Physical Organic Chemistry of Transition Metal Carbene Complexes. 7. Kinetics of Hydrolysis of (CO)5MC(OR)Ph (M = Cr, W; R = Me, Et) and (CO)5Cr(OMe)CHCHPh in Aqueous Acetonitrile

A study of the hydrolysis of (CO)5CrC(OMe)Ph (1a), (CO)5CrC(OEt)Ph (1b), (CO)5CrC(OMe)CHCHPh (1c), (CO)5WC(OMe)Ph (1d), and (CO)5WC(OEt)Ph (1e) in 50% MeCN−50% water (v/v) at 25 °C is reported. The reaction occurs in two stages; the first is the formation of (CO)5MC(O-)R or (CO)5MC(OH)R (M = Cr or W, R = Ph or CHCHPh) while the second, much slower stage is the formation of RCHO and (CO)5MOH-. This paper reports a kinetic investigation of the first stage. It is shown that nucleophilic attack by OH- at high pH and by water at low pH, presumably to form a tetrahedral intermediate, is rate limiting; in the presence of buffers general base catalysis of water addition is observed. Kinetic solvent isotope effects of ca. 1 for the OH- and of 3 to 4.7 for the water pathways are also reported. The results lead to insights regarding the following points: (1) effect of changing the metal (Cr vs W), the alkoxy group (MeO vs EtO), and the R group (Ph vs CHCHPh) on reactivity; (2) relative reactivity of Fischer carbene complexes vs carboxylic esters; (3) reasons why a tetrahedral intermediate is not detectable even though the equilibrium for its formation is probably favorable at high pH; and (4) reasons why the hydrolysis of Fischer carbene complexes containing an acidic proton such as (CO)5CrC(OMe)CH3 follow an entirely different mechanism.