PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES .5. KINETICS AND MECHANISM OF HYDROLYSIS OF (CO)(5)CR=C(OCH3)CH3 AND (CO)(5)CR=C(OCH2CH3)CH3 IN AQUEOUS ACETONITRILE

This paper reports a product and kinetic study of the hydrolysis of (CO)(5)Cr=C(OCH3)CH3 (1a) and (CO)(5)Cr=C(OCH2CH3)CH3 (1b) in 50% aqueous acetonitrile at 25 degrees C. The organic products are acetaldehyde and methanol in the case of la and acetaldehyde and ethanol in the case of 1b. The reactions are general base catalyzed and subject to a primary solvent kinetic isotope effect. This indicates a mechanism which includes a rate limiting proton transfer. A number of plausible mechanisms which are consistent with the experimental data are scrutinized. The preferred mechanism involves rapid deprotonation of the methyl group of the carbene complexes to form 1a(-) or 1b(-), respectively, followed by rate limiting Cr-C bond cleavage of 1a(-) (1b(-)) concerted with protonation on the carbene carbon to yield (CO)(5)Cr(CH2=CHOR), which is rapidly hydrolyzed to the final products.