Alkyne Insertion into the Ru−C Bond of a Four-Membered Metallacycle. Insertion Rate and Reaction Pathway

The insertion of HOCH 2 C⋮CH (ha) into the Ru−C bond of Ru II (RL 1 )(PPh 3 ) 2 (CO)Cl, 1, has afforded Ru II (RL 3 )(PPh 3 ) 2 (CO)Cl, 3, which has been structurally characterized. Insertion rates in CH 2 Cl 2 −MeOH for ha as well as for PhC⋮CH (pa), which inserts similarly into 1 affording Ru II (RL 2 )(PPh 3 ) 2 (CO)Cl, 2, are proportional to the product of the concentrations of alkyne and methanol. The insertion rate of ha is nearly 5 times faster than that of pa, and for a given alkyne the rate increases as R becomes more electron-withdrawing (OMe < Me < Cl). A reaction model implicating the adduct 1 ·MeOH, which binds and activates the alkyne via displacement of MeOH, is proposed.