STEREOELECTRONIC EFFECT OF THE TRIMETHYLSILYL SUBSTITUENT UPON C-O BOND LENGTHS AT THE BETA-POSITION - SOME STRUCTURAL STUDIES

Results of low-temperature (130 K) crystal structure analyses for seven beta-trimethylsilyl-substituted cyclo-hexylnitrobenzoate esters are reported. For those molecules (three) with the Si-C and C-O bonds antiperiplanar the C-O bond lengths are increased by 0.014 angstrom av (DELTA/sigma min = 2.9) compared with that in the silicon-free For those molecules (four) with the Si-C and C-O bonds gauche no such systematic lengthening of the C-O bonds is observed. The result is in qualitative agreement with that [DELTA-l is-proportional-to cos2 (Si-C-C-O)] predicted from semiempirical MO calculations on a simple model complex and is attributed to the effects of interactions between the Si-C sigma and C-O sigma* orbitals. It is suggested that existence of the observed ground-state effect constitutes persuasive, if circumstantial, evidence that the major kinetic effects known to result from the presence of a beta silicon substituent also have their genesis in the same sigma-sigma* interactions.