AB-INITIO THEORETICAL INVESTIGATION ON THE WRIGHT-WEST AND WITTIG ANIONIC MIGRATION REACTIONS

The carbanion to oxyanion rearrangements in the (H2COSiH3)- and (H2COCH3)- model systems have been studied. In the case of silicon the direct [1,2] migration takes place in two steps with low energy barriers, passing through a cyclic intermediate species in which silicon is pentacoordinate. In contrast, the model carbon [1,2] migration goes through a single transition structure and, although very exothermic, presents a significantly higher energy barrier. An estimate of the energy barriers for methyl and silyl dissociation shows that the former is preferred over a methyl [1,2] shift, while the latter is much more difficult than silyl [1,2] migration. The remarkable differences between the two reaction energy profiles lie in the stability of the silicon cyclic intermediate. The significant strain introduced as a result of pentacoordination within a cyclic structure is overcome by the ability of the silyl group as a whole to delocalize to some extent the negative charge of the system through bonding interactions with the H2CO group.