Ruthenium-Catalyzed Carboxylative Cyclization of 1,6-Diynes

A ruthenium-catalyzed carboxylative cyclization of 1,6-diynes has been developed. In the presence of catalytic amounts of [Ru( p -cymeme)Cl 2 ] 2 (2.5 mol %), P(4-F-C 6 H 4 ) 3 (7.5 mol %) and 4-dimethylaminopyridine (10 mol %), a variety of carboxylic acids condense with 1,6-terminal diynes to give rise to cyclohexylidene enol carboxylates as the products with exclusive ( E )-selectivity. A set of control experiments suggests that the reaction proceeds through a mechanism involving an anti attack of the carboxylate nucleophile on the π-alkyne coordinated to a ruthenium vinylidene complex. Thus, the new reaction achieves C−C bond formation through a dual mode of alkyne activation vinylidene formation and π-complexation mediated by a single ruthenium(II) catalyst.