A review. 2-(Alkynyl)benzaldehyde derivs. are attractive substrates for org. reactions, because they possess two reactive functional groups, a formyl group and an alkynyl group. It is well known that cationic rhodium(I) complexes are able to activate the aldehyde carbon-hydrogen bon d (C-H bond) through oxidative addn., and the alkyne triple bond through ﾏ�complexation. In this account, the author describes a rhodium(I)-catalyzed [4+2] annulation reaction of 2-alkynylbenzaldehydes with unsatd. compds. which proceed by formation of a benzo-fused five-membered acyl-rhodacyclic compd., generated through aldehyde C-H bond activation followed by intramol. cis-addn. of rhodium acyl hydrides to an alkyne triple bond. Analogous [4+2] annulation reactions of 2-vinylbenzaldehyde with unsatd. compds. are also described. Furthermore, an asym. annulation of 2-alkynylbenzaldehydes with cyclic dicarbonyl compds. catalyzed cooperatively by rhodium(I) and silver(I) complexes is also described. This unique annulation presumably proceeds by formation of benzopyrylium intermediates after alkyne activation and subsequent ketone hydroacylation through aldehyde C-H bond activation. [on SciFinder(R)]
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