GROUND-STATE STEREOELECTRONIC EFFECTS OF SILICON - A COMPARISON OF THE EFFECTS OF SYNPERIPLANAR AND ANTIPERIPLANAR SILICON ON C-O BOND LENGTHS AT THE BETA-POSITION

Results from low-temperature crystal structure analyses of beta-trimethylsilyl esters with synperiplanar and antiperiplanar geometry are reported. These studies reveal a significant response of the C(alkyl)-O(ester) bond length to the electron demand of the ester function when the beta-trimethylsilyl substituent is antiperiplanar to the ester. However, when the geometry is synperiplanar, the C-O bond lengths are not significantly different from those of the corresponding nonsilylated derivatives. This is interpreted in terms of more effective overlap between the sigma(C-Si) orbital and the sigma*(C-O) antibonding orbital in the antiperiplanar geometry than in the synperiplanar geometry.