| Popis: |
Rates of cleavage by NaOMEMeOH at 25°C have been determined for (2-thienyl) 2 CHSiMe 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 2 CH 2 , 33.4; Ph(2-thienyl)CH 2 , 30.0; Ph(2-furyl)CH 2 , 29.6; (2-thienyl) 2 CH 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. |
