Experimental and Theoretical Study of Oxygen Insertion into Trialkylsilanes by Methyltrioxorhenium Catalyst
Among the reactions of hydrogen peroxide that are catalyzed by methyltrioxorhenium, the oxidation of alkylsilanes is unique. It is not a reaction in which an oxygen atom is added to a substrate, but one featuring a net insertion, R3Si−H + H2O2 → R3Si−OH + H2O. Kinetics studies were carried out on 10 compounds. Rate constant were determined for the bimolecular reaction of the silane with the peroxo compound CH3Re(O)(η2-O2)2(H2O). The variation of rate constant with the alkyl groups R follows two trends: the values of log(k) are linear functions of (a) the stretching frequency of the Si−H group and (b) the total Taft constant for these substituents. The reactions of (n-Bu)3Si−H and (n-Bu)3Si−D exhibit a kinetic isotope effect of 2.1 at 0 °C. From these data, a model for the transition state was formulated in which O−H and Si−O bond making accompany Si−H bond breaking. Quantum mechanical calculations have been carried out on the gas-phase reaction between Et3SiH and CH3Re(O)2(η2-O2). These results support this structure, calculating a structure and energy that are in agreement. The theoretical activation energy is 28.5 kcal mol-1, twice the experimental value in aqueous acetonitrile, 12.4 kcal mol-1. The difference can be attributed to the solvation of the polar transition state in this medium.
Reprinted (adapted) with permission from Organometallics 18 (1999): 4753, doi:10.1021/om990579d. Copyright 1999 American Chemical Society.