Hydrovinylation and [2+2] Cycloaddition Reactions of Alkynes and Alkenes Catalyzed by a Well-Defined Cationic Ruthenium−Alkylidene Complex

The cationic ruthenium−alkylidene complex [(PCy 3 ) 2 (CO)(Cl)Ru CHCH C(CH 3 ) 2 ] + BF 4 - ( 1 ) was found to catalyze both hydrovinylation and [2+2] cycloaddition reactions of alkynes and alkenes. The reaction of R‘C⋮CR‘ ‘ (R‘ = H, Ph, n -Pr; R‘ ‘ = Ph, n -Pr, p -Tol, SiEt 3, CH 2 CH 2 OH) with ethylene in the presence of 1 (3 mol %) produced the hydrovinylation products 2 and 3 . The analogous reaction of dimethyl acetylenedicarboxylate (R‘ = R‘ ‘ = CO 2 Me) with ethylene and norbornene resulted in the formation of the [2+2] cycloaddition products 4 . Based on the experimental evidence, a plausible mechanism of the hydrovinylation reaction has been proposed via a sequential insertion of alkyne and ethylene to the hydride complex 7 .