J. Phys. Org. Chem., submitted.

© Wiley-Blackwell

Kinetics and Mechanisms of the Reactions of Transient Silylenes with Amines

Svetlana S. Kostina, Tishaan Singh, and William J. Leigh*

Contribution from the Department of Chemistry & Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

The N-H insertion reactions of dimethyl-, diphenyl-, and dimesitylsilylene (SiMe₂, SiPh₂, and SiMes₂, respectively) with n-butylamine (BuNH₂) and diethylamine (Et₂NH) have been studied in hexanes by steady state and laser photolysis methods. The process begins with the formation of the corresponding Lewis acid-base complexes, which decay with second order kinetics at rates that show modest sensitivity to silylene and amine structure. The complexation process, which has also been studied with triethylamine (Et₃N), proceeds at close to the diffusion limit but the rates vary systematically with steric bulk in the amine. Equilibrium constants have been determined for the complexation of Et₂NH and Et₃N with SiMes₂, which proceeds reversibly. The complexes of SiMe₂ and SiPh₂ with BuNH₂ and Et₂NH decay with pseudo-first order rate coefficients in the 10⁴ – 10⁵ s^-1 range. The variation in the decay rates of the SiMe₂-BuNH₂ and –Et₂NH complexes with [amine] affords upper limits of ca. 10⁶ M^-1s^-1 for the rate constants for amine-catalyzed H-migration, which is thought to be the dominant mechanism for product formation from the complexes. The results are compared to recent kinetic data for the O-H insertion reactions of these silylenes with alcohols, which also proceeds via initial complexation followed by catalytic proton transfer. The results indicate that amine-catalysis in the amine complexes is at least 10⁴-times slower than the analogous process in silylene-MeOH complexes. The data are compared to the results of theoretical calculations of the SiMe₂+NH₂Me and SiMe₂+MeOH potential energy surfaces, carried out at the G4 and B3LYP/6-311+G(d,p) levels of theory.

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