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Effects of Ether Solvents on the Reactivity of Transient Silenes

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journal contribution
posted on 19.02.2002, 00:00 by William J. Leigh, Xiaojing Li
Absolute rate constants and Arrhenius parameters have been determined for reaction of two transient silenes, 1,1-diphenylsilene (2a) and 1,1-bis(4-trifluoromethylphenyl)silene (2e), with methanol, tert-butanol, acetic acid, acetone, and methoxytrimethylsilane in tetrahydrofuran (THF) solution over the 0−60 °C temperature range. The results are compared to previously reported data for the same reactions in hydrocarbon and/or acetonitrile solution. In the latter solvents, 2e is significantly more reactive than 2a toward all of these reagents, and in most cases the Arrhenius activation energies for reaction are negative, a result of the common stepwise mechanism by which these reactions proceed. In contrast, the reactivities of both silenes are reduced in THF solution, their relative reactivities are reversed, and positive Arrhenius activation energies for reaction are obtained in every case. The UV absorption spectra of the two silenes in THF at various temperatures between 0 and 60 °C show that this is due mainly to the effects of complexation of the silenes with the ether solvent, the equilibrium constant for which enters the expression for the observed overall second-order rate constant for reaction. The positive activation energies result from the effect of temperature on the equilibrium constant for solvent complexation, which increases with decreasing temperature and accordingly reduces the concentration of free silene present in solution. For nonacidic nucleophiles like acetone, this leads to particularly large reductions in the overall rate constant for reaction. The effects are smaller with more acidic reagents, consistent with competing addition via initial protonation of the solvent complex. Weak complexation appears to be present even in acetonitrile, which partially accounts for the reduced reactivity of the silenes in this solvent relative to that in hexane.