0-Nitrobiphenyls, reductive cyclization

Amino-2 -nitrobiphenyls, which may be intermediates in some of the reductions, cyclize to benzo[c]cinnoline oxides on treatment with bases. This reaction provides a route to certain substituted monoxides of known orientation [Eq. (3)]. ... 

This paragraph details carbazole syntheses that involve the formation of new bonds to nitrogen. A direct C-H functionalization/amination of 2-acetaminobiphenyl compounds give carbazoles in one step <05JA14560>. For example, treatment of biphenyl 130 with palladium acetate and copper(II) acetate gave carbazole 131. A short s)mthesis of carbazoles involved the reductive cyclization of 2-nitrobiphenyl compounds mediated by triphenylphosphine... 

Sanz, Amaiz, and colleagues discovered that a dichlorodioxomolybdenum(VI) complex can reductively cyclize nitrobiphenyls to carbazoles and nitrostyrenes to indoles (Scheme 3, equations 1 and 2) [ 12]. The Mo-catalyzed cyclization was applied to the synthesis of 2-carbonyl indoles in one pot from o-nitrobenzaldehydes (equation 3) [12]. Waldvogel and colleagues used MoCl,/ TiCl to synthesize carbazoles via an oxidative cyclization of diaryl amines (equation 4) [13]. These conditions were superior to TiCl, FeClj, and Pd(OAc)j/HOAc. A Pauson-Khand reaction using Co(CO) was employed by Saito s team to prepare lff-pyrrolo[2,3-(>]indol-2-ones (equation 5) [8]. 

Davies reported that carbazoles 39 were obtained from 2-nitrobiphenyls 38, which are the same substrates in Cadogan cyclization, via the palladium-catalyzed reductive cyclization under carbon monoxide (70 psi) as a reductant (Scheme 23.16) [23]. Amide and chloride groups were tolerated, leading to the corresponding carbazoles. Carbon dioxide is the only stoichiometric by-product, which is an advantage over Cadogan cyclization. 

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