Structure and Reactivity of Hypercoordinate Silicon Compounds Mechanism of Hydrogen Peroxide Oxidation of Silicon-Carbon Bonds

Abstract

We have previously reported that the silicon-carbon bonds in silafunctional compounds are cleaved by hydrogen peroxide in the presence of a fluoride ion to form the corresponding alcohols. We have proposed a plausible mechanism which involves penta-coordinate silicon species as intermediates and hexa-coordinate silicon species in the transition state. Thus, tetra-coordinate silicon compounds are converted into penta-coordinate silicon species via the interaction with a fluoride ion, which are attacked by hydrogen peroxide as the sixth ligand in the rate-determining step: In the hexa-coordinate species, the organic group may migrate with a carbanionic character from silicon to the adjacent oxygen of the hydroperoxy ligand. The supporting evidence includes [1] activation by a fluoride ion and high reactivity of penta-coordinate diorganotrifluorosilicates, [2] rate-dependence on the concentration of hydrogen peroxide, [ 3] retention of configuration of the carbon center, (4) acceleration by an electron-withdrawing substituent on the organic group, [5] dual steric effects, steric acceleration and steric hindrance effects. Based on the last steric effects and X-ray structures of pertinent penta-coordinate silicon compounds, attempts have been made to analyze the lines of attack by hydrogen peroxide on penta-coordinate silicon species.