J. Am. Chem. Soc., in press.

American Chemical Society

A Combined Experimental and Theoretical Study of the Kinetics and Mechanism of the Addition of Alcohols to Electronically Stabilized Silenes. A New Mechanism for the Addition of Alcohols to the Si=C Bond.

William J. Leigh,* Thomas R. Owens, Michael Bendikov,* Sanjio S. Sade, and Yitzhak Apeloig

Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1, the Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot Israel, and the Department of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32 000, Israel.

The stabilized silene 1,1-bis(trimethylsilyl)-2-adamantylidenesilene (4) has been generated by photolysis of a novel trisilacyclobutane derivative in various solvents and studied directly by kinetic UV spectrophotometry. Silene 4 decays with second-order kinetics in degassed hexane solution at 23 oC (k/e = 8.6 x 10-6 cm s-1), due to head-to-head dimerization. It reacts rapidly with oxygen [k(25 oC) ~ 3 x 105 M-1s-1], but ca. 10 orders of magnitude more slowly with methanol (MeOH) than other silenes that have been studied previously. The data are consistent with a mechanism involving reaction with the hydrogen-bonded dimer of the alcohol, (MeOH)2 (k = 40 3 M-1s-1; kH/kD = 1.7 0.2). The stable analogue of silene 4, 1-tert-butyldimethylsilyl-1-trimethylsilyl-2-adamantylidenesilene (5) reacts ca. 50 times more slowly, but via the same mechanism. The mechanism for addition of water and methanol (ROH; R = H, Me) to 4, 5, and the model compound 1,1-bis(silyl)-2,2-dimethylsilene (3a) has been studied computationally at the B3LYP/6-31G(d) and MP2/6-31G(d) levels of theory. Hydrogen-bonded complexes with monomeric and dimeric methanol, in which the Si=C bond plays the role of nucleophile, have been located computationally for all three silenes. Reaction pathways have been characterized for reaction of the three silenes with monomeric and dimeric ROH, and reveal significantly lower barriers for reaction with the dimeric form of the alcohol in each case. The calculations indicate that 5 should be ca. 40-fold less reactive toward dimeric MeOH than 4, in excellent agreement with the ca. 50-fold difference in the experimental rate constants for reaction in hexane solution.


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