Reprinted from the Journal of the American Chemical Society, 116, 10468-10476 (1994).
©American Chemical Society, 1994.
William J. Leigh* and Gregory W. Sluggett
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: Steady state and nanosecond laser flash photolysis techniques have been employed to investigate the mechanism of the reaction of transient silenes with alcohols in polar solvents. The photolysis of a homologous series of three aryldisilanes PhRR'Si-SiMe3 (R, R' = methyl or phenyl) has been employed to generate transient 1,3,5-(1-sila)hexatriene derivatives which differ in the degree of aryl/alkyl-substitution at trivalent silicon. Rate constants for reaction of the silatreienes with methanol, methanol-O-d, trifluoroethanol, and acetic acid have been determined in acetonitrile, tetrahydrofuran, and isooctane solution. For the silatriene obtained from photolysis of pentamethylphenyldisilane, rate constants have also been measured for acetic acid-O-d, ethylene glycol, and 1,3-propanediol in acetonitrile solution. The results are consistent with a mechanism involving reversible formation of a silatriene-alcohol complex, followed by competing intracomplex and extracomplex proton transfer. The proton transfer steps are rate-determining when the alcohol is only weakly acidic, while complex formation is rate-determining for acidic alcohols or carboxylic acids. It is concluded that the extracomplex proton transfer reaction most likely proceeds by a general base catalysis mechanism involving deprotonation of the complex by alcohol or solvent, followed by rapid reprotonation. The products of [1,2]-, [1,4]-, and [1,6]-addition of methanol to the silatriene obtained from photolysis of pentamethylphenyldisilane in acetonitrile containing 0.15 M methanol have been isolated and identified, and the variation in product distribution with methanol concentration has been determined. The [1,2]-adduct predominates at very low methanol concentrations (<= 0.01 M), where addition proceeds predominantly via the intracomplex proton transfer pathway. The [1,4]-adduct predominates at very high concentrations (2-5 M), which is proposed to be due to the involvement of methanol oligomers in the final, product-determining protonation step of the extracomplex proton transfer pathway.