Base cleavage of substituted [phenyl(2-thienyl)methyl]- and [phenyl(2-furyl)methyl]-trimethylsilane. Stabilization of carbanionic centres by 2-thienyl and 2-furyl groups

Abstract

Rates of cleavage by NaOMEMeOH at 25°C have been determined for (2-thienyl) 2CHSiMe 3 and for the compounds Ph(2-thienyl)CHSiMe 3 and Ph(2-furyl)CHSiMe 3 and some of their derivatives with a substituent in the m- or p-position of the phenyl group or the 5-position of the heterocyclic group. The results indicate that the 2-thienyl and 2-furyl groups stabilize a carbanionic centre more effectively than a phenyl group, and the following approximate p K a values can be derived: Ph 2CH 2, 33.4; Ph(2-thienyl)CH 2, 30.0; Ph(2-furyl)CH 2, 29.6; (2-thienyl) 2CH 2, 27.1. The effect of the 2-Cl substituent in the thiophen ring is close to that of the p-Cl substituent in the benzene ring, and the effects of the p-Me substituents on the benzene ring are very close to those of the 2-Me substituents on the thiophen or furan rings. The product and rate isotope effects (determined by use of MeOD) are consistent with separation of the carbanion in the rate-determining step.