7 Photochemistry By J. D. COYLE Chemistry Department The Open University Milton Keynes MK7 6AA 1 General The involvement of exciplexes excited complexes and radical ions in photochemical cycloadditions has been reviewed,’ and this reflects the increasing frequency with which electron-transfer steps are postulated in photoreaction mechanisms. Typical experimental criteria for justifying such hypotheses are given in a report2 of the quenching of aromatic hydrocarbon fluorescence by 2,3-bis(methoxycarbonyl)nor-bornadiene to give a radical-ion pair (and then diene triplet) and in a paper3 demonstrating that the photoisomerization of p -nitrobenzaldehyde to p -nitrosoben-zoic acid proceeds by way of electron transfer from water. In effect water acts as a catalyst carrying an electron ‘hole’; this is related to the use of semiconductors in photoreactions (e.g.for water-splitting) and one of the few examples of this in non-aqueous solvent is the cadmium sulphide catalysed photodimerization of phenyl vinyl ethere4 Photocycloreversion of a caged compound sensitized by a pyrylium salt goes by way of electron transfer and a radical-cation chain process,’ with a limiting quantum yield of 80. A review6 of photochemical reactions in organized assemblies (micelles vesicles films multilayers interfaces) is timely in view of a number of reports of the effects of a micellar medium on known photoreactions. One group’ has begun to put on a more quantitative basis a description of the photochemistry of molecules on silica-gel surfaces.Another growth area in organic photochemistry is the use of lasers to provide narrow-band radiation for synthetic purposes. An example is the successive use of 254nm (mercury arc) and 350 or 355 nm (Rayonet or Nd-YAG laser) light to improve the yields of pre-vitamin D3from 7-dehydrocholesterol (83% at 95% conversion analytically; 66% isolated on a preparative scale).’ ’ S. L. Mattes and S. Farid Acc. Chem. Res. 1982 15 80. * G. Jones 11 W. Schwarz and V. Malba. J. Phys. Chem. 1982,86 2286. G. G. Wubbels T. F. Kalhorn D. E. Johnson and D. Campbell J. Org. Chem. 1982,47,4664. R. A. Barber P. DeMayo and K. Okada J. Chem. SOC.,Chem. Commun. 1982 1073. K. Okada K. Hisamitsu T. Miyashi andT. Mukai J. Chem. SOC.,Chem. Commun.1982,974. ‘D. G. Whitten J. C. Russell and R. H. Schmehl Tetrahedron 1982 38 2455. ’R. K. Bauer R. Borenstein P. DeMayo K. Okada M. Rafalska W. R. Ware and K. C. Wu J. Am. Chem. SOC.,1982 104,4635. * W. G. Dauben and R. B. Phillips J. Am. Chem. SOC.,1982,104,355. 126 J. D. Coyle Finally in this introductory survey a collection of short articles has been pub- lished,' aiming to provide a general account of the very widespread applications of photochemistry. The continuing contribution of the subject to the well-being of mankind is highlighted" in a patent describing the formulation of a photochromic nail-polish (colourless indoors maroon outdoors) ! 2 Alkenes Direct ultraviolet irradiation of simple trans acyclic alkenes is not a good method for preparing the corresponding cis compounds because of limited conversion and competing side-reactions.However using pulsed infrared radiation high conversion can be achieved (90% cis for pent-2-ene)" because small differences in absorption cross-section are amplified in multiphoton absorption. The photochemical conver- sion of the A8"4'-16-ketosteroid (1)into the corresponding A'-compound is thought to go by way of a transoid intermediate,'* and this is the first example of a Py to yS enone photoisomerization. The excited states of styrenes and stilbenes have been studied intensively. Two significant new findings are that styrene undergoes quite efficient intersystem crossing (4 = 0.4 k = 3 x lo' s -') contrary to the most widely held assumptions about the excited states of alkene~;'~ and that the directly measured rate constants for photoisomerization of stilbene are very high (k,,, = 1.2 x 10'0s-'),'4 which strongly supports the view that the reaction occurs entirely on the singlet potential surface.The cis-trans photoisomerization of azobenzenes is different from that of stilbenes and a st~dy'~ of two azobenzenophanes for which nitrogen inversion is thought to be the only likely isomerization mechanism suggests that in normal azobenzenes different excited states isomerize by different routes. The use of metal trifluoromethylsulphonates to promote alkene photoreactions is well established. This has been used to produce high yields of the norbornene- acetonitrile adduct (2);16 the mechanism involves electron transfer from Ag' to co-ordinated alkene and subsequent formation of the CH2CN radical which adds to the alkene.Copper(1) triflate promotes the internal photocycloaddition of 1,6- 'Light Chemical Change and Life' ed. J. D. Coyle R. R. Hill and D. R. Roberts Open University Press 1982. I" JP 81 100 709 (Chem.Abstr. 1982.96 11 514). P. P. Teng. E.Weitz and F. D. Lewis J. Am. Chem. SOC., 1982 104 5518. l2 J. R. Williams A.Abdel-Magid W. Ricker and H. Salama J. Org. Chem. 1982 47 2536. I3 R. Bonneau J. Am. Chem. SOC.,1982,104,2921. '' M. Sumitani and K. Yoshihara Bull. Chem. SOC.Jpn. 1982 55 85. Is H. Rau and E. Luddecke J. Am. Chem. SOC.,1982,104 1616. I6 J. W. Bruno T. J. Marks and F. D. Lewis J. Am. Chem. SOC.,1982,104 5580.Photochemistry 127 dienes and the scope of this reaction has been extended" to provide routes to a variety of substituted bicyclo[3.2.0]heptanes [e.g. (3)]. Singlet oxygen reactions with alkenes and dienes continue to attract the interest of synthetic and mechanistic chemists. Myrcene can be converted into perillenal" by a double attack of singlet oxygen carried out under reducing conditions followed by oxidation of the allylic alcohol and then iron(I1)-promoted dehydration of the 1,2-dioxene (Scheme 1).The lack of stereospecificity in the formation of hydroper- oxides from the isomers of the constrained alkene (4) is taken as evidence against a concerted mechanism in this system." hv 02 sensitizer ' Bu~NBH~ X" :" CHO Scheme 1 3 Aromatics The full complexity of aromatic photonucleophilic substitution reactions is probably not yet revealed.In the reactions of the chloroanisoles with alcohols three mechan- isms are proposed;20 via aryl radical cations to give substitution products via aryl R. G. Salarnon D. J. Coughlin S. Ghosh and M. G. Zagorski J. Am. Chem. SOC.,1982 104 998. '* P. Baeckstrom S. Okecha N. DeSilva D. Wijekoon and T. Norin. Acra Chem. Scand. Ser. B,1982 36 31. '9 E. W. H. Asveld and R. M. Kellogg J. Org. Chem. 1982,47 1250. '"J. E. Siegman and J. J. Houser J. Org. Chem. 1982 47 2773. 128 J. D. Coyle radical anions to give reduction (dechlorination) products and uia aryl radicals also to give reduction products. The excited states involved in such reactions have not always been identified and although exciplex intermediates have often been invoked only recently have they been detected directly.With 3,5-dinitroanisole and nucleophiles exciplexes can be seen by flash spectroscopy,” as well as u-complexes that revert to starting materials. Arguments are presented for the involvement of a second type of exciplex and u-complex as well as substrate radical-anions which do not give rise to substitution products. The photochemistry of phenols and phenolates can lead to ring-contracted products. In the presence of trifluoromethanesulphonic acid phenol itself gives bicyclo[3.l.0]hex-3-en-2-one(5) in a method that overcomes the problems (low temperature low conversion extremely strong acid) associated with the previous use of fluorosulphuric acid.” 0-Chlorophenolates give a dimer derived from a cyclopentadiene-5-carboxylate(Scheme 2);’3 the mechanism is probably related to that of the reaction of 0-hydroxybenzenediazonium salts.0‘’ [0c02R] +dimer Scheme 2 The photochemical ring-contraction in 2-pyridones has been to make functionalized p -1actams (Scheme 3) by subsequent silylation ozonization and reduction of the initial bicyclic compound. An unrelated but fascinating method based on the irradiation of (pentacarbony1)chromium carbene complexes and imines gives good yields of p lac tarn^,^^ including those derived from thiazolines (Scheme 4). OMe Me02C. -do -+++ OHC SiMe,Bu’ H Scheme 3 ” C. A. G. 0.Varrna J. T.Tarnminga and J. Cornelisse J. Chem. SOC., Faraday Trans. 2 1982,265. 22 R. F. Childs G. S. Shaw and A. Varadarajan Synthesis 1982 198. 23 C. Guyon P. Boule and J. Lemaire Tetrahedron Lett. 1982 23 1581. 24 T. Kametani T. Mochizuki and T. Honda Heterocycles 1982 19 89. 25 M. A. McGuire and L. S. Hegedus J. Am. Chem. SOC.,1982,104,5538. Photochemistry 129 Scheme 4 Photocycloadditions of alkenes dienes or alkynes to aromatic compounds have provided a rich ground for mechanistic speculation but synthetically useful reactions are not common. 1,2-Cycloaddition (via excited-state alkyne) is the major reaction for alkynes with benzene and the bicyclo[4.2.0]octatriene products normally isomerize to cyclo-octatetraenes. With bulky groups (Scheme 5)the relative stability of the bicyclo[4.2.O]octatriene allows a caged isomer to be formed in reasonable yield,26 in a process that resembles the reactions of naphthalenes with alkynes.The 1,3-cycloaddition of vinyl acetate to indane gives a product (6) that has been elaborated into other [3.3.3]propellane~.*~ 0 Bu' + Bu'CECC0,Me -% 40% Scheme 5 &0Ac v 21% 1,3-Cycloadducts of alkenes with naphthalenes are uncommon but it is reported2* that trans (but not cis) cyclo-octene gives a mixture of 1,3-and 1,4-photoaddition products with naphthalene. The mixtures of products formed from anthracenes and dienes have provided apparent inconsistencies between results from different laboratories but this is now ascribed29 to the fact that the [4 + 41 adducts can be converted into the [4 + 21 adducts in a triplet-sensitized process.It is possible to prepare each type of adduct separately and it appears that they are not formed by way of a common biradical intermediate. Benzenoid compounds do not undergo photodimerization involving the ring as anthracenes do but the first such (intramolecular) reaction is reported3' based on a constrained multilayered cyclo- phane (7). 26 Y. Hanzawa and L. Paquette Synthesis 1982 661. " P. A. Wender and G. B. Dreyer J. Am. Chem. SOC.,1982 104,5805. *' Y. Inoue K. Nishida K. Ishibe T. Hakushi and N. J. Turro Chem. Lett. 1982,471. 29 T.-Y. Wang J.-D. Ni J. Masnovi and N. C. Yang Tetrahedron Lett. 1982 23 1231. 30 H. Higuchi K. Takatsu T. Otsubo Y. Sakata and S.Misumi Tetrahedron Lett. 1982,23,671. 130 J. D. Coyle Photocyclization reactions of aromatic compounds provide many useful routes to polycyclic products. The scope of the reaction of N-allylpyridinium (and quino- Mum) salts has been investigated (limitations arise because internal electron- transfer is involved) and the reaction can be used to prepare indolizidine derivatives (Scheme 6).31Aromatic enethioamides (8) give isoquinoline- 1-thiones and their 3,4-dihydro-derivatives on irradiati~n,~~ and these can be converted into isoquinolones and 1,2,3,4-tetrahydroisoquinolines respectively. The corresponding enamides undergo a 1,3-acyl shift and do not cyclize on irradiation. ROH 33 (& OR OR 90% (R= Me) Scheme 6 4 Carbonyl Compounds The photoreduction of benzophenone by alcohols has previously provided photo- chemists with surprises.Now it is clearly dem~nstrated~~ that with methanol ethanol or isopropanol three products are formed -benzpinacol a mixed pinacol and the oxidized para-adduct (9) -in ratios and with quantum yields that vary U. C. Yoon S. L. Quillen P. S. Mariano R. Swanson J. L. Stavinoha and E. Bay Tetrahedron Lett. 1982,23,919. 32 A. Couture R. Dubiez and A. Lablache-Combier J. Chem. SOC.,Chem. Commun. 1982 842. 33 M. B. Rubin. Tetrahedron Lett. 1982,23 4615. Photochemistry 131 0 significantly with light intensity. The intramolecular photochemical hydrogen abstraction and cyclization of carbonyl compounds has been used many times in synthesis but a particularly appealing application is in its repeated use to build up the dodecahedrane skeleton in twenty-three stages from ~yclopentadiene.~~ The mechanism for cis-trans isomerization of thioindigoid dyes (with an enedione grouping) has been shown3' to be unusual in that the trans +cis reaction occurs by way of the trans-triplet whereas the cis +trans process foIlows a singlet route.Substituted enediones (lo) obtained from furans give high yields of 2,5-dialkoxy- (or diacetoxy-) dihydrofurans on irradiation in hydroxylic and these are of interest for the synthesis of other heterocyclic systems. The triplet-sensitized oxa-di-7r-methane rearrangement of bicyclo[2.2.2]oct-5-en-2-ones(1l),usually prepared by way of the Diels-Alder cycloaddition of cyclohexa- 1,3-dienes and acrylonitriles gives products that can be elaborated to make bi- and tri-cyclic cyclopentanoid natural products and this process has been re~iewed.~' Mechanistic on the monocyclic P,y-unsaturated ketone (12) show that the oxa-di-r- methane reaction occurs through the TI state (which is not available by internal conversion from T2),and the 1,3-acyl shift through S or T2; unusually for a ketone xenon enhances S1*T2 intersystem crossing.hu 0 R'O 34 R. J. Ternansky D. W. Balogh and L. A. Paquette J. Am. Chem. SOC.,1982,104,4503. 35 C. P. Klages K. Kobs and R. Memming Chem. Phys. Len. 1982,90,46. 36 R. Antonioletti M. D'Auria G. Piancatelli S. Santucci and A. Scattri Tetrahedron Lett. 1982,23,2981. '7 M.Demuth and K. Schaffner Angew. Chem. Int. Ed. Engl. 1982,21,820. 38 D. I. Schuster and L. T. Calcaterra J. Am. Chem. SOC.,1982,104 6397. 132 J. D. Coyle Irradiation of N-methylphthalimide with allyltrimethylsilane gives a substituted hydroxyisoindolinone (Scheme 7) together with a smaller amount (1 1 Oh) of the corresponding -OSiMe derivati~e.~~ This is an unusual type of reaction in that phthalimides and alkenes normally give benzazepinediones but the difference can be rationalized on the basis of a similar process observed with a cyclic iminium salt (Scheme 8) where a mechanism is proposed4' that involves electron transfer from the allylsilane and subsequent generation of ally1 radicals. Scheme 7 + &SiMe3 A @Ph I I Scheme 8 42% Dioxetanes are normally obtained by cycloaddition of an alkene and singlet oxygen although they have occasionally been postulated as intermediates in the photoreactions of carbonyl compounds (e.g.1,3-dioxetanes in oxygen isotope exchange). Now a species (13) has been obtained by irradiation of N-methylacridone and acetone at -78"C that has the characteristics of a 1,2-dioxetane and gives rise to cherniluminescence on warming.41 The exchange of methyl groups that occurs on irradiation of a mixture of hexadeuterioacetone and the N-cyclohexyl- imine of acetone is thought to occur by way of a 1,3-oxazetidine (14).42 The formation of oxetanes from benzophenone or benzoquinone and alkenes is quenched by oxygen and 1,2,4-trioxanes [e.g. (15)] can be isolated.43 It is suggested that the species quenched is a charge-transfer exciplex rather than a biradical and that the quinone process is a model relevant to the photo-oxidation of vitamin K.y 3 ,C,Hl1 H3C-C-N I II Oao>Ph & 0-C-CD3 0-0 I CD3 (14) 39 JP 91 125 365 (Chem. Abstr. 1982,96 35 085). 40 K. Ohga and P. S. Mariano J. Am. Chem. SOC.,1982,104,617. 4' N. Suzuki Y. Kazui and Y. Izawa Tetrahedron Lett. 1982,23,95. 42 P. Margaretha Helv. Chim.Acta 1982 65 290. 43 R. M. Wilson S. W. Wunderly T. S. Walsh A. K. Musser R. Outcalt F. Geiser S. K. Gee W. Brabender L. Yerino J. T. Conrad and G. A. Tharp J. Am. Chem. Soc. 1982,104,4429. Photochemistry Of the many photochemical reactions that have been employed in synthesis the largest group is the photocycloaddition of alkenes to a,@-unsaturated carbonyl compounds.A review44 this year describes such reactions that can be followed by ring-opening in an alternative mode to the cycloaddition with special emphasis on intramolecular examples involving substituted cyclopentenones or cyclohexenones. A typically valuable example of such a reaction is the key stage (Scheme 9) in a seven-stage synthesis of sarracenin from methyl lactate,45 which is to be compared with the earlier fifteen-stage synthesis. 3-Cyanocyclohexenone and related systems give rise to a major alternative product (Scheme 10) that is formed by way of the normal intermediate biradical and an (isolable) irni~~e.~~ hu Me0 CHO C0,Me major Scheme 9 0 0 ah- CN 0 NH 8 1'/o 12% Scheme 10 44 W. Oppolzer Acc. Chem. Res. 1982 15 135. 4s S. W. Baldwin and M. T. Crimmins J. Am. Chem. SOC.,1982,104 1132. 46 I. Saito K. Shimozono and T. Matsuura Tetrahedron Lett. 1982 23 5439.