Germyl Wright - 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 (Delta E(double dagger) = 2 kcal mol(-1)) to a flat zone on the energy hypersurface, corresponding to a cyclic arrangement of the C, O, and Ge atoms, 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 silicon the cyclic structure is stable). The overall migration is exothermic by 32 kcal mol(-1) (only 20 kcal mol(-1) for silicon). At variance with silicon, the importance of a cyclic intermediate is thus quite dubious in the germanium case. This result can explain the experimentally observed absence of products originating from the open-chain carbanion, when the migrating system is exposed to an electrophile. Finally, the energy barrier for oxygen-germanium bond cleavage is found to be ca. 10 kcal mol(-1): therefore, the dissociative process is not likely to be significantly competitive with the direct [1,2] shift.