Mechanism of carbene insertion into hydrides of Group 4 via chromium carbene complexes. Part I. Organosilicon hydrides

Abstract

Measurements of the kinetics of reaction between chromium carbene complexes, [Cr(CO)₅{C(X)C₆H₄Y}](X = OMe or NC₄H₈; Y =p-OMe, H, or p-Cl), and organosilicon hydrides, SiR₃H (R = Et, Prⁱ, Buⁿ, or Ph), in hexane solution in the presence of pyridine, to give [Cr(CO)₅(py)] and SiR₃[CH(X)(C₆H₄Y)] as the primary products, have been made. The reactions have been studied by i.r. spectrophotometry under pseudo-first-order conditions as a function of reagent concentration and temperature. In general, a three-term rate equation is obeyed in which the kinetically significant contributions to the insertion process comprise a first-order dissociative path, which involves a major internal rearrangement of the carbene complex, and a second-order associative path in which the organosilicon hydride acts as a nucleophile. The pattern of substituent effects at the silicon atom and at the carbene-carbon atom support this picture. The reaction between [Cr(CO)₅(py)] and pyridine to give cis-[Cr(CO)₄(py)₂] is first order in [Cr(CO)₅(py)] and a predominantly dissociative mechanism is indicated.