WRIGHT,GERMYL - WEST ANIONIC MIGRATION - AB-INITIO THEORETICAL-STUDY OF THE REACTION-MECHANISM IN THE CASE OF A FREE ANION

The mechanism of the nondissociative [1,2] germyl rearrangement in the (H2COGeH3)(-) model system appears to be significantly different from its silicon analogue. A first exothermic step leads quite easily (Del ta E(double dagger) = 2 kcal mol(-1)) to a flat zone on the energy hyp ersurface, corresponding to a cyclic arrangement of the C, O, and Ge a toms, in which Ge is pentacoordinate. However, this kind of structure (ca. 12 kcal mol(-1) below the carbanion; 18 kcal mol(-1) for silicon) is unstable with respect to ring opening to the final oxyanion (for s ilicon the cyclic structure is stable). The overall migration is exoth ermic by 32 kcal mol(-1) (only 20 kcal mol(-1) for silicon). At varian ce with silicon, the importance of a cyclic intermediate is thus quite dubious in the germanium case. This result can explain the experiment ally observed absence of products originating from the open-chain carb anion, when the migrating system is exposed to an electrophile. Finall y, the energy barrier for oxygen-germanium bond cleavage is found to b e ca. 10 kcal mol(-1): therefore, the dissociative process is not like ly to be significantly competitive with the direct [1,2] shift.