Aryldisilane photochemistry. A kinetic and product study of the mechanism of alcohol additions to transient silenes

Abstract

Steady state and nanosecond laser flash photolysis techniques have been employed to investigate the mechanism af the reaction of transient silenes with alcohols in polar solvents. The photolysis of a homologous series of three aryldisilanes PhRR'SiSiMe3 (R, R' = methyl or phenyl) has been employed to generate transient 1,334 1-si1a)hexatriene derivatives which differ in the degree of aryYalky1-substitution at trivalent silicon. Rate constants for reaction of the silatrienes with methanol, methanol-0-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-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, followed by rapid protonation. 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 (50.01 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.