Cleavage of furan-2-yl, 2-thienyl-, benzo[b]furan-2-yl-, and benzo[b]thiophen-2-yl-methyl(trimethyl)silanes (RCH2SiMe3) by methanolic sodium methoxide; acidities of the corresponding RCH3 species

Abstract

Rates of cleavage of RCH₂SiMe₃ by 2.00M-NaOMe in MeOH or MeOD at 50 °C have been determined for R = furan-2-yl, 2-thienyl, benzo[b]furan-2-yl, and benzo[b]thiophen-2-yl; the values of 10⁵k_s in MeOH and (in parentheses) in MeOD, where k_s is the specific second order rate constant, are, respectively: 0.070 (0.167), 0.26 (0.52), 165 (310), and 70 (130) dm³ mol^–1 s^–1. The values of the ratios of k_s in MeOH to that in MeOD are in the range 0.42–0.54, consistent with rate-determining separation of the carbanion RCH₂^–. [In contrast; for 2-thienylmethyl(trimethyl)stannane, for which 10⁵k_s is 1.57 (1.55), the ratio is 1.01, consistent with electrophilic assistance by proton transfer from the solvent to the separating carbon centre.] The results are discussed in the light of the deprotonation energies calculated (STO-3G) for the corresponding carbon acids RCH₃, and approximate pK_a values are derived for the latter, viz.(R =) furan-2-yl, 40.6; 2-thienyl, 39.7; benzo[b]furan-2-yl, 35.2; benzo[b]thiophen-2-yl, 35.8.