Abstract
Exposure of aldehydes or α-ketoesters to equal volumes of acetylene and hydrogen gas at ambient temperature and pressure in the presence of cationic rhodium catalysts provides products of carbonyl Z-butadienylation, which arise via multicomponent coupling of four molecules: two molecules of acetylene, a molecule of vicinal dicarbonyl compound, and a molecule of elemental hydrogen. The collective data suggest a catalytic mechanism involving carbonyl insertion into a cationic rhodacyclopentadiene intermediate derived via oxidative dimerization of acetylene. Hydrogenolytic cleavage of the resulting oxarhodacycloheptadiene via formal σ-bond metathesis provides the product of carbonyl addition and cationic rhodium(I) to close the catalytic cycle. Studies involving the hydrogenation of 1,6-diyne 14a in the presence of α-ketoester 6a corroborate the proposed catalytic mechanism. These multicomponent couplings represent the first use of acetylene gas, a basic chemical feedstock, in metal-catalyzed reductive C−C bond formation.

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