The Photochemistry of Imines By A. C. Pratt DEPARTMENT OF CHEMISTRY, UNIVERSITY OF MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY, MANCHESTER M60 1QD 1 Introduction During the past two decades organic photochemistry has developed into a major branch of organic chemistry. Considerable advances have been made in understanding the mechanisms of light-induced reactions, particularly those of olefinsl and carbonyl compounds,2 and in exploiting these for synthetic pur- poses.3 Under photochemical conditions, a reacting molecule in the excited state has considerably more energy than in the ground state. This, allied to the alteration in electron density distribution which accompanies electronic exci- tation, typically causes it to undergo chemical reactions which are quite different from those encountered for ground-state reactions under thermal conditions.This review is aimed at surveying the major photochemical reactions associated with the C=N group in imines. In general, attention has been confined to com- pounds which have alkyl, aryl, or acyl substituents at nitrogen. The photo- chemistry of hydrazones, azines, oximes and other classes of compounds containing the C-N-X grouping (where X represents a heteroatom) has been excluded, as has that of molecules containing the C=N group as part of an aromatic heterocycle. Mechanistic details are discussed where appropriate. Nitrogen lies between carbon and oxygen in the Periodic Table and it might be expected that the photochemistry of the C=N chromophore would be inter- mediate between that of C=C and C=O.However, though some resemblances do exist, imine photochemistry differs in many respects from that of olefins and carbonyl compounds. For reasons of greater stability and ease of handling, most work in this area has been with C-aryl or N-aryl imines. 2 Excitation Alkyl imines exhibit two major absorption bands in the U.V. region of the spectrum. The short-wavelength absorption at 170-1 80 nm is of high intensity and is believed to involve a v -+ r*transition.4 The less intense band at 230-260 nm is considered to result from an n -+ r* transition.4 The latter wavelength For a review of olefin photochemistry see J. D. Coyle. Chem. SOC.Rev.. 1974, 3, 329. For reviews of carbonyl photochemistry see (a)J.D. Coyle and H. A. J. Carless, Chem. SOC. Rev., 1972, 1, 465; (6) N. J. Turro, J. C. Dalton, K. Dawes, G. Farrington, R. Hautala, D. Morton, M. Niemczyk, and N. Schore, account^ Chem. Res., 1972. 5, 92. For reviews of photochemical reactions in synthesis see (a)P. G. Sammes, Quarr. Rev., 1970, 24. 37; (b) T. Kametani and K. Fukumoto, Accounts Chem. ReJ., 1972, 5, 212; (c) N. J. Turro and G. Schuster, Science, 1975, 187, 303. * G. Wettermark in 'The Chemistry of the Carbon-Nitrogen Double Bond', ed. S. Patai, Interscience, New York. 1970, p. 566. The Photochemistry of Imines region is much more commonly used in photochemistry and the n,r* excited state is the most readily accessible one. Aryl substituted imines absorb at longer wavelengths.4 They exhibit complex spectroscopic behaviour, the number of bands observed and their positions depending on the aromatic substituents present. These bands have been inter- preted as being of the n -+ n* and charge-transfer type.5 Benzylideneaniline (1) exhibits an intense maximum at 252 nm and a shoulder at 315 nm.The expected n -+ n* transition has not been unambiguously assigned and may be concealed by more intense absorption bands.5 The lack of similarity between the absorption spectra of benzylideneaniline and stilbene is quite marked. There is general agreement that this is because benzylideneaniline exists in the E-form in a conformation (1) in which the PhC-N moiety is planar, whilst the plane of the N-phenyl ring makes an angle of approximately 60" with the remainder of the molecule.This allows conju- gation with the lone pair electrons on the nitrogen atom but considerably reduces stilbene-like conjugation between the phenyl rings through the C-N linkage. 3 Geometrical Isomerisation Disubstituted imines exist in the more stable E-form (2).617 Irradiation can be used to effect equilibration with the Z-form (3) in a reaction analogous to olefin photoisomerisation.1 However, the barrier to interconversion is normally so small for imines that at ambient temperatures very rapid complete thermal reversion to the more stable E-form (2) occurs.6 R' R' Ra\C=N \c=N / H/ H/12 Spectroscopic observation of imine photoisomerisation was first noted 20 years ago.8 Irradiation of some diarylimines in solution at -100 "C induced initial dramatic changes in the U.V.spectra of such solutions. Extended irradi- ation led to a photoequilibrium. The spectrum of the E-isomer (2) was restored (a) M. A. El-Bayoumi, M. El-Aasser, and F. Abdel-Halim, J. Anter. Chem. Soc., 1971, 93, G. Wettermark, Svensk Kem. Tidskr., 1967, 79, 249.586; (b) M. El-Aasser, F. Abdel-Halim and M. A. El-Bayoumi, ibid.,1971, 93, 590. 'G. G. McCarthy in ref. 4, p. 363. * (a) E. Fischer and Y. Frei, J. Chern. Phys., 1957, 27, 808; (b) A recent n.m.r. investigation supports the assignment of the Z-configuration to the photoisomers produced from p-di- methylamino-benzylideneanilinesbelow -70 "C: M. Kobayashi, M.Yoshida and H. Minato, J. Org. Chem., 1976, 41, 3323. Pratt on allowing the solutions to warm up. No changes occurred for benzhydryli- deneaniline (4), for which only a single isomeric form can exist.8 Flash photolysis techniques have been used at room temperature to observe geometrical isomer- isation of benzylideneanilines and salicylideneanilines and to follow the rapid thermal reformation of the more stable E-isomers.9 The photochemistry of salicylideneaniline derivatives (5) is complicated by competing reversible photoreactions involving transfer of the phenolic proton to the imino nitrogen with formation of the corresponding o-quinoid forms (6). H IPh CcN\ Rac*NAfPhI \ph OH Photochemical techniques have been used as a practical method for obtaining solutions rich in the less favoured Z-isomer of aryl-N-alkylimines (3; R1 = aryl, R2= alkyl).1° The fact that the configurational stability of oxime ethers is in marked contrast to that of N-aryl and N-alkyl imines, the rate for the thermal Z --+ E intercon-version being very slow, has proved useful in studies of C=N geometrical isomerisation.ll It has been shown from quenching studies with the oxime ethers (7) and (8) that photoequilibration occurred from the singlet excited state on direct irradiation.A triplet-initiated equilibrium could be achieved by sensitization. Ar Ar OMe (7) (8) Ar =Ph,P-naphthyl The question of whether photoisomerization occurs by a torsion mechanism, involving rotation about the carbon-nitrogen double bond, or by an inversion mechanism, which involves transfer of the substituent at nitrogen from one side ¶((a> D.G. Anderson and G. Wettermark, J. Amer. Chem. Soc., 1965, 87, 1433; (b) G. Wettermark, J. Weinstein, J. Sousa, and L. Dogliotti, J. Phys. Chem., 1965, 69, 1584; (c) G. Wettermark, Ark. Kemi, 1967, 27, 159; (d) N. Kanamaru and K. Kimura, Mol. Phofochem., 1973, 5, 427; (e) R. Potashnik and M. Ottolenghi, J. Chem. Phys., 1969, 51, 3671;cf) T. Rosenfeld, M. Ottolenghi, and A. Y. Meyer, Mol. Photochem., 1973, 5, 39. lo H. J. C. Yeh, H. Ziffer, D. M. Jerina, and D. R. Boyd, J. Amer. Chem. SOC.,1973,95,2741. l1 (a) A. Padwa and F. Albrecht, J. Amer. Chem. Soc., 1974, 96,4849; (b) A. Padwa and F-Albrecht, J.Org. Chem., 1974, 39, 2361. 65 The Photochemistry of Imines of the molecule to the other via a linear transition state which retains the carbon- nitrogen double bond intact, is difficult to resolve. It has been proposed12 that the torsion mechanism is facilitated by electron-donating substituents and the inversion mechanism by electron-withdrawing substituents in the aryl ring attached to the imino nitrogen of the azomethine dyes (9) and (10). 4 Photoreduction and Hydrogen Abstraction The photoreduction commonly observed for ketones is initiated by the carbonyl n,n* triplet excited state.2 Aryl-N-alkylimines are also readily photoreduced in hydrogen-donating solvents, e.g. propan-2-01 and ethanol, though imine excited states are not intermediates.Benzophenone imines (1 1 ;R1= Ph) are converted to the corresponding amines (12), whereas benzaldehyde imines (11 ;R1 = H) yield reductive dimerization products (13).13 hvlMe,CHOH hv/Me,CHOH PhCH-NHR' Ph,CH-NHR2 -PhCR1=NRe 1R' = Ph R' = H PhCH-NH R' A 'chemical sensitization' mechanism, involving ground-state reaction of the imines with ketyl radicals, is believed to operate for these photoreductions.l3 Carbonyl compounds present in the reaction mixture as impurity (e.g. from hydrolysis), an added sensitizer, or as a species produced by photo-oxidation, generate the ketyl radicals. Scheme 1 outlines the mechanism for benzophenone- sensitized reductions in propan-2-01. Triplet excited imine could be excluded as a hydrogen abstracting species in the photoreduction of benzophenone-N-methylimine (1 1 ;R1 = Ph, R2 = Me) since, when generated by benzonitrile sensitization, reduction did not OCCUT.~~~In the photoreduction of benzaldehyde-N-benzylimine(1 1 ;R1 = H, R2= CHzPh) using benzophenone sensitization in propan-2-01, the imine quenched benzo- phenone photoreduction but did not quench benzophenone phosphorescence.la W. G. Herkstroeter, J. Amer Chem. SOL.,1976, 98, 330. l3 (a)M. Fischer, Chem. Ber., 1967,100,3599; (b)A. Padwa, W. Bergmark, and D. Pashayan,J. Amw. Chem. SOC.,1968,90,4458; (c) A. Padwa, W. Bergmark, and D. Pashayan, ibid., 1969, 91,2653. Pratt This implies that an intermediate of benzophenone photoreduction (PhzdOH), rather than a triplet excited state of the imine, initiates the red~cti0n.l~~ Thermally generated free radical initiators could also effect reduction, verifying that the hydrogen abstracting species need not be an excited state of the imine.13C Two opposing factors determine whether coupling or disproportionation is observed.13c Coupling introduces steric strain as the trigonal carbon atom in (14) becomes tetrahedral during formation of (15), an effect which increases in severity with increasing steric bulk of the group R1 at the imino carbon atom.Disproportionation, on the other hand, has a high activation energy owing to the extensive electron reorganisation which has to occur as the hydrogen atom is transferred from the radical (14), which is stabilised by overlap of the singly- occupied orbital at carbon with the 'non-bonding' orbital at nitrogen, to form the carbon-nitrogen double bond in (16).This activation energy should be largely hv Me,CHOHPh2C0 3Ph,C0 > Ph,kOH +Me,kOH Ph,dQH + PhCR1=NR2 Ph,CO + PhkRLNHR'_______f (1 1) Me,kOH + PhCR1=NR2 P Me,CO + PheR1-NHRZ R' = Ph2 PhdR1-NHR2 Disproportionation' PhCHRl-NHR' + PhCR'=NRe (14) , (16) R1= H PhCRl-NHR' PhCRl-NHR' Scheme 1 independent of the size of the substituent R1. For the hindered diphenylamino- methyl radicals (14; R1 = Ph) strain due to coupling would be severe, so the disproportionation pathway is followed. With the much less hindered phenyl- aminomethyl radicals (14; R1 = H) there is negligible coupling strain, so coupling is observed.To explain the low r2activity of excited imines in hydrogen abstraction, Fischer suggested that the rate of the hydrogen abstraction reaction was very sIow.~~~Padwa and co-workers have suggested, as an alternative explanation, that exited imines undergo very rapid radiationless decay to ground state, possibly because of ready E-Z isomerism about the carbon-nitrogen double bond, with the result that hydrogen abstraction is prevented.14 Incorporation of the imino-group into a ring system should prevent deacti- vation of the imine excited state by twisting. Hornback and co-workers compared l4 A. Padwa, M.Dharan, J. Smolanoff, and S. I. Wetmore, Pure Appl. Cheni., 1973, 33,269. 67 The Photochemistry of Imines the photoreduction of the acylic imine (17) with that of the cyclic compound (18) and showed that both imines underwent very inefficient excited state reductive H dimerization at essentially equal rates under conditions designed to exclude a ‘chemical sensitization’ mechanism.This suggests that the low reactivity of these imines is not due to rapid radiationless decay involving twisting about the carbon-nitrogen double bond but is more likely a consequence of low rates for the hydrogen abstraction reaction.15 Phosphorescence studies have suggested16 that the cyclic imines (19) and (20), which do not undergo excited state hydrogen abstraction, have T,T* lowest Ph R (19) (20) R = CH2Ph, Ph, Me triplet excited states. It has been proposed that possession of a T,T*lowest triplet excited state may account for the low reactivity of excited imines in hydrogen abstraction reactions.16 Inefficient intramolecular 7-hydrogen abstraction may occur for the sub- stituted cyclic imine (21). Low yields of 1-methyl-3,4-dihydroisoquinoline(22) were suggested1’ to arise by a process (Scheme 2) analogous to the Norrish Type IT photocleavage of carbonyl compounds.2 In an inert solvent in the presence of a sensitizer, an imine may act as the hydrogen donor.18 Benzonitrile and tert-butylbenzene were obtained when imine (23) was irradiated in benzene in the presence of benzophenone.18a They were proposed to result from cleavage of the radical (24) produced by abstraction of the imino hydrogen by triplet excited benzophenone. In a related reaction, Baum and Karnischky have isolated oxazolidines (25) from irradiation of aryl ketones and isopropylidene isopropylamine (26) in benzene.Initial abstraction of the tertiary hydrogen atom of (26) produced the stabilized radical (27). Coupling of l5 J. M. Hornback, G. S. Proehl, and 1. J. Starner, J. Org. Chem., 1975, 40, 1077. l8 H. Ohta and K. Tokumaru, Tetrahedron Lefters, 1974, 2965. l7 Y.Ogata and K. Takagi, Tetrahedron, 1971, 27, 2785. (a) H. Ohta and K. Tokumaru, Chem. and Znd. (London), 1971, 1301; (6) A. A. Baum and L. A. Karnischky, J. Amer. Chem. SOC.,1973, 95, 3072. Pratt hV H H Me Scheme 2 hv -PhCN bCnZene PhCH=NCMe, 2PhC=NCMe, Me& -PhCMe,Ph&O (23) (24) (27) with the ketyl radical (28), followed by cyclization of imino alcohol (29) produced the observed oxazolidines (25).lgb Me,C=NCHMe, + 'PhCOR -MeaC=NeMe, + PheROH N-Acylimines contain the C-N-C=O chromophore and are aza analogues of a,P-unsaturated carbonyl compounds whose photochemistry has been extensively investigated.lg N-Acylphenylketimines (30) undergo saturation of the carbon-nitrogen double bond when irradiated in propan-2-oL20 l8 P.E. Eaton, Accounts Chem. Res., 1968, 1, 50. *O (a) T. Okada, K. Saeki, M. Kawanisi and H. Nozaki, Tetruhedrm, 1970, 26, 3661 ;(6) A. Padwa and W. P. Koehn, J. Org. Chem., 1975,40, 1896. The Photochemistry of Imines hvPhCR1=NCOR2 PhCHRtNHCORS (30) The mechanism operating in the case of N-aroylphenylketimines of the type (30; R2 = aryl) appears to depend on the identity of the group R1 at the imino carbon atom.Thus, Padwa and co-workers206 demonstrated a ‘chemical sensitization’ mechanism (cf.Scheme 1) for compounds with a phenyl substituent at this position, whereas with alkyl substitution the acylimine n,n* triplet excited state was believed206 to be responsible for initiating the observed photo- reduction, with transfer of hydrogen from the solvent to the carbonyl oxygen initially rather than to the imino nitrogen. The low quantum yields observed for these imine excited state reactions were shown to be due to small bimolecular hydrogen-abstraction rates (ca. 100 times less than that of benzophenone) operating in conjunction with very rapid triplet aecay rates (ca.50 times greater than that of benzophenone).zob This situation is similar to that found for a$-unsaturated ketones.lg Those N-acylimines which are photoreduced by a ‘chemical sensitization’ mechanism may possibly have lowest T,T*triplet excited states or a negligible intersystem crossing efficiency from the singlet excited state. In solutions of hydrocarbons containing allylic or benzylic hydrogens, e.g. cyclohexene or toluene, N-acetyldiphenylketimine(3 1) underwent photoaddition of the hydrocarbon to the carbon-nitrogen double bond.21 In toluene, the adduct (32) was obtained. An intramolecular version of this hydrogen abstraction OH hv I PhaC=NCOMe ___) PhaC-N-CMe + Ph6HI -Ph2C-NHCOMePhMe I (3 1) CH,Ph reaction has been reported. Irradiation of o-methyl or o-benzyl compounds (33) in [0-2H]methanol led to incorporation of deuterium.22 A mechanism similar to that known to operate for the photoenolization reactions of o-alkyl-substituted aromatic carbonyl compounds2 has been proposed (Scheme 3).7-Hydrogen abstraction transfers a hydrogen from the o-alkyl substituent to the imino nitrogen with formation of the o-quinodimethane (34). Subsequent exchange *I (a)N. Toshima, S. Asao, K. Takada and H. Hirai, Tetrahedron Letters, 1970, 5123; (h) H. Hirai, N. Toshima, S. Asao, and K. Takada, Chem. AbA., 1973, 78, 124309; (c) H. Hirai, N. Toshima and S. Asao, ibid., 1973, 79, 5145; (d)H. Hirai, N. Kojima, and S.Asao, ibid., 1974, 81, 13247; (e) S. Asao, N. Toshima and H. Hirai, Bull. Chem. SOC.,Japan, 1976, 49, 224 report that a triplet excited state of (31) is involved in the photoaddition of tolu-ene. ‘a M. Saeki, N. Toshima, and H. Hirai, Bull. Chem. SOC.,Japan, 1975, 48 ,478. Pratt Ph Ph I (33) R = H, Ph hvi Ph Ph MeOD, ,'@ NDAc ' 'CHR (34) (35) Scheme 3 of the mobile amino hydrogen for deuterium forms the species (35) which, by rapid thermal 1,5-transfer of deuterium, generates (36) differing from imine (33) only by the presence of deuterium at the benzylic position of the o-alkyl group.2z 5 Photocyclization of N-Benzylideneanilines In olefin photochemistry the singlet r,r*excited state cyclization of stilbenes to dihydrophenanthrenes is well known, and under oxidative conditions provides a useful synthetic route to phenanthrenes.1 Early efforts to achieve a similar oxidative photoconversion of N-benzylideneaniline (37) to phenanthridine (38) in conventional organic solvents were unsucce~sful.~~ N-Benzylideneaniline (37) exists as the E-isomer, and the concentration of Z-isomer (39) necessary to capture sufficient light for successful cyclization to the dihydro-compound (40) could not be achieved by room temperature photoequilibration. At lower temperatures the rate of the thermal Z -, E reaction is decreased and the steady- state concentration of the Z-isomer is increased.Irradiation at -10 "C,in the presence of dissolved oxygen or iodine as oxidizing agent, achieved a 2% yield of phenanthridine (38).24 N-Benzhydrylideneaniline(41), which does not need to isomerize prior to cyclizat ion, cyclized succe~sfully~~ to yield the expected 2-phenylbenzo [3,4]quinoline (42).The quantum yield for this conversion was, however, merely times that for oxidative photocyclization of the related triphenylethylene (431, which must, of necessity, undergo cyclization from a v,v* excited state. It was suggested24 that, by analogy with N-benzylideneaniline (37) whose lowest singlet excited state is probably n,r* in ~haracter,~~ the imine (41) "(a) E. V. Blackburn and C. J. Timmons, Quart. Rev., 1969, 23, 482; (6) P. Hugelshofer, J. Kalvoda, and K. Schaffner. Hdv. Chim. Acta, 1960, 43, 1322. p4 F. B. Mallory and C.S. Wood, Tetrahedron Letiers, 1965, 2643. 96 (a)H. H. Jaffk.,S. J. Yeh, and R. W. Gardner, J. Mof.Specrroscopy, 1958,2, 120; (b)K.H. Grellmann and E. Tauer, J. Amer. Chem. SOC.,1973, 95, 3104. The Photochemistry of Imines (37) (39) Ph Ph Phhv Ph Ph has a lowest ln,rr* excited state and that cyclization from the h,n* state has to compete with rapid internal conversion to the lower energy inactive In,n* state. This suggestion has received support from the reports that N-benzylideneaniline (37)26 and many substituted N-benzylideneaniline~~~ cyclize readily to the corres- ponding benzo [3,4]quinoline derivatives in strongly acidic media in which the protonated imine (44)is undergoing cyclisation. Since the nitrogen lone-pair electrons are occupied in bonding to the proton, cation (44)must have a lowest k,n* excited state.Not all substituted N-benzylideneanilines, however, require acidic media for successful cyclization. Thus N-(1 -naphthylidene)-1 -naphthylamine (45) and the trialkoxy-compound (46) readily form the expected oxidative ring closure products in aerated ethanol and diethyl ether, respectively.2* These compounds may possibly possess lowest 1n,n* excited states. A related photocyclization which yields the 1 ,Zdihydrobenzo [3,4]quinoline (47) has been reported for N-(2-propylidene)-2-aminobiphenyl(48). The isolated 26 G. M. Badger, C. P. Joshua, and G. E. Lewis, Tetrahedron Letters, 1964, 371 1. (u) M. Scholz, F. Dietz, and M. Miihlstiidt, Tetrahedron Letters, 1970, 2835; (b) H.H. Perkampus and B. Behjati, J. Heterocyclic Chem., 1974, 11, 511. 2s (a) M. P. Cava and R. H. Schlessinger, Tetrahedron Letters, 1964, 2109; (b) T. Onaka, Y. Randa and M. Natsume, ibid., 1974, 1179. Pratt + OMePhCH=NPh H (4) product (47) may be formed by a thermally-allowed 1,5-hydrogen shift from (49), the initial cyclisation product.29 Me Me Me Me Me 6 Alkylative Photocyclizations of Imines When N-arylidene-2-naphthylamines(50) are irradiated in alcohols of the type RCH2CHzOH under oxidative conditions, formation of 2-arylbenzo[5,6]-quinolines (51) occurs with incorporation of the alkyl group of the solvent and hv IICHAr cyclization on to the N-substituted ring exclusively in the a-position.30 Where the a-position is blocked, reaction occurs at the p-position, e.g.(52) -+ (53).30d Incorporation and cyclization on to a phenyl ring, rather than naphthyl, may be achieved by using an aniline-derived imine, e.g. (54) 3 (55).30@ The two-carbon fragment incorporated as part of the new six-membered ring 2D J. S. Swenton, T. J. Ikeler, and G. L. Smyser, J. Org. Chem., 1973, 38, 1157. ao (a) J. S. Shannon, H. Silbennan, and S. Sternhell, Tetrahedron Letters, 1964, 659; (b) P.f. Collin, H. Silberman, S. Sternhell, and G. Sugowdz, ibid., 1965, 2063; (c) P. J. Collin, J. S. Shannon, H. Silberman, S. Sternhell, and G. Sugowdz, Tetrahedron, 1968, 24, 3069; (d) M. Scholz. H. Herzschuh, and M. Miihlstadt, Tetrahedron Letters, 1968, 3685.The Photochemistry of Imines I I Ph Ph (52) (53) in these alkylative cyclizations is probably derived from the aldehyde RCH2CHO formed by photo-oxidation of the solvent RCHBCH~OH.~~ A detailed mechanism involving an initial 'chemical sensitization' step has been proposed31 for these reactions. 7 (2 + 2) Photocycloadditions to Imines In contrast to the situation found for olefinsl and carbonyl compounds,2 which readily undergo (2 + 2) photocycloaddition reactions to form cyclobutanes and oxetans respectively, there appears to be no undisputed example of four-membered ring formation by intermolecular photocycloaddition to a simple alkyl or aryl substituted imine.32 It has been suggested36 that the presence of a conjugated electron-withdrawing group may be necessary for successful photocycloaddition to the carbon-nitrogen double bond.N-Acylimines, aza-analogues of a,P-unsaturated ketones, have been shown to undergo (2 + 2) photoaddition to olefins, a reaction typical of enone photo- cherni~try.~~For example, the keto-imine (56) undergoes triplet excited-state addition to 0lefins,3~1,l-dimethoxyethylene adding to form the azetidine (57). A similar reaction with olefins is observed for (58);38 in the absence of added olefin an a-cleavage reaction (see Section 8) occurs with formation of epoxyisocyanate (59).38 s1 F. R. Stermitz, R. P. Seiber, and D. E. Nicodem, J. Org. Chem., 1968,33, 1136. 'I A claim33 that a 1,3-diazetidine was formed by (2 + 2) photodimerisation of benzaldehyde- N-cyclohexylimine has been disproved, 136 and the intermediacy of a 1 ,2-diazetidine in the photofragmentation of N-(4-dimethylamin0benzyIidene)-aniline~~is in doubt since an attempt to repeat the reaction met with fai1u1-e.~~ 35 R.0.Kan and R.L. Furey, J. Amer. Chem. SOC.,1968.90, 1666. 34 S. Searles and R. A. Clasen, TetrahPdron Letters. 1965. 1627. as D.R.Arnold, V. Y.Abraitys, and D. McLeod, Canad.J. Clzrm., 1971, 49, 923. ssJ. S. Swenton, and J. A. Hyatt, J. Amer. Chem. SOC.,1974,96, 4879. ST K.A. Howard and T. H. Koch, J. Amer. Chem. SOC.,1975.97, 7288. R. M. Rodenhorst and T. H. Koch, J. Amer. Chem. SOC.,1975,97, 7298. Pratt It has been suggested that N-acylimines which undergo (2 + 2) photocyclo-addition do so from reactive T,T*excited states.38 8 a-Cleavage of N-Acylimines In contrast to the acylimines (56) and (58), imines (60) and (61) (Scheme 4) do n= 1.2 iH2 + +N=C=O CHa OEt (60;n = 1) (63) (61; n = 2) 0 0 CH, (65) Scheme 4 not undergo photoaddition reactions.Instead, a-cleavage reactions occur from the singlet n,m* excited state.39The n -+ n* transition for N-acylimines is in the 265-285 nm region and hence sufficient energy is available for cleavage of the carbon-carbon single bond adjacent to the carbonyl group in (60) and (61).39 a,P-Unsaturated ketones have lower ln,m* excited state energies and do not normally exhibit a-cleavage.lg The acylimine (60) is converted to the cyclopropyl isocyanate (62) and imine (61) yields an unsaturated isocyanate (63) on irradiation.39 The first step in each 39 T.H. Koch, R.J. Sluski, and R. H. Moseley, J. Amer. Chem. SOC.,1973, 95, 3957. The Photochemistry of Imines of these reactions (Scheme 4) is analogous to the Norrish Type I reaction often observed in the photochemistry of non-conjugated carbonyl compounds2 and typically initiated from an n,n* excited state. In the case of (61) a competing reaction, which yields glutarimide (64), is also observed.39 This is initiated by y-hydrogen abstraction from the alkyl side chain, followed by cleavage of the 1,4-biradical (65) so formed, in a manner analogous to the Norrish Type I1 reaction2 of n,r* excited carbonyl compounds. The aroylimine (56), because of aryl conjugation with the carbonyl group, has a strong a-bond and does not exhibit a-cleavage reactions.37 However, imine (58) does undergo a-cleavage to form epoxyisocyanate (59),38 though the reaction, analogous to the conversion of (60) -+ (62) in Scheme 4, cannot compete with the much faster (2 + 2) photoaddition which is observed in the presence of added 1,l-dimethoxyethylene.38 The N-acylimine (66)undergoes an analogous a-cleavage reaction with formation of (67).40 Me Me Me /Me N=C=O Me Me 9 Electrocyclic Reactions of Conjugated Imines Conjugated imines may be considered as aza-analogues of butadiene and hexa- triene.Theory predicts41 that the electrocyclic reactions of 2-azabutadiene and of aza- and diaza-hexatrienes should proceed along pathways similar to those found for the corresponding hydrocarbons. The conjugated cyclic imine (68) has been reported42 to undergo photocycli- zation to the l-azetine (69).In a related reaction, the imine (70) was postulated43 to undergo triplet excited state ring closure to yield (71). The l-azetine (71) was 40 T. Sasaki, S. Eguchi, and M. Ohno, J. Anter. Chem. SOC.,1970, 92, 3192. 41 Z. Neiman, J.C.S. Perkin ZI, 1972, 1746. C. Lohse, Tetrahedron Letters, 1968, 5625. 43T.H. Koch and D. A. Brown, J. Org. Chem., 1971,36, 1934 Pratt not isolated but was converted to the products (72) or (73) under acidic con- ditions or when formed in the presence of methoxide ion, respectively.43 The OEt MeMea:HO IMe Me imine (74), which is closely related to (70), undergoes singlet excited state conversion to the products (75) and (76).These compounds were believed44 to be formed from the bicyclic intermediate (77). The l-azabutadiene (78) does not give products of electrocyclic ring closure. Rather, on irradiation in the presence of a sensitizer, geometrical isomerization about the carbon-carbon double bond is believed to occur and the products obtained arise from ground-state reactions of the severely strained, and consequently highly reactive, isomer (79).45 cq:rMeooMe ->hv ___) Products Me Me In the case of the 3H-azepines (80),two modes of closure are possible. On irradiation, the cyclobutene (81) was the sole product isolated.46 The retention of the N=C-X resonance energy in the formation of (81) may have been the decisive factor in determining the observed ~electivity.4~ Formation of the 4* E.Lerner, R. A. Odum, and B. Schmall, J.C.S. Chent. Cumm., 1973, 327. 4b T. H. Koch, M. A. Geigel, and C. Tsai, J. Org. Chem., 1973, 38, 1090. 40 R.A. Odum and B. Schmall, J.C.S. Chem. Cumm.,1969, 1299. The Photochemistry of Imines alternative product (82) would have necessitated loss of such resonance energy. (80; X = NMe,, NH,, OEt) Irradiation of cyclohexa-l,3-dienes commonly results in electrocyclic ring opening to form linear hexatrienes.l A similar conversion occurred in the excited state reactions of 2,3-dihydropyrazines (83).35947 Photochemically-allowed conrotatory ring-opening formed the corresponding enedi-imines (84) in benzene solution.In methanol or ethanol, rapid cyclization of the enedi-imines (84) occurred with formation of ylide intermediates (85). A proton transfer sub- sequently completed formation of imidazoles (86), the isolated products. Cycli- zation of the enedi-imines (84) was facilitated by the ability of the hydroxylic solvents to stabilize the developing charge centres by solvation. The optically active 2,5-dihydropyrazine (87) underwent photoracemization in methanoL4* A concerted (2 + 2 + 2 + 2) reaction may be involved in the conversion of (87) to its enantiomer (88). 10 1,3-Dipole Formation by 2H-Azirines The photochemistry of 2H-azirines (89) has been extensively studied by the schools of Padwal4 and Schmid.49 The bulk of the work reported has been with ’’(a)P.Beak and J. L. Miesel, J. Amer. Chem. Soc., 1967, 89, 2375; (b) A. Padwa, S. Clough,and E. Glazer, ibid., 1970, 92, 1778; (c) A. Padwa and E. Glazer, ihid., 1972, 94, 7788. 48 D. G. Farnum and G. R. Carlson, J. Amer. Chem. SOC.,1970, 92, 6700. Claus, Th. Doppler, N. Gaskis, M. Georgarakis, H. Giezendanner, P. Gilgen, H. Heimgartner, B. Jackson, M. Marky,N. S. Narasimhan, H. P. Rosenkranz, A. Wunderli, H.-J. Hansen, and H. Schmid, Pure Appl. Chem., 1973,33, 339. Pratt 2-substituted-3-phenyl-2H-azirines(89; R1 = Ph), though the photochemistry appears to be largely independent of the presence or absence of aryl or alkyl substitution on the three-membered ring.Cleavage of the carbon-carbon bond in the electronically excited 2H-azirine ring forms a nitrile ylide, (89) 4 (90).14149 The reaction is initiated from the 2H-azirine singlet excited state50 and the 1,3-dipolar species (90)has been observed spectroscopically at low temperatures in a hydrocarbon rnatrix.5l The 1,Zdipole (90) may be intercepted by a wide variety of dip~larophiles,~~ X=Y,present during irradiation of the 2H-azirine (89). Formation of numerous five-membered nitrogen heterocycles (91) has been achieved in this manner.53 Alternatively, a protic solvent may be used to trap the lY3-dipole; e.g. methanol converts (90) to (92).54 H OMe (92)11 Photo-oxidation Ketones, amides, and benzoic acid have been isolated from the products of irradiation of C-arylimines in oxygenated propan-2-01.55 A chemical sensitization mechanism (Scheme 9,related to that proposed for imine photoreduction, has PhC=NR2 Phk-NHR' Ph R' = H I -I -% R~+NHR~ -PhC0.NHR2 R1 R' 0-0' amide PhPhC0,H R' = H f--k0OPbenzoic acid R' ketone (R1# H) Scheme 5 60A.Padwa, M.Dharan, J. Smolanoff, and S. I. Wetmore, J. Amer. Chem. SOC.,1973, 95, 1945. s1 W. Sieber, P. Gilgen, S. Chaloupka, H.-J. Hansen, and H. Schmid, Helv. Chim. Acta, 1973, 56, 1679. 62 R. Huisgen, Helv. Chim. Acta, 1967, 50, 2421. 63 For leading references see (a) W. Stegmann, P. Gilgen, H. Heimgartner and H. Schmid, Helv. Chim. Acta, 1976, 59, 1018 and (b)A. Padwa, A. Ku, A. Mazxu, and S. I. Wetmore, J. Amer. Chem.SOC., 1976, 98, 1048. 64 A. Padwa, J. K. Rasmussen, and A. Tremper, J. Amer. Chem. SOC.,1976, 98, 2605. 65 N. Toshima and H. Hirai, Tetrahedron Letters, 1970, 433. 4 79 The Photochemistry of Imines been proposed. The observation that acetophenone, which possesses a hydrogen- abstracting triplet excited state, sensitizes the oxidations whereas l-acetyl- naphthalene, which is inactive in hydrogen-abstracting reactions, fails to act as a sensitizer is consistent with this mechanism. The intramolecular oxidation of the carbon-nitrogen double bond of 2- nitrobenzylideneanilines (93) is one of the earliest photochemical reactions reported for imine~.~6 Formation of the corresponding 2-nitrosobenzanilide (94) is believed to involve the intermediacy of a short-lived o-quinone imine derivative (93.57 12 Photohydrolysis Benzylideneaniline yields benzaldehyde and aniline when irradiated in the presence of benzophenone in oxygenated ethanol or propan-2-01.58 Hydrogen peroxide is produced under these conditions and it, or hydroxyl radicals derived from it, was believed to be the agent responsible for the hydrolysis (Scheme 6).PhCH-NPh PhCH-NHPh PhCHOHZO, PhCH= NPh* -Me,CHOH --3+or -I I OH OH PhNHzOH Scheme 6 13 Vision In vision, one of the key steps in the conversion of a light signal received at the photoreceptors in the retina to an electrical pulse to be transmitted to the brain by the optic nerve, involves photoisomerization of the protonated imine (96) to the geometrical isomer (97).The imine (96) is formed following condensation of 1l-cis-retinal with the visual protein ~psin.~~ (a) F. Sachs and R. Kempf, Ber., 1902,35,2704; (b)A. Senier and R. Clarke, J. Chem. SOC., 1914, 105, 1917. s7 E. Hadjoudis and E. Hayon, J. Phys. Chem., 1970,74,2225. I6 R. L. Furey and R. 0.Kan, Tetrahedron, 1968.24, 3085. ss Aspects of the chemistry of vision are discussed in a series of five review articles in Accounts Chem. Res., 1975, 8, 81-112. Pratt Me Me Me Me Me Me hv4 Me