15003-82-4Relevant articles and documents
Kinetics and mechanisms of the reactions of transient silylenes with amines
Kostina, Svetlana S.,Singh, Tishaan,Leigh, William J.
experimental part, p. 937 - 946 (2012/01/13)
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.
Fast kinetics study of the reactions of transient silylenes with alcohols. Direct detection of silylene-alcohol complexes in solution
Leigh, William J.,Kostina, Svetlana S.,Bhattacharya, Adroha,Moiseev, Andrey G.
experimental part, p. 662 - 670 (2010/05/15)
The kinetic behavior of dimethyl-, diphenyl-, and dimesitylsilylene in hexanes solution in the presence of methanol (MeOH), tert-butanol (t-BuOH), and the respective O-deuterated isotopomers has been studied, with the goal of elucidating a detailed mechanism for the formal O-H insertion reaction of transient silylenes with alcohols in solution. The data are in all cases consistent with a mechanism involving the intermediacy of the corresponding silylene-alcohol Lewis acid-base complexes, which have been detected directly for each of the SiMe2-ROL and SiPh2-ROL (L = H or D) systems that were studied. Complexation proceeds effectively irreversibly (Keq ≥ 2 x 105 M-1) and at close to the diffusion-controlled rate in these cases. In contrast, the kinetic and spectroscopic behavior observed for SiMeS2 in the presence of these alcohols indicates the SiMeS2-ROL complexes are involved as steady-state intermediates, formed reversibly and 10-100 times more slowly than is the case with SiMe2 and SiPh2. Product formation from the silylene-alcohol complexes is shown to proceed via catalytic proton transfer by a second molecule of alcohol, the rate of which exceeds that of unimolecular intracomplex H-migration in all cases, even at submillimolar alcohol concentrations. The catalytic rate constants range from 109 to 1010 M-1 s-1 for the SiMe2-ROH and SiPh2-ROH complexes, sufficiently fast that the isotope effect ranges from ca. 2.5 to close to unity for all but the SiPh2-t-BuOL complex, where it is remarkably large (kHH/kDD = 10.8 ± 2.4). The value is consistent with a mechanism for catalysis involving double proton transfer within a cyclic five-membered transition state. The isotope effects on the ratio of the rate constants for catalytic proton transfer and dissociation of the SiMeS2-MeOH and SiMeS2-t-BuOH complexes suggest that a different mechanism for catalytic proton transfer is involved in the case of the sterically hindered diarylsilylene.
