Further studies of solvent isotope effects in the cleavage of substituted benzyltrimethylsilanes by methanolic sodium methoxide. Intermediate kinetic isotope effects for reactions of carbanions with methanol

Abstract

Product isotope effects (p.i.e.), given by the product ratio RH : RD obtained in 1 : 1 MeOH–MeOD, have been determined for cleavages of some substituted benzyltrimethylsilanes by methanolic sodium methoxide; values (with estimated uncertainties of ±20%) given in the sequence of decreasing ease of cleavage, are: (X or XY in XC₆H₄– or XYC₆H₃CH₂–SiMe₃)p-NO₂, 2-Me-4-NO₂, or o-NO₂, 10 (25 °C); p-COPh, 7 (50 °C); p-SO₂Ph, 2.9 (50 °C); 2-Me-6-NO₂, 10 (25 °C); p-CN, 2.0 (50 °C); 3,5-Cl₂, 1.3 (50 °C); m-NO₂, 1.3 (25 °C); m-CN, m-CF₃, or H, 1.2 (50 °C). These values are believed to represent the kinetic isotope effects for interactions of the carbanions R^– with MeOH and MeOD; as expected, except when there is unusual steric hindrance, they show an overall increase with increasing reactivity of the RSiMe₃ compound, and thus with the pK_a of the carbon acid RH, but remain effectively constant over a large pK_a range before increasing steeply over a small pK_a range. The p.i.e. values (at 50 °C) for the compounds Ph₂CHSiMe₃ and Ph₃CSiMe₃ have been redetermined, and found to be ca. 1.3 ± 0.2, consistent with the values for substituted benzyl compounds of comparable reactivity. The value for the triphenyl compound is incompatible with a reported value of the kinetic isotope effect for proton abstraction from triphenylmethane by sodium methoxide in methanol.