SYNTHESES AND THEORETICAL-STUDIES OF EXOCYCLIC GAMMA-OXOALKENYLTRIMETHYLSILANES - AN APPROACH TO THE STEREODEFINED EXOCYCLIC TETRASUBSTITUTED ALKENES

The Z-selectivity in the dehydration of alpha-hydroxy-gamma-oxoalkyltr imethylsilanes under acidic conditions was studied from experimental a nd theoretical points of view. Experimental results showed that (Z)-ga mma-oxoalkenyltrimethylsilanes were thermodynamic products under these conditions. The dehydration studies of the compound involving the ter t-butyl group instead of the TMS group pointed out that not only steri c but also electronic effects of Si could contribute to the distributi on of the products as well as their stereochemistries. Theoretical stu dies using ab initio calculation at the 6-31G level indicated that th e (Z)-isomer was thermodynamically more stable than the corresponding (E)-isomer. Detail examinations of the optimized structures showed tha t the configuration of Si in the (Z)-isomer was slightly distorted fro m tetrahedral. Interpretation of the geometrical change of Si to ratio nalization of the thermodynamic preference was discussed fi om the vie wpoint of possible coordination of the carbonyl oxygen to Si. Those (Z )-gamma-oxoalkenyltrimethylsilanes would have potential to be the nove l type of alkenylmetal compounds in organic synthesis as we demonstrat ed in the construction of stereodefined exocyclic tetrasubstituted alk ene.