Photolysis/photochemical synthesis carbazoles

Some additional examples are depicted in Scheme 2, such as a convenient synthesis of 1 l f-benzo[a]carbazole (equation 1) [34], synthesis of A-substituted carbazoles (equations 2 and 3) [18, 35], and the use of AA -diarylsul-fonamides (equation 4) to construct ellipticine precursors [36,37]. Interestingly, photolysis of 2-cyanodiphenylamine (254nm, EtOH, MeCN) afforded carbazole in 87% yield, but a similar photochemical reaction of 2-aminophenyl-acetylene gave only a trace of indole (l%-2%) [38]. 

As we saw briefly in Chapter 53, the photochemical version of the Graebe-Ullmann carbazole-carboUne synthesis can be an excellent route to these heterocycles. Similar to this benzotriazole reaction is the photolysis of aryl azides to give carbazoles and related fused indoles. 

In 1951, Smith and Brown were the first to show that the photolysis of aryl azides led to carbazoles (Scheme 17, equation 1) [114], a reaction that also occurs upon heating. Other carbazoles so prepared are 36 to 39. This reaction has been reviewed [115-119], and has been of more interest mechanistically than synthetically, by the groups of Reiser [120, 121], Swenton [122-124], Berry [125], Sundberg [126, 127], Yabe [128-130], Meth-Cohn [131], and Spagnolo [132]. Sundberg provides an excellent summary of the possible mechanisms involved in the photolysis of 2-azidobiphenyl to carbazole [126, 127], and his own work indicates that a triplet nitrene may not be the sole or major carbazole precursor [127]. In any event, the photochemical transformation of aryl azides is a useful synthesis of carbazoles. Some additional examples are shown in Scheme 17 [114, 115, 124, 128, 129, 133]. In addition, Sauer and Engels obtained, as expected, a nearly 1 1 ratio of 1- and 3-methylcarbazoles (85%) on photolysis of 2-azido-3 -methylbiphenyl [134]. 

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