Evidence for formation of carbanions in cleavage of carbon–silicon bonds by methanolic sodium methoxide. Some large 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 measured for cleavage of Me₃SiR compounds by methanolic sodium methoxide; values (with estimated uncertainties of ±20%) are (R =), Ph₂CH, 2.20 (50 °C); Ph₃C, 2.3 (50 °C); fluoren-9-yl, 9.0 (22 °C); 9-methylfluoren-9-yl, 10.8 (22 °C); inden-1-yl, 10.0 (22 °C): o-NO₂C₆H₄CH₂, 14 (22 °C); o-NO₂C₆H₄CH₂, 15.6 (22 °C). For some compounds, values of the ratio of the rate in MeOH to that in MeOD have also been measured; they are (R =). Ph₂CH, 0.48 (50 °C); Ph₃C, 0.51 (50 °C); fluoren-9-yl, 0.57 (25 °C); p-NO₂C₆H₄CH₂. 0.50 (25 °C). It is concluded that at least for the more reactive compounds, and probably for all, the rate-determining step is the separation of the carbanion R–, and that the p.i.e. values represent the isotope effects for interaction of the carbanion with MeOH and MeOD. A p.i.e. value of ca. 1.5 for interaction of ethereal fluoren-9-yl-lithium with methanol is interpreted in terms of inadequate mixing.