Experimental and Theoretical Study of Oxygen Insertion into Trialkylsilanes by Methyltrioxorhenium Catalyst
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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.