplet vinylmethylene, generated by triplet-photosensitized decomposition of diazopropene, also readily participates in hydrogen abstraction reactions.30 Irradiation in cyclohexane, for example, yields allylcyclohexane by a radical pair mechanism. Studies with cyclohexene, however, have cast doubt on the view that addition is the preferred mode of reaction of triplet carbenes with alkenes. The major product of reaction of triplet vinylmethylene with cyclohexene is 3-allylcyclohex-l-ene, formed presumably by hydrogen abstraction and coupling I4 26
I6 27
I9
C. W. Rees, R. C. Storr, and P. J. Whittle, J.C.S. Chem. Comm., 1976, 411. P. Cadman, W. J. Engelbrecht, S. Lotz, and S. W. J. Van der Merwe, J. S. African Chem. Inst., 1974, 27, 149. G. Vitt, E. Hadicke, and G. Quinkert, Chem. Ber., 1976, 109, 518. H. Diirr, Topics Current Chem., 1975, 55, 87. J. M. Figuera, J. M. PerCz, and A. P. Wolf, J.C.S. Faraduy I, 1975, 71, 1905. G. Cauguis and G. Reverdy, Bull. SOC.chim. France, 1975, 1841. M. L. Manion and H. D. Roth, J. Amer. Chem. SOC.,1975, 97, 6919.
510
Photochemistry of the resulting radicals. Singlet excited carbene, generated by photodecomposition of l-phenyldiazoethane (44), undergoes addition to cis-but-2-ene to give the cis-cyclopropanes (45) and (46).31 Styrene (47) is also formed and arises by an hv
Ph-C-Me II
-N*
Ph-c-Me
N2
(44)
uncommon 1,Zhydrogen migration to the carbene centre. As with diphenylcarbene, there is apparently an equilibrium between singlet and triplet carbene, the triplet-derived products being the corresponding trans-cyclopropane, ethylbenzene, and, in the presence of oxygen, acetophenone. The effect of p-substitution on the addition of diarylcarbenes to alkenes has been studied with a view to clarifying the electronic effect of the substituent on the stereochemistry of the addition.32 Photodecomposition of diaryldiazomethanes (48) in cyclopentadiene gave adducts (49) and (50). In all cases, the major product has the
C=N2
Ph’
hv
-N2 +
Ar\C: Ph’
>
3’:
electron-rich aryl group endo to the five-membered ring. The stereoselectivities of these carbenes and corresponding carbenoids, generated by zinc chloridecatalysed decomposition of the diazomethanes, are similar. Attempts to obtain evidence for the existence of singlet bis(methoxycarbony1)carbene as a discrete intermediate in the photoreaction of dimethyl diazomalonate with cyclohexene by analysis of the activation parameters has been U ~ S U C C ~ S S ~ U ~ . ~ The role of the carbene in the direct photodecomposition is still in some doubt, and an explanation involving the formation of a complex between the excited diazo-compound and the alkene is preferred. The product of direct photolysis of 2-diazomethyl-l,3,5-triazine( 5 1) in cyclohexane is the cyclohexylmethyltriazine (52).34 Photodecomposition of diphenyldiazomethane (53) in hexane in the presence of 2,6-dimethylphenyl isocyanide (54) affords the ketenimine (55), presumably by electrophilic attack of carbene on the isocyanide, the amide (39, and a small amount of the indene (57);35the formation of the indene is the result 81 s2 88
8p 85
Y. Yamamoto, S . 4 . Murahashi, and I. Moritani, Tetrahedron, 1975, 31, 2663. D. S. Crumrine and H.-H. B. Yen, J. Amer. Chem. Soc., 1976, 98, 297. D. S. Wulfman, B. Poling, and R. S. McDaniel, Tetrahedron Letters, 1975, 4519. A. Kumagai, S. Sekiguchi, and K. Matsui, Bull. Chem. SOC.Jupan, 1975, 48, 3409. N. Obata, H. Mizuno, T. Koitabashi, and T. Takizawa, Bull. Chem. SOC.Japan, 1975, 48, 2287.
51 1
Photoelimination
hv, -Nz
0
II
NH-C-
CHPh,
of a thermal addition of the ketenimine to the isocyanide. Aryl and hydrogen migration compete in the carbenes generated by photodecomposition of a-diazo/3-hydroxyphosphine oxides (58) to give products (59) and (60) respectively, the latter pred~minating.~~ The OH insertion product (61) and dimethyl malonate (62) are the major products of direct irradiation of dimethyl diazomalonate (63)
(58) R = Ph,p-MeC,H,, p-CNC,H,, 2-naphthyl, 2-thienyi, or CH=CHPh C0,Me
/
N2C\ C02Me (63) s6
hv, ROI-I -N, +
C0,Me I R-0-CH I CO, Me (61)
W. Disteldorfand M. Regitz, Chem. Ber., 1976, 109, 546.
+
/CO, Me C\H2 C0,Me (62)
512
Photochemistry
in The yield of dimethyl malonate increases with the hydrogendonating ability of the alcohol, and it is the only major product in the corresponding benzophenone-photosensitized decomposition. These results give support for a singlet excited state in the insertion reaction and a triplet excited state in the hydrogen abstraction process. The photoelimination of nitrogen from diazo-ketones and the fate of the resulting a-oxocarbene is still an area of major interest. The Wolff rearrangement A Wolff of the diazo-ketone (64) could only be accomplished photo~hemically.~~
C~CHN, (64)
CI-I,CO,Me
hv
&cH
-N2’
Ph
rearrangement to keten (65) is also thought to be responsible for the conversion of the a,/l-epoxydiazo-ketone (66) into the ester (67) on irradiation in methan01.~~ Nucleophilic addition of methanol to the keten is presumably accompanied by ring cleavage of the epoxide. Analogous transformations in cyclic diazoketones result in ring contraction. Thus, irradiation of 2-diazo[1-13C]naphthalenl(2H)-one (68) in dioxan-water leads to the formation of the labelled carboxylic acid (69).40 The absence of any isotope scrambling in this and closely related transformations excludes the possibility of an oxiren intermediate, a species frequently proposed in the decomposition of other diazo-ketones. Ring con,02J’]traction has also been reported in 8-diazo-endo-benzo[c]tricyclo[4y2,1 non-3-en-7-0ne.~~ 87 s8
40
41
W. Ando, T. Hagiwara, and T. Migita, Bull. Chem. SOC.Japan, 1975, 48, 1951. A. J. H. Klunder and B. Zwanenburg, Tetrahedron, 1975,31, 1419. N. F. Woolsey and M. H. Khalil, J. Org. Chem., 1975, 40, 3521. K.-P. Zeller, Chem. Ber., 1975, 108, 3566. L. Enescu, F. Chiraleu, and M. Avram, Rev. Roumaine Chem., 1975,20, 957.
513
Photoelimination
(69)
Irradiation of 3-diazobenzofuranone (70) at low temperature yields two primary products, the keten (71) and the ring-opened product (72).42 These products can be photochemically interconverted, and the keten undergoes further photodecomposition with short-wavelength light to give benzyne (73), presumably
J via the carbene (74). 3-Diazobenzofuranone is therefore an ideal precursor for the preparation of matrix-isolated benzyne. The formation of the keten is easily viewed as the result of a Wolff rearrangement, whereas the formation of (72) probably involves a concerted ring cleavage in the intermediate carbene. Identical interconversions have been reported in 2-diazoindan-1-one, and the first synthesis of the 2I~-l-thiacyclobutabenzenesystem has been accomplished by irradiation of the thia-analogue (75).43 The aza-analogues (76) behave quite differently and provide the first example of reversible photochromic valence isomerization between diazo-compounds and diazirine~.~* Although a-oxocarbenes are known to undergo addition reactions with alkenes, the addition of such a species to an aromatic system is so far unrecorded. 42
0. L. Chapman, C.-C. Chang, J. Kolc, N. R. Rosequist, and H. Tomioka, J. Amer. Chem. SOC.,1975, 97, 6586.
O3 O4
E. Voigt and H. Meier, Angew. Chem. Internat. Edn., 1976, 15, 117. E. Voigt and H. Meier, Chem. Ber., 1975, 108, 3326.
514
Photochemistry
a‘’ 0
hv,
___, MeOH
> 290 nm
- Na
\
\
R
R
(76)
R
=
H or Me
Irradiation of 2-diazoacenaphthen-l-one(77) in benzene, however, affords the spiro-compound (78) in 84% yield.46 Similar reactions were effected with toluene and p-xylene. A carbene insertion reaction has again been used in the synthesis
hv, benzene
- Na (77)
Et02C H hv
0
of a penam analogue; photodecomposition of the diazo-ketone (79)in carbon tetrachloride gave the 7-oxa-l-azabicyclo[3,2,0]heptane(80) in an estimated yield of 55%.4s The key step in a new synthesis of (+)-glaziovine (81) is provided by the
(82)
(8 1)
C. G. F. Bannerman, J. I. G. Cadogan, I. Gosney, and N. H. Wilson, J.C.S. Chem. Cumm., 1975, 618. 46
B. T. Golding and D. R. Hall, J.C.S. Perkin 1, 1975, 1517.
Pho toelim ination
515
photolysis of the o-diazo-oxide (82).47 This reaction is a significant improvement over the previously reported synthesis in which the final cyclization is accomplished by photoelimination of HBr from ( k )-8-bromo-N-methylcoclaurine. 3 Elimination of Nitrogen from Azides The photoreactions of azides can almost without exception be rationalized in terms of an intermediate nitrene, and the decomposition provides an easy and efficient method for the generation of these species. The reactions of photochemically generated nitrenes have been reviewed.48 Photolysis of aromatic azides is claimed to produce nitrenes primarily in the singlet state; the lifetimes formed by photoof these nitrenes have been d e t e ~ m i n e d . 1-Pyrenylnitrene, ~~ decomposition of 1-azidopyrene,reacts to give 1-aminopyrene and 1,l ’-azopyrene in degassed methanol, whereas only 1,l’-azopyrene is formed in benzene.50 The quantity of 1-aminopyrene formed was measured and was used to calculate rate constants for hydrogen abstraction by 1-pyrenylnitrene from a wide variety of solvents. Hydrogen abstraction does not occur from benzene. When phenol was present in the degassed benzene solution, N-( 1-pyreny1)-p-benzoquinonemonoimine was formed in addition to the other products. Photoelimination of nitrogen from azidoacetonitrile (83) at - 196 “C gave formimidoyl cyanide (84), undoubtedly via the nitrene, and on further irradiation this was converted into
formimidoyl isocyanide (85).51 The mechanism for this cyanide to isocyanide rearrangement has been the subject of much discussion, and a three-membered ‘zwitterionic’ azirinimine intermediate is now suggested. 9-Azidotriptycenes (86), on irradiation in methanol, are converted into the azahomotriptycenes (87) which can be regarded as solvent adducts of the unstable imines (88).52 Irradiation in cyclohexane affords imine dimers. The formation of imine is presumably the result of a 1,2-aryl migration in the nitrene. The parallel photochemical behaviour of such diverse bridgehead azides as 1-azidoadamantane and 9-azidotriptycene indicates that solvation effects do not play a decisive role in this reaction and that bridgehead azides can in general be regarded as simple precursors of highly strained bridgehead imines. Products arising by 1,2-hydrogen transfer and 1 ,2-alkyl migration in nitrene intermediates were formed by photolysis of a variety of 3-, 6-, 17-, and 20-steroidal a z i d e ~ In . ~ only ~ one case, that of 6P-azido-5a-pregnane (89), was pyrrolidine formation observed and then in only 6% yield. The major products of this photodecomposition are the imine (go), the azepines (91) and (92), and pyrrolidine (93). p7 48
48 6o 61 62
63
C. Casagrande and L. Canonica, J.C.S. Perkin I, 1975, 1647. 1. F. Goryainova and Y. A. Ershov, Khim. uysok. Energii, 1975, 9, 99. A. V. Oleinik, V. M. Treushnikov, and N. N. Gessen, 2hur.fiz. Khim., 1976, 50, 202. T. Tsunoda, T. Yamaoka, and M. Takayama, Nippon Kagaku Kaishi, 1975,12,2074. J. H. Boyer, J. Dunn, and J. Kooi, J.C.S. Perkin I, 1975, 1743. H. Quast and P. Eckert, Angew. Chem. Internat. Edn., 1976, 15, 168. A. Pancrazi and Q. Khuong-Huu, Tetrahedron, 1975, 31, 2041.
516
Photochemistry
hv __3
(86) R
=
H or Me
1
MeOH
The principal products of photodecomposition of azidothiopyran (94) are the pyridines (95) and (96) and the thiophen (97).64 Details of the mechanism of this transformation are not clear, although thiazepine intermediates have been proposed.
The photoreactions of vinyl azides have been reviewed.65 Decomposition leads at least initially to 1-azirines; on the basis of kinetic results, it is clear that a nitrene is not involved in the formation of this azirine. A concerted process 64
b5
J. P. LeRoux, J. C. Cherton, and P. L. Desbene, Compt. rend., 1975, 280, C, 37. G. Labbe, Angew. Chem. Internat. Edn., 1975, 14, 775.
517
Photoelimination
involving cyclization with simultaneous loss of nitrogen is the most likely pathway for this reaction, but the possibility of an unstable intermediate 1,2,3-4Htriazole must still be considered. l-Azirine intermediates have been proposed to account for a variety of photoreactions of unsaturated and aryl azides. The formation of the nitrile (98) by photodecomposition of 2,5-diazido-3,6-di-t-butyl1,Cbenzoquinone (99) in benzene is believed to occur via the azirine
+ hv But
N3
But&N
d
- N2
But
N3
N3
0
0
0
(99)
( 100)
(98)
( 102)
In the photodecomposition of 6-, 7-, and 8-azidoquinolines and in 2-azidonaphthalene, the azirine intermediates can be trapped as 1,Zdiamines with secondary amine~.~'7-Azidoquinoline (101), for example, is converted into the diamine (102) in 80% yield on irradiation in the presence of diethylamine. Ring expansion of the azirine to the corresponding 1H-azepine competes with diamine
aN3 -N2' hv
(103)
68
57
W. Weyler, W. G. Duncan, and H. W. Moore, J. Amer. Chem. SOC., 1975, 97, 6187. S. E. Carroll, B. Nay, E. F. V. Scriven, and H. Suschitzky, Synthesis, 1975, 11, 710.
518 Photochemistry formation in phenyl azide. Following many failures, this ring-expansion process has now been extended to azido-naphthalenes and -anthracenes, but only in the presence of concentrated potassium methoxide in Thus, irradiation followed by gentle heating under reflux of 2-azidoanthracene (103) affords a nearly quantitative yield of 3-methoxy- 1H-napht ho [2,3-c]azepine (104), whereas irradiation followed by immediate neutralization affords 1-amino-2-methoxyanthracene (105). These results are easily rationalized in terms of an unstable methoxyaziridine (106). Ring expansion to 8H-thieno[2,3-c]azepines has been observed in 6-azidobenzo[b]thiophensbut not in the corresponding 4- or S-azidoderivatives.69 Substituted carbomoyl azides have been found to undergo a photo-Curtius rearrangement to give aminoisocyanates;60these can be trapped by nucleophiles. Thus, phenylcarbamoyl azide (107) on irradiation in methanol is readily converted into methyl 2-phenylhydrazinecarboxylate(108). Attempts to detect intermediate 0
II PhNH-C-NB
hv -Nzh
PhNH-NHC02 Me
PhNH-N=C=O
Me
Me (1 10)
nitrenes on photolysis in benzene, cyclohexene, and cyclohexane were unsuccessful. The reasons for the difference in behaviour between these carbamoyl azides and diarylcarbamoyl azides (which afford relatively stable nitrenes in aprotic solvents) are not clear. NN-Dimethylaminoisocyanate (109) was detected spectroscopically on neon-matrix-isolated photolysis of dimethylcarbamoyl azide (1 10). Doubts have been raised concerning the proposal that triplet biphenylnitrene is the sole carbazole precursor in the photocyclization of 2-a~idobiphenyl.~~ A difference in rate constants for the formation of carbazole and for the disappearance of a short-lived intermediate absorbing at X 360nm was observed, suggesting that two separate processes are involved in the formation of carbazole. Photolysis of a series of pyrazole azides (1 11; R = 5-C1,4-CF8, or 5-NMe2)in the presence of acetophenone gave products (112) and (113) derived solely from triplet nitrene.62 These were also formed on direct irradiation, but the ylides (114) were also obtained by what appears to be a singlet-derived process. The electrophilic character of singlet nitrene is further demonstrated by its facility I*
6o
J. Rigandy, C. Igier, and J. Barcelo, Tetrahedron Letters, 1975, 3845. B. Iddon, M. W. Pickering, H. Suschitzky, and D. S. Taylor, J.C.S. Perkin I, 1975, 1686. W. Lwowski, R. A. deMauriac, M. Thompson, R. E. Wilde, and S.-Y. Chen, J. Org. Chem., 1975,40,2608.
R. J. Sundberg, D. W. Gillespie, and B. A. DeGraff, J. Amer. Chem. SOC.,1975, 97, 6193. I. M. McRobbie, 0. Meth-Cohn, and H. Suschitzky, Tetrahedron Letters, 1976, 925.
519
Photoelimination
I
to attack nitrogen nucleophiles, a reaction which is preferred to attack on sulphur nucleophiles as shown for the azidobenzothiazole (115).63 Competitive singletand triplet-derived cyclizationswere also observed in 2-azidophenylbenzimidazole.
Qc)-p
-=? v I1
N3
(115)
Pentafluorophenyl nitrene, generated photochemically from the corresponding azide, adds stereospecifically to cis- and trans-1,Zdichloroethylene to afford a ~ i r i d i n e s . The ~ ~ aziridine (116) is presumably an intermediate in the addition of pentafluorophenyl nitrene to thiophen to give the 2-aminothiophen
(117). Pivaloyl nitrene, generated by photodecomposition of pivaloyl azide, adds to alkenes stereospecifically in its singlet state and stereoselectively in its triplet state.6S The addition of triplet nitrene to 4-methylpent-2-ene, for example, is highly stereoselective, producing the cis-aziridine from both cis- and transalkenes. Addition and C-H bond insertion are observed on reaction of the I. M. McRobbie, 0. Meth-Cohn, and H. Suschitzky, Tetrahedron Letters, 1976, 929. R. A. Abramovitch, S. R. Challand, and Y. Yamada, J. Org. Chem., 1975,40, 1541. G. R. Felt and W. Lwowski,J. Org. Chem., 1976,41, 96.
520 Photochemistry nitrene generated by the photolysis of 2-azido-4,6-dimethoxy-l,3,5-triazine with cyclohexene and cyclohexane re~pectively.~~ Reaction with ketones, however, leads to the formation of 5,7-dimethoxy-3H-1,2,4-oxadiazolo[4,3-a]-s-triazines by a pathway which is thought to involve electrophilic attack of singlet nitrene on the carbonyl oxygen atom as outlined in Scheme 2.
Me0
)(
Me0
Me0
R1R2 Scheme 2
The first convincing evidence for the formation of silaimines in the condensed phase has been de~cribed.~'The two silaimines (118) and (119), arising respectively by methyl and t-butyl migration in the nitrene derived from the silyl azide (120), have been trapped as the adducts (121) and (122) with t-butyl ButMe,SiN,
I
kv, 254 nm -N,
Me, Si =NBut (1 19)
i
hv, 254 nm
------+ - N,
ButMeSi=NMe
1
ButOH
ButMcSi -NHMe I OBut (121)
B ~ O H
Me, Si -N H But
I
OBut (122)
alcohol. Silylation of the alcohol by the azide, presumably accompanied by the formation of hydrazoic acid, is a competing reaction. Sulphamoyl nitrenes do not appear to be obtained on photodecomposition of sulphamoyl azides; complex reactions are reported to occur involving mainly S-N bond cleavage 66
6'
R. Kayama, H. Shizuka, S. Sekiguchi, and K. Matsui, Bull. Chem. SOC.Japan, 1975,48, 3309. D. R. Parker and L. H. Sommer, J . Amer. Chern. Soc., 1976,98, 618.
Photoeliminat ion
52 1
but with some C-N cleavage.68 Certain aryl azides have been used as photoaffinity labels because of the ease with which reactive nitrenes can be generated.6g-71 4 Photodecomposition of other Compounds having N-N Bonds Photodecomposition of sodium salts of toluene-p-sulphonylhydrazonesis a well established and mild route for the generation of carbenes. In many instances, intermediate diazo-compounds can be detected.72 2,3-Homocycloheptatri-
1 (126)
enylidene (123), formed by photodecomposition of the sodium salt of 2,3-homotropone toluene-p-sulphonylhydrazone(124), undergoes rapid ring-opening to afford the reactive highly strained cyclic allene cyclo-octa-l,2,4,6-tetraene(125).73 Na'
68
(128) R. A. Abramovitch and K. Miyashita, J.C.S. Perkin I, 1975, 2413. D. F. Wilson, Y. Miyata, M. Erecinska, and J. M. Vanderkooi, Arch. Biochem. Biophys., 1975,171, 104.
70
71 72
73
S. H. Hixson and S. S. Hixson, Biochemistry, 1975, 14, 4251. F. Seela and F. Cramer, Z. Physiol. Chem., 1975, 356, 1185. R. Siegfried, Tetrahedron Letters, 1975, 4669. M. Oda, Y. Ito, and Y . Kitahara, Tetrahedron Letters, 1975, 2587.
522
4
Photochemistry
N-NHTs
MeOH-MeONa
&C€-IN2 CHO
I
J-f
HO
hv, MeOH
(130)
(1 32)
Ph o toe lim inat ion
523
The tetraene, which is rapidly converted into the dimer (126), can be trapped as an adduct with cyclopentadiene. A carbene-carbene rearrangement is observed on low-temperature photodecomposition of the tosylhydrazone salt (127) to give as final product the fulvalene (128).74 This constitutes the first reported example of a low-temperature rearrangement of an arylcarbene to an aromatic carbene, and, by analogy with other carbene-carbene rearrangements in solution, an intermediate cyclopropene (129) is proposed. Unexpectedly, the tosylhydrazone of 4-hydroxy-endo-tricyclo[4,2,102~6]non-7-en-3-one (130) underwent photodecomposition involving retroaldol cleavage exclusively on irradiation in methanol containing sodium methoxide to yield the ethers (131) and (132).75 The corresponding 4-methoxy-derivatives gave the ring-contracted product expected from the carbene. Evidence for the formation of a dibenzobicyclo[4,1,O]heptatriene (133) has been r e p ~ r t e d Irradiation .~~ of the tosylhydrazone salt (134) at - 110 "C affords the diazoalkane (135) which in turn partitions into the 3H-pyrazole (136) and the carbene (137). The latter is converted into the bicycloheptatriene (133) by intramolecular addition of the carbene to the triple bond; a stable adduct (138) is formed with butadiene. Intramolecular electrophilic addition of carbenes to sulphur has been observed in photochemically generated j?-arylthioalkyl carbenes, leading to the formation of products derived from novel thietanonium ylides.?' Further evidence for the intervention of thietanonium ylides comes from a study of the photodecomposition of tosyl salts (139) as illustrated in Scheme 3.
W
R
[2,31
% & P h
I
YYPh R Scheme 3 74
7s 70
77
U. H. Brinker and W. M. Jones, Tetrahedron Letters, 1976, 577. W. Kirmse and T. Olbricht, Chem. Ber., 1975, 108, 2629. J. P. Mykytka and W. M. Jones, J. Amer. Chem. SOC.,1975,97, 5933. K. Kondo and I. Ojima, Bull. Chem. SOC.Japan, 1975,48, 1490.
524 Photochemistry The synthesis of a series of fluoropyrazoles and 3-fluoro-l,2,4-triazole has been accomplished by irradiation of the corresponding diazonium salts in HBF4.78 5 Photoelimination of Carbon Dioxide Elimination of carbon dioxide is one of a variety of reaction pathways observed on irradiation (A = 254 nm) of monochloroacetic acid in aqueous ~ o l u t i o n . ~ ~ Semidione radicals have been detected in the photofragmentation of a-oxocarboxylic acids in aqueous solution, and they appear to arise by decarboxylative substitution of a-oxo-carboxylic acids by acyl radicals.80* Decarboxylation is also observed on irradiation of nalidixic acid (140) in alkaline oxygen-free solution;82the mechanism proposed to account for the decarboxylation and for the formation of the lactam (141) is outlined in Scheme 4. 7,8-Dimethylisoalloxazine-10-acetic acid yields 7,8-dimethylalloxazine (lumichrome), carbon 0
Naf
co,
fJ?J
+ *co,-
hv ___,
-
coz
Et
Et
Scheme 4
dioxide, and formaldehyde by an intermolecular triplet process.83 3-Methyland 3,/3-dimethyl-isoalloxazine-1O-propanoicacids (142), on the other hand, undergo intramolecular photodecarboxylation and cyclization to give the tetracyclic products (143), again confirming the photoreactivity of the N-1 position in this system. Photoexcited l-cyanonaphthalene reacts with substituted phenylacetic acids to give excited complexes which deactivate preferentially via exciplex emission in benzene, but via electron transfer followed by chemical reaction in acetonitrile ;84 photoreduction and reductive alkylation of l-cyanonaphthalene 78
79 8a
82
83 84
J. Vilarrasa, C. Galvez, and M. Calafell, Anales de Quim., 1975, 71, 631. M. Neumann-Spallart and N. Getoff, Monatsh., 1975, 106, 1359. S. Steenken, E. D. Sprague, and D. Schulte-Frohlinde, Photochem. and Photobiol., 1975, 22, 19. S. Steenken, Photochem. and Photobiol., 1975,22, 157. N. Detzer and B. Huber, Tetrahedron, 1975, 31, 1937. W.-R. Knappe, Chem. Ber., 1975,108, 2422. J. Libman, J. Amer. Chem. SOC.,1975, 97, 4139.
Photoelimination
525
accompanied by the elimination of carbon dioxide is observed. Analogous reactions have been reported for l-methoxynaphthalene.8s Examples of the photoelimination of carbon dioxide from esters and lactones have again been widely reported. Photodecomposition of 4-acetoxysantonene (144) in benzene proceeds by way of a triplet excited state and affords 4-methylsantonene (145), the 1l-methyl isomer (146), and 4-methylphotosantonene
q
0
*+ o
0
0
(147)
(146)
(147).86 Santonenyl and acetoxyl radicals are formed initially, the latter undergoing loss of carbon dioxide followed by radical recombination to give (145) and (146). In arylmethyl esters, a novel oxygen scrambling reaction has been shown to compete with photodecarb~xylation.~~ This process probably arises by recombination of initially formed radical pairs, although other explanations are possible; in arylmethyl phenylacetates, it is a major pathway. A new synthesis of unsymmetrical biphenyls has been accomplished by irradiation of l-aroyloxy3,5-dinitro-2(1H)-pyridones in benzene.88 These pyridones are a useful source of aryl radicals which arise by homolytic N-0 bond cleavage followed by loss of carbon dioxide from the aroyloxy-radical. The photolyses of perfluoroacetic anhydride and perfluoropropionic anhydride in the gas phase are quantitatively described by the equation:8g (RC0),0 86
*’
88
hv
R,
+ C 0 2 + CO
J. Libman, Tetrahedron Letters, 1975, 2507. T. B. H. McMurray and R. R. Talekar, J.C.S. Perkin I, 1976, 442. R. S. Givens and B. Matuszewski, J. Amer. Chem. SOC.,1975, 97, 5617. E. C. Taylor, H. W. Altland, F. Kienzle, and A. McKillop, J. Org. Chem., 1976, 41, 24. G . A. Chamberlain and E. Whittle, J.C.S. Faruduy I, 1975, 71, 1978. 18
526 Photochemistry Evidence for the intermediacy of unstable tetrafluorocyclobutadiene (148) in the photodecomposition of the anhydride (149) has been reported.90 Irradiation in the presence of furan leads to the formation of the cyclobutadiene adduct (150), whereas irradiation alone yields the cyclobutadiene dimer (1 51).
11v - co,, - co+
J
Fw F F
Cyclic carbonates have been found to be useful precursors for arylcarbenes (Scheme 5).01 A photoelimination process is involved with elimination of carbon dioxide; the properties of phenylcarbene obtained from meso- and dl-hydrobenzoin carbonates are virtually identical with those of phenylcarbene obtained 0
from conventional precursors such as trans-2,3-diphenyloxiransand phenyldiazomethane. Photodecomposition of silver trifluoroacetate in solution yields silver, carbon dioxide, and trifluoromethyl radicals.92 This reaction, therefore, can be used as a convenient source of trifluoromethyl radicals and on irradiation in benzene a 57% yield of benzotrifluoride was obtained. 6 Fragmentation of Organosulphur Compounds The photolyses of methanethiol and ethanethiol at 185 nm have been studied;93 the quantum yields for the formation of hydrogen and methane from methanethiol are 0.70 and 0.26 respectively. Ethyl and ethylthiyl radicals are the principal products of triplet-mercury-photosensitized decomposition of diethyl sulphide O0 g1
ga
93
M. J. Gerace, D. M. Lemal, and H. Ertl, J. Amer. Chem. Soc., 1975, 97, 5584. G. W. Griffen, R. L. Smith, and A. Marmade, J . Org. Chem., 1976,41, 338. E. K. Fields and S. Meyerson, J. Org. Chem., 1976, 41, 916. D. Kamra and J. M. White, J. Photochem., 1975, 4, 361.
Photoelimination
527
in the vapour phase at 25 O C . 0 4 The products of irradiation of thietan, 3-ethyl2-propylthietanYand 3-methylthietan in the vapour phase, in solution, and in glassy matrices at low temperature can be explained in terms of an initial C-S cleavage to give 1,4-biradical~.~~ Ring contraction is observed on irradiation of the 1-thiacycloheptan-4-one derivatives (152) with the formation of 3,3-dimethyly-butyrothiolactones (153) as the major ~ T O ~ U C ~These S . ~ ~results can best be accounted for by postulating a one-electron-transfer quenching process as outlined in Scheme 6 in preference to C-S bond cleavage. Other products obtained in lower yield appeared to be the result of Type I processes.
(152) R
= H, AcO, or CI,CCH,OCO,
Scheme 6
The first preparation of a stable, crystalline dithiet tautomer of a dithioo-quinone has been reported.g7 Photodecomposition of the 2,3-dihydro-lY4benzodithiin (154) in n-heptane gave the dithiet (155) and ethylene in virtually quantitative yield. The exceptional stability of this dithiet must, at least in part, be attributed to the steric protection afforded it by the host lanostane molecule. The synthesis of fluoranthene (156) has been accomplished in low yield by photoelimination of sulphur from the spirodihydrothiopyran (157).08 Further examples of the extrusion of sulphur from cyclic sulphides in the presence of trimethyl or triethyl phosphite have been described. This approach is of particular value in the synthesis of cyclophanes,gB-loland the extrusion of sulphur from the sulphide (158) is a key step in a new coronene synthesis.102 Fragmentation is the only pathway observed on irradiation of bis(dipheny1methyl) sulphide (159) ;lo3 diphenylmethane (160), 1,lY2,2-tetraphenylethane(161), and bis(diphenylmethy1) disulphide (162) were obtained in yields of 28, 44, and 14% respectively. The major sulphur-containing products of irradiation of C. S. Smith and A. R. Knight, Canad. J. Chem., 1976, 54, 1290. D. R. Dice and R. P. Steer, Canad. J. Chem., 1975,53, 1744. P. Y. Johnson and M. Berman, J. Org. Chem., 1975,40, 3046. R. B. Boar, D. W. Hawkins, J. F. McGhie, S. C. Misra, D. H. R. Barton, M. F. C. Ladd, and D. C. Povey, J.C.S. Chem. Comm., 1975,756. K. Praefcke and Ch. Weichsel, Tetrahedron Letters, 1976, 1787. H. Tatemitsu, T. Otsubo, Y. Sakata, and S. Misumi, Tetrahedron Letters, 1975, 3059. l o o T. Umemoto, S. Satani, Y . Sakata, and S. Misumi, Tetrahedron Letters, 1975, 3159. lol K. Galuszko, Roczniki Chem., 1975, 49, 1597. loa J. T. Craig, B. Halton, and S.-F. Lo, Austral. J. Chem., 1975, 28, 913. Io3 R. W. Binkley, S.-C. Chen, and D. G. Hehemann, J. Org. Chem., 1975,40,2406. g4
M
528
Photochemistry
AcO
(155)
(154)
v + I1
\
& /
\
/
(156)
(157)
-@ I1v
\
(MeO),P
I’ll
Ph
PI:
PI1
S-(cis-l-propeny1)-L-cysteine are prop-l-ene-l-thiol, 2,4-dimethylthiophen, 3,4dimethylthiophen, and 3-methy1thi0phen.l~~All of these appear to arise directly from photochemically generated l-propenylthiyl radicals. Evidence for methyleneoxaziridine radical intermediates in the photolysis of sulphur-containing nitrones has been reported.loS Irradiation of the nitrone (163), for example, gave three products, benzophenone (164), the oxaziridine (165), and the 1,Zthiazine (166) in approximately equal amounts. These are viewed as arising via the methyleneoxaziridine radical (167) as shown in Scheme 7. The photodecomposition of 5-nitrosoimino-4-phenyl-3-phenylimino1,2,4-thiadiazolidine is also thought to be the result of an initial C-S homolytic bond cleavage.lo6 The photochemically induced conversion of the thioesters (168) into the same 1-p-tolylmercapto-7-methylthioxanthone(169) appears to involve a photoH. Nishimura and J. Mizutani, J. Org. Chem., 1975, 40, 1567. W. M. Leyshon and D. A. Wilson, J.C.S. Perkins I, 1975, 1925. lo6 K. Akiba, T. Tsuchiya, I. Fukawa, and N. Inamoto, Bull. G e m . SOC.Japan, 1976, 49, 550. lo4
lo6
529
Pho toelimination
substitution of the intermediate 1 -halogenothioxanthone (1 7O).lo7 The products of the photodecomposition of dithiocarbamic anhydrides and of acyl xanthanes can also be rationalized in terms of an initial C-S bond cleavage.lo8 S-S and S-N bond cleavage compete on irradiation of bis-(2,2,6,6-tetramethylpiperidl-yl) disulphide in a solid matrix at low temperature.log
(168) R = F or C1
( 1 70)
( 169) G. Buchholz, J. Martens, and K. Praefcke, Tetrahedron Letters, 1975, 3213. lo* S. N. Singh and M. V. George, Tetrahedron, 1975, 31, 2029. lo# B. Maillard and K. U. Ingold, J. Amer. Chem. SOC.,1976, 98, 520. lo7
530 Photochemistry Direct but not triplet-sensitized photolysis of 5-phenyl-1,2,3,4-thiatriazole (171) leads to the formation of phenyl isothiocyanate (172), benzonitrile (173), sulphur, and nitrogen.l1° Phenyl isothiocyanate is apparently formed directly from the thiatriazole, whereas benzonitrile sulphide (174) was identified as an intermediate in the formation of benzonitrile and may itself arise via phenylthiazirene (175). The related 5-phenyl-l,2,3,4-thiatriazole 3-oxide affords
I
R v -N2
Ph
\pS
--+
(175)
R3
(176)
+
PhC-N-S-
---+
PhC=N
+
S
(173)
(174)
R3
(177)
benzonitrile and phenyl isothiocyanate on irradiation in ethanol.lll Photolysis of a series of N-(N-arylimidoy1)sulphimides (176) results in cleavage of the S-N bond and formation of 2-substituted benzimidazoles (177) by cyclization of the resulting imidoyl nitrene.l12 The 3aH-benzimidazoles formed by photodecomposition of analogous ortho-blocked sulphimides undergo further rearrangement to derivatives of 5H-~yclopenta[d]pyrirnidine.~l~Benzimidazole and benzimidazole-2-carboxamideare photodegradation products of the fungicide thiabendaz01e.l~~ The sulphoxide (178) is converted into the isoquinolone (179) on irradiation in benzene.l16 In contrast to this, photolysis of the sulphoxide (180) gave the ketone (181) with elimination of sulphur.ll6 Photoelimination of sulphur from certain cyclohexa-l,4-diene-3-thioneS-oxides to give the corresponding 1,4-dien3-ones has also been re~0rted.l~' A. Holm, N. Harrit, and N . H. Toubro, J. Amer. Chem. SOC.,1975,97, 6197. A. Holm, L. Carlsen, S.-0. Lawesson, and H. Kolind-Anderson, Tetrahedron, 1975,31, 1783. 118 T. L. Gilchrist, C. J. Moody, and C. W. Rees, J.C.S. Perkin Z, 1975, 1964. T. L. Gilchrist, C. J. Moody, and C. W. Rees, J.C.S. Chem. Comm., 1976, 44. 114 T.A. Jacob, J. R. Carlin, R. W. Walker, F. J. Wolf, and W. J. A. Vanden Heuvel, J. Agric. Food Chem., 1975,23, 704. 116 H. Kato, S. Nakazawa, T. Kiyosawa, and K. Hirakawa, J.C.S. Perkin I, 1976, 672. K. Praefcke and Ch. Weichsel, Tetrahedron Letters, 1976, 2229. 117 D. H. R. Barton, L. S. L. Choi, R. H. Hesse, M. M. Pechet, and C. Wilshire, J.C.S. Chem. Comm., 1975, 557. ll1
531
Photoelimination Ph
Ph
(179)
fi
,s=o
The photolysis of methyl benzenesulphonate in methanol has been described and yields benzene, biphenyl, and aniso1e.ll8 The photodecomposition of toluene-p-sulphinamides has also been studied ; in methanol, photoalcoholysis took place yielding methyl sulphinates, whereas in aprotic solvents such as benzene or acetonitrile products arising by S-N bond homolysis were obtained.lle The photodecomposition of N-arylsulphonyl-SS-dimethylsulphoximides (182) appears to proceed by a radical mechanism with no evidence for the
04-
I
Me,S=NH +
+
Me,SO
+
Me,SO,
intermediacy of singlet sulphonylnitrene.120 S-N Bond homolysis is also implicated in the dye-sensitized photolysis of an aryldiazo-sulphone.121 The formation of nitriles (183) in addition to hydroxamic acids (184) and acetylenes (185) on irradiation of the 1,2,3-thiadiazole 1,l,Ztrioxides (186) suggests that rearrangement occurs in the heterocyclic nucleus before fragmentation.lzZ The primary step in the photodecomposition of the hitherto unknown ethoxy(diphenylmethy1ene)sulphonium tetrafluoroborate to benzophenone is probably dealkylation with the formation of ethyl fluoride and thiobenzophenone A Type I1 photoelimination has been described in thiobenzoic acid 1-0xide.l~~ O-esters.lZ4 11* 119 120
lZ1 lZ2 12*
lZ4
Y.Izawa and N. Kuromiya, Bull. Chem. SOC.Japan, 1975,48, 3197. H. Tsuda, H. Minato, and M. Kobayashi, Chem. Letters, 1976, 149. R. A. Abramovitch and T. Takaya, J.C.S. Perkin I , 1975, 1806. T. Yamase, H. Hisada, S. Suzuki, and T. Ikawa, Bull. Chem. SOC.Japan, 1976,49, 351. H. Meier, G . Trickes, and H. P. Braun, Tetrahedron Letters, 1976, 171. L. Carlsen and A. Holm, Acra Chem. Scand., 1976,30B, 277. Y. Ogata, K. Takagi, and S. Ihda, J.C.S. Perkin I, 1975, 1725.
Photochemistry
532
7 Miscellaneous Decomposition and Elimination Reactions Fragmentation and elimination reactions which cannot be included in any of the above categories are briefly reviewed in this section. It has not proved possible to classify these processes, although analogous reactions are grouped together. A common intermediate is proposed in the sensitized photodecomposition of benzylamine with a variety of sensitizers.126 A novel photochemical N-demethylation has been observed in cocaine and related bicyclic amines;126 the reaction appears to be intermolecular in character. Photochemical N-aryl bond cleavage is less common than N-alkyl bond cleavage. The photolysis of alkylated 0- and p-phenylenediamine derivatives in acidic methanol, however, has been reported to lead via homolytic bond cleavage to the corresponding aniline.12’ Photochemically induced C-0 bond homolysis in alkyl ethers has
R3 R 2 0 0 R 1
hv Ph,CO
(187) R1 = Me or MeCHCOMe R2, R3 = H, Me, or M e 0
lz6
’
lRH
Z. A. Sinitsyna, Y.I. Kiryukhin, and K. S. Bagdasaryan, Doklady Akad. Nauk S.S.S.R., 1975,225, 361.
120
lZ7
V. I. Stenberg, S. P. Singh,N. K. Narain, and S . S . Parmar, J.C.S. Chem. Comm., 1976, 262. D. P. Specht, J. L. R. Williams, T.-H. Chen, and S . Farid, J.C.S. Chem. Comm., 1975, 705.
Photoelimination
533 been observed in ( + )-O-methyl mandelate,128 in 2,4-diphenyl-S-ethoxyvinyl5,6,7,8-tetrahydrobenzopyrylium p e r ~ h l o r a t e ,and ~ ~ ~in 3,3-dietho~ypropene.~~O Aryl glycosides with an a-linkage undergo photochemical cleavage more rapidly than those with a P-linkage.l3l The benzophenone-sensitizedphotodecomposition of 2-alkoxytetrahydropyrans (187) is initiated by hydrogen abstraction and not C-0 cleavage; the ratio of products (188) and (189) is dependent on the nature of the s ~ b s t i t u e n t s . ~ ~ ~ New examples of the well known [02-+ ,2] photocleavage of cyclobutane derivatives have been reported. Bond cleavages which release the highest amount
of steric interaction are preferred. Thus, the cyclobutane (190) affords only the biphenyl (191), whereas the all-cis-isomer undergoes the alternative cleavage resulting in fragmentation to phenanthrene and t r a n ~ t i 1 b e n e . lThe ~ ~ formation
Ph
lz8 lZ9 130
131 132
13a
4
+ (-jJ;+co
M. Yoshida and R. G. Weiss, Tetrahedron, 1975, 31, 1801. V. P. Karmazin, E. P. Olekhnovich, M. I. Knyazhanskii, and G. N. Dorofeenko, Zhur. org. Khim., 1975, 11, 1137. R. Sastre, M. V. Dabrio, and Y. J. L. Mateo, Anales de Quim., 1974, 70, 905. T. Yamada, M. Sawada, and M. Taki, Agric. and Biol. Chem. (Japan), 1975, 39, 909. C. Barnasconi and G. Descotes, Compt. rend., 1975, 280, C , 469. G . Kaupp and W. H. Laarhoven, Tetrahedron Letters, 1976, 941.
Photochemistry
534
of phenanthrene from a tetrabenzo[a,c,g,i]cyclododecene seems to involve an A new and efficient synthesis of barrelene (192) analogous photo~1eavage.l~~ has been accomplished in this way by irradiation of the cyclobutane (193) in t e t r a h y d r ~ f u r a n ,and ~ ~ ~ conclusive evidence for the intermediacy of cyclobutadiene in the photodecomposition of cyclobutene (194) has now been A number of substituted c&dimethoxycarbonylstilbene oxides p~b1ished.l~~ 11 photocycloelimination reaction to give aryl methoxyundergo a 13 -+2 carbonylcarbene~.~~~ In alkyl amides, a Type I1 elimination process is inefficient in comparison with Type I ~1eavage.l~~ Triarylmethane leuconitriles undergo heterolytic cleavage on irradiation in ethanol to form a dye cation and cyanide The cyano-radical, rarely encountered in a liquid-phase organic system, has been generated by photolysis of benzoyl cyanide in cyclohexane or benzene Photodecomposition of the a-peracetoxynitrile (195) in benzene or t-butyl alcohol yields the 8-ketonitrile (196) regioselectively in 52% ~ i e 1 d . l A ~ ~mechanism consistent with this
+
1
(195)
- EO II
- H.
(196) Scheme 8
observation is outlined in Scheme 8. Irradiation of the isomeric triphenylmethylA%oxazolines (1 97) is accompanied by elimination of the triphenylmethyl radical followed by fragmentation of the isoxazoline radical to give the ketone (198) and the nitriles (199).142 Photochemical intra- and inter-molecular elimination of HCI, HBr, and HI, arising in many cases by initial carbon-halogen bond cleavage, has again been widely described. In this way, the photodecomposition of a variety of substituted 2-iodobenzylamine hydrochlorides (200) in aqueous solution provides a convenient route to the corresponding 6,7-dihydro-5H-dibenz[c,e]azepines la4 135
G. Wittig and G . Skipka, Annalen, 1975, 1157. W. G. Dauben, G . T. Rivers, R. J. Twieg, and W. T. Zimmerman, J. Org. Chem., 1976,41, 887.
lS6 lS7 lS8
lSo 140 142
R. D. Miller, D. L. Dolce, and V. Y. Merritt, Tetrahedron Letters, 1976, 1845. G. W. Griffin, D. M. Gibson, and K. Ishikawa, J.C.S. Chem. Comm., 1975, 595. P. H. Mazzocchi and M. Bowen, J. Org. Chem., 1976,41, 1279. M. L. Herz, J. Amer. Chem. SOC.,1975,97, 6777. J. Kooi and J. H. Boyer, J.C.S. Perkin I, 1975, 2374. D. S. Watt, J. Amer. Chem. SOC.,1976, 98, 271. H. Kaufmann and J. Kalvoda, J.C.S. Chem. Comm., 1976,210.
Photoelimination
535 OAc
Ph3C
H
"0
(197)
I
536
Photochemistry
(201).143An analogous approach has been successfully employed in the synthesis of many alkaloids including ( k )-n~rpredicentrine,l~~ ( k )-actin~daphnine,l~~ a t h e r ~ l i n eprotoberberine,14' ,~~~ and ( k )-bo1dine.l4* Similarly, irradiation of the metacyclophane (202) affords the tricycle (203).149 Numerous examples of the photocyclization of chloroacetamide derivatives have again been described. Thus, for example, the indole (204) is converted into the azepinone (2O5).l5O The cyclizations of N-chloroacetyl-3-methoxyphenethylarnine,l5l N-~hloroacetyl-2,5-dimethoxyphenethylamine,~~~ and N-chloroacetyl derivatives of indolylethylamines153 have also been reported, and the cyclization has been used in the synthesis of the quebrachamine-dihydrocleavamine skeleton 154 and of benzazocine derivatives.155 (206) Elimination of HBr from 3,5-dibromo-2,6-dimethylhepta-2,5-dien-4-one by irradiation (300nm) in hexane yields the cyclopentenone (207) as the major
product together with a cyclobutane dimer.166 The photoelimination of HBr from trans,trans-2,4-dibromo-l,5-diphenylpenta-l,4-dien-3-one has also been examined.15' On irradiation of dibromomaleic anhydride (208) with N-phenylpyrrole (209), photosubstitution and photocyclization took place in successive
143 144
146
P. W. Jeffs, J. F. Hansen, and G. A. Brine, J. Org. Chem., 1975, 40, 2883. M. S. Premila and B. R. Pai, Indian J. Chem., 1975, 13, 13. M. S. Premila, B. R. Pai, and P. C. Parthasarathy, Indian J. Chem., 1975, 13, 945. T. Kametani, R. Nitadori, H. Terasawa, K. Takahashi, and M. Ihara, Heterocycles, 1975, 3, 821.
T. Kametani, K. Fukurnoto, M. Ihara, M. Takernura, H. Matsumoto, B. R. Pai, K. Nagarajan, M. S. Premila, and H. Suguna, Heterocycles, 1975, 3, 811. S. M. Kupchan, C.-K. Kim, and K. Miyano, J.C.S. Chem. Comm., 1976,91. 149 S . Hirano, H. Hara, T. Hiyama, S. Fujita, and H. Nozaki, Tetrahedron, 1975, 31, 2219. lri0 R. J. Sundberg and F. X. Smith, J. Org. Chem., 1975, 40, 2613. 151 Y. Okuno and 0. Yonemitsu, Chem. and Pharm. Bull. (Japan), 1975,23, 1039. lS2 Y. Okuno, M. Kawamori, K. Hirao, and 0. Yonemitsu, Chem. and Pharm. Bull. (Japan), 1975,23, 2584. I b 9 S. Naruto and 0 . Yonernitsu, Tetrahedron Letters, 1975, 3399. 164 R. J. Sundberg and R. L. Paton, Tetrahedron Letters, 1976, 1163. lS6 Y. Sawa, T. Kato, A. Morimoto, M. Toru, M. Hori, and H. Fujimura, Yakugaku Zasshi, 1975, 95, 261 (Chem. A h . , 1975, 83,28 074). lri6C. W. Shoppee and Y . Wang, J.C.S. Perkin I, 1975, 1595. lS7 C. W. Shoppee and Y . Wang, J.C.S. Perkin I, 1976, 695. 14?
Photoelimination
537
steps to yield pyrrolo[l,2-a]quinoline-4,5-dicarboxylic anhydride (210) as the final product.15* The photoinduced benzoylation of anthracene with benzoyl chloride has been described.159 Many other decomposition reactions arising by carbon-halogen homolytic bond cleavage have been described, but these are essentially radical processes having no special photochemical significance, and so are not included in this Report. lS8 lsS
T. Matsuo and S . Mihara, Bull. Chem. SOC.Japan, 1975, 48, 3660. T. Tamaki, J.C.S. Chem. Comm., 1976, 3 3 5 .
Part IV POLYMER PHOTOCHEMISTRY By D. PHILLIPS
1 Introduction The severe limitations of available space in this volume have necessitated drastic curtailment of this section this year: thus radiation effects are now excluded.
2 Photopolymerization Two useful reviews of photopolymerization have appeared,lV pertinent to the coatings, printing ink, and lithography industries.
the former
Photoinitiation of Addition Polymerization.-A thorough review of the chemistry involved in the widely used aromatic carbonyl-type photoinitiators has been presented by an authority in this field.3 A review of novel photoinitiators of the metal carbonyl type, such as [Mn,(CO),,], [Re,(CO),,], etc., has appeared:, the rhenium compound is the best for polymerization of fluoro-olefins, facilitating the formation of block copolymers.6 The Lewis acids VCI,, TiCl,, TiBr,, SnCl,, and AIBr, photoinitiate radical-cation polymerization of isobutylene with excitation in the 400-480 nm region.s Charge-transfer complex formation between monomer and metal compound prior to excitation is implicated. Vinyl polymerization by iron(~~~)-salt-saccharide,~ iron(r~~)-amine-CCI,,~and poly(viny1amine)-copper(r1) O a systems has been reported. Vanadium(v) and platinum(r1) chelates have also been used to initiate addition polymerization.9b Complexes of molecular chlorine with vinyl monomers such as methyl methacrylate (MMA), ethyl methacrylate, vinyl acetate, styrene, and methyl acrylate upon photoexcitation exhibit initiation of polymerization with efficiencies of monomers increasing in the order shown.lo Benzoin-pyridine and CCl, l1 and 1,1,l-trichloro-3-phenylpropanel2 have been used to initiate 1
4 6
9
7 8
0)
10 l1
12
R. B. Cundall, J. Oil Colour Chemists' Assoc., 1976, 59, 95. S. S. Labana, J . Macromol. Sci. (Chem.), 1974, C11, 299. A. Ledwith, J . Oil Colour Chemists' ASSOC.,1976, 59, 157. S. M. Aliwi, C. H. Bamford, and S. U. Mullik, J. Polymer Sci.,Polymer Symposia, 1975,50,33. C. H. Bamford, Polymer, 1976,17, 321; C . H. Bamford and S. U. Mullik, ibid., p. 225; J.C.S. Farady I, 1976, 72, 368. M. Marek, L. Toman, and J. Pilar, J. Polymer Sci., Polymer Chem., 1975, 13, 1565. T. Okimoto and Y . Inaki, Angew. Makromol. Chem., 1974, 36, 27. Y . Inaki, M. Takahashi, and K. Takemoto, J . Macromol. Sci. (Chem.), 1975, A9, 1133. K. Kimura, Y . Inaki, and K. Takemoto, J. Macromol. Sci.(Chem.), 1975, A9, 1399. S. M. Aliwi and C. H. Bamford, J.C.S. Faraday I, 1975, 71, 1733; C. H. Bamford, S. U. Mullik, and R. J. Puddephatt, ibid., p. 2213. P. Ghosh and S. Chakraborty, J. Polymer Sci., Polymer Chem., 1975, 13, 1531. K. Inoue, N. Nakagawa, and T. Tanigaki, Polymer J., 1976, 8, 254. C.A. Barson, R. A. Batten, and J. C. Robb, European Polymer J., 1975, 11, 381.
541
542 Photochemistry polymerization of methyl methacrylate and styrene respectively. Br, (for MMA),13 the pyridine-Br, charge-transfer (CT) complex (for MMA and other vinyl monom e r ~ )the , ~ ~quinoline-Br, CT complex (for MMA),15 N-bromosuccinimide (for MMA and other vinyl monomers),lS 2,4,6-tribromophenol (for MMA),17 and 1,2-dibromotetrafluoroethane (for tetrafluoroethylene, TFE) have been used successfully as photoinitiators. Photopolymerization of the following monomers has been reported : ethylene,lg ethylene with formamide,20vinyl fluoride,21viny1 acetate (under high pressure),22 3-0xaperfluorobutene,~~ MMA with an acriflavine dye,24MMA in the presence of saccharides,2Sacrylonitrile with aromatic hydrocarbons and benzophenone 28 (summarized in Scheme l), acrylonitrile with substituted triphenyl ph~sphites,~' (u)
N
+ hv@>
310 nm)
AN + lN* + *(N
(b) B
+ hu(A>310nm)
__+
'B
4 3B
R* + BH. B
intermediates giving freeradical polymerization
N
B
N
+ Iwl,
3
N
N
AN)'$
AN
> 3(N.. . - . AN)
+ hVl,
AN = acrylonitrile, N = naphthalene, B = benzophenone; (a) case where naphthalene alone is excited ; (b) benzophenone-naphthalene mixtures with B preferentially excited
Scheme 1
methacrylonitrile,28acrylamide, 29 NN-bis-(2-cyanoethyla~rylarnide),~~ and N-cycloalkylacrylamides of types (1) and (2).31 l3
l4 l6
l6 17 la
20
21 22
ad 25 26
27 28
z9 30
31
P. Ghosh, J. Polymer Sci., Polymer Letters, 1975, 13, 439. P. Ghosh and P. S. Mitra, J . Polymer Sci., Polymer Chem., 1976. 14, 981. P. Ghosh and P. S. Mitra, J . Polymer Sci., Polymer Chem., 1975, 13, 921. P. Ghosh and P. S . Mitra, J. Polymer Sci., Polymer Chem., 1976, 14, 993. T. Tanigaki and S . Asami, Nippon Kagaku Kaishi, 1975, 1076. W. S. Mungall, C. L. Martin, and G. C. Borgeson, Macromolecules, 1975,8, 934. T. J. Pullukat, Mukromol. Chem., 1975, 176, 2479. H. P. Rath, A. Saus, and B. Dederichs, 2.Nuturforsch., 1975, 30b, 740. D. Raucher and M.Levy, J. Polymer Sci., Polymer Chem., 1975, 13, 1339. M. Yokawa and Y. Ogo, Makromol. Chem., 1976, 177,429. V. A. Novikov, E. A. Manuilova, L. F. Sokolov, and S. V. Sokolov, Vysokomol. Soedineniya Ser. (B), 1975, 17, 82. K. P. Chakrabarti, J. Polymer. Sci., Polymer Chem., 1975, 13, 2051. H. Kubota, Y. Ogiwara, and K. Matsuzaki, J. Appl. Polymer Sci., 1976, 20, 1405. J. Barton, I. Capek, and P. Hrdlovic, J. Polymer Sci., Polymer Chem., 1975,13,2671; J. Capek and J. Barton, ibid., p. 2691. T. Taninaka, T. Ogawa, and Y. Minoura, J. Polymer. Sci., Polymer Chem., 1975, 13, 2353. P. Smith, R. D. Stevens, and L. B. Gilman, J. Phys. Chem., 1975, 79, 2688. T. Yamase and T. Ikawa, Bull. Chem. SOC.Japan, 1975, 48, 3738. C. Azuma and N. Ogata, J. Polymer Sci., Polymer Chem., 1975, 13, 741. N. Ogata, C. Azuma, and H. Itsubo, J. Polymer Sci., Polymer Chem., 1975, 13, 1959.
543
Polymer Photochemistry
H,C =CHCO
la 0
5
H,C=CHCON 0
(1) N-acrylylpyrrolidone (2) N-acrylylsuccinimide
A laser flash photolysis study of the interaction of triplet benzophenone with the monomers styrene ( S ) , methyl methacrylate (MMA), acrylonitrile (AN), vinyl acetate (VA), and the solvent THF gave measured second-order rate constants of 3.3 x lo8, 6.9 x lo7, 3.4 x lo7, 5.4 x lo6, and 3 x lo61 mol-1 s-l re~pectively.~~ The rate constant for styrene is so high (presumably because of efficient exothermic energy transfer) that benzophenone is an inefficient photoinitiator for this monomer in THF. The reaction of ketyl radicals with VA, AN, and MMA had rate constants of 5.5 x lo3, 3.8 x lo3, and 9.0 x lo3 I mol-1 s-l respectively, meaning that pinacol is not a major reaction product when monomer concentrations are less than 1 mol I-1 and incident light intensities are low. The benzophenone-amine initiator 33 and alkyl aryl benzoin ether,36 disulphide~,~~ and peroxidic initiators 37 have been discussed. In the last study, luminescence in MMA polymerization was observed. Photopolymerization in the presence of pigments 38 has been discussed, and many useful references on technological applications will be found in ref. 39. The patent literature is summarized in the Appendix. Ionic polymerization has some advantages over free-radical polymerization, 41 N-Vinylcarbazole is easily and the field has been reviewed briefly polymerized by a radical-cation mechanism, and recent studies on this monomer , ~ ~ halogens and halogenusing Rhodamine 6G,42bromanil and ~ h l o r a n i l other ated and metal salts45 have been reported. The production of photosensitive polymers by the cationic polymerization of vinyl ethoxyacrylate has been described.46a The benzaldehyde-sensitized photopolymerization of penta-1,3-dienes from the gas phase on to surfaces has been reported.46b R. KuhImann and W. Schnabel, Polymer, 1976, 17,419. J. F. Kinstle and S. L. Watson, jun., J. Radiation Curing, 1976, 3, 2. a4 M. Hamity and J. C. Scaiano, J. Photochem., 1975, 4, 229. 36 S. P. Pappas and A. K. Chattopadhyay, J . Polymer Sci., Polymer Letters, 1975, 13, 483. 38 G. V. Leplyanin, S. R. Rafikov, E. G. Varisova, 0. I. Korchev, and F. Z. Galin, Vysokomol. Soedineniya Ser. A , 1976, 18, 597. 37 L. Matisovarychla, J. Rychly, and M. Lazar, Makromol. Chem., 1975, 176, 2701. 38 P. S. Pappas and W. Kuhhirt, J. Paint Technol., 1975, 47, 42; Z . W. Wicks, jun, and W. Kuhhirt, ibid., p. 49. 39 ‘Non-silver Photographic Processes’, ed. R. J. Cox, Academic Press, London, 1975. 40 M. Irie, Y. Yamamoto, and K. Hayashi, J. Macromol. Sci. (Chem.), 1975, A9, 817. 4 1 D. Phillips, J. Oil Colour Chemists’ ASSOC., 1976, 59, 202. p 2 R. A. Crellin and A. Ledwith, Macromolecules, 1975, 8, 93. 45 M. Shimizu, K. Tada, Y. Shirota, S. Kusabayashi, and H. Mikawa, Makromol. Chem., 1975, 176, 1953. 44 M. Biswas, J. Macromol. Sci. (Chem.), 1976, C14, 1. 46 M. Asai, Y. Takeda, S. Tazuke, and S. Okamura, Polymer. J., 1975, 7, 359; Y. Takeda, M. Asai, and S. Tazuke, ibid., p. 366. 4e0 T. Nishikubo and T. Ichijyo, J. Appl. Polymer Sci., 1976, 20, 1133. 466 G. R. de Mare, J. R. Fox, M. Termonia, and B. Tshibangila, European Polymer J., 1976, 12, 119. sa
33
544
Photochemistry
Photocondensation Polymerization and Photochemical Cross-linking.-The Paterno-Biichi reaction of aromatic diketones with tetramethylallene forms a photopolymer (Scheme 2) through oxetan formation.47 Attempts to form 1 : 1
J-? c-c, ,c-c'y ?-7 X Scheme 2
&
adducts of aromatic diketones with furan were unsuccessful, but 2 : 1 furan : diketone adducts were formed more easily. Photosensitive poly(amin0-acids) with acrolyl, methacrolyl, and cinnamoyl side-chains have been 49 Four-centre photopolymerization in the solid state of m-phenylene diacrylic acid dimethyl ester proceeds as in Scheme 3.60 The formation of amorphous oligomer is in contrast with the related case of distyrylpyridazine where a crystalline polymer results. The reaction in
,(MeO,CHC=HC O
C
H =CHC0,Me 1
C0,Me z ! : H = C C0,Me H C O , M e ) MeO,CHC=CH Scheme 3
Scheme 3 has been shown to proceed in two stages, i.e. the formation of a topochemical dimer in a regular lattice followed by subsequent random cycloaddition in a disordered lattice. Solid-state polymerization has been reviewed.61
49
D. J. Andrews and W. J. Feast, J. Polymer Sci., Polymer Chem., 1976, 14, 319, 331. M. Nanasawa and H. Kamogawa, Bull. Chem. SOC.Japan, 1975,48,2588. Y. Kadoma, A. Ueno, K. Takeda, K. Uno, and Y. Iwakura, J. Polymer Sci.,Polymer Chem.,
so
F. Nakanishi, H. Nakanishi, M. Hasegawa, and Y. Yamada, J . Polymer Sci., Polymer Chem.,
s1
M. Nishii and K. Hayashi, Ann. Rev. Materials Sci., 1975, 5 , 135.
0
1975, 13, 1545. 1975,13,2499.
Polymer Photochemistry
545
Photocross-linkable resins based upon the benzylideneacetophenone (chalcone) 6 2 and expoxide 63 moieties have been discussed, and cross-linking of poly-(Zphenylbutadiene) through charge-transfer interactions with tetracyanobenzene 54 and poly(isopropeny1 styryl ketone) 66 has been investigated. The surface photopolymerization of TFE 6g and u.v.-induced self-adhesions of poly(ethy1ene terephthalate) (PET) 67 have been reported. Photografting.-The photografting of MMA on to poly(viny1 alcohol) fibres,6a acrylic monomers on to fibrous substrates using b i a ~ e t y l ,amine~~ and ketosubstituted polystyrenes,60AN,gf and other polymers g2 to silica, and grafting of poly(styrene-ah-acrylonitrile) g3 have been discussed. 3 Optical Properties and Luminescence of Polymers An excellent review of photoluminescence in synthetic polymers and its many uses has appeared.g4 Excitons and polaritons in polymers have been discussed,g5 as has the influence of molecular conformation upon intramolecular energy transfer in macromolecules reviewed by leading authorities in the field.sg Observation of depolarization of luminescence from polymeric species (often with luminescent probe molecules attached) yields important information about rotational relaxation in solution. A new technique to improve such measurements, in which simultaneous measurements of fluorescence polarization and quenching are made, has been reportedg7 and a simplified theory describing orientational relaxation by fluorescence correlation described.gs The use of such techniques on PMMA,69 PS,'O and other systems 7 2 including copolymers of 4-vinylpyridine and anthrylmethyl methacrylate 73 has been reported. The use of fluorescence depolarization in this manner limits the range of environments for study owing to the short duration of this luminescence. The time-scale for rotational relaxation can be extended, permitting the study of more viscous 71t
63
63 64 66 66 I'
eo 62
63 64 66
66
67
'O
71 72
73
S. P. Panda, J. Polymer Sci., Polymer Chem., 1975, 13, 1757. S. P. Panda and D. S. Sadafule, J. Polymer Sci., Polymer Chem., 1975, 13, 2415. K. Kato, S. Okamura, and H. Yamaoka, J. Polymer Sci., Polymer Letters, 1976, 14, 211. I. Naito and A. Kinoshita, Kobunshi Ronbunshu, 1975, 32, 321. D. H. Maylotte and A. N. Wright, Faraday Discuss. Chem. SOC.,1974, No. 58, p. 292. D. K. Owens, J. Appl. Polymer Sci., 1975, 19, 3315. Y. Ogiwara, T. Yasunaga, and H. Kubota, J . Appl. Polymer Sci., 1976,20, 1413; Y. Ogiwara and T. Yasunaga, ibid., p. 1119. R. P. Seiber and H. L. Needles, J, Appl. Polymer Sci., 1975, 19, 2185. J. F. Kinstle and S. L. Watson, jun., J . Radiation Curing, 1975, 2, 7. N. I. Litsov and A. A. Kachan, Vysokomol. Soedineniya Ser. B, 1976, 18, 182. E. Papirer, V. T. Nguyen, J.-C. Morawski, and J. B. Donnet, European Polymer J., 1975, 11, 597. N . G. Gaylord and T. Tomono, J. Polymer Sci., Polymer Letters, 1975, 13, 697. A. C. Somersall and J. E. Guillet, J. Macromol. Sci. (Chem.), 1975, C13, 135. M. R. Philpott, J. Chem. Phys., 1975, 63, 485. R. E. Dale and J. Eisinger, Proc. Nut. Acad. Sci. U S A . , 1976, 73, 271. J. P. Bentz, J. P. Beyl, G. Beinert, and G. Weill, European Polymer J., 1975, 11, 711. J. T. Yardley and L. T. Specht, Chem. Phys. Letters, 1976, 537, 43. E. V. Anufrieva, Yu. Ya. Gotlib, and I. A. Torchinskii, Vysokomol. Soedineniya Ser. A , 1975, 17, 1169; M. G . Krakovyak and E. V. Anufrieva, Izuest. Akad. Nauk S.S.S.R., ser.$z.,l 975, 39, 2354. B. Valeur and L. Monnerie, J . Polymer Sci., Polymer Pliys., 1976, 14, 11, 29. I. A. Torchinskii and A. A. Darinskii, Vysokomol. Soedineniya Ser. A , 1976, 18, 413. G. Beck, J. Kiwi, D . Lindenau, W. Schnabel, Colloid and Polymer Sci., 1976, 254, 162. Yu. E. Kirsh, N. R. Pavlova, and V. A. Kabanov, European Polymer J., 1975, 11, 495.
Photochemistry media if the longer-lived phosphorescence is used as the probe, and such studies on the MS time-scale have recently been reported on PVA, PEMA, PS, poly(buty1 methacrylate), and PMMA,74using benzophenone and anthrone as probes. Energy migration may also contribute to fluorescence depolarization, and phosphorescence studies on copolymers of vinylbenzophenone and styrene and fluorescence studies on S-MMA, S-MA, and 1-vinylnaphthalene-MMA copolymers show that this does not occur in copolymers containing less than a few mole per cent of the luminescent comonomer, but increases (i.e. p-l, where p is the polarization, increases) linearly with the percentage of fluorophore up to 50-65%.76 At very high levels of fluorescent comonomer, fluorescence depolarization is complete. A comparison has been made of fluorescence, birefringence, and X-ray methods to study molecular orientation in poly(ethy1ene terephthalate) (PET) drawn fibres, and it was shown that use of 4,4’-dibenzoxazolylstilbene as a fluorescent probe provides a method capable of characterizing quantitatively the distribution of chain orientations only in the non-crystalline regions of semi-crystalline polymers, since fluorescent molecules are excluded from crystalline regions.76 It was further shown that PET behaves as a rubber with 5.6 freely jointed links between cross-link points. Polymer conformation through monitoring of magnetic field modulation of delayed fluorescence 7 7 and the use of oriented polymer films to determine optical transition moment directions in solute molecules 78 have been discussed. Relaxation processes near the glass transition temperature (T,)in polymers by means of excimer fluorescence 79 and the effects of tacticity on excimer formation in poly-(p-methylstyrene) 8o have been reported. Exciplex formation in poly(vinylnaphthalene) and poly(acenaphtha1ene) has been studied and compared with the model compounds ethylnaphthalene and acenaphthalene.sl In one series of experiments, the aromatic models and polymers were electron acceptors, with diethylaniline as donor, and in another the same aromatic molecules were used as electron donors with dicyanoanthracene as acceptor. In these latter experiments no exciplex emission from the aromatic polymers was observed. There have been several other reports of luminescence in polymers. In one, the distribution of localized electronic states in atactic poly(styrene) was discussed,82and in another the fluorescence of styrene as a model for lignin compounds was c ~ n s i d e r e d . The ~ ~ luminescence of the amide chromophore in poly(amides) such as Nylon 66 is still a matter of controversy. Strong emission in commercial samples of the polyamide has been attributed to ap-unsaturated carbonyls arising from aldol condensation reactions, and upon prolonged irradiation this type of emission disappears, implicating such species in photo-oxidation mechanism^.^^ One recent report, however, suggests that the amide chromophore 546
74 76 76
77 78 70 80 81
82
83 84
L. J. Miller and A. M. North, J.C.S. Faraday IZ, 1975, 71, 1233. C. David, D. Baeyens-Volant, and G. Gueskens, European Polymer J., 1976, 12, 71. J. H. Nobbs, D. 1. Bower, and I. M. Ward, Polymer, 1976, 17,25. P. Avakian, R. P. Groff, A. Suna, and H. N. Cripps, Chem. Phys. Letters, 1975, 32, 466. C. C. Bott and T. Kurucsev, J.C.S. Faraday ZI, 1975, 71,749. C.W. Frank, Macromolecules, 1975, 8, 305. T. Ishii and S. Matsunaga, Makromol. Chem., 1976, 177,283. C. David, N. Putman de Lavareille, and G. Gueskens, European Polymer J., 1976, 12, 365. T.J. Fabish, H. M. Satsburg, and M. L. Hair, J. Appl. Phys., 1976, 47, 940. H. Konschin, F. Sundholm, and G. Sundholm, Actu Chem. Scand., 1976, B30, 262. N. S. Allen, J. F. McKellar, and G. 0. Phillips, J. Polymer Sci., Polymer Chem., 1975,13,2857.
547
Polymer Photochemistry
itself is phosphorescent, albeit weakly compared with impurities and oxidation products, in the 443-470 nm region with a decay time of 0.2-0.6 s in ethanol at 77 K.ss These authors suggest that such phosphorescence can be excited via direct So+TI absorption, although such a process is too improbable to be of importance in photodegradation. Dye-polymer interactions in Nylon 66 have been discussed.86
1
Exclhtion
Wavelength n m
Figure Fluorescence excitation and emission spectra of polymer films (Reproduced by permission from Chem. and Ind., 1976, 692)
a/3-Unsaturated carbonyl groups, diagnosed earlier as of importance in the are believed also to photo-oxidation of poly(butadiene) and related be of importance in poly(propylene), as a new study reveals.88 Thus luminescence attributed wrongly earlier 89 to naphthalene contamination from exposure to urban atmospheres has been shown not to be due to this source, as the excitation spectra in the Figure clearly demonstrate, and is probably due to unsaturated c a r b o n y l ~which , ~ ~ may well render the polymer light-sensitive. A thorough study has shown that triplet energy migration in copolymers of styrene and vinylbenzophenone is efficient in both films and glassy solutions at 77 K.gl The results show that exchange interactions are not sufficient to account for observed efficiencies, and that more efficient transfer in ordered regions of the polymer must be of importance. The effect of PS on the quenching rate of benzil phosphores~ence,~~ fluorescence quenching of PS by OZyg3 and, more 85 86
n2
n3
J. A . Dellinger and C. W. Roberts, J. Polymer Sci. Polymer Letters, 1976, 14, 167. A. M. Athale and M. R. Padhye, J. Appl. Polymer Sci., 1976, 20, 403. S. W. Beaven and D. Phillips, European Polymer J., 1974, 10, 593; Rubber Chem. Technol., 1975, 48, 692; J. Photochem., 1975, 3, 349; S. W. Beavan, P. A. Hackett, and D. Phillips, European Polymer J., 1974, 10, 925. N . S. Allen, R. B. Cundall, M. W. Jones, and J. F. McKellar, Chem. and Ind., 1976, 110. D. J. Carlsson and D. M. Wiles, J. Polymer Sci.,Polymer Letters, 1973, 11, 759. N. S. Allen, J. Homer, and J. F. McKellar, Chem. and Znd., 1976, 692. C. David, V. Naegelen, W. Piret, and G. Gueskens, European Polymer J., 1975,11, 569. K. Horie and 1. Mita, Polymer J., 1976, 8, 227. M. Nowakowska, J. Najbar, and B. Waligora, European Polymer J., 1976, 12, 387.
Photochemistry
548
interestingly, fluorescence enhancement in organic polymers by o2(lAg)through reaction (1),94 where A is an aromatic chromophore, have been discussed, O,(lA,)
+ 3A
-
lA
+ 02(3Eg-)
(1)
E-Type delayed fluorescence of 1,l’-diacronyl in a plastic matrix g5 and the behaviour of fluorescent molecules in a polymerizing medium 96 have been investigated. There have been many studies on poly-(N-vinylcarbazole) (PVCZ) and related photoconducting polymers. MO calculations on the electronic structure of PVCZ 9 7 and energy migration in the solid state in this polymer 98-100 have been reported. In PVCZ, poly-(N-ethyl-2-vinyIcarbazole),and poly-(N-ethyl-3-vinyIcarbazole),lol an intrachain excimer fluorescence some 5400 cm-l to the red of the monomer 0-0 band is seen, but PVCZ is unique in exhibiting a second, higher-energy excimer band in addition. Results show that the geometrical arrangement required for this higher-energy band exists in the gound state of the polymer. Excimer formation in PVCZ in solution has also been studied,lo2and triplet energy migration and delayed luminescence in the species have been investigated.lo3 Excimer formation appears to require pendant carbazole groups on adjacent chromophores separated by three carbon atoms.lo2 Charge-transfer complexes of PVCZ and related polymers of the types (3) and (4) have been widely studied because of their p h o t o c ~ n d ~ ~ t i ~ i tTyYPe . ~ ~(3)~ - ~ ~ ~ was found to be a very inefficient photoconductor, probably owing to the poor electron-donor character and short lifetime of singlet excitons in the presence of R. D. Kenner and A. U. Khan, Chem. Phys. Letters, 1975,36,643; J. Chem. Phys., 1976,64, 1877. 96 M. Zander, 2.Naturforsch., 1975, 30a, 1097. 9B R. D. M. Neilson, I. Soutar, and W. Steedman, J. Polymer Sci., Polymer Chem., 1976, 14, 1005. 97 K. Hattori and Y. Wada, J. Polymer Sci., Polymer Phys., 1975, 13, 1863. gs B. Jezek, J. Pospisil, and I. Chudacek, Czech. J. Phys., 1975, 25, 1176. O0 R. M. Siegoczynski, J. Jedrzejewski, and A. Kawski, Acta Phys. Polon., 1975, A47, 707. l o o G. E. Venikouas and R. C. Powell, Chem. Phys. Letters, 1975, 34, 601. lol G. E. Johnson, J. Chem. Phys., 1975, 62,4697. lo2 M. Yokoyama, T. Tamamura, M. Atsumi, M. Yoshimura, Y. Shirota, and H. Mikawa, Macromolecules, 1975, 8 , 101. l o 3 R. D. Burkhart, Macromolecules, 1976, 9, 234. lo4 K. Okamoto, M. Ozeki, A. Itaya, S. Kusabayashi, and H. Mikawa, Bull. Chem. SOC.Japan, 1975,48, 1362. lo5 K. Okamoto, A. Itaya, and S. Kusabayashi, Polymer J., 1975, 7 , 622. log K. Okamoto, A. Itaya, and S. Kusabayashi, J. Polymer Sci., Polymer Phys., 1976, 14, 869. lo’ S. Moriwaki, K. Okamoto, S. Kusabayashi, and H. Mikawa, Bull. Chem. SOC.Japan, 1975, 48, 2623. l o 8 H. Ito, S. Tazake, and M. Okawara, Makromol. Chem., 1976, 177, 621. loo P. K. C. Pillai and R. C. Ahaja, Polymer, 1976, 17, 192. S. Tazuke and Y . Matsuyama, Macromolecules, 1975, 8, 280. M. Yokoyama, Y. Endo, and H. Mikawa, J. Luminescence, 1976, 12, 865. P. J. Reucroft, S. K. Ghosh, and K. Takahashi, J. Polymer Sci., Polymer Phys., 1975, 13, 1275. 113 M. F. Froix, D. J. Williams, and A. 0. Goedde, Macromolecules, 1976, 9, 81. 114 Y. A. Cherkasov, A. D. Lopatko, M. S. Borodkina, and T. V. Cheltsova, Zhur. nauch. priklad. Fotograf. Kinemat., 1975, 20, 370. 115 V. Gaidyalis, I. Vapshinskaite, A. Undzenas, and A. Lyudkyavichyus, Zhur. nauch. priklad. Fotograf. Kinemat., 1976, 21, 57. 116 K. Okamoto, N. Oda, A. Itaya, and S. Kusabayashi, Chem. Phys. Letters, 1975, 35, 483; K. Okamoto and A. Itaya, Chem. Letters, 1976, 99. 94
Polymer Photochemistry
549
the carbonyl group.1os The PVCZ-trifluoroenone CT complex was found to be a better photoconductor than PVCZ.log Electric ll1 and magnetic fields were found to increase the photocurrent by assisting the dissociation into free carriers from the non-relaxed exciplex state. EDA properties of thin polymer films of PTFE on silicon,l17 and photoconduction in poly(olefins), poly(ethylene-CO),l18 e p o x y - r e s i n ~ ,and ~ ~ ~ poly(diacetylene) single crystals 120 ( 5 ) have been investigated. There have been reports of photoelasticity in poly-(n-alkyl acrylates), poly(methacrylamide), and poly(methacry1amide-co-2-hydroxyethyl methacrylate) gels,121poly(viny1idene fluoride),12, and styrene-2-ethylhexyl acrylate copolymer fiims.123 Phot ochromic polymers have been further discussed.124-126
4 Photochemical Reactions in Polymers reactions in Photochemical Reactions in the Absence of O,.-Photochemical polymeric systems have been 128 Specific subjects covered in recent papers include photolysis of poly(forma1dehyde)12@ and glutaric anhydride-type 11'
H. R. Anderson, jun., F. M. Fowkes, and F. H. Hielscher, J. Polymer Sci., Polymer Phys.,
118
1976, 14, 879. G. Y. C. Chan and H. J. Wintle, J . Polymer Sci., Polymer Phys., 1975, 13, 1187.
G. E. Golubkov, V. I. Krainyukov, and B. N. Satyukov, Vysokomol. Soedineniya Ser. B, 1975, 17, 133. R. R. Chance and R. H. Baughman, J . Chem. Phys., 1976, 64, 3889. lal M. Ilavsky, J. Hasa, and K. Dusek, J . Polymer Sci., Polymer Symposia, 1975, 53, 239; M. Ilavsky and K. Dusek, ibid., p. 257. laa H. Ohigahi, J. Appl. Phys., 1976, 47, 949. l Z 3 A. E. Grishchenko, E. P. Vorob'eva, and V. T. Surkov, Vysokomol. Soedineniya Ser. B, 1975, 17, 820s. la' G. Smets, J . Polymer Sci., Polymer Chem., 1975, 13, 2223. J. Verborgt and G . Smets, J. Polymer Sci., Polymer Chem., 1975, 13, 2415. 126 M. Kryszewski, B. Nadolski, and A. Fabrycy, Rocznilci Chem., 1975, 49, 2077. la' G. Smets, Pure Appl. Chem., 1975, 42, 509. 12* G. Smets, J. Thoen, and A. Aerts, J . Polymer Sci., Polymer Symposia, 1975, 51, 119. la* L. L. Yasina and V. S. Pudov, Vysokomol. Soedineniya Ser. B, 1975, 17, 153. lrO
Photochemistry and photochemical transformations of methylox in p r ~ p y l e n e , ~ ~ ~ poly(viny1-p-azidobenzoate)in the presence of ole fin^,^^^ naphthalene in cellulose t r i a ~ e t a t e ,134 ~ ~photointeractions ~' of SO, with poly(viny1 photoisomerization of stilbene by a phenyl vinyl ketone-2-vinylnaphthalene copolymer,136photoreactions of fumaric and maleic derivatives in m ~ l t i l a y e r s and ,~~~ ferocene containing polymers.138 The photolysis of poly(propy1ene) (PP) under vacuum at 253.7 nm results in the formation of methane and ethane as additional products compared with thermal d e c o m p o ~ i t i o n . ~The ~ ~ photoreactions are caused by absorption of 550
Ph CONHPh
p-MeC,H,NHCOPh
(6)
(7)
P h C O N H o N HCOPh
e H C O P h PhCONH
253.7 nm radiation by Ti02 pigment residues. It was found that blending PMMA with PP stabilizes the former. Thermal and photoinduced phenomena in PMMA have been further 141 The quenching of chain-scission processes in copolymers of biphenyl methacrylate and 2-naphthyl methacrylate PhCONHPh
'"
[PhkO
+ fiHPhlGege-
PhkO
+ rjHPh
Scheme 4 lao
H.Hiraoka, Macromolecules, 1976, 9, 359.
E. M. Slobodetskaya, M. G. Vorob'ev, and 0. N. Karpukhin, Vysokomol. Soedineniya Ser. A , 1975, 17, 2533. lsa A. G. Filimoshkin, R. N. Nevedomskaya, I. P. Zherebtsov, and R. M. Livshits, Vysokomol. Soedineniya Ser. A, 1975, 17, 2260. lS3 L. N.Guseva, Yu. A. Mikheev, and D. A. Toptygin, Bull. Acad. Sci. U.S.S.R., 1974,23, 1910. n4 A.A. Degtyareva, A. A. Kachan, L. N. Sharovol'skaya, and V. A. Shrubovich, Vysokomol. Soedineniya Ser. A , 1975, 17, 2144. A. V. Oleinik, V. M. Treushnikov, and N. V. Frolova, Vysokomol. Soedineniya Ser. A , 1975, 17, 1989; 1975,17, 361. la6 S. Irie, M. Irie, Y. Yamamoto, and K. Hayashi, Macromolecules, 1975, 8, 424. la' R. Ackerman and D. Naegele, Makromol. Chem., 1974,175, 699. lS8 K. Kojima, S. Iwabuchi, T. Nakahira, T. Uchiyama, and Y. Koshiyama, J. Polymer Sci., Polymer Letters, 1976, 14, 143. lS0 N. Grassie and W. B. H. Leeming, European Polymer J., 1975, 11, 809, 819; N. Grassie, A. Scotney, and T. I. Davis, Makromol. Chem., 1975, 176, 963. 140 A. Torikai, T.Asai, and T. Suzuki, J. Polymer Sci., Polymer Chem., 1975, 13, 797. 141 E. Ya. Davydov, G. B. Pariiskii, and D. Ya. Toptygin, Vysokomol. Soedineniya Ser. A, 1975, 17, 1504. lal
55 1
Polymer Photochemistry
-
has been ~ e p 0 r t e d . l Using ~~ the Perrin model, effective radii of -16 A were found for these copolymers, compared with 13 A for quenching in copolymers of 1- and 2-vinylnaphthalene and acrylophenone and 10 A for related small model compounds. The photolysis of the fully aromatic amides (6)-(9) is believed to occur through the free-radical mechanism exemplified in Scheme 4, rather than the a1ternat ive molecular concerted react ion.143 Other photoreactions in poly(amides) have been d i s c ~ s s e d , l ~and ~ - ~photo~~ processes in aromatic poly(su1phones) 14' and n-alkylpyrrolidones 148 [as models for photoreactions of poly(vinylpyrrolidone)] investigated. N
Photo-oxidation and Weathering.-There have been several useful reviews of photodegradation (and stabilization) of p ~ l y m e r s , ~ specifically ~ ~ - ~ ~ ~ poly( o l e f i n ~ )lSo . ~ ~Reports ~~ on photodegradation in particular polymers are collated below. PoZy(oZefins) (PE, PP). The photo-oxidation of PE sensitized by ferric acetylacetonate-tristearyl phosphate has been described.153 Photoinitiation processes in PE 164 and PP 164s lS6 have been investigated. Titania pigments were found to be important initiators, and quenching of the anatase form by Nil1 chelates was found to be effective as a stabilizing method. The kinetics of photo-oxidation of isotactic PP 166 and the influence of light intensity 16' and 1-benzoyl-2-naphthol and 6-hydroxybenzanthrone lS8on this process have been investigated. PoZy(styrene) (PS). A review of photodegradation in styrene polymers has appeared lSQ and the effects of this upon permeability in PS and poly-(p-methylstyrene) have been reported.lso In copolymers of PS with poly(viny1 acetate) (PVA) it was found that the presence of PS does not affect the photodegradation of PVA, whereas PS is stabilized by the presence of PVA, probably because the PVA prevents extensive energy migration in PS.lS1 14%
I. Lukac and P. Hrdlovic, European Polymer J., 1975, 11, 767.
llS
D. J. Carlsson, L. H. Gan, and D. M. Wiles, Canad. J. Chem., 1975, 53,2337.
144
H.S. Koenig and C. W. Roberts, J . Appl. Polymer Sci., 1975, 19, 1847.
G. S. Zhdanov and U. K. Milinchuk, Vysokomol. Soedineniya Ser. A , 1976, 18, 3. S. Caccamese, P. Maravigna, G. Montaudo, A. Recca, and E. Scamporrino, J. Polymer. Sci., Polymer Letters, 1975, 13, 51. 147 N. V. Eliseeva, L. T. Danilina, and A. N. Pravednikov, Vysokomol. Soedineniya Ser. B, 1976, 18, 189. P. H. Mazzochi, F. Danisi, and J. J. Thomas, J. Polymer Sci., Polymer Letters, 1975, 13, 737. 149 N. S. Allen and J. F. McKellar, Chem. SOC.Rev., 1975, 4, 533. lS0 E. Cernia, E. Mantovani, and W. Marconi, J. Appl. Polymer Sci., 1975, 19, 15. lS1 V. Ya. Shlyapintokh, Plast. Massy, 1976, 47. lS2 D. J. Carlsson and D. M. Wiles, J. Radiation Curing, 1975, 2, 2. lSs T.Sato, H. Tamai, H. Deura, and K. Oba, Kobunshi Ronbunshi, 1975, 32, 598. lS4 N. S. Allen, J. F. McKellar, and D. G. M. Wood, J. Polymer Sci., Polymer Chem., 1975, 13, 23 19. lS5 D.J. Carlsson and D. M. Wiles, J. Macromol. Sci. (Chem.), 1976, C14, 65. 145
146
ls6 lS7
lS8
lS9 lE0
E. M. Slobodetskaya, 0. N. Karpukhin, and V. V. Amerik, Vysokomol. Soedineniya Ser. B, 1976, 18, 184. 0.N. Karpukhin, E. M. Slobodetskaya, V. V. Amerik, T. M. Fes'kova, and M. G . Vorob'ev, Vysokomol. Soedineniya Ser. B, 1975, 17, 749. P. Bentley and J. F. McKellar, J. Appl. Polymer Sci., 1976, 20, 1145. G. E. Sheldrick and 0. Vogl, Polymer Eng. Sci., 1976, 16, 65. R. Greenwood and N. Weir, Makromol. Chem., 1975, 176,2041 ;J. Appl. Polymer Sci., 1975, 19, 1409. A. Jamieson and I. C. McNeill, J. Polymer Sci., Polymer Chem., 1976, 14, 603.
552
Photochemistry
PuZy(amides)and PoZy(uretharzes). The role of ap-unsaturated carbonyls in amide d e g r a d a t i ~ nand , ~ ~ luminescence 84-86 and direct photolysis 145, 146 of amides have been discussed earlier. The effects of heat pretreatment and orientation in the photo-oxidative degradation of poly(caproamide) have been reported.ls2 The effects of metal acetylacetonates,ls3 adamantane,ls4 ethyl phenyl carbamates,ls5 and the structure of urethane groups 166 on the photo-oxidation of poly(urethanes) have been investigated. PuZy(vinyl chloride) (PVC). 1.r. and U.V. spectroscopy and gel permeation chromatography have been used in a recent study of the photo-oxidation of unprocessed PVC.le7 It was concluded that the mechanism is an autocatalytic chain scission initiated by carbonyl species which gives carboxylic acids among the main products (Scheme 5). Additives such as calcium stearate and an
c1
Cl 0 CI Cl JIPCH-CH,-CH--C-CHJVI I I I1 I Norrish Type I1
I’ypc 1
I
c1
c1 I
mC=CH2
Noi-ri~h I1 v +
+
I
I
c1
CICH,COCHm
I
c1
I
mcH-cH2&H-eo
J
c1 c1 I
mCH-CH,CH-CO,H a-chloro-acid
+
I
*CH-
I
c1
.1
I
HOO-CHm
CI I
mc=o p-chloro-acid chloride
Scheme 5
organotin stabilizer alter the rate but not the mechanism of degradation. The light and weather resistance of PVC,lSEthe role of THF in photodegradation of PVC,le9and the interaction of thermal stabilizers and U.V. absorbers in PVC 170 have been discussed. Elastomers. Photodegradation of chlorinated rubbers 171, 172 and the effect of zinc mercaptobenzothiazolate and its derived basic zinc salt in vulcanized rubbers 162 163 164
166
168
leS 170 171 172
E. V. Vichutinskaya and L. M. Postnikov, Vysokomol. Soedineniya Ser. B, 1976,18,279. E.-L. Cheu and Z. Osawa, J. Appl. Polymer Sci., 1975, 19, 2947. S. S. Novikov, A. P. Khardin, N. G. Gureev, and S. S. Radchenko, Vysokomol. Soedineniya Ser. A , 1976, 18, 619. 2. Osawa, E.-L. Cheu, and Y. Ogiwara, J. Polymer Sci., Polymer Letters, 1975, 13, 535. 0.G. Tarakanov, M. N. Kurganova, E. K. Anisimova, and L. V. Nevskii, Vysokomol. Soedineniya Ser. B, 1975, 17, 461. G. Scott and M. Tahan, European Polymer J., 1975, 11, 535. G. Menzel, Angew. Makromol. Chem., 1975, 47, 181 ;K. V. Bassewitz and G. Menzel, ibid., p. 201. J. F. Rabek, J. Shur, and B. Ranby, J. Polymer Sci., Polymer Letters, 1975, 13, 1285. J. Wypych, J. Appl. Polymer Sci., 1976, 20, 279. C. More and H. Valot, Compt. rend., 1976, 282 C, 113. R. A. Petrosyan, K. A. Ordukhanyan, and R. V. Bagdasaryan, Vysokomol. Soedineniya Ser. A, 1975, 17, 1831.
553
Polymer Photochemistry
in preventing oxidation 173 have been investigated. Reactions of 02(lAg) with 176 unsaturated polymers have been Cellulose. There have been a series of papers reporting an extensive e.s.r. study of radicals produced in photo-irradiated c e l l u l o ~ e . ~ Other ~ ~ - ~ papers ~~ on cellulose triacetate 179s 180 and lignin have appeared. Wool. In a thorough study on the triplet state of tryptophan in the solid environments of poly(viny1 alcohol) (PVAL) film and in the wool protein keratin, it has been concluded that triplet-triplet interactions play a major role in the deactivation of this species in PVAL films, whereas in wool keratin in the presence of air the major loss mechanism appears to involve interaction of triplet tryptophan with oxygen.182 The effects of metal ions18S and of 2-pyrazoline-type fluorescent whitening agents lE4on the photo-yellowing of wool have been discussed. Photodegradable Polymers. Photodegradable vinyl lE6 poly(ethy1ene),lE7and a degradable polymer containing a pyrazine moiety 188 (10) have been reported. The patent literature is surveyed in Table A1 (Appendix).
(10)
U.V.-Stabilization.-A polymeric u . v . - a b s ~ r b e r , ~ 2-(-2’-hydroxypheny1)benzo~~ triazole absorbers,lQoand the interaction of thermal antioxidants and U.V. absorbers in PVC170 have been discussed. Other papers have reported the ~~~ testing of diffusion of 2-hydroxy-4-octoxybenzophenonein p o l y ( ~ l e f i n s ) ,the F. A. A. Ingham, G. Scott, and J. E. Stuckey, European Polymer J., 1975,11, 783. N. B. Zolotoi, M. N. Kuznetsova, V. B. Ivanov, G. V. Karpov, V. E. Skurat, and V. Ya. Shlyapintokh, Vysokomol. Soedineniya Ser. A , 1976, 18, 658. 176 A. Zweig and W. A. Henderson, jun., J. Polymer Sci., Polymer Chem., 1975, 13, 717, 993. 176 N . 4 . Hon, J. Polymer Sci., Polymer Chem., 1975, 13, 1933, 2363, 2416, 2641, 2653. N.-S. Hon, J. Polymer Sci., Polymer Letters, 1976, 14, 225. 178 N . 4 . Hon, J. Appl. Polymer Sci., 1975, 19, 2789. 17@ L. N. Guseva, L. E. Mikheeva, Yu. A. Mikheev, D. Ya. Toptygin, and V. F. Shubnyakov, Vysokomol. Soedineniya Ser. B, 1975,17, 117. l a 0 V. I. Gol’denberg, E. V. Bystritskaya, V. I. Yustl, 0. A. In, V. Ya. Shlyapintokh, and I. Ya. Kalontarov, Vysokomol. Soedineniya Ser. A , 1975, 17, 2779. lB1 G . Gellerstedt and E. L. Pettersson, Acta Chem. Scand., 1975, B29, 1005. la* K. P. Ghiggino, C. H. Nicholls, and M. T. Pailthorpe, Photochem. and Photobiol., 1975, 22. 169. lS3 G. H. Smith, Textile Res. J., 1975, 45, 483. u4 N. A. Evans, D. E. Rivett, and P. J. Waters, Textile Res. J., 1976, 46, 214. la6 B. Freedman and M. J. Diamond, J. Appl. Polymer Sci., 1976, 20, 463. la8 B. Freedman, J. Appl. Polymer Sci.,1976, 20, 911, 921. lS7 V. Pozzi, A. E. Silvers, and L. Giuffre, J. Appl. Polymer Sci., 1975, 19, 923. laBM. Sakuragi, M. Hasegawa, and M. Nishigaki, J. Polymer Sci., Polymer Chem., 1976, 14, 521. lB9 Y. Mizutani and K. Kusumoto, J. Appl. Polymer Sci., 1975, 19, 713. l B 0 M. N. Volkotrub, T. A. Rubstova, and A. F. Lukovnikov, Vysokomol. Soedineniya Ser. A , 1976, 18, 62. lg1 M. Johnson and J. F. Westlake, J. Appl. Polymer Sci.,1975, 19, 1745. 173 17’
554 Photochemistry materials as potential stabilizersyfQ2the measurement of U.V. radiation in accelerated weathering and the use of poly(pheny1ene oxide) as a dosimeter for U.V. r a d i a t i ~ n . ~ IBS ~*~ The patent literature is surveyed in Table A2 (Appendix).
5 Appendix: Review of Patent Literature Photopolymerizable Systems.-Pa tents of interest concerning pho topolymerizable and U.V. curing systems can be found in references 196-296* and under the following British patent numbers : 1400504 1404378 1 406 467 1407795 1 408 466 1 411 966 1 413 410 1 415 378 1417750 1 420 888 1 422 192
1400798 1404497 1 406 741 1407898 1 409 832 1 412 015 1 414 065 1 415 883 1418804 1 420 958 1 422 778
1400978 1404687 1 406 742 1408265 1 409 833 1 412 252 1 414 521 1 417 088 1419187 1 421 078 1 423 548
1400979 1405324 1 406 780 1408412 1 411 295 1 412 290 1 414 671 1 417 396 1420064 1 421 538 1 424 443
1400 988 1 405 865 1 407 069 1 408 413 1 411 677 1 412 754 1 414 837 1 417 404 1 420 351 1421 854
V. F. Tsepalov, Uspekhi Khim., 1975,44, 1830. E. Capron and J. R. Crowder, J. Oil Colour Chemists’ ASSOC.,1975, 58, 9. lg4 A. Davis, G. H. W. Deane, and B. L. Diffey, Nature, 1976, 261, 169. lor,A. Davis, G. H. W. Deane, D. Gordon, G. V. Howell, and K. J. Ledbury, J. Appl. Polymer lg2 lgs
Sci., 1976, 20, 1165.
Agency of Industrial Sciences and Technology, JA 75 24 393. Agency of Industrial Sciences and Technology, JA 75 45 076. lg8 Agency of Industrial Sciences and Technology, JA 75 123 138. leg Agency of Industrial Sciences and Technology, JA 74 128 992. aoo Agency of Industrial Sciences and Technology, JA 75 24 392. 201 Arakawa Forest Chemical Industries Ltd., JA 74 15 633. 2oa Dainippon Ink and Chemicals Inc., JA 74 130 983. 2os Dainippon Ink and Chemicals Inc., JA 75 72 990. 204 Dainippon Printing Co. Ltd., JA 75 105 729. 2os Fuji Systems Co. Ltd., JA 76 20 788. 206 Grace W. R. and Co., JA 75 103 536. 807 Hokuetsu Paper M.F.G. Co. Ltd., JA 75 112 506. 208 Japan Oil Seal Co. Ltd., JA 75 114 489. Japan Oil Seal Co. Ltd., JA 75 124 983. 210 Kansai Paint Co. Ltd., JA 75 02 189. 211 Leben Utility Co. Ltd., JA 75 154 378. 212 Leben Utility Co. Ltd., JA 75 154 390. *lS Leben Utility Co. Ltd., JA 75 154 391. 214 Leben Utility Co. Ltd., JA 75 154 333. 216 Matsuda and Haruo, JA 75 67 885. 216 Matsushita Electric Works Ltd., JA 75 123 150. *I7 Mitsubishi Rayon Co. Ltd., JA 75 17 435. 218 Mitsubishi Rayon Co. Ltd., JA 75 92 341. 219 Mitsui Toatsu Chemicals Inc., JA 74 26 061. e20 Mitsui Toatsu Chemicals Inc., JA 75 56 425. 221 Nippon Oil Seal Industry Co. Ltd., JA 74 115 134. 222 Nippon Oil Seal Industry Co. Ltd., JA 75 63 087. 22s Nippon Oil Seal Industry Co. Ltd., JA 75 70 101. * Patents references: CA Canada, CZ Czechoslovakia, DT West Germany, GB Great Britain, IS Israel, JA Japan, NL Netherlands, SU Soviet Union, S W Switzerland, US United States. lg6
lg7
Polymer Photochemistry Nippon Oil Seal Industry Co. Ltd., JA 75 67 886. 225 Nippon Oil Seal Industry Co. Ltd., JA 75 29 600. 226 Nippon Paint Co. Ltd., JA 75 112 435. 227 Nippon Paint Co. Ltd., JA 74 115 128. 228 Nippon Paint Co. Ltd., JA 75 126 072. 229 Nippon Paint Co. Ltd., JA 75 129 650. 230 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 151 981. 2s1 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 149 73 1. Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 133 238. 233 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 04 152. 2s4 Nippon Synthetic Chemical Ind. Co. Ltd., JA 75 66 596. 2ss Nippon Telegraph and Telephone Public Corp., JA 75 98 832. 236 Research Institute for Production Development, JA 74 15 633. Sakuranomiya Kaguku K.K., JA 75 137 206. Shiotsu et al., JA 75 514 392. Shiotsu et Tutsumi, JA 75 116 539. 240 Teijin Ltd., JA 74 33 995. 241 Teijin Ltd., JA 74 44 935. 24a Toa Paint Co. Ltd., JA 74 47 889. 243 Toa Gosei Chemical Industry Co. Ltd., JA 75 56 423. a44 Toray Industries Inc., JA 74 23 304. 245 Toyo Ink M.G.F. Co. Ltd., JA 75 50 440. a46 Toyo Ink M.F.G. Co. Ltd., JA 75 50 441. Toyo Ink M.F.G. Co. Ltd., JA 75 59 487. 248 Toyo Ink M.F.G. Co. Ltd., JA 75 59 497. 24g Toyota Auto Body Co. Ltd., JA 75 67 871. 2 6 0 Wako Pure Chemical Industries Ltd., JA 74 85 174. 2s1 Bayer A.G., DT 2 430 081. 26a Bayer A.G., DT 2 349 979. Bayer A.G., DT Prog. Org. Coal. 32-115-39 1975. 264 Cellophane S.A. (France) DT 2 510 873. 2s6 Ciba-Geigy A.G., DT 2 507 008. 266 Ciba-Geigy A.G., DT 2 528 358. Continental Can Co., DT 2 505 448. Felten und Guilleaume Kabelwerke A.G., DT 2 459 320. 26,B Finna Michael Huber Miinchen, DT 2 438 724. 2Eo W. R. Grace and Co., DT 2 402 390. General Electric Co., DT 2 518 639. a6a I.C.I. Ltd., DT 2 457 575. 268 I.C.I. Ltd., DT 2 454 800. 264 I.C.I. Ltd., DT 2 522 756. 266 Knonos Titan G.m.b.H., DT 2 350 468. 266 Matsumoto et al., DT 2 420 409. 267 Mobil Oil Co., DT 2 521 986. 268 National Starch and Chemical Corp., DT 2 512 642. 26g Nippon Paint Co. Ltd., DT 2 442 879. 2 7 0 Nippon Paint Co. Ltd., DT 2 514 249. 271 Oce-Van der Grinten N.V., DT 2 503 526. 272 Reliance Universal Inc., DT 2 437 885. 273 Unisearch Ltd., DT 2 458 959. 274 Agency of Industrial Sciences and Technology, US 3 882 084. 276 Bridgestone Tyre Co. Ltd., US 3 870 620. 276 E. I. Du Pont de Nemours and Co., US 3 926 643. 277 W. R. Grace and Co., US 3 900 594. 278 W. R. Grace and Co., US 3 877 971. 279 W. R. Grace and Co., US 3 908 039. 280 Keuffel and Esser Co., US 3 909 273. 281 P.P.G. Industries Inc., US 3 861 945. 282 S.C.M. Corp., US 3 876 519. 283 S.C.M. Corp., US 3 878 075. 284 Sun Chemical Corp., US 3 926 641. 286 Sun Chemical Corp., US 3 926 638. 288 Sun Chemical Corp.,US 3 926 640. 287 University of California, US 3 933 607. 224
555
556 288
2sn 2n0
281 292
2n4 296
286
Photochemistry
Western Ltiho Plate and Supply Co., US 3 852 256. Western Litho Plate and Supply Co., US 3 923 761. Hoechst A.G., GB 1 377 526. W. R. Grace and Co., Fr. Demande 2 258 436. J. Parrein and E. Marechel (Fr.), Chim. Peint 22 238, 2-77-83. Muanyagipan Kuto. Intez., Budapest, Hungary, Magyar Kdm.Lapja, 1975,30, 241. Polychrome Corp., NL (Appl.) 73 17 187. All-Union Scientific Research Institute of the Chemical Industry (U.S.S.R.), SU 465 384, 488 113. Akhonedor and Tulyuganov, Russ. Khim. Drev., 1975, H3643.
Prodegradants and u.v.-sensitizers are given in Table A1 and references 297385. Sekisui Kagaku Kogyo K.K., GB 1 409 439. Hoechst A.G., GB 1 411 539. 299 Badische Anilin- and Soda-Fabrik A.G., GB 1 412 335. Badische Anilin- and Soda-Fabrik A.G., GB 1 424 620. *01 I.C.I. Ltd., GB 1 400 570. Mitsui Toatsu Chemicals Inc., GB 1 408 307. Sekisui Kagaku Kogyo K.K., GB 1 409 439. Hoechst A.G., GB 1 410 641. Mitsubishi Chemical Ind., GB 1 404 927. ao6 Hoeschts A.G., GB 1 411 538. 807 Union Carbide Corp., GB 1 412 021. Union Carbide Corp., GB 1 412 396. Badische Anilin- and Soda-Fabrik A.G., GB 1 412 861. *lo Badische Anilin- and Soda-Fabrik A.G., GB 1 412 877. Hoechst A.G., GB 1 414 693. s12 Hoechst A.G., GB 1 416 604. Konishiroku Photo. Ind. Co. Ltd., GB 1 418 216. 814 Daicel Ltd., GB 1 420 008. I.C.I. Ltd., GB 1421 913. s16 I.C.I. Ltd., GB 1423 655. s17 I.C.I. Ltd., GB 1 423 657. sls Sahi Chem. Ind. Co. Ltd., JA 75 100 141. sln Canon, K. K., JA 74 33 659. s20 Dain ichiseika Color and Chemicals M.F.G. Co. Ltd., JA 75 60 523. 821 Fr. SociCte Anono Jet, JA 74 71 030. a22 Ibonai and Masaru, JA 75 65 592. s2s Kayiya et al., JA 75 10 376. 324 Kureha Chemical Ind. Co. Ltd., JA 75 24 340. s25 Kureha Chemical Ind. Co. Ltd., JA 74 133 438. s26 Kureha Chemical Ind. Co. Ltd., JA 75 09 643. s27 Kureha Chemical Ind. Co. Ltd., JA 75 24 338. s28 Japan Oil Seal Ind. Co. Ltd., JA 75 70 485. s29 Mitsubishi Monsanto Chemical Co.. JA 74 61 234. 330 Mitsubishi Monsanto Chemical Co., JA 75 37 882. s31 Mitsubishi Monsanto Chemical Co., JA 75 38 741. s32 Mitsubishi Petrochemical Co. Ltd., JA 75 67 889. s3s Mitsubishi Plastics Industries Ltd., JA 74 52 243. ss4 Mitsui Toatsa Chemicals Inc., JA 75 61 444. sss Mitsui Toatsa Chemicals Inc., JA 75 16 741. 3s6 Mitsubishi Rayon Co. Ltd., JA 74 114 660. Nippon Soda Co. Ltd., JA 75 18 596. ss8 Nissek Plastic Chem. Co. Ltd., JA 74 78 740. s8n Nippon Zeon Co. Ltd., JA 76 06 242. s40 Sagami Chemical Research Center, JA 74 117 600. s41 Shiseido Co. Ltd., JA 75 158 630. s42 Shiseido Co. Ltd., JA 75 67 861. s43 Shiseido Co. Ltd., JA 75 82 152. s44 Shiseido Co. Ltd., JA 75 67 346. 345 Shiseido Co. Ltd., JA 75 52 153. 346 Shiseido Co. Ltd., JA 75 113 550. 2Q7
28s
557
PoZymer Photochemistry 347 348
34Q 350
351 35a
353 354 365
356 367
368 350
361 362
363 364 365
366 367
360 370
371 s72
373 374
876 876
377 378 37g 880
381 883
88s
Sumitomo Chemical Co. Ltd., JA 75 34 087. Sumitomo Chemical Co. Ltd., JA 75 34 044. Sumitomo Chemical Co. Ltd., JA 75 34 047. Toyobo Co. Ltd., JA 75 15 827. Bayer A.G., DT 2 554 534. Bayer A.G., DT 2 436 260. Bayer A.G., DT 3 915 823. Ciba-Geigy A.G., DT 2 516 168. Ethylene-Plastique S.A., DT 2 432 689. Fuji Photo Film Co. Ltd., DT 2 445 038. Hoechst A.G., DT 2 400 418. Hoechst A.G., DT 2 418 834. Kureha Chemical Industry Co. Ltd., DT 2 364 875. I.C.I. America Inc., DT 2 410 219. Montedison S.P.A. (Italy), DT 2 529 617. Ruhrchemie A.G., DT 2 357 035. Snama Progetti S.P.A. (Italy), DT 2 450 359. Snama Progetti S.P.A. (Italy), DT 2 450 367. Snama Progetti S.P.A. (Italy), DT 2 450 398. Solray et Cie. (Belg.), DT 2 513 200. Arco Polymers Inc., US 3 917 545. Arc0 Polymers Inc., US 3 903 024. Arc0 Polymers Inc., US 3 929 690. Continental Oil Co., US 3 808 272. Eastman Kodak Co., US 3 871 887. Eastman Kodak Co., US 3 912 697. I.C.I. Ltd., U.S. Patent Office T 921 026. Owens-Illinois Inc., US 3 941 759. Polaroid Corp., US 3 929 829. U.O.P. Inc., US Publ. Pat. (appl.) B 596 692. United States Dept. of Agriculture, US 3 932 352. United States Dept. of Agriculture, US 3 932 338. J. E. Guillet, CA 983 200. Bio-degradable Plastics Inc., Fr. Demande 2 234 337. Lion Fat and Oil Co. Ltd., Fr. Demande 2 249 903. Shin-Etsa Chemical Ind. Co. Ltd., Fr. Demande 2 235 353. Lion Fat and Oil Co. Ltd., NL (appl.) 73 14 895. Dainippon Ink and Chemicals Inc., JA 75 59 431. Koga et al., Japan Sumitomo Chem. Co. Ltd., and Kyodo Chem. Co. Ltd., JA 75 159 482.
U.V.stabilizers are collated in references 386-452 386 387
388 38B 3B0
3Q1 3Q2 3Q3 394
3Q5 3Q6 3g7 3g8
30* 400 401
*02 403
Oo5 406 407
and Table A2.
Koga er al., Japan Sumitomo Chem. Co. Ltd., and Kyodo Chem. Co. Ltd., JA 75 159 483. Koga et al., Japan Sumitomo Chem. Co. Ltd., and Kyodo Chem. Co. Ltd., JA 75 159 494. Kyodo Chem. Co. Ltd. and Sumitomo Chem. Co. Ltd. JA 75 136 291. Mitsubishi Paper Mills Co. Ltd., JA 74 121 893. Nippon Shokubai Kagaku Kogyu Co. Ltd., JA 75 35 287. Nippon Shokubai Kagaku Kogyu Co. Ltd., JA 75 41 988. Sumitomo Chemical Co. Ltd., JA 75 121 178. Sumitomo Chemical Co. Ltd., JA 75 120 486. Sumitomo Chemical Co. Ltd., JA 74 61 070. Teijin Ltd., JA 75 54 670. Yoshitomi Pharmaceutical Industries Ltd., JA 75 125 978. Sandoz Ltd., DT 2 432 098. Cincinnati Milacron Inc., US 3 888 823. Eastman Kodak Co., US 3 936 419. Eastman Kodak Co., US 3 939 115. Eastman Kodak Co., US 3 900 442. Martin Processing Co. Inc., US 3 943 105. Monsanto Co., US 3 928 264. Phillips Petroleum Co., US 3 867 342. R. F. Reinisch and G. R. Hermilo, US (appl.) 414 043. Weston Chemical Corp., US Publ. Pat. (appl.) B 54 859. Canadian Titanium Pigments Ltd., CA 962 142.
19
General formula
U.V.
sensitizers Specification R is a G-C, alkyl which may be substituted by halogen C1--6alkoxy, or nitro or may be bonded to the C atom at the ortho position of the benzene nucleus directly or in a CO group or a group of formula
R2
R3
II
provided that the C atom adjacent to the CO group is bonded to the cation at the ortho position of the benzene nucleus directly or via the CO group and R1--Bis either H, halogen, OH, C,, alkyl, C14 alkoxy, or nitro, provided that when at least one of the said group is not bonded to the C atom at the ortho position of the benzene nucleus
R1ac-R
0
Table A1 Prodegradants and
Application Used in conjunction with at least one aliphatic carboxylic acid or a Zn, Mg, Al, Ca, or Ba salt thereof for the photodegradation of polyolefins
297
Ref.
V
Oxymethane polymers
0.1/5% used by weight as a light sensitizer for polystyrene, polyolefin, poly(viny1 chloride) or polylactam, polyurethane, polyester, or polyamide systems Used as a sensitizer in the above system, especially for polyolefins such as polyethylene, polypropylene, and polybut-1-ene
A light sensitizer which is acenaphthene quinone and/or aceanthrene quinone or a mixture thereof with anthrone and/or naphthaquinone X is H, CH20R3[R3 = H, COCR4=CH,, or CO(CH,),,Me]; R, R1,R2,and R4 are H or C14 alkyl
R is H, ClW3alkyl or alkoxy, sulpho, nitro, or halogen; A = ammonium or alkali-metal ion; n = 2 or 3, a = 2or3,andn - a = Oorl
300
299
298
4
2> 2F
0
ca
3
3
&*cr
560 408
409
410
411 412
413 414
416 416 417 418
'19 OZ0
421 422
423 424 425 426
427 428
430
431 432 433
434 435 436
4s7 438
43s 440 441
442 443 444 445 446
447 448 44g
450
451 452
Photochemistry
Luston et al., CZ 159 526. Manasek et al., CZ 159 525. Kvuzat Poalim Lehityashret Shitufit B.M. (Israel), IS 39 037. U.S.S.R. Institute of Chemistry, Academy of Sciences, Tadzhik, U.S.S.R., SU 480 780. Institut FranGais du Petrole, GB 1 401 234. Unilika Ltd., GB 1401 895. Sanyko Co. Ltd., GB 1 401 924. Ciba-Geigy A.G., GB 1402 888. Ciba-Giegy A.G., GB 1 402 889. Ciba-Giegy A.G., GB 1 403 942. Ciba-Giegy A.G., GB 1401 163. Ciba-Geigy A.G., GB 1 411 301. I.C.I. Ltd., GB 1 411 436. Ciba-Giegy A.G., GB 1 411 515. Ciba-Giegy A.G., GB 1 411 656. Ciba-Geigy A.G., GB 1 411 657. Ciba-Geigy A.G., GB 1 415 266. Sanyko Co. Ltd., GB 1415 741. Bayer A.G., GB 1 416 415. Sanyko Co. Ltd., GB 1 417 835. Ciba-Geigy A.G., GB 1 418 701. Ciba-Geigy A.G., GB 1 418 783. Ciba-Geigy A.G., GB 1420 882. Daihto Chemical Industry Co. Ltd., JA 74 61 482. Chisso Corp., JA 76 11 839. Kanebo Ltd., JA 75 22 155. Mitsui Toatsu Chemicals Inc., JA 76 07 026. Nippon Kayaku Co. Ltd., JA 75 25 877. Nippon Kayaku Co. Ltd., JA 75 25 876. Showa Chemicals Industries Ltd., JA 75 200 79. Sumitomo Chemical Co. Ltd., JA 74 59 844. Toray Indust. Inc., JA 75 03 813. Uehara et al., JA 74 53 219. Bayer A.G., DT 2 419 766. Bayer A.G., DT 2 412 785. Ciba-Geigy A.G., DT 2 441 102. Ciba-Geigy A.G., DT 2 529 564. Ciba-Geigy A.G., DT 2 529 568. Ciba-Geigy A.G., DT 2 538 816. Ciba-Geigy A.G., DT 2 538 815. Ciba-Geigy A.G., DT 2 454 946. Eastman Kodak Co., DT 2 427 404. Henckel and Cie. G.m.b.H., DT 2263 940. Hercules Inc., DT 2 452 870. Hoechst A.G., DT 2 442 514.
Optical brighteners are collated in Table A 3 and references 453-475. 453
454
455 466 467 458 459
460 461 462 463
464 466
466 467
468
Sterling Drug Inc., US 3 935 195. Ciba-Geigy A.G., SW 560 277. Ciba-Geigy A.G., SW 567 607. Ciba-Geigy A.G., SW 560 236. Ciba-Geigy A.G., SW 557 917. Ciba-Geigy A.G., SW 561 746. Ciba-Geigy A.G., SW 566 420. Ciba-Geigy A.G., SW 561 327. Ciba-Geigy A.G., GB 1 400 963. Hoechst A.G., GB 1 402 326. Bayer A.G., GB 1 402 371. Ciba-Geigy A.G., GB 1 402 803. Sandoz Ltd., GB 1 403 564. Bayer A.G., GB 1 410 31 1. Sandoz Ltd., GB 1 411 989. Ciba-Geigy A.G., GB 1 412 049.
Polymer Photochemistry 468 470 471
47a 473
m 476
Sandoz Ltd., GB 1 414 155. Hoechst A.G., GB 1 414 669. Ciba-Geigy A.G., GB 1 416 116. Sandoz Ltd., GB 1 417 019. Ciba-Geigy A.G., GB 1 418 572. Ciba-Geigy A.G., GB 1 422 530. Bayer A.G., GB 1422 621.
561
R 2
R'
Y
I
6;:
R3- CH-CH,OH
Me
-
Me
CIass and general formula
Table A2 U.U.absorbers and stabilizers
R1 is q4alkyl; R2 is H or G4 alkyl; R3 is Cl-20 alkyl, alkenyl, phenylbenzyl, alkyl-phenyl, or alkylbenzyl, there being no more than two alkyl substituents, each having 1-8 C atoms. Others are based similarly on the same formula
R1 and R2 are the same or different and each is Me or Et or R1 and R2 having 1 to 12 C atoms, together with the C atom to which they are bound, form a Cs-12 cycloalkyl residue. Y is zero, H is a straight- or short branched-chain C1-,, alkyl residue, a C3-12 alkenyl or alkynyl residue, a C,-12 aralkyl residue, or a group having the general formula CH,CH(R)OH, where R = H, Me, or Ph R3 is H or a straight- or branchedchain C1-,, alkyl residue
Specification R1 and R2 are straight- or branchedchain alkyl residues. A full description can be found in the relevant patent
U.V.
by weight, used in the stabilization of polyolefins 0.01-5%
for example
Stabilization of polypropylene
AppIication Used as a stabilizer in many polymer systems
418
416
415
Ref.
4
5
g
2 2 Q
/N- .A-!0{R3
HN\
11
R2 0
R1 I
,c-c
F-F
II
R (Disc 265.1Q1This is due to increased photochemistry or internal conversion to So from the ‘cis’ band at 265 nm compared with internal conversion to S1. 9-cis- and 11-trans-Retinal have a constant (Disc but a wavelengthdependent fluorescence yield as described above. A common triplet state produced by sensitization for dienes and trienes of the vitamin A series has been invoked to explain the high yields (up to 100%) of the formation of the sterically hindered 7-cis-isomer;Isz g-cis-, 7,9-di-cis-, and all-trans-isomers are also produced, depending on the sensitizer energy. I. Ozolina and A. Mochalkin, Izuest. Akad. Nauk S.S.S.R., Ser. Biol., 1975, 3, 387. E. Dawe and E. Land, J.C.S. Faraday I, 1975, 11,2162. lS4 D. Frankowiak and G. Bialet, Bull. Acad. Polon. Sci., S b . Math. Astron. Phys., 1975,23,355. A.Campillo, R. Hyer, V. Kollman, S. Shapiro, and H. Sutphin, Biochim. Biophys. Acta, 1975, 387,533. lE8 P A . Song and Q. Chae, J. Luminescence, 1976,12113,831. lS7 P . 4 . Song, Q. Chae, and W. Briggs, Photochem. and Photobiol., 1975,22, 77. lE8 L.Pratt, Photochem. and Photobiol., 1975,22,33. lSo P . 4 . Song, Q. Chae, M. Fujita, and H. Baba, J. Amer. Chem. SOC.,1976,98,819. loo S. Georghiou, Nature, 1976,259,423; S. Georghiou and J. Churchich, International Quantum Chemistry and Quantum Biology Symposium, 1975, 2, 331. R. Bensasson, E. Land, and T. Truscott, Photochem. and Photobiol., 1975,21,419. lea (a) V. Ramamurthy and R. Liu, J. Amer. Chem. SOC.,1976, 10, 2935; (b) V. Ramamurthy, C. Tustin, C. Yan, and R. Liu, Tetrahedron, 1975, 31, 193.
lS2
ls3
608
Photochemistry
Non-classical behaviour (similar to that of the styrenes) is observed with endothermic sensitizers.la2 Higher members of the vitamin A series have planar triplets which, except the 7-cis-tripletYcan equilibrate at room temperature producing a mixture of isomers although not the 7-cis-isomer.la2 On direct irradiation, only singlet state isomerization occurs but in 11-cis-retinal both sub-nanosecond singlet and slower triplet isomerizations occur.la3 The 11-cis-Schiff's base isomerizes in