9 Photochemistry By A. GILBERT Chemistry Department The University Reading As with the previous two Reports in this series the present review of the literature is restricted to publications which in the author’s opinion either have general interest to organic photochemists or are of significance in a particular area of the subject. Even so lack of assigned space has meant that only about half of the originally selected reports from 1972 are mentioned here and the choice whether to include particular work or not was often arbitrary. The trend noted last year of general concern by organic photochemists with reaction mechanisms intermediates etc.,has again been evident in a large number of publications which appeared in 1972 and the detailed description of develop-ment in apparatus design and usage is to be welcomed.Recent publications in the latter area have spanned the range from the interesting account of preliminary experiments on frequency doubling of an output of 520-650 nm 1*2 to provide a CW laser light in the 260-325 nm region at an expected level of a few mW,3 to descriptions of simple apparati for quantitative photochemical studies4 and for the kinetic investigation of such reaction^.^ It is also encouraging that photo- chemical techniques and experiments are being described for teaching purposes.6 Stilbene continues to be the molecule most commonly used in cis Strans isomerization studies.’ One of the problems encountered in this area is the com- parative lack of efficiency of energy transfer to it by certain sensitizers.A re-examination of this aspect has revealed that in the cases of acetophenone benzophenone and fluorenone quenching of the triplet ketone by stilbene does not always occur with energy transfer to the olefin.8 Thus on collision with acetophenone triplets 5% of the time the stilbene does not become excited. Stilbene has also been used in a mechanistic study of the quenching of triplet states by di-t-b~tylnitroxide.~ The nitroxide quenches the stilbene in a way ’ S. A. Tuccio and F. C. Strome Appl. Optics 1972 11 64. A. Dienes E. P. Ippen and C. V. Shank IEEE J. Quantum Electron. 1972 QE-8 388. C. Gabel and M. Hercher iEEE J. Quantum Electron. 1972 QE-8 524. J. Rennert and P. H. Ginsburg Mol. Photochem. 1972,4 235. V. Rehak F.Novak and I. Cepciansky Chem. listy 1972 66 875. D. M. Goodall P. W. Harrison and J. H. M. Wedderburn J. Chem. Educ. 1972 49 669. J. Saltiel and E. D. Magarity J. Amer. Chem. SOC. 1972,94 2742. D. Valentine and G. S. Hammond J. Amer. Chem. Soc. 1972,94 3449. ’ R. A. Caldwell and R. E. Schwerzel J. Amer. Chem. Soc.. 1972,94 1035. 315 3 16 A. Gilbert which favours decay to the cis-isomer. Although the mode of reaction is still unknown an attractive theory is that the nitroxide reacts with the phantom twisted triplet of stilbene so that a different range of twist is adopted. The complex so formed may well prefer a cisoid conformation thus influencing the amount of cis-stilbene eventually formed. It has been suggested that a cyclic biradical intermediate could rationalize formation of some of the photoproducts of linear conjugated dienes.Boue and Srinivasan have now applied kinetic analysis to recent data of the mercury- sensitized isomerization of several alkyl-substituted buta-1'3-dienes and thus obtained evidence for the existence of such an intermediate." Further it is considered that the chemical properties of the species are best accommodated by a cyclopropyl biradical structure (1). The structure of the products as well as their subsequent reactions are greatly influenced by the vibrational energy which (1) possesses. Thus 'hot' (1) favours 1,4- and 1,2-shift of hydrogen over the normally preferred 1,3-shift and 'hot' alkyl cyclopropene reopens to a 1,2-diene a 1,3-diene or an acetylene.Since cyclopropenes are not detected as products from the sensitized reactions of dienes in solution it is suggested that in the triplet state vibrational energy is necessary for cyclization of the excited diene to (1). The light-induced ring-opening of substituted cyclohexa-1,3-dienes to geo- metric isomers of hexatrienes is still of interest and a study of the reaction with compounds of type (2) has been made with the view to confirming a postulate that the ground-state conformation of a molecule undergoing con- and dis-rota- tory ring-opening plays a major part in reaction mode. l1 The results in conjunc- tion with the observation that 5-alkyl substituents adopt pseudo-axial or pseudo-equatorial positions dependent upon their bulk appear to substantiate the original suggestion.Thus in derivatives with groups such as 5-t-butyl there is a distinct preference for pseudo-equatorial positioning and hence the cis,trans-hexatriene product is favoured whereas with the 5-methyl compound there is little to choose between the two conformers and more equal mixtures of the cis,cis-and cis,trans-hexatrienes are formed. H (2) The di-n-methane rearrangement is now a well-established reaction although further work concerning the fine details of the process remains to be done. Three centres of stereochemistry are involved in the conversion of the penta- 1,4-dienes lo S.Boue and R. Srinivasan Mol. Photochem. 1972,4 93. 'I C. W. Spangler and R. P. Hennis J.C.S. Chem. Comm. 1972,24. Photochemistry into the vinylcyclopropanes.It is known that the configuration at C-5in the cis-and trans-isomers of l,l-diphenyl-3,3-dimethylhexa-1,4-diene is retained and preliminary evidence has suggested that inversion of configuration is preferred at C-3 the stereochemistry at C-1 was however uncertain [see (3)]. The latter (3) aspect of the reaction has now been studied in detail by the direct photolysis of cis-and trans- 1-phenyl-3,3,5-trimethylhexa-1,4-dienes and most interestingly the di-n-methane rearrangement of the two dienes is stereospecific the situation at C-1paralleling that at C-5 in that cis-diene yields cis-product.12 Benzo- phenone-sensitized irradiation of the dienes only resulted in cis trans inter- conversion. The di-n-methane reaction appears to require central methyl groups and terminal phenyl substitution since derivatives without these features rearrange with reluctance.In order to determine the role of the methyl group the photochemistry of 1,1,5,5-tetraphenylpenta-1,4-diene(4)has been studied. l3 Again the desirability of methyl groups for facile reaction is noted and although the expected product (5)is observed its formation is via the alternative C-H CT + n mechanism. It is also demonstrated that the low reactivity of (4) is not merely due to rapid competitive decay but rather because of inherently low excited-state reactivity. Di-n-ethane and di-n-propane rearrangements are con- sidered to account for the products from the light-induced reactions of 1,1,2,2- tetra~henylethane'~ l5 and 1,1,3,3-tetraphenylpropanes.Kramer and Bartlett have studied in detail the p-acetonaphthone-sensitized cycloaddition of cis-and trans-but-2-enes to cyclopentadiene. l6 The product is composed of three stereoisomers of 5,6-dimethylnorbornene and four 6,7-dimethylbicyclo[3,2,O]heptenes. The olefins are reported to yield the same ratio of total cis-to total trans-isomers and of 1,2- to 1,4-cycloadducts but differences I* H. E. Zimmerman P. Baeckstrom T. Johnson and D. W. Kurtz J. Amer. Chem. SOC.,1972 94 5504. H. E. Zimmerman and J. A. Pincock J. Amer. Chem. SOC.,1972,94 6208. l4 W. C. Schumann D. B. Vashi J. A. Ross and R.W. Binkley J. Org. Chem. 1972,37 21. l5 R.W. Binkley and W. C. Schumann J. Amer. Chem. SOC.,1972,94 1769.B. D. Kramer and P. D. Bartlett J. Amer. Chem. SOC.,1972 94 3934. 318 A. Gilbert are observed in the products from the two butenes in the relative amounts of those having erythro and threo configurations with respect to C-5 and C-6 in the bicycloheptenes and C-1 and C-6 in the norbornenes. It is shown that the trans-1,4-adduct which could arise from either an erythro-or threo-biradical at 30°C is almost exclusively formed via the threo-intermediate and mainly so at -15 "C. Following the formation of the two diastereoisomeric triplet bi- radicals rotational equilibrium is established in such before spin inversion allows the final ring-closure. Similar types of product were reported from 5-bromo- acenaphthylene and cyclopentadiene but here the interest lay in the influence of heavy atoms on photochemical reactions.A kinetic analysis of the addition showed that surprisingly the reaction is insensitive to heavy-atom solvents the data are rationalized by proposing that the rate constant for triplet decay (kd) to ground state is important and it is concluded that the intramolecular heavy- atom effect increases kd. Such (2 + 4) addition to that noted above has also been observed in photo- dimerization in the solid state of compound (6).'* The authors point out that whereas (2 + 2) light-induced dimerizations are known in crystals theirs is the first example of a (2 + 4) process occurring in the solid state rather than in solution. 0,Me OMe MeOCH,OMOM' The stereochemistry of the 'Diels-Alder-like' cyclization of hexa-1,3,5-trienes to bicyclohexenes has been investigated with derivatives which have no strong steric constraints on cyclization are adequately labelled to indicate stereo- selectivity and which undergo cis trans conversions far slower than closure reactions." The process was found to be non-concerted although there appears '' B.F. Plummer and W. I. Ferree J.C.S. Chem. Comm. 1972 306. '' H. Achenback W. Karl and E. Schaller Angew. Chem. Internat. Edn. 1972 11 434. lYA. Seeley J. Amer. Chem. Soc. 1972 94 4378. Photochemistry to be no reason why this should be so. That in other instances at least partial electronic control has been demonstrated has led to the conclusion in the present work that 'one must not depend on orbital symmetry control of selectivity in exploiting this reaction'.Worthy of note in any consideration of Diels-Alder reactions are the reports which appeared this year describing both the thermal and photochemical reaction with thiophens.20 The benzenoid products reflect initial 1,6cycloaddi- tion followed by loss of sulphur. Again this year there have been numerous publications concerned with the many and varied aspects of carbonyl photochemistry ; in particular Norrish Type I1 and competing processes (Scheme 1) have received considerable atten- tion. It seems to be fairly well established that the Type I1 elimination and HO R' /A phG: PhU R ' \* OH R' \ Scheme 1 cyclization reactions of aryl alkyl ketones having y-hydrogens involve the inter- mediacy of a 1,4-biradical.Further studies however continue to provide interesting information concerning the nature of the 1'4-biradical and to this end the photochemistry of methyl-substituted butyrophenones has been investi- gated.21 The data show a pronounced substituent effect on biradical reaction and the results are explained in terms of the transition state for biradical cyclization and elimination. It appears that a-substituents increase the percentage of cyclization products whereas P-substituents have the opposite effect. Wagner and co-workers report on several aspects of the Type I1 process in a series of comprehensive papers the first of which offers further evidence for a biradical intermediate.22 Irradiation of P,y-diphenylbutyrophenoneled to no triplet stilbene which would have been formed in a concerted elimination ;from this it is concluded that y-hydrogen abstraction to give the 1,4-biradical is faster than the energetically allowed elimination.Further it is suggested that the 'triplet state leading to biradical and then on to product' sequence presents the facile path for triplet to singlet spin inversion which a concerted elimination cannot. An extensive study has been made on the effects of y-and 6-substituents on the rate of y-hydrogen abstraction by triplet states of such ketones.23 For example a y-methoxycarbony1 substituent gives little enhancement of the rate whereas 2o H. J. Kuhn and K. Gollnick Tetrahedron Letters 1972 1909; R. Helder and H. Wynberg ibid.p. 605. " F. D. Lewis and T. A. Hillard J. Amer. Chem. SOC.,1972,94,3852. '' P. J. Wagner P. A. Kelso and R. G. Zepp J. Amer. Chem. SOC.,1972,94,7480. 23 P. J. Wagner and A. E. Kemppainen J. Amer. Chem. SOC.,1972 94 7495. 320 A. Gilbert y-vinyl phenyl and cyano-groups increase the rate by a factor of ten and y-alkoxy-groups by approximately fifty-fold :this is presumed to reflect the charge- transfer stabilization of the transition state for hydrogen abstraction. That no correlation is obtained between quantum yields for Type I1 reactions and triplet- state reactivities is taken to provide kinetic evidence for the intermediacy of a 1,cbiradical. The same workers have also suggested that the biradical may be trappable by a thiol the evidence for this is provided from a kinetic study of the quenching of the Type I1 process of y-methoxybutyrophenone by dodecyl mercaptan and S-deuteriated n-butyl mer~aptan.~~ The observed effects on the photochemistry of phenyl alkyl ketones by electron-withdrawing and -donating groups in the phenyl ring and alkyl chain are suggested to result from charge transfer from the y-alkyl position to the oxygen in the transition state for reverse transfer of hydrogen back to the y-carbon which in fact is the major reaction of most l-hydroxy-1,4-biradicals.25 The steric effects which are observed are interpreted as reflecting the restricted rotation in the biradical elimination and cyclization processes occur before rotational equilibrium is established.Both competitive charge-transfer in a- y- and 6-dialkylamino-ketonesz6 and competi- tive &hydrogen abstraction” have received attention.The latter publication also describes work concerning the geometry of intramolecular hydrogen-atom transfers. For example geminal dimethyl substitution as in (7) significantly retards &hydrogen abstraction and this is considered to be due to the torsional strain in a seven-membered cyclic transition state. Such ring-strain considera- tions are suggested to account for the 1,5-rather than 1,6-hydrogen atom trans- fers. Other workers have commented upon the high dependence of excited-state reactivity towards y-hydrogen abstractions on conformational factors for both rigid (bicycloalkyl) and non-rigid (cyclopentyl and acyclic) phenyl ketones.’* These workers have also investigated the prediction that by careful choice of substituents a degree of control over product composition and stereochemistry could be obtained and have studied the photolysis of bicycloalkyl phenyl ketones.Support for the supposition was obtained and high-yield stereoselective syntheses of several novel bridged polycyclic alcohols [e.g.(8)] are reported. 24 R. G. Zepp and P. J. Wagner J.C.S.Chem. Comm. 1972 167; P. J. Wagner and R. G. Zepp J. Amer. Chem. SOC.,1972 94 287. 25 P. J. Wagner H. N. Schott and R. G.Zepp J. Amer. Chem. SOC.,1972,94,7506. 26 P. J. Wagner A. E. Kemppainen and T. Jelliner J. Amer. Chem. SOC.,1972 94 7512. 27 P. J. Wagner P. A. Kelso A. E. Kemppainen and R. G. Zepp J. Amer.Chem. SOC. 1972,94,7500. 28 F. D. Lewis R. W. Johnson and R. A. Ruden J. Amer. Chem. SOC.,1972,94,4292. Photochemistry 32 1 Reactions of the above type have been studied with aliphatic ketones in the presence of triplet q~enchers.~~ Photolysis of threo- and erythro-4-methyl- [5-’H]hexan-2-one leads to non-stereospecific formation of butenes from the singlet state. Olefin formation by hydrogen transfer is calculated to be 95 and 90 % from the erythro- and threo-ketones respectively. The results are interpreted in terms of a short-lived singlet biradical intermediate. Attempts have been made to determine the synthetic prospects of the Norrish Type I1 process with cyclopropane derivatives for example a-cyclopropoxy- acetophenone yields acetophenone and cyclopr~panone.~~ Introduction of two methyl groups into the cyclopropane residue however dramatically affects the reactivity of the system and (9) does not yield any dimethylcyclopropanone or significant amounts of acetophenone :the prin.cipa1 reactions are cyclization and isomerization of the cyclopropane ring to yield (10) and (1 1) respectively.The authors draw attention to the fact that such ‘remote’ methyl groups produce a surprising increase in the rate of radiationless decay to the ground-state ketone and suggest that this may result from electronic vibronic coupling in which the stretching of the C-2-C-3 bond induced by excited ketone ?-hydrogen inter- action acts as the energy sink. Pertinent here are studies on the selective photolysis of thiobenzoic acid 0-esters which leads to the formation of olefins and thioacids (Scheme 2).Attempts to Scheme 2 trap possible reaction intermediates failed but from nanosecond flash photolysis quenching and phosphorescence studies it has now been shown that the process proceeds mainly via the lowest nTt* triplet state :chemical evidence for the forma- tion of the 1,4-biradical intermediate is pre~ented.~ * Formation of oxetans continues to attract interest and two theoretical papers have dealt with donor-acceptor interactions in photocycloaddition reactions involving the carbonyl Irradiation of barbaralone in the presence of benzophenone affords an addition product but instead of the possibly expected 29 C. P. Casey and R.A. Boggs J. Amer. Chem. SOC., 1972,94 6457. 30 T. R. Darling and N. J. Turro J. Amer. Chem. SOC.,1972. 94 4366. 3’ D. H. R. Barton M. Bolton P. D. Magnus and P. J. West J.C.S. Chem. Comm. 1972 632. 32 N. D. Epiotis J. Amer. Chem. SOC.,1972 94 1941 1946. 322 A. Gilbert oxetan compound (12)is formed.33 Reports on the photolysis of py-unsaturated ketones usually describe 1,2-and 1,3-acyl shifts from the triplet and singlet states respectively and cyclobutanol formation if there is an abstractable y-hydrogen. Intramolecular oxetan formation has now been observed in such systems and 2-cyclo-oct-1-enylcyclo-octanone is reported to yield the novel compound (I 3).34 0 Ph ph@ In contrast the cyclobutanol (14) is formed from 2-cyclohex-1-enylcyclohexa-none.Further examples of the intramolecular process have been observed with the enone (15)35and bichromophoric molecules of type (16).36The former yields the oxetan and the py-unsaturated isomer of (1 5) whereas (16) forms the cis-isomer and the oxetan (17). In the latter case singlet excited states appear to be involved and the importance of intramolecular complexation is recognized. lVlC Me-Me Two groups of workers have been concerned with the photochemistry of cyclo- pentenone derivatives. Rearrangement of aryl-substituted compounds in a mode similar to the di-n-methane reactions of 4,4-diarylcyclohexenones has been de- ~ribed.~’ The process proceeds via the 1,3-biradical (18) and although experi- ments suggest triplet intermediates the singlet-triplet splitting of only 1-2 kcal mol-in cyclopentenes is noted.Zimmerman and Little have studied the reactions of (19) and in benzene solution have obtained evidence for formation of the keten (20).38 33 K. Kurabayashi and T. Mukai J.C.S. Chem. Comm. 1972 1016. 34 R. C. Cookson and N. R. Rogers J.C.S. Chem. Comm. 1972 809. ’’ L. E. Friedrich and G. B. Schuster J. Amer. Chem. SOC.,1972,94 1193. 36 S. R. Kurowsky and H. Morrison J. Amer. Chem. SOC.,1972,94 507. 3’ S. Wolff and W. C. Agosta J.C.S. Chem. Comm. 1972 226. 38 H. E. Zimmerman and R. D. Little J.C.S. Chem. Comm. 1972 698. Photochemistry The photolysis of 6,6-diphenylbicyclo[3,l,O]hex-3-en-2-oneis known to yiefd 2,3- and 3,4diphenylphenols the ratios of which are a function of soIvent polarity.6-Bromo-5,5-diphenylcyclohex-2-ene-1-one has been synthesized as a precursor for the postulated zwitterionic intermediate in this phot~reaction.~ Indeed reaction of the bromo-compound with potassium-t-butoxide is now reported to yield the phenols in ratios which have the same solvent dependence as in the light-induced reaction and thus convincing proof is provided for the intermediacy of the zwitterion in the rearrangement of bicyclo[3,l,0]hexenone derivatives. Steroidal and other naturally occurring enones are still extensively studied but the area is too large for comment here. The photochemistry of the former class has been re~iewed,~' and the usual incredible number of publications concerned with light-induced reactions of pyrimidine derivatives have again appeared.* Seemingly cyclohexa-2,5-dienone photochemistry has been far from exhaust- ively studied and one particularly interesting example within the year has con-cerned the light-induced processes of the dienone (21).41 Novel in this area is the formation of the cyclopentadienone (22) ;formation of this is accompanied BU'O 5 + Bu' BU' OMe Me0 OMe (22) Bu' OMe $.Dimer by the isomeric dienone (23). It is suggested that (22) arises by concerted cheleo- tropic elimination of dimethylcarbene from either (21) or the usual zwitterionic intermediate or the latter's triplet precursor. Surprisingly the reaction is not general and the isomer (24) gave onIy the ester (25) in methanol. * The reader is referred to Chapters 2 and 6 in Part 111 of Volume 4 of the Chemical Society's Specialist Periodical Report on Photochemistry (ed.D. Bryce-Smith) for reviews of the literature in this area. 39 H. E. Zimmerman and G. A. Epling J. Amer. Chem. SOC.,1972,94 7806. 'O J. A. Waters Y. Koudo and B. Witkop J. Pharm. Sci. 1972,61 321. " D. G. Hewitt and R. F. Taylor J.C.S.Chem. Comm. 1972,493. 324 A. Gilbert Once again the ubiquitous benzophenone and its derivatives have been the subject of a variety of reports. Its photoreactivity in water has been reinvesti- gated.42 Quantum yields are low and in oxygen-free solution benzpinacol is formed whereas in aerated solution 2- 3- and 4-hydroxybenzophenones are produced very inefficiently. The primary products are the ketyl radical and a radical adduct of benzophenone.Approximate thermochemical calculations indicate that simple hydrogen abstraction from water by triplet ketone may indeed be occurring. Continuing with his extensive studies on the photoreduc- tion of aryl ketones by amines Cohen has reported his findings on the process with 2-naphthaldehyde and 2-acetonaphthone which both have lowest m* triplet states.43 The aldehyde is far more efficiently reduced than the ketone and only a tertiary amine gives a rapid rate of reaction. Generally the results parallel those reported earlier for fiuorenone and p-aminobenzophenone and appear to be characteristic of carbonyl compounds with low-lying m* triplet states. Davidson and co-workers have also contributed many worthwhile studies to this area and have now reported details of their kinetic and flash photolysis evidence for the electron-transfer mechanism of interaction between tertiary aryl amines and ben~ophenone.~~ A new technique has been described for the determination of intersystem crossing yields.45 The method is really a combination of two previously reported procedures involving cis Strans isomerism and the flash spectroscopic tech- nique which uses heavy-atom enhancement of intersystem crossing.This combination is suggested to be most useful for benzenoid compounds since in these cases the earlier methods are only marginally applicable. Basically the technique involves measuring the relative fluorescence intensities of the benzene derivative along with the yields of isomerization of the ‘triplet counter’ in solu- tion.Concentrations of the counter and aromatic compound are held constant while that of the heavy-atom fluorescence quencher is varied. New examples of 1,2-? 1,3- and 1,4-light-induced cycloadditions of olefins and dienes to aromatic compounds continue to appear. The 1,3-process with norbor- nene and benzene has been described in and the reaction with 3,4- dimethylcyclobutene is reported to yield both 1,3- and 1,4-adducts in the ratio 42 M. B. Ledger and G. Porter J.C.S. Faraday I 1972,68 539. 43 S. G. Cohen G. A. Davis and W. D. K. Clark J. Amer. Chem. SOC.,1972,94 869. 44 R. F. Bartholomew R. S. Davidson P. F. Lambeth J. F. McKellar and P. H. Turner J.C.S.Perkin II 1972 577. 45 1972 94 6246. F. A. Carroll and F. H. Quina J. Amer. Chem. SOC. 46 R. Srinivasan J. Phys. Chem. 1972 76 15. Photochemistry of 4 l.47 On the other hand when allenes are used as the olefin the 1,4-cyclo- adduct (26) predominates over the 1,3-adduct (27):48 this is unusual in such pro- cesses. The photoreaction of benzene with 1,2-dichloroethylenes leads to cis-and trans-P-chlorostyrenes olefin dimers and the two 1 1 adducts (28) and (29):the latter are suggested to arise via acid-catalysed rearrangement of the primary products although these were not i~olated.~’ (28) (2% Two groups have reported on the photoaddition of conjugated dienes to anthracene and its derivative^.^^-^^ Both describe the (4+ 4) cycloaddition of cyclohexa-1,3-diene but Kaupp’s main concern was with the (2 + 4)and (4+ 4) cycloadditions of cyclopentadiene to anthracene.” The American workers also report on the reactions of dienes with anthracenes substituted with electron- withdrawing groups stereospecific 1,2-additions of the diene are observed and exciplex intermediates are in~oked.’~ The basic stilbene-phenanthrene conversion has been applied to numerous specialized examples.Such usage for the photochemical synthesis of isoquinoline alkaloids has been re~iewed.’~ Reports continue to appear concerned with preparation of helicenes by this method,54 and recent descriptions of the further success of formation of helicene optical isomers using circularly polarized light are of great intere~t.~’ The mechanism of the reaction has been studied and convincing evidence has been obtained for the ‘bond rotation’ hypothesis as against the earlier proposed pathway of ‘partial photoresolution’ of the dihydro- intermediates.” The cyclization process with aryl dienes trienes tetraenes and pentaenes has received detailed examination from Leznoff and Hayward.’ As 47 R.Srinivasan J. Amer. Chem. SOC.,1972 94 81 17. 48 D. Bryce-Smith B. E. Foulger and A. Gilbert J.C.S. Chem. Comm. 1972 664. 49 D. Bryce-Smith B. E. Foulger and A. Gilbert J.C.S. Chem. Comm. 1972 769. 5o G. Kaupp Angew. Chem. Internat. Edn. 1972. 11. 718. ’’ N. C. Yang and J. Libman J. Amer. Chem. Sac. 1972,94 1405. 52 N. C. Yang J. Libman L. W. Barrett M. H. Hui and R. L. Loeschen J.Amer. Chem. Soc.,1972 94 1406. s3 T. Kametani and K. Fukumoto Accounrs Chem. Res. 1972 5 212. 54 W. H. Laarhoven and R. G. M. Veldhuis Tetrahedron 1972,28 181 I 1823. 55 W. J. Bernstein M. Calvin and 0.Buchardt J. Amer. Chem. SOC. 1972,94,494,2195. s6 R. J. Hayward A. C. Hopkinson and C. C. Leznoff Tetrahedron 1972 28,439; C. C. Leznoff and R. J. Hayward Canad. J. Chem. 1972,50 528. 326 A. Gilbert with the stilbenes the CF* values for the terminal atoms involved in the cycliza- tion are accurate guides in predicting modes of reaction and again for reaction to occur the value must be greater than unity. Recently there has been some discussion concerning the mechanism of the formation of carbazole from di- phenylamine. It now however seems clear that the product is derived from the earlier described ‘610 nm’ transient5’ and further the same workers have described a new transient from their flash studies of this system.58 This transient absorbs at 430 nm and is assigned to the excited triplet state of the intermediate photoproduct 11,12-dihydrocarbazole.Problems concerning the mechanism of the photo-Friedel-Crafts reaction have also apparently been resolved. In a most comprehensive paper Meyer and Hammond describe their efforts in this area and clearly demonstrate that the reaction pathway followed is one of cage combination of radicals to yield re- arrangement products and diffusion from the solvent cage by the radicals to form phenols.59 In these studies attempts to observe short-lived intermedidates by both e.s.r.and flash photolysis were unsuccessful but other workers using phenyl acetate 2,6-dimethylphenyl acetate and acetanilide were able to observe transient species.60 Since the earlier reports by Buchardt and co-workers on the photolability of heteroaromatic N-oxides each year more workers seem to become interested in some aspect of the process. Much effort continues to be directed towards identifi- cation of intermediates,6 and both conventional and laser photolytic studies have been applied to the reaction of isoquinoline N-oxides.62 It is difficult to choose highlights from the vast amount of literature concerned with photoelimination reactions but Chapman’s successful synthesis of the a-lactone (31) by loss of carbon dioxide from (30)is of outstanding interest.63 (30) The year has been most eventful in the area of photo-oxidation reactions particularly those involving simple olefins.Discussions of the mechanism(s) involved in the formation of allylic peroxides continue with the ‘ene’ mechanism seemingly coming back into favour possibly at the expense of the perepoxide ” E. W. Foerster and K. H. Grellmann J. Amer. Chem. SOC.,1972 94 634. 58 E. W. Foerster and K. H. Grellmann Chem. Phys. Letters 1972 14 536. 59 J. W. Meyer and G. S. Hammond J. Amer. Chem. Sue. 1972,94 2219. ‘O C. E. Kalmus and D. M. Hercules Tetrahedron Letters 1972 1575. 61 For recent work in this area see 0.Buchardt C. L. Pedersen and N. Harrit J. Org. Chem. 1972 37 3592; I.Ono and N. Hata Bull. Chem. SOC.Japan 1972 45 2951 ; S. Yamada M. Ishikawa and C. Kaneko Tetrahedron Letters 1972 971 977. 62 C. Lohse J.C.S. Perkin II 1972 229. 63 0. L. Chapman P. W. Wojkowski W. Adam 0. Rodriquez and R. Ruckstaschel J. Amer. Chem. SOC.,1972 94 1365. Pholochemistry pathway. The groups of both Gollnick and Foote have contributed to this topic but these reports are considered to be too important to summarize in a sentence or so and the reader is referred to the original articles.64 Other noteworthy accounts within the year describe the use of a pulsed ruby laser to investigate the effect of azide ion on singlet oxygen reactions6’ and further results on the stereo- specificity of addition of singlet oxygen to vinylidene diethers.66 Within the year there have been several accounts of the more applied and unusual aspects of photochemistry.These have included a description of the identification of organo-chlorine pesticide residues by U.V. solid-phase irradia- ti~n,~’ photochemical oxidation of organic pollutants in waste water,68 and photochemical smog and its related problems.69 A most readable and interest- ing account of the photochemistry of interstellar molecules has appeared and this includes a brief description of an interstellar photochemical labor at or^.^' The work involving the photochemistry in the atmospheres of comets has been described and further experiments are suggested for laboratory and astronomical observations in order to obtain a more detailed understanding of comets.’* Finally the importance of sunlight in bringing about not only photosynthesis but also other photo-effects has been re~iewed.’~ In this all too short account more effort is urged to be devoted to storage of the sun’s light energy.‘Ground rules’ for searching for suitable and convenient solar photoreactions are presented and future possibilities are suggested. 64 K. Gollnick D. Haisch and G. Schade J. Amer. Chem. Soc. 1972,94 1747; C. S. Foote T. T. Fujimoto and Y. C. Chang Tetrahedron Letters 1972 45. 65 N. Hasty P. B. Merkel P. Radlick and D. R. Kearns Tetrahedron Letters 1972 49. A. P. Schaap and N. Tontapanish J.C.S. Chem. Comm. 1972 490. b7 D. E. Glotfelty Ana/yt. Chem. 1972 44 1250. C. Y. Cha and J. M. Sjith Ind.and Eng. Chem. (Process Design),1972 11 451. 69 N. Yamaki J. Jap. Petrol. Inst. 1972 15 540. ’O L. J. Stief Mol. Photochem. 1972 4 153. ” W. M. Jackson Mol. Photochem. 1972,4 135. ’’ F. Daniels,Biophys. J. 1972 12 723.