Solvent, substituent, and temperature effects on the absolute rate constants for addition of methoxytrimethylsilane to 1,1-diarylsilenes

Absolute rate constants have been measured for the reaction of methoxytrime thylsilane with a series of transient, para-substituted 1,1-diphenylsilenes (H2C=Si(C6H4X)(2), where X = H, Me, F, Cl, and CF3) in hexane solution at room temperature. The data correlate with Hammett substituent constants, af fording the reaction constant rho = +0.9 +/- 0.2. For the parent compound a nd the 4, 4'-bis(trifluoromethyl) derivative, rate constants have been dete rmined over the 0-60 degreesC temperature range in hexane, 1,2-dichloroetha ne, and acetonitrile solution. The rate constants for reaction of the paren t compound increase in the order hexane approximate to 1,2-dichloroethane < MeCN, but are relatively insensitive to solvent in the other case. However , differences are revealed in the Arrhenius activation energies for reactio n, which are negative for both compounds in all three solvents. Those for t he 4,4'-bis(trifluoromethyl) compound in particular change to more positive values with increasing solvent polarity. The data are consistent with a tw o-step mechanism involving reversible preassociation of the silene and the alkoxysilane to form a Lewis acid-base complex, which collapses to product by intramolecular transfer of trimethylsilyl from oxygen to the silenic car bon.