Mechanism of the germyl Wright-West anionic migration. Ab initio theoretical study of counterion effects and comparison with the analogous silyl and methyl (Wittig) rearrangements

The mechanism of the [1,2] germyl rearrangement in the (H2COGeH3)Li model s ystem is studied and compared with the results previously obtained for (1) the corresponding free anion model and (2) the two formally similar silyl a nd methyl migrations. The mechanism of the germanium rearrangement is found to diverge from both of these. The nondissociative rearrangement pathway i s a one-step [1,2] Ge shift and does not involve a cyclic intermediate, in contrast with the two-step mechanism of the silicon migration. A transition structure with pentacoordinate Ge, similar to the cyclic intermediate foun d for Si, is located ca. 17 kcal mol(-1) above the initial lithiated carban ion. The energy shoulder previously found in the free anion Ge system, in c orrespondence of such a geometry, completely disappears when Lif interacts with the anionic system. As the energy barrier is raised with respect to th e anion, where it was only 2 kcal mol(-1) high, the transition structure be comes later, in a geometrical sense. Concurrently, the exoergicity (Delta E = -24 kcal mol(-1)) is reduced to some extent with respect to the free ani on (Delta E = -30 kcal mol(-1)). A dissociation/reassociation pathway is th en explored, which leads, via O-Ge bond cleavage, to a rather stable comple x between the two H2CO and LiGeH3 moieties (-10 kcal mol(-1)). However, sim ilarly to silicon, the energy barrier for the dissociative process (ca. 30 kcal mol(-1)) is higher (by 13 kcal mol(-1)) than that for direct germyl mi gration, which is as a consequence the preferred pathway. Also in the free anion the dissociation pathway was required to overcome an energy barrier h igher than that for direct [1,2] shift by 8 kcal mol(-1). A similar situati on had been found for silicon too. This result contrasts the description re cently obtained for the Wittig carbon re arrangement that shows a sharp pre ference for a dissociation/reassociation mechanism.