COUNTERION EFFECTS ON THE WRIGHT-WEST ANIONIC MIGRATION REACTION - AN AB-INITIO THEORETICAL-STUDY

The [1,2] silyl migration in the (H2COSiH3)Li model system has been studied, following a first study on the related free anion, to investigate the effects of the lithium counterion on the reaction mechanism. As was the case in the absence of lithium, the nondissociative rearrangement takes place in two steps, passing through a cyclic intermediate in which silicon is pentacoordinate. Lithium stabilizes the intermediate less than the carbanionic ''reactant'' and the oxyanionic ''product'' and raises as a consequence the energy barrier, but has a small effect on the reaction exothermicity. As an alternative pathway, the initial lithiated carbanion can undergo heterolytic dissociation of the O-Si bond without complete separation of the H2CO and SiH3- fragments, which are held together by the interaction with Lie: an electrostatic complex is then formed between formaldehyde and silyllithium. From this intermediate the same product of the nondissociative [1,2] shift could be easily obtained through a reassociation step. However, the energy barrier for the first dissociative step is much higher than that for the direct [1,2] migration, which is then the preferred pathway. Comparison of these results with those obtained for the free anions leads to the conclusion that the Wright-West migration takes place in a two-step process where the C-Si bond is formed before the O-Si bond is cleaved, and not vice versa.