METALLACARBORANES IN CATALYSIS .9. CATALYTIC HYDROSILANOLYSIS OF ALKENYL ACETATES BY TRIETHYLSILANE

Reaction of an alkenyl acetate, [CH3CO2CR=CH2] (R = CH3, C6H5), with triethylsilane in the presence of either the closo-rhodacarborane [closo-3.3-PPh3)2-3-H-3,1,2-RhC2B9H11] (I) or the exo-nido-rhodacarborane [exo-nido-(PPh3)2Rh-7,8(μ-(CH2)3)-7,8-C2B9H10] (II) as catalyst precursors in CH2Cl2at 40 °C for 42 h yielded triethylsilyl acetate and the alkene derived from the alkenyl acetate rather than the hydrosilylation product. The closo complex I was shown to be a superior catalyst precursor, giving over 80 turnovers, which corresponds to 80% conversion of alkenyl acetate to trialkylsilyl acetate under experimental conditions. The exo-nido complex II, however, showed limited catalytic activity and formed the closo bidentate acetato complex [closo-3-(PPh3)-3,3-(η2-CH3CO2)-1,2-(μ-(CH2)3)-3,1,2-RhC2B9H9] (III) as a detectable reaction intermediate. Under mild conditions, tris(triphenylphosphine)chlororhodium, [(PPh3)3RhCl] (IV), was found to catalyze the hydrosilanolysis as well as the hydrosilylation of isopropenyl acetate. However, hydrosilanolysis predominated over hydrosilylation in a ratio of 20:1. Reaction of III with triethylsilane gave triethylsilyl acetate and the exo-nido-rhodacarborane II in the presence of triphenylphosphine. The mechanisms of the hydrosilanolysis and related reactions of alkenyl acetates are discussed. © 1990, American Chemical Society. All rights reserved.