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 acetylide anions, i.e., RC α ⋮C β - → - C α ⋮C β R, were calculated with ab initio methods. The barrier for the rearrangement depends on the capability of R to form a hypervalent-type bonding, for which 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 than their first-row counterparts. The gas-phase acidity of the acetylenes RC⋮CH was found to be dependent mainly on the inductive and resonance effects of R; influence of negative hyperconjugation and polarizability were also observed. Some migrations in the ethenyl (RCH CH - ) and ethyl (RCH 2 CH 2 - ) anions were also calculated. The major finding was that the barriers for the migration of SiH 3, GeH 3 and AlH 2 in RC⋮C - are relatively low and these hitherto unknown rearrangements should presumably be observed experimentally. Moreover, silicon, which has only been found to migrate in saturated carbanions, could do so also in vinylic anions.