Organometallics, 21, 1197-1207 (2002).
© 2002 American Chemical Society
William J. Leigh* and Xiaojing Li
Contribution from the Department of Chemistry, McMaster University, Hamilton, ON Canada L8S 4M1
Abstract: 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 oC temperature range. In hexane or acetonitrile solution, 2e is significantly more reactive than 2a towards 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 in almost every case. This is shown to be 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 constants for reaction. Transient UV absorption spectra have been recorded in THF as a function of temperature, and verify that complexation is stronger with the more electrophilic of the two silenes (2e). The positive activation energies for reaction of the silenes in this solvent results from the effect of temperature on the equilibrium constant for solvent complexation, which is sufficiently large with these silenes that it overshadows the negative temperature dependences for reaction that are observed in non-complexing solvents. The results suggest that the more intrinsically electrophilic a given silene is, the more it is stabilized (in an absolute sense) in THF relative to hexane solution. This leads to interesting variations in the relative reactivities of transient silenes in solvents of different Lewis basicity and in several cases, a change in reaction mechanism. Weak complexation is shown to be present even in acetonitrile, which partially accounts for the reduced reactivity of the silenes in this solvent relative to that in hexane.
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