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Kinetics and Mechanism of the Addition of Aliphatic Amines to Transient Silenes

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journal contribution
posted on 12.02.2003, 00:00 by William J. Leigh, Xiaojing Li
Primary and secondary amines such as n-butyl-, t-butyl-, and N,N-diethylamine add across the SiC bond of transient silenes such as 1,1-diphenylsilene (1a) and 1,1-bis(4-trifluoromethylphenyl)silene (1b) to yield the corresponding amino(methyl)diarylsilanes as the only products of reaction. The kinetics and mechanism of reaction of these three amines with the two 1,1-diarylsilene derivatives have been studied in hexane, acetonitrile, and tetrahydrofuran (THF) solution by laser flash photolysis techniques, using the corresponding 1,1-diarylsilacyclobutanes as photochemical precursors to the silenes. The reactions proceed with clean second-order kinetics and bimolecular rate constants in excess of 5 × 108 M-1 s-1 in hexane and MeCN, with 1b being up to four times more reactive than 1a. Arrhenius plots for reaction in hexane and/or acetonitrile solution show strong curvature over the 0−60 °C temperature range, consistent with an addition mechanism involving the intermediacy of a zwitterionic silene−amine addition complex, which collapses to product by intramolecular proton transfer from nitrogen to the silenic carbon. The reactions are substantially slower in THF, where rate reductions on the order of 5−10-fold and 50−70-fold are observed for 1a and 1b, respectively, as compared to MeCN solution. This is due to the effects of complexation of the silenes with the ether solvent, the equilibrium constant for which enters the expression for the reaction rate constant in the complexing solvent. In contrast to the situation in hexane and MeCN, Arrhenius plots for reaction of n-BuNH2 with 1a,b in THF solution are linear and lead to positive activation energies. Addition of amines in THF solution is proposed to occur predominantly via the free silenes but with minor contributions from a pathway involving nucleophilic displacement of the solvent from the silene−solvent complex.