Photocyclizations forming carbazoles

Nonoxidative photocyclization also occurs in 2-diphenyl isocyanate (367) to form carbazole (368) and phenanthridone (369).390 The mechanism of this transformation is uncertain, and, in fact, little attention has been given to the photochemistry of isocyanates. Carbazole is also the principal product of nonsensitized photolysis of 2-azidobiphenyl391 the reaction presumably involves the cyclization of an intermediate nitrene. 

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

In the presence of tetracyanoethylene (TCNE), photocyclization of 2-azido-biphenyi gives carbazole as the main product, but an appreciable amount of a spiroazepine is also formed. Reviews of the chemistry [3746] of TCNE and of its use in cycloadditions [3244] are available. The synthesis of an indole by thermolysis of a previously prepared side-chain azide (see p. 301) is a widely used method but such an azide may also be formed in situ, as in this conversion of a benzaldehyde to an indole-2-carboxylic ester. 

Two classes of photocyclization reactions of indoles are well exemplified in the literature. One is based on a 6-ti electrocyclic closure of trienes to cyclohexadienes where the indole C2-C3 bond forms part of the triene. The other involves photolysis of m-haloalkylindoles whereupon the halogen-substituted carbon becomes attached to an indole ring position. The first example of the former reaction was reported by Szmuszkovicz [75], who found that UV light irradiation of 2,3-bis-(para-methoxyphenyl)indole in the presence of iodine gave the carbazole 71 (Scheme 37). By analogy with the photocyclization reaction of other 1,2-diarylethenes [76], this reaction presumably proceeds via conrotatory closure of the indole singlet excited... 

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