AB-INITIO CALCULATIONS OF [1,2]-SHIFTS IN CARBANIONS - THEORETICAL PREDICTIONS OF CERTAIN FACILE REARRANGEMENTS

The activation barriers for the [1,2]-migration of several groups in a cetylide anions, i.e., RCalpha=C-beta(-)-->C--(alpha)=CbetaR, were cal culated with ab initio methods. The barrier for the rearrangement depe nds on the capability of R to form a hypervalent-type bonding, for whi ch its ability to accomplish negative hyperconjugation as well as its polarizability are important. For these reasons, substituents derived from second-row elements are significantly better migrating groups tha n their first-row counterparts. The gas-phase acidity of the acetylene s RC=CH was found to be dependent mainly on the inductive and resonanc e effects of R; influence of negative hyperconjugation and polarizabil ity were also observed. Some migrations in the ethenyl (RCH=CH-) and e thyl (RCH2CH2-) anions were also calculated. The major finding was tha t the barriers for the migration of SiH3, GeH3 and AlH2 in RC=C- are r elatively low and these hitherto unknown rearrangements should presuma bly be observed experimentally. Moreover, silicon, which has only been found to migrate in saturated carbanions, could do so also in vinylic anions.