Kinetics and mechanisms of the reactions of transient silylenes with amines

Article abstract of DOI:10.1002/poc.1908

The N-H insertion reactions of dimethyl-, diphenyl-, and dimesitylsilylene (SiMe₂, SiPh₂, and SiMes₂, respectively) with n-butylamine (BuNH₂) and diethylamine (Et₂NH) were studied in hexanes by steady-state and laser photolysis methods. The process begins with the formation of the corresponding Lewis acid-base complexes, which decayed with second-order kinetics at rates that show modest sensitivity to silylene and amine structures. The complexation process, which was also studied using triethylamine (Et₃N), proceeds at rates close to the diffusion limit, but the rate constants vary systematically with steric bulk in the amine. Equilibrium constants were determined for the complexation of Et₂NH and Et₃N with SiMes₂, which proceeds reversibly. The complexes of SiMe₂ and SiPh₂ with BuNH₂ and Et₂NH decayed with pseudo-first-order rate coefficients in the 10⁴-10⁵s^-1 range. This is consistent with upper limits of about 10⁶M^-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 published 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 catalyzed H-transfer in the amine complexes is at least 10⁴ times slower than the analogous process in silylene-MeOH complexes. The experimental data are compared to the results of theoretical calculations of the SiMe₂+NH₂Me and SiMe ₂+MeOH potential energy surfaces, carried out at the Gaussian-4 and B3LYP/6-311+G(d,p) levels of theory.