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-C6H4)(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 pi-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 pi-complexation-mediated by a single ruthenium(II) catalyst.