Organometallics 20, 932-936 (2001).
© 2001 American Chemical Society
William J. Leigh,* Christine J. Bradaric, Tracy L. Morkin and Xiaojing Li
Contribution from the Department of Chemistry, McMaster University, Hamilton, ON Canada L8S 4M1
Abstract: Absolute rate constants have been measured for the reaction of methoxytrimethylsilane 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, affording the reaction constantr = +0.9 ± 0.2. For the parent compound and the 4,4-bis(trifluoromethyl) derivative, rate constants have been determined in hexane, 1,2-dichloroethane, and acetonitrile over the 0-60 oC range. The rate constants for reaction of the parent compound increase in the order hexane ~ 1,2-dichloroethane < MeCN, but are relatively insensitive to solvent in the other case. However, differences are revealed in the Arrhenius activation energies for reaction, which are negative for both compounds in all three solvents. Those for the 4,4-bis(trifluoromethyl) compound in particular change to more positive values with increasing solvent polarity. The data are consistent with a two-step mechanism involving reversible pre-association 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 carbon.
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