PHYSICAL ORGANIC-CHEMISTRY OF TRANSITION-METAL CARBENE COMPLEXES .6. KINETICS AND MECHANISM OF THE THERMAL AND PHOTOCHEMICAL HYDROLYSIS OF (CO)(5)M=C(OME)CH(2)PH (M=CR,W) IN AQUEOUS ACETONITRILE

An extensive kinetic study of the hydrolysis of (benzylmethoxycarbene)pentacarbonylchromium(0), 7-Cr, and a more limited study of the hydrolysis of its tungsten analog, 7-W, in 50% acetonitrile-50% water is reported. Both lead to beta-methoxystyrene as the main organic product; with 7-W, phenylacetaldehyde is a significant second product, while with 7-Cr only traces of phenylacetaldehyde are found. The kinetic results which include the demonstration of general base catalysis and of a kinetic solvent isotope effect are consistent with a mechanism in which 7-M (NI = Cr, W) undergoes a rapid, reversible deprotonation of the alpha-carbon to form 7-M-, followed by rate-limiting reaction of 7-M(-) with water, H3O+, or buffer acids. This latter step most likely involves protonation of the carbene carbon concerted with cleavage of the C-Cr bond. The hydrolysis of both 7-Cr and 7-M is strongly accelerated by light; a limited study of this photochemical pathway at high pH, which also leads to beta-methoxystyrene as the principal organic product, suggests a mechanism which most likely involves a ligand exchange between CO and acetonitrile prior to the water-induced rate-limiting product-forming step.