J. Phys. Org. Chem., submitted.
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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