Abstract
The inductive contribution to the β-effect of silicon has been investigated through the preparation of 1-(trimethylsilyl)bicyclo[2.2.2]octan-2-ol (5-OH) and its sulfonate derivatives. The dihedral relationship in 5 between the trimethylsilyl group and the bond to the leaving group approaches orthogonality, preventing any contribution to the β-effect from either hyperconjugation or bridging. Any residual effect of silicon should come from induction. The rate of the tosylate, compared with that of the silicon-free model bicyclo[2.2.2]oct-2-yl tosylate (7-OTs), was found to be 1.2–1.3 in 80–97% aqueous trifluoroethanol at 20–35°C. This negligible β-effect compares with values of about 10 5 at the synperiplanar geometry, 10 4 at clinal geometries, and 10 12 at the antiperiplanar geometry. The complete dihedral dependence resembles the cosine-squared curve expected for a hyperconjugative mechanism rather than for bridging, with little contribution from induction.
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