E-Selective dimerization of phenylacetylene catalyzed by cationic tris(μ-hydroxo)diruthenium(II) complex and the mechanistic insight: The role of two ruthenium centers in catalysis

A dinuclear complex [(Me₃P)₃Ru(μ-OH)₃Ru(PMe₃)₃]⁺[OPh]⁻ (1) (0.5 mol%) catalyzes E-selective dimerization of phenylacetylene, which involves the C–H bond cleavage of phenylacetylene and resulting stereospecific C–C bond forming reaction, at 100 °C for 2 h to give (E)-1,4-diphenylbut-3-en-1-yne in quantitative yield (E/Z = 91/9). Similar reactions using 4-nitro-, 4-cyano-, 4-trifluoromethyl-, 4-acetyl-, and 4-methylphenylacetylene give the corresponding enynes. The kinetic study for dimerization of phenylacetylene shows the second-order and first-order reactions with regard to the phenylacetylene and 1 concentrations, respectively, suggesting this reaction to be catalyzed by a dinuclear ruthenium complex. Addition of PMe₃ to the catalytic system strongly discourages the dimerization. These features are consistent with the scenario, where dissociation of a PMe₃ ligand from 1 gives a coordinatively unsaturated diruthenium species and one of the ruthenium centers performs as a reaction site for the enyne formation and the other ruthenium center behaves as a spectator in the catalysis.