Substituent effects on the reactivity of the silicon-carbon double bond. Arrhenius parameters for the reaction of 1,1-diarylsilenes with alcohols and acetic acid

Absolute rate constants for the reaction of a series of ring-substituted 1,1-diphenylsilene derivatives with methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined using nanosecond laser flash photolysis techniques. The three reactions exhibit small positive Hammett rho-values at 23 degrees C, consistent with a mechanism involving initial, reversible nucleophilic attack at silicon to form a a-bonded complex that collapses to product via rate-limiting proton transfer. Deuterium kinetic isotope effects and Arrhenius parameters have been determined for the reactions of 1,1-di-(4-methylphenyl)silene and 1,1-di-(4-trifluoromethylphenyl)silene with methanol, and are compared to those for the parent compound. Proton transfer within the complex is dominated by entropic factors, resulting in negative activation energies for reaction. The trends in the data can be rationalized in terms of variations in the relative rate constants for reversion to reactants and proton transfer as a function of temperature and substituent. A comparison of the Arrhenius activation energies for reaction of acetic acid with 1,1-diphenylsilene (E-a = + 1.9 +/- 0.3 kcal/mol) and the more reactive di-trifluoromethyl analogue (E-a = + 3.6 +/- 0.5 kcal/mol) suggests that carboxylic acids also add by a stepwise mechanism, but with formation of the complex being rate determining.