THE BETA-EFFECT OF SILICON IN THE SYNPERIPLANAR GEOMETRY

The effect of silicon on the development of β positive charge has been measured for the synperiplanar geometry of the Si-C-C-X fragment in endo3-(trimethylsilyl)-endo-2-norbornyl esters (3). Solvent effects were used to demonstrate that solvolysis took place by a carbocation mechanism. When the leaving group X was mesylate (3-OMs) and the solvent was 97% trifluoroethanol, the β effect was found to be about 105 by comparison of the solvolysis rate with the analogous structure lacking the trimethylsilyl group, endo-2-norbornyl mesylate (5-OMs). Thus, the synperiplanar β effect is much smaller than the antiperiplanar β effect (about 1012 under similar conditions). The effect may be smaller because of poorer vertical overlap in the synperiplanar geometry or because the syn leaving group prevents optimal vertical overlap. Alternatively, the antiperiplanar effect may be larger because of contributions from nonvertical participation. © 1990, American Chemical Society. All rights reserved.