8 Photochemistry By H. A. J. CARLESS Department of Chemistry Birkbeck College Malet Street London WClE 7HX Photochemistry is the subject of a well-established annual Specialist Periodical Report the most recent volume (Vol. 7) of which contains over 1300 references to papers on organic photochemistry. Consequently the present report is highly selective. The main processes already observed on direct irradiation of simple alkenes are cis-trans isomerization [2 + 21 cycloaddition and carbene formation. Kropp and co-workers’ have added yet another possibility to this list by their observation of a 1,34gmatropic hydrogen shift in alkenes [e.g. (1)to (2)]. Migration is intramolecu- lar as shown by the lack of incorporation of deuterium in the isomer (2) on irradiation of (1) in [2H12]cyclohexane,and by the specific rearrangement of the deuteriated alkylidenecyclopentane (3).The photochemical cyclisation of allyl Grignard reagents such as (4) leads to cyclopropyl Grignards (5) in good yield.’ This interesting example of the photo- chemical closure of an allyl anion to a cyclopropyl anion might be expected to be a disrotatory process but unfortunately the authors could not establish the stereochemistry of the ring closure. An investigation of the stereochemistry of the acetone-sensitized photorearrangement of allylic chlorides to chlorocyclopropanes has shown that this triplet process is stereoselective but certainly not stereo~pecific.~ R (1) (2) (3) (4) a; R=H (5) b; R=Ph Kropp and Poindexter4 have previously reported that the irradiation of alkyl iodides in solution is a method for the generation of carbocations which are probably formed via homolytic C-I bond cleavage followed by electron transfer to produce cation and iodide ion.Kropp and McNeely’ have now used the photolysis of vinyl iodides in solution as a simple method for the generation of vinyl cations. The reaction appears especially useful for the generation of cyclic vinyl cations otherwise P. J. Kropp H. G. Fravel and T. R. Fields J. Arner. Chem. SOC.,1976,98 840. S. Cohen and A. Yogev J. Amer. Chem. SOC.,1976,98,2013. S. J. Cristoi and C. S. Ilenda Tetrahedron Letters 1976 3681. G. S. Poindexter and P. J. Kropp J. Amer. Chem. SOC.,1974 % 7142. S. A. McNeely and P. J. Kropp J. Amer.Chem. SOC.,1976,98,4319. 155 156 H. A.J.Carless accessible with difficulty. Thus irradiation of 1-iodocyclohexene in methanol or dichloromethane solution gives products (65-87% yield) arising from nucleophilic trapping of the 1-cyclohexenyl cation. A vinyl cation may also be generated on irradiation of the dianisylvinyl bromides (6a-c) which produce anisyl-group- migrated products (7a-~).~ Interestingly the less electron-rich diphenylvinyl bromide (6d) does not rearrange in this manner. (6) a; R=p-OMe (7) b; R=m-OMe c; R=o-OMe d; R=H The belief that production of a twisted trans -like cyclohexene intermediate occurs on irradiation of cyclohexenes in methanol has received support. Laser flash photolysis of 1-phenylcyclohexene in methanol produces a transient species (absorp- tion maximum 380nm lifetime 9ps) which is quenched by hydrogen ions and is assigned the structure trans-1-phenylcy~lohexene.~ Morrison and Nylund' report that along with triplet di-n-methane rearrangement products an unusual anti- Markovnikov addition occurs to the double bond of alkene (8)on irradiation in acidic methanol yielding the ether (9).The technique of adding xenon' to quench the production of (9)suggests that a singlet excited state is the species responsible for the above reaction. CH,OMe A further example of the photochemical di-n-methane rearrangement now provides the first case in which reaction proceeds in one direction on direct irradiationlo*" and in the other direction on sensitized irradiation.lo Thus direct irradiation of the diene (10) (cis-trans mixture) in hexane ether or dioxan produces predominantly the chrysanthemate (11) whereas sensitized irradiation in acetone or benzene produces (12). Hou~'~,~~ has presented a molecular orbital method to 6 T. Suzuki T. Sonoda S.Kobayashi and H. Taniguchi J.C.S. Chem. Comm. 1976 180. R. Bonneau J. Joussot-Dubien L. Salem and A. J. Yarwood J. Amer. Chem. SOC.,1976 98,4329. 8 H. Morrison and T. Nylund J.C.S. Chem. Comm. 1976 785. 9 H. Morrison T. Nylund and F. Palensky J.C.S. Chem. Comm. 1976 4. 10 P. Baeckstrom J.C.S. Chem. Comm. 1976,476. 11 M. J. Bullivant and G. Pattenden J.C.S. Perkin I 1976 256. C. Santiago and K. N. Houk J. Amer. Chem. SOC.,1976,98 3380. C. Santiago K. N. Houk,R.A. Snow and L. A. Paquette J. Amer. Chem. SOC.,1976,98 7443. Photochemistry 157 explain regiospecificity in the di-rr -methane rearrangements of benzonorbor-nadienes recently observed by Paq~ette,'~ but there is evidence from deuterium labelling studies that the rearrangement of 2-cyanobenzobarralene to 1-cyanobenzosemibullvalene does not proceed by Houk's predicted pathway. l5 Irradiation of o-divinylbenzenes such as (13a) is known to lead to benzobicyc- lohexenes [e.g. (14a)l which are formed by [4+2] addition followed by rearrange- ment.16 The [2+2] cycloaddition products [e.g. (15a)l normally found for hexa-1,5- dienes are not formed. However it is now reported that 2-vinylstilbene (13b) gives [2+2] cycloaddition to yield (15b) as the major process on irradiation and the alternative product (14b) is absent from the reaction mixture." The effect of the phenyl group in determining the conformational preference of the vinylstilbene (13b) probably lies at the root of this marked difference in behaviour between (13a) and (13b).Hixson" has measured rate constants for the photochemical 1,2- migration of hydrogen methyl and phenyl in the isomerization of substituted styrenes to arylcyclopropanes. An investigation of the photorearrangement of optically active 2-phenyl-3-methylmethylenecyclopropaneshows that chirality is maintained during rearrangement. l9 Although the results are somewhat equivocal on this point there does seem to be an inversion of configuration at the migrating centre similar to that observed in the thermal methylenecyclopropane rearrangement.Certainly the present results exclude the formation of a planar trimethylenemethane intermediate on photolysis. (13) a; R=H b; R=Ph The photocyclization of stilbenes to phenanthrenes involves the well-known processes of trans to cis isomerization followed by cyclization and oxidation of an unstable dihydrophenanthrene intermediate; such intermediates have been observed spectroscopically but never isolated. Filipescu et ul.*O have now obtained the first stable dihydrophenanthrene from diethylstilbestrol(l6) photocyclization in 14 L. A. Paquette D. M. Cottrell R. A. Snow K. B. Gifkins and J. Clardy J. Amer. Chem. Soc. 1975,97 3275. 15 C. 0.Bender and E. H. King-Brown J.C.S.Chem. Comm. 1976,878. 16 M. Pomerantz J. Amer. Chem. Soc. 1967,89,694;J. Meinwald and P. H. Mazzochi ibid. 1967,89,696. M. Sindler-Kulyk and W. H. Laarhoven J. Amer. Chem. Soc. 1976,98 1052. S. S. Hixson J. Amer. Chem. SOC.,1976,98 1271. 19 W. A. Gros T. Luo and J. C. Gilbert J. Amer. Chem. SOC.,1976,98,2019. to T. D. Doyle W. R. Benson and N. Filipescu J. Amer. Chem. Soc. 1976,98 3262. H.A.J. Carless which it appears that the extra stability results from the double tautomerism of the dienol to a keto-structure (17). (16) (17) 1,5-Hydrogen shifts have been noted in alkene photochemistry. For example an intramolecular hydrogen abstraction (similar to that observed in carbonyl com- pounds) occurs on irradiation of 1-0-tolyl-1-phenylethylene leading to an o -xylylene intermediate (18) which may be trapped in a Diels-Alder reaction by cyclohexene.21 However the author does not mention the similar trapping experi- ments recently carried out by Pratt.22 The predominant products formed on photolysis of cis-cyclo-octene vapour appear to arise by intramolecular hydrogen abstraction of the vibrationally excited triplet state of the alke~~e.~~ The epoxy-diene (19) undergoes a novel photoreaction on irradiation (254 nm) in pentane producing the cyclopropenyl ketone (20) in 90% yield.24 A mechanism is proposed which involves homolytic C-0 bond-breaking of the epoxide ring as the initial step.(18) (19) (20) This year has seen some very interesting examples of reactions which ensue because of the conformational mobility of the biradicals generated by a-cleavage of cyclic ketones.Weiss and ~o-workers~~ have noted a stereospecific decarbonylation in the photolysis of the bicyclic ketone (21) to produce the ester (22) in which the stereochemistry of the methoxycarbonyl group is changed from ex0 to endo. This unusual inversion process is thought to reflect a radical substitution reaction whereby the LY -cleaved alkyl acyl biradical changes conformation allowing the alkyl radical to assist displacement of carbon monoxide from the acyl radical centre. The biradical (23) generated by photolysis of bicyclo[3,2,l]octan-6-one(24) disproportionates almost entirely to the unsaturated aldehyde (25).26 From a study of the rearrange- ment on photolysis of two specifically deuteriated isomers of (24) it is concluded that 21 J.M. Hornback Tetrahedron Letters 1976 3389. 22 A. C. Pratt J.C.S. Chem. Comm. 1974 183. 23 Y. Inoue K. Moritsugu S. Takamuku and H. Sakurai J.C.S. Perkin II. 1976 569. 24 A. P. Alder H. R. Wolf and 0.Jeger Helu. Chim. Acta 1976,59,907. 25 D. S. Weiss M. Haslanger and R. G. Lawton J. Amer. Chem. SOC.,1976,98 1050. 26 W. C. Agosta and S. Wolff J. Amer. Chem. SOC.,1976,98,4182. Photochemistry 159 the transfer of axial hydrogen in the biradical(23) is about 20 times more favoured than the transfer of equatorial hydrogen.27 H 0 eCH2CH0 - (23) (24) (25) The Norrish Type I1 reaction of aryl alkyl ketones such as (26a) generally leads via a 1,4-biradical to predominant elimination with a small amount of cyclization to cyclobutanol.Wagner and Thomas28find that a-fluorine substitution in the ketones (26b) and (26c) leads to a striking enhancement of cyclobutanol formation so that cyclobutanol is the only significant product formed from ketone (26c). An explana-0 I1 Ph-C-CXYCH,CH,Me (26) a; X=Y=H b; X=H,Y=F c; X=Y=F tion which will need further substantiation is given whereby fluorine substitution alters the preferred conformation of the 1,4-biradical to prevent C-C bond cleavage. In ester photochemistry although similar examples of elimination to alkene and carboxylic acid are well known there have been no reports of the cyclization to yield oxetanols. Marron and Gan~~~ now present this unobserved process as responsible for the photochemical conversion of adamant-2-yl trifluoroacetate (27) into the isomer (28) and they propose this reaction as a convenient high-yield route to various 1,2-disubstituted adamantanes.(27) (28) Double decarboxylation by irradiation of the diester (29) in methanol provides an ingenious route to [2,2]paracyclophane (30) in yields of up to 70% .30 Kaupp3' has observed three new examples of [6+61photocleavage of [2,2]cyclophanes including 27 W. C. Agosta and S. Wolff J. Amer. Chem. SOC.,1976,98 4316. 28 P. J. Wagner and M. J. Thomas J. Amer. Chem. SOC.,1976,98 241. 29 N. A. Marron and J. E. Gano J. Amer. Chem. Soc. 1976,98,4653. 30 M. L. Kaplan and E. A. Truesdale Tetrahedron Letters 1976 3665. 31 G. Kaupp Angew.Chem. Internat. Edn. 1976,15 442. 160 H. A. J. Carless the formation of the tetraene (31) on irradiation of the cyclophane (30) at low temperatures. (29) (30) (31) Irradiation of cyclobutanones in solution generally leads to ring expansion decarbonylation and cycloelimination reactions. However Jones and M~Donnell~~ describe a novel pathway from the photolysis in methanol solution of chlorocyc- lobutanones such as (32a) which produces the ring-contraction products (33) in competition with cycloelimination to form (34a). The ratio of ring contraction to (32) a; R1=Me,R2=C1 (33) (34) b; R1=C1 R2=Me elimination depends on the stereochemistry and conformation adopted by the particular bicyclic ketone. Theoretical considerations suggest that the photo- chemistry of cyclobutanones may differ so markedly from that of unstrained cycloal- kanones because of the ability of the former to generate a linear rather than bent acyl radical by cu-~leavage.~~ Attempts have been made to cause bis-decarbonylation (loss of C20,)by photo- lysis of cyclic unsaturated a-diketones.A more careful study of this reaction has now shown that bicyclic a-diketones such as (35) do not bis-decarbonylate but instead rearrange via a 1,3-shift to form cyclobutanediones [e.g. (36)].34 It is these latter compounds which give rise to the observed photochemical decarbonylations previ- ously ascribed to their bridged precursors. The overall reaction presents a useful synthetic route from the benzonorbornenedione (37) to the highly reactive isoindene (38).35 Photoenolization which has been reviewed by Samme~,~~ has provided some significant results this year.Ullman and Tseng3' have shown that the photoenoliza- 32 G. Jones and L. P. McDonnell J.C.S. Chem. Comm. 1976 18; J. Amer. Chem. SOC.,1976,98,6203. 33 N.J. Turro W. E. Farneth and A. Devaquet J. Amer. Chem. SOC.,1976,98 7425. 34 M. B.Rubin M. Weiner and H.-D. Scharf J. Amer. Chem. SOC.,1976,98 5699. 35 R.N.Warrener R. A. Russell and T. S. Lee Tetrahedron Letters 1977,49. 36 P.G.Sammes Tetrahedron,1976,32 405. 37 S.-S. Tseng and E. F. Ullrnan J. Amer. Chem. SOC.,1976 98 541. Photochemistry 161 (35) (36) (37) (38) tion of o-alkylbenzophenones such as (39) can be followed by efficient elimination reactions to produce o-vinylbenzophenone.Wagner and Chen3' report quenching studies which implicate both short-lived (0.2 ns) and long-lived (30 ns) triplets in the photoenolization of o-methylacetophenone (40). The long-lived triplets are thought to be anti-isomers [derived from anti-(40)] that rearrange to short-lived enolizable syn-isomers [also derivable from syn-(40)]. There does also appear to be singlet- state photoenolization. In a series of o -alkyl-phenyl alkyl ketones the authors3* present evidence that the measured apparent rate constant for enolization is independent of CFH bond strength so that they interpret this rate (ca. lo7 s-') instead as that of rotation of the anti-triplet to the syn-conformation. Ph 0A (39) anti -(40) syn -(40) Enone photochemistry remains a controversial subject.Hou~~~ has reviewed comprehensively the photochemistry of By -unsaturated carbonyl compounds much of which is dominated by 1,2-shifts (oxa-di-.n-methane rearrangements) occurring from excited triplet states and 1,3-shifts occurring from excited singlet states. This latter generalization is questioned in independent paper^.^^.^^ Dalton et aL41prop-ose on the basis of enone fluorescence quantum yields and lifetimes that some of the 1,3-acyl shifts observed on irradiation of alkyl py-enones may be occurring from the triplet nr*(T,) state rather than from the accepted singlet n.n*(S1) state. Such arguments would certainly help to explain some of the sensitized 1,3-acyl shifts 2f py-enones mentioned in last year's Report and also those recently observed by Parker and In principle the study of a triplet oxa-di-wmethane rearrangement of an optically active enone should allow a choice between the various radical and concerted pathways available for the 1,2-~hift.~~ Two group^^^,^^ have studied such a rearrangement but the results are not clear-cut.Schaffner and co-worker~~~ report that irradiation of the optically active enone (41) produces the two diastereoisomers (42)and (43) each essentially retaining the optical purity of the starting enone; in this example both a radical (a-cleavage) mechanism and a fully concerted [,2 +,2,] route are ruled out. However the optically active enone (44) 38 P. J. Wagner and C.-P. Chen J. Amer. Chem. SOC.,1976,98 239.39 K. N. Houk Chem. Rev. 1976,76 1. 4O K. Schaffner Tetrahedron 1976 32 641. 41 J. C. Dalton M. Shen and J. J. Snyder J. Amer. Chem. SOC.,1976,98 5023. 42 S. D. Parker and N. A. J. Rogers Tetrahedron Letters 1976,4389. 43 R. L. Coffin R. S. Givens and R. G. Carlson J. Amer. Chem. Soc. 1974 96 7554. 44 B. Winter and K. Schaffner J. Amer. Chem. SOC.,1976,98 2022. H.A.J. Carless has been photolysed by Dauben et QI.,~~ and in this case both oxa-di-rr-methane diastereoisomers formed are optically active but only about 10%optically pure. In conclusion the acyl-shifted biradical [e.g. (45)]is the intermediate most consistent with all these observations but its formation and selectivity of ring closure may depend very much on the substitution pattern of the &-enone from which it is derived.(41) R =CH(OMe)* (42) (43) (44) (45) In another example of the photochemical rearrangements of optically active ketones Schuster and have investigated the cyclohexenones (46).In each case the bicyclic ketones (47)and (48)are formed without any detectable loss of (46) a; R=Pr” (47) (48) b; R=Ph optical purity and the absolute stereochemistry of the products has been determined. The results prove that these cyclohexenone rearrangements proceed with inversion at the chiral centre (as expected for a concerted [,2 +,2,] process) and show that a biradical mechanism is not permissible. Photolysis of 6-hydroxycyclohexenones gives the first examples of products derived from a-cleavage of the cyclohexenone ring.47 There are very few molecules known where an upper excited state has a lifetime long enough to show different chemical reactions to those of the lowest excited state.Thiones are amongst such molecules and de Mayo has presented clear examples showing how two excited states can differ in their chemistry.48 Thus irradiation into the S2(?r?r*)band of aryl alkyl thiones such as (49)containing a 6-Chydrogen atom in the alkyl chain produces the cyclopentanethiol (50) from 6-hydrogen abstrac- ti~n.~~ Irradiation of (49)in the long-wavelength band [Sl(n.rr*)],however produces different unidentified products with an efficiency reduced by a factor of lo3. Likewise adamantanethione undergoes [2 +21 photocycloaddition to alkenes;” on irradiation in the Sz band addition is stereospecific but not regiospecific whereas irradiation in the S1band leads to regiospecific but not stereospecific reaction.These 45 W. G. Dauben G. Lodder and J. D. Robbins J. Amer. Chem. Soc. 1976,98 3030. 46 D. I. Schuster and R. H. Brown J.C.S. Chem. Cornm. 1976 28. 47 M. Jeffares and T. B. H. McMurry J.C.S. Gem. Comm. 1976 793. 48 P. de Mayo Accounts Chem. Res. 1976,9 52. 49 A. Couture K. Ho M. Hoshino P. de Mayo R. Suau and W. R. Ware J. Amer. Chem. Soc. 1976,98 6218. SO A. H. Lawrence C. C. Liao P. de Mayo and V. Ramamurthy,J. Amer. Chem. Soc. 1976,98,2219,3572. Photochemistry 163 differences are reminiscent of the singlet and triplet cycloadditions observed by Lewis and Hirsch’l (see below) and suggest that the reactive excited states are Sz (TT*) and TI (n?r*) respectively.Aryl alkyl ketones having an activated /3-C hydrogen such as (51) cyclize on irradiation in the S1 band to produce cyclo- propanethiols (52) in high yield.52 (49) (50) (51) a; R=Ph (52) b; R = 2-naphthyl c; R=SMe Fluorine substitution seems to exert a powerful effect in determining the orienta- tion of photochemical addition of alkenes to 5-fluorouracil (53),53 in contrast to the normally rather unselective [2 +21 additions of enones to alkenes which have been observed. For example isobutene reacts with (53) to form the adduct (54) as the exclusive photoproduct. Treatment of such adducts with base converts them into potentially useful 5-substituted uracil~.~~ (53) (54) Lewis and Hirsch” report a detailed investigation of the [2 +21 photocycloaddi-tion of diphenylvinylene carbonate (55) with conjugated dienes which gives much information on the role of the singlet and triplet excited states of (55) in the reaction.H Ph Ph (55) The singlet reaction occurs oiu an observable ex~iplex,~~ with complete retention of diene stereochemistry but only modest regioselectivity in attack at an unsymmetrical diene. In contrast the triplet cycloaddition occurs with substantial loss of diene stereochemistry but with high regioselectivity at the less substituted terminus of the diene. The important fact emerges that this high triplet regioselectivity is due to preferential collapse of the more substituted 1,4-biradical intermediate rather than to selective bonding to the less substituted diene terminus.Koch and co-workers 51 F. D. Lewis and R. H. Hirsch J. Amer. Chem. Soc. 1976,98 5914. 52 A. Couture M. Hoshino and P. de Mayo J.C.S. Chem. Comm. 1976 131. 53 A.Wexler and J. S. Swenton J. Amer. Chem. SOC.,1976,9& 1602. s4 Cf. A. Wexler R. J. Balchunis and J. S. Swenton J.C.S. Chem. Comm. 1975 601. 55 F.D.Lewis and C. E. Hoyle J. Amer. Chern. SOC.,1976,98,4338. 164 H.A. J.Carless have found further uncommon examples of the reactivity of the carbon-nitrogen double bond towards photocycloaddition in the reactions of 3-ethoxyisoindolone (56)56 and 2-oxazolin-4-ones (57)57with alkenes. (56) (57) Photocycloaddition reactions of the nitrile group are rare. Cant~eIl~~ has suggested a 1-azetine (58a) as an intermediate in the photoaddition of benzonitrile with 2,3-dimethylbut-2-ehe to yield the azadiene (59a).Yang and co-w~rkers~~ have now been able to isolate such 1-azetines (58a-c) from [2+21 photoadditions of A Ar * Ar $ (58) a; Ar=Ph (59) b; Ar =1-naphthyl c; Ar =2-naphthyl the corresponding aromatic nitriles to the same alkene. For the naphthonitriles [2+2] addition between the aromatic ring and the alkene also forms a competing reaction.60 A very different route to azadienes (60) is provided by the irradiation of derivatives of 2-hydroxymethylazirines (6 1) containing good leaving groups.6' A mechanism is proposed which involves photochemical generation of a nitrile ylide (62) followed by its 1,4-substituent shift to yield the azadiene (60).*-(60) a; X=C1 (61) b; X=Br c; X=OCOMe d; X=OCOPh Photo-oxidations are generally carried out by the use of light a dye photosen- sitizer and ground-state oxygen (30,) to generate the reactive species singlet oxygen ('0,). Amines are sometimes used to test for the participation of '0 in such dye-sensitized reactions although Davidson and Trethewey6 now point out that caution must be used in interpreting such experiments because amines (and also 56 K. A. Howard and T. H. Koch J. Amer. Chem. SOC.,1975,97 7288. 57 R. M. Rodehorst and T. H. Koch J. Amer. Chem. SOC., 1975,97,7298. 58 T. S. Cantrell J. Amer. Chem. SOC.,1972 94 5929. 59 N. C. Yang B. Kim W. Chiang and T. Hamada J.C.S. Chem. Comm. 1976,729.60 J. J. McCullough R. C. Miller D. Fung and W.-S. Wu J. Amer. Chem. SOC.,1975 97 5942. 61 A. Padwa J. K. Rasmussen and A. Tremper J.C.S. Chem. Comm. 1976 10. 62 R. S. Davidson and K. R. Trethewey J. Amer. Chem. SOC.,1976,98 4008. Photochemistry p-carotene) quench not only '0,but also the singlet excited state of the dye. Cyclic conjugated dienes generally react with '0, in a [4 +21 addition reaction to yield 1,4-endo-peroxides. In an alternative route Barton and co-~orkers~~ have used the triphenylmethyl cation and other electrophiles as catalysts in the photo-oxygenation of ergosteryl acetate to the peroxide excitation of a diene-catalyst complex allows the formally spin-forbidden reaction of 302with cisoid dienes to yield endo- peroxides.The oxidation of the strained acetylene (63)with triplet oxygen is another example of a reaction involving the conversion of 302 into singlet Attack by '0,on (63)is also feasible and low-temperature reaction produces evidence to suggest a dioxeten intermediate (64),which decomposes with fluorescence above ca. -30"Cto the corresponding a-diketone. One synthetic use of photo-oxidation is the recently reported conversion in high yield of ketones into LY -diketones uia the formation of enamino-ketones and their reaction with 0-0 (63) (64) The role of exciplex formation in the photochemical cycloadditions of aromatic molecules continues to evoke interest. 9,lO-Dicyanoanthracene and methyl-1,2- diphenylcyclopropene-3-carboxylate form an emitting exciplex in benzene solution and irradiation under these conditions leads to formation of the [4 + 21 adduct (65).66 In contrast different products are observed on irradiation in acetonitrile solution and no exciplex emission is seen.As in other example^,^' polar solvents are believed to assist electron transfer in the exciplex producing radical ions. Yang6' presents evidence that the two adducts arising from photocycloaddition of 9-cyanoanthracene to cycloheptatriene are formed by different reaction pathways uiz. a [4+4]con-certed route and a [4 + 21 biradical pathway. The first example of what purports to be an allowed photochemical [m4+,2 +,2] cycloaddition has been reported in the formation of compound (66) (along with anthracene photodimer) by irradiation of anthracene in benzene solution in the presence of q~adricyclane.~~ Exciplex forma- tion is again important in the photochemical behaviour of octafluoronaphthalene (65) (66) 63 D.H. R. Barton R. K. Haynes G. Leclerc P. D. Magnus andI. D. Menzies J.C.S. Perkinl. 1975,2055. 64 N. J. Turro V. Ramamurthy K.-C. Liu A. Krebs and R. Kemper J. Amer. Chem. SOC.,1976,98,6758. 65 H. H. Wasserman and J. L. Ives J. Amer. Chem. SOC.,1976,98 7868. 66 S. Farid and K. A. Brown,J.C.S. Chem. Comm. 1976 564. 67 A. Weller Pure Appl. Chem. 1968,16 11.5. 68 N. C. Yang and K. Srinivasachar J.C.S. Chem. Comm. 1976 48. b9 T. Sasaki K. Kanematsu I. Ando and 0.Yamashita J. Amer. Chem. Soc. 1976,98 2686. 166 H. A.J. Carless towards conjugated diene~.~’ In non-polar solvents (e.g.cyclohexane) 1 :1 adducts of aromatic with diene are formed whereas in polar solvents (e.g.acetonitrile) the yield of adducts is reduced and the formation of diene dimers becomes an important reaction. There is strong evidence that the diene dimers do not result from enhanced intersystem crossing of the exciplex. In contrast with the complex cycloaddition products formed on irradiation of hexafluorobenzene in the presence of cis-cyclo- ~ctene,~~ the photoaddition of hexafluorobenzene in cyclohexane solution to indene or 1,2-dihydronaphthalene results in the stereospecific formation of a single adduct (67) in each case.72 &GF HFF F (67) a; n = 1 b; n=2 Notable examples of aromatic substitution include the first instances of photo- chemical cine-substitution in the reactions of primary and secondary amines with aryl fluoride~’~ and the use of crown ethers and potassium cyanide in acetonitrile for the photochemical cyanation of aromatic^.^^ Two groups have succeeded in intercepting a zwitterionic intermediate in the photorearrangement of 4-pyrones to 2-pyrones by nucleophilic trapping with trifluoroethan01.~~”~ Thus the 4-pyrone (68)leads to the adduct (69) uia trapping of the zwitterion (70).Additionally Barltrop and co-w~rkers~~ have trapped the same species (70) by cycloaddition to furan. In protonated form the zwitterion (70) is also involved in the photorearrangement of hydroxypyrylium cations and can be trapped by sulphuric ac as the cyclic ~ulphate.~~ The first photochemical rearrangement of homotropyliuw cations has been noted in the conversion of the hydroxy-derivative id (71) on irradiation in fluorosulphonic acid at -70 “C into the cation (72).78The 0 0-(68) (69) (70) (71) (72) stereospecific nature of such cyclopropyl migrations around the periphery of the ring is in accord with orbital symmetry predictions.70 J. Libman J.C.S. Chem. Comm. 1976 361 and 363. 71 D. Bryce-Smith A. Gilbert and B. H. Orger Chem. Comm. 1969 800. 72 B. Sket and M. Zupan J.C.S. Chem. Comm. 1976 1053. 73 D. Bryce-Smith A. Gilbert and S. Krestonosich J.C.S. Chem. Comm. 1976,405. 74 R. Beugelmans M.-T. Le Goff J. Pusset and G. Roussi J.C.S. Chem. Comm. 1976 377; Tetrahedron Letters 1976 2305. 75 J.W. Pavlik and L. T. Pauliukonis Tetrahedron Letters 1976 1939. 76 J. A. Barltrop A. C. Day and C. J. Samuel J.C.S. Chem. Comm. 1976,822. 77 J. A. Barltrop A. C. Day and C. J. Samuel J.C.S. Chem. Comm. 1976 823. 7* R. F. Childs and C. V. Rogerson J. Amer. Chem. Soc. 1976,98 6391.