INTERACTION OF TRIMETHYLSILYL AND TRIMETHYLSTANNYL SUBSTITUENTS WITH C-O BONDS AT THE GAMMA-POSITION - RESULTS FROM X-RAY STRUCTURAL STUDIES AND SOLVOLYSIS STUDIES

Accurate X-ray structural studies have been carried on the gamma-Me3M (M = Si, Sn)-substituted cyclohexyl esters 21-Ns, 22-Ns, 23-Ns, 24-PNB , and 24-PNB. Analysis of the structural parameters reveals that the C (alkyl)-O(ester) bond distance is lengthened slightly compared to nonm etal-substituted analogues provided that the C-M, C-O, and intervening C-C bonds are in the W arrangement as is the case for 22-Ns, 23-Ns, a nd 24-Ns. However, the C-O bond distances for 22-PNB and 22-Ns, which do not have the W geometry, are not significantly lengthened. These st ructural effects are consistent with the presence of a percaudel inter action between the back lobe of the C-M sigma bond and the C-O sigma orbital. The gamma-Me3Sn cyclohexyl nosylates 21-Ns and 23-Ns undergo unimolecular solvolysis in 97% TFE/H2O with relative rates ca. 1:>10 0 00.