7 Photochemistry By J. D.COYLE Chemistry Department The Open University Milton Keynes MK7 6AA 1 General The term 'photocatalysis' and related phrases have been used with different mean- ings and sometimes they have clearly been misused. In a review' of catalysis of photochemical reactions a definition has been given of a catalyst for a photochemical reaction together with kinetic and other criteria for establishing that catalysis is operating. Photoreactions in which species other than the substrate play an essential mechanistic role are of widespread importance and both organic heterogeneous photocatalysis* (i. e. chemical reactions sensitized by irradiated semiconductors) and homogeneous metal catalysis in organic photochemistry3 have been reviewed. A more general article4 on photoelectron-transfer catalysis covers both organic and inorganic photochemistry.Interest in electron-transfer mechanisms for organic photoreactions continues unabated and a comprehensive account' of the chemistry of excited complexes provides a good grounding in the photophysical aspects of systems that might undergo reaction by such mechanisms. A new tool in the investigation of photochemical mechanisms is holography,6 which can be used to give information about the photophysical or photochemical processes occurring in certain solid-state reactions. 2 Alkenes The trans isomers of cycloheptenes and cyclohexenes have often been proposed as reaction intermediates but now a species stable at -78 "C has been prepared7 by the sensitized irradiation of cis-cycloheptene and assigned the trans-cycloheptene structure.The use of copper(1) chloride to promote the photochemical reactions of alkenes has been extended to the photoaddition of halogenoalkanes to electron-deficient alkenes* to give for example a-halonitriles (Scheme 1). If dichloromethane is employed as the addend,' 1,3-dichloro-compounds are formed that can be converted ' G. G. Wuebbels Acc. Chem. Res. 1983 16 285. M. A. Fox Acc. Chem. Res. 1983 16 314. R. G. Salomon Tetrahedron 1983 39 485. 'M. Juillard and M. Chanon Chem. Rev. 1983 83 425. ' R. S. Davidson,.Adv. Phys. Org. Chem. 1983 19 1. 'D. M. Burland Ace. Chem. Res. 1983,16 218; Angew. Chem. Znt. Ed. Engl. 1983 22 582. 'Y. Inoue T. Ueoka T. Kuroda and T. Hakushi J.Chem. Soc. Perkin Trans. 2 1983 983. ' M. Mitani I. Kata and K. Koyarna J. Am. Chem. SOC. 1983 105 6719. M. Mitani Y. Yamamoto and K. Koyama J. Chem. SOC.,Chem. Commun. 1983 1446. 141 142 J. D. Coyle hu,BuBr dcN &CN cuci 62'/O Scheme 1 c1 &CN hv,cUci CH,CI 'CI &CN FCN 90% 83% Scheme 2 electrochemically into cyclopropanes (Scheme 2); this provides a useful alternative to the Simmons-Smith reaction which is not successful for electron-poor alkenes. A different metal-promoted photoreaction of alkenes involves oxidation in the presence of iron( 111) chloride. lo Cyclohexene leads to 2-chlorocyclohexanone but 1,2- and 1,6-dimethylcyclohexene give ring-opened dichloroketones (Scheme 3)that can be usefully modified for synthetic purposes.'" The reaction probably involves electron transfer to Fe3+ from the alkene and a similar electron-transfer mechanism is proposed' to account for the photochemical cyclization of a 5-hydroxypentene in the presence of 9,lO-dicyanoanthracene. The product (Scheme 4)is a tetrahy- drofuran compared with the 2,2-diphenyltetrahydropyran obtained by acid-promoted cyclization. c1 c1 0 Scheme 3 Ph Ph-OH up to 60% &Ph Ph Scheme 4 Direct irradiation of simple alkenes or dienes often leads to complex mixtures of dimers and the problem is exacerbated when two alkenes are irradiated together. However when a mixture of cyclohexene and cycloheptene reacts in the presence of copper(1) triflate,12 one stereoisomer of the cross-adduct (1) can be isolated in yields up to 37%.The photodimerization of cyclohexa- 1,3-diene normally gives 10 A. Kohda K. Nagayoshi K. Maemoto and T. Sato J. Org. Chern. 1983 48 425. 'I Z. Q. Jiang and C. S. Foote Tetrahedron Lett. 1983 24 461. 12 P. J. J. A. Tinnemans G. M. T. de Ruiter A. H. A. Tinnemans and A. Mackor Tetrahedron Lett. 1983,24 1419. Photochemistry [2 + 21 cycloadducts as major products but when 9,lO-dicyanoanthracene is present as an electron-acceptor sensitizer [4 + 21 dimers can be predominant (>60%).13 It is likely that the sensitizer generates the radical cation of the diene as the reactive species. In a related reaction radical cations from 1-arylpropenes can be produced non-photochemically using (Ar3N.)+(SbCl6)- and are found14 to lead to [2 + 21 cycloadducts.The reverse of alkene [2 + 21 cycloaddition namely the cleavage of cyclobutanes can also be achieved using electron-transfer photosensitization and irradiation of a mixture of 1,2-diarylcyclobutane and dicyanoanthracene in the presence of oxygen givesI5 a 1,2-dioxan (2) in 60-70% yield by trapping of a ring-opened radical cation. Under similar conditions 1-butyl-2,3-diphenylaziridine yields (83%) a 1,2,4-dioxazolidine (3).16 The di- v-methane reaction of 3-arylpropenes gives arylcyclopropanes and this has been used" in a patent description of the production of arylcyclopropanes with oxygen substituents in the aryl group for use in fragrances. 3-Vinylcyclopropenes are converted photochemically into cyclopentadienes and the most likely mechanism for the rearrangement has been elucidated" by careful study of suitably substituted systems.3 Aromatics Substituted thiophenes furans and pyrroles undergo photochemical ring-transposi- tion or ring-contraction reactions often in low yield. Furans substituted with trimethylsilyl groups are now shown" to give allenic aldehydes or ketones (Scheme 5) in good yield in a clean reaction at -78 "C. Scheme 5 The mechanisms of aromatic photosubstitution reactions have been reviewed.*' An unusual nucleophilic photosubstitution by cyanide ion in a tricyclic pyrrole gives rise (Scheme@ to a product in which the leaving group (hydride ion) has been 13 C. R. Jones B. J. Allman A. Mooring and B.Spahic J. Am. Chem. SOC.,1983,105 652. 14 N. L. Bauld and R. Pabon J. Am. Chem. SOC.,1983,105 633. 1s K. Mizuno K. Murakami N. Kamiyama and Y. Otsuji J. Chem. Soc. Chem. Comrnun. 1983 462. I6 A. P. Schaap G. Prasad and S. D. Gagnon Tetrahedron Lett. 1983 24 3047. Jpn. Kokai Tokkyo Koho JP 58 21,634 (Chem. Abstr. 1983 98 178935). 18 H. E. Zimmerman and S. A. Fleming J. Am. Chem. SOC.,1983,105 622. 19 T. J. Barton and G. P. Hussmann J. Am. Chem. SOC.,1983 105 6316. 20 C. Parkanyi Pure Appl. Chem. 1983 55 331. 144 J. D. Coyle transferred within the molecule to effect the reduction of a ketone.2' A p-chloro-phenylurea has been employed2* as a radical source to effect substitution at the 3-position in pyridine (Scheme 7); the resultant 3-arylpyridine can be converted in two stages into 3-phenylpyridine which is not readily accessible by more conven- tional routes.Scheme 6 Scheme 7 Aryl or vinyl halides irradiated with visible light in the presence of carbon monoxide and dicobalt octacarbonyl are converted into arylcarboxylate salts;23 if there is an orfho substituent with a suitably placed amino or hydroxyl (Scheme 8) group lactams or lactones are readily formed. *mo T O H 95% 0 Scheme 8 Although the mefu photocycloaddition of alkenes to benzenes can give complex mixtures of isomeric adducts each in fairly low yield there have been a number of useful applications of the reaction in natural product synthesis. Another example is the synthesis of c~riolin~~ in eleven stages from the intramolecular photoadduct obtained from a 5-phenylpentene (Scheme 9).Photocycloaddition of conjugated dienes to anthracenes no longer offers the original fairly straightforward picture with major products derived by addition across the 9,lO-positions. If the initial adducts are protected from shorter-wavelength radiation it is now apparent that for 9,lO-dichloroanthracene and cyclohexadiene the major product (57%) involves [2 + 21 addition across the 1,2-positions of the anthra~ene.~~ 21 Y. Girard J. G. Atkinson P. C. BClanger J. J. Fuentes J. Rokach C. S. Rooney D. C. Remy and C. A. Hunt J. Org. Chem. 1983 48 3220. 22 F. S. Tanaka R. G. Wien and B. L. Hoffer Synth. Commun. 1983 951. 23 J.-J. Brunet C. Sidot and P.Caubere J. Org. Chem. 1983 48 1166. 24 P. A. Wender and J. J. Howbert Tetrahedron Lett. 1983 24 5325. 25 W. K. Smothers M. C. Meyer and J. Saltiel J. Am. Chem. Soc. 1983 105 545. Photochemistry Ac$ +-I= H Scheme 9 A number of interesting photocyclizations to aromatic rings have been reported. 2,6-Dichlorocinnamate esters or the corresponding amides undergo photochemical ring-closure’“ to give 5-chlorocoumarin (Scheme lo) in near quantitative yield if the solution is dilute enough to prevent significant photodimerization of the product. Such aryl-oxygen bond formation is unusual in a photochemical reaction and the cyclization step is formally an oxa-analogue of the photocyclization of 1-phenyl-butadienes. Also rather unexpected is the ring-clo~ure~’ of 2-alkoxy-3-arylcyclohex-2-enones (Scheme 11); this process cannot be considered as a 67-electron cycliz- ation and the reaction is initiated by intramolecular hydrogen abstraction involving the oxygen of the enone group.C1 CI Scheme 10 Scheme 11 When benzophenone is irradiated with acetone oxime in methanol fluoren- 1-01 (4)is formed in 60% yield.28 Irradiation of the oxime of benzophenone alone produces the same compound in 34% yield,’” increasing to 60% if benzophenone 26 R.Arad-Yellin B. S. Green and K. A. Muszkat J. Org. Chem. 1983 48 2578. 27 J.-P. Pete and D. Scholler Tetrahedron Lett. 1983 24 5887. 28 B. Kumar N. Kaur and G. Kaur Synthesis 1983 115. 2Y B. Kumar and N. Kaur J. Org. Chem. 1983,48 2281.146 J. D. Coyle is added. The mechanism proposed to account for this process involves attack by the oxygen of excited benzophenone on the nitrogen of ground-state oxime to give an intermediate 1,4-biradical with a new N-0 bond which undergoes cleavage and then cyclization to fluorenol. Finally in this section the application of a photo- chemical 1,3-shift related to the photo-Fries reaction provides an efficient route to fused medium-ring lactams [e.g. the model compound (5)],which are starting points for the synthesis of a variety of alkaloids of the strychnos aspidosperma schizozy- gane or eburnamine fa mi lie^.^' Off (4) H 90% H 0x-/ (5) 4 Carbonyl Compounds Two of the best known photochemical reactions of ketones and other organic carbonyl compounds are the Norrish type 1 and type 2 reactions and these can often be useful in syntheses of systems that are not readily obtainable by other routes.Photochemical extrusion of carbon dioxide from lactones can yield small-ring compounds and from fused oxazolidinones 1-azabicyclo[ 1.1 .O]butanes [e.g. (6)]can be ~btained.~’ The epoxides of cyclohex-3-enols are not easily oxidized to the corresponding cyclohexanones because of their sensitivity to acid reagents but photolysis of the pyruvate esters (7) achieves this oxidation by a Norrish type 2 ph~toelimination.~’ The pyruvic part of the substrate is converted into a hydroxy-ketene and the analogous species from a cyclohexyl benzoylformate can be trapped by an imine (Scheme 12) to give a P-la~tam.~~ 0 -Ph I I0 U rii 0 30 Y.Ban K. Yoshida J. Goto T. Oishi and E. Takeda Tetrahedron 1983 39 3657. 31 R. Bartnik Z. Cebulska and A. Laurent Tetrahedron Lett. 1983 24 4197. 32 H. A. J. Carless and G. K. Fekarurhobo Tetrahedron Lett. 1983 24 107. 33 H. Aoyama M. Sakamoto K. Yoshida and Y. Omote J. Heterocycl. Chem. 1983 20 1099. Photochemistry OH Ph P h A o o *Ho>=o PhCH=NCH,Ph Ph-cf:, Ph 0 76% Scheme 12 Irradiation of nonan-5-one in fluid solution gives cyclobutanols and elimination products (hexan-2-one and propene) in a ratio of 0.32 1;when the urea inclusion complex of the ketone is irradiated this ratio increases to 0.67 :1 and mainly one stereoisomer of the cyclobutanol is formed.34 Similarly N,N-dialkylpyruvamides give P-lactams (42-74%) when irradiated as their inclusion complex with desoxycholic acid,35 whereas in solution these are at best minor products.Some a#-unsaturated ketones undergo photochemical isomerization to P,y-unsaturated isomers by way of intramolecular hydrogen abstraction. However many enones are inert under normal conditions but when irradiated in the presence of a mild base (e.g.pyridine) the isomerization can be achieved su~cessfully.~~ 2-Vinyl-lactones (8) can be obtained in an analogous process by photoisomerization of the more readily available 2-methylenela~tones.~~ Photoreduction of benzophenone in acidified methanol gives Ph,CHCH( OMe), the acetal of diphenyla~etaldehyde,~' by way of the initially formed mixed pinacol 1,l-diphenylethane-172-diol; the formation of the acetal from the diol is a thermal process.Some cyclic ketones have long been known to give their ketals on irradiation in methanol and it is now suggested39 that this is a genuine photoreaction involving attack by solvent on the ketone excited state. do%& (8) The oxetanes formed photochemically from furans and aldehydes undergo facile ring-opening and the overall process (Scheme 13)is equivalent to an aldol condensa- ti01-1.~' The stereochemical course of the reaction is well defined and the highly functionalized products are useful for subsequent transformations. Aromatic imides -d: hw + R1dR2 RIGR2 R3CHo R1 R2 HO R3 Scheme 13 34 H.L. Casal P. de Mayo J. F. Miranda and J. C. Scaiano J. Am. Chem. Soc. 1983 105 5155. 35 H. Aoyarna K. Miyazaki M. Sakarnoto and Y. Ornote J. Chem. SOC.,Chem. Commun. 1983 333. 36 S. L. Eng R. Ricard C. S. K. Wan and A. C. Weedon J. Chem. Soc. Chem. Commun. 1983 236. 37 F. HCnin R. Mortezaei and J.-P. Pete Synthesis 1983 1019. 38 N. J. Bunce and E. J. Toone J. Chem. Res. (S) 1983 115. 39 V. Malatesta M. Jennings and P. Hackett Can. J. Chem. 1983 61 366. 40 S. L. Schreiber A. H. Hoveyda and H.-J. Wu J. Am. Chem. Soc. 1983 105 660. 148 J. D. Coyle do not normally give oxetanes on irradiation with alkenes and the course of the reaction depends on the solvent. A 5-phthalimidopent-1-ene (Scheme 14) gives a fused 2-benzazepinedione in acetonitrile but the reaction takes a different course in methan01.~' ,OMe -k 0 Scheme 14 [2 + 21 Photocycloadditions involving a,P-unsaturated ketones are some of the best ways of making fused cyclobutane systems.Intramolecular reaction in 1-or 2-acylhexa-l,5-dienes or in hexa-l,5-dien-3-ones can give products derived from either initial 1,5-or initial 1,6-bond formation and an extensive study4* has high- lighted the substituent and ring effects that can promote the 1,&mode of ring-closure. 3-Alkenylcyclohex-2-enones have a 1 -acylhexa- 1,Sdiene unit and an intramolecular photocycloaddition in such an enone has been employed43 in a synthesis of acoradiene (Scheme 15); the overall synthesis requires eight stages from 3-methoxycyclohex-2-enone.Coumarin dimerizes on irradiation to give a mixture of cyclobutane products; addition of BF increases the quantum yield by a factor of a hundred and changes the ratio of isomers formed.44 Scheme 15 5 Nitrogen Compounds Many aliphatic azo-compounds undergo &-trans isomerization and/or eliminate nitrogen on irradiation but some of the products formed with loss of nitrogen seem 41 P. H. Mazzocchi P. Wilson. F. Khachik. L. Klingler and S. Minamikawa J. Org. Chem. 1983.48. 2981. 42 S. Wolff and W. C. Agosta. J. Am. Chem. SOC.,1983 105. 1292 1299. 43 W. Oppolzer F. Zutterman and K. Baettig Helv. Chim. Am 1983 66,522. 44 F. D. Lewis. D. K. Howard and J. D. Oxman. J. Am. Chem. Soc. 1983 105 3344. Photochemistry to require a carbene rather than a radical intermediate.This suggests that initial azo-diazo-compound isomerization may occur and by using a laser source for the irradiation it has now been demonstrated4’ that a diazo-compound is generated in solution at room temperature from the tricyclic azo-compound (9). & hunm) +6’ (333.6 (9) Useful cyclization reactions involving the formation of heterocycles from iminium salts have been reported in recent years and these have now been reviewed.46 Photocycloadditions of alkenes to C=N compounds have become familiar and 2-phenylbenzoxazole gives a high yield of a 1,3-diazetidine dimer (Scheme 16) on irradiati~n.~’ Scheme 16 45 W. Adam N. Carballeira and W. D. Gillaspey Tetrahedron Lett. 1983 24 5473. 4h P. S. Mariano Ace. Chem. Rex 1983 16 130; Tetrahedron 1983 39 3845. 47 J. Roussilhe E. Fargin A. Lopez B. Despax and N. Paillons J. Org. Chem. 1983 48 3736.