Rhodium-catalyzed annulation reactions of 2-alkynylbenzaldehydes and 2-vinylbenzaldehyde with unsaturated compounds

Abstract

2-Alkynylbenzaldehydes are attractive substrates for organic reactions, because they possess two reactive functional groups, the formyl and alkynyl groups. It is well known that cationic rhodium(I) complexes are able to activate the aldehyde C-H bond through oxidative addition, and the alkyne triple bond through π - complexation. In this account, I describe the rhodium(I)-catalyzed [4+2] annulation reactions of 2-alkynylbenzaldehydes with unsaturated compounds which proceed via benzo-fused five-membered acylrhodacycles, generated through aldehyde C-H bond activation followed by intramolecular cis addition of rhodium acyl hydrides to the alkyne triple bond. Analogous [4+2] annulation reactions of 2-vinylbenzaldehyde with unsaturated compounds are also described. Furthermore, the asymmetric annulation of 2-alkynylbenzaldehydes with cyclic dicarbonyl compounds catalyzed cooperatively by rhodium(I) and silver(I) complexes is also described. This unique annulation presumably proceeds via formation of benzopyrilium intermediates after alkyne activation and subsequent ketone hydroacylation through aldehyde C-H bond activation.