Improved synthesis of pyrroles and indoles via Lewis acid-catalyzed Mukaiyama-Michael-type addition/heterocyclization of enolsilyl derivatives on 1,2-diaza-1,3-butadienes. Role of the catalyst in the reaction mechanism

Abstract

The Mukaiyama-Michael-type addition of various silyl ketene acetáis or silyl enol ethers on some 1,2-diaza-1,3-butadienes proceeds at room temperature in the presence of catalytic amounts of Lewis acid affording by heterocyclization 1-aminopyrrol-2-ones and 1-aminopyrroles, respectively. 1Aminoindoles have been also obtained by the same addition of 2-(trimethylsilyloxy)-1,3-cyclohexadiene on some 1,2-diaza-1,3-butadienes and subsequent aromatization. Mechanistic investigations indicate the coordination by Lewis acid of the enolsilyl derivative and its 1,4-addition on the azo-ene system of 1,2-diaza-1,3-butadienes. The migration of the silyl group from a hydrazonic to an amidic nitrogen, its acidic cleavage and the final internal heterocyclization give the final products. Based on NMR studies and ab initio calculations, a plausible explanation for the migration of the silyl protecting group is presented. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.