THE QUESTION OF VERTICAL OR NONVERTICAL PARTICIPATION OF SILICON-BETA TO A CATION IN THE ANTIPERIPLANAR STEREOCHEMISTRY

The mechanism of stabilization of a beta positive charge by silicon in the antiperiplanar stereochemistry has been clarified by the alpha secondary deuterium kinetic isotope effect. A k(H)/k(D) of 1.17 +/- 0.01 has been observed for the trifluoroethanolysis of r-5-tert-butyl-c-2-(trimethylsilyl)cyclohex-t-yl-1,3,3-d3 3,5-dinitrobenzoate (3) at 25-degrees-C. This value is consistent with vertical (hyperconjugative) stabilization (1) probably with rate-determining conversion of an intimate ion pair to a solvent-separated ion pair. The isotope effect is inconsistent with nonvertical participation to form a bridged intermediate (2).