Formation of Acyl-Substituted Nitrile Ylides by Rh2(OAc)4-Catalyzed Decomposition of α-Diazocarbonyl Compounds in Nitriles

The Rh2(OAc)4-catalyzed reactions of α-diazocarbonyl compounds in nitrile in the presence of dimethyl acetylenedicarboxylate (DMAD) gave oxazole and pyrrole derivatives. The formation of the oxazole derivatives is explained in terms of the 1,5-cyclization of an acyl-substituted nitrile ylide intermediate, and the formation of the pyrrole derivatives is explained by the 1,3-dipolar cycloaddition of the same intermediate with DMAD. The regiochemistry of the cycloaddition of the acyl-substituted nitrile ylide with methyl propiolate showed that the contribution of an allenyl-type resonance structure plays an important role in the acyl-substituted nitrile ylide reaction.