Investigation of the possible role of arylamine formation in the ortho-substituted nitroarenes reductive cyclization reactions to afford heterocycles

Reductive carbonylation of nitroarenes with an unsaturated group in the ortho position generally affords high yields of heterocyclic compounds, including indoles, chalcones, quinolines, benzimidazoles, benzotriazoles, and indazoles. There is, however, no direct information on the organic intermediates formed during the reaction. In this paper, we have examined the possible formation of anilines as intermediates in the synthesis of indoles, acylindoles, quinolines, and quinolones with several catalytic systems based on palladium, ruthenium and rhodium. It transpired that, independent of the metal complex used as a catalyst, the formation of aniline as an intermediate is essential in the intra-molecular condensation reactions of nitroarenes involving an aldehydic or a keto group (including Michael addition reactions), but plays at best a very limited role in the addition reactions involving a C=C double bond. (C) 1999 Elsevier Science S.A. All rights reserved.