The mechanism of the [1,2] germyl rearrangement in the (H(2)COGeH(3))Li model system is studied and compared with the results previously obtained for (1) the corresponding free anion model and (2) the two formally similar silyl and methyl migrations. The mechanism of the germanium rearrangement is found to diverge from both of these. The nondissociative rearrangement pathway is 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 found for Si, is located ca. 17 kcal mol(-1) above the initial lithiated carbanion. The energy shoulder previously found in the free anion Ge system, in correspondence of such a geometry, completely disappears when Lif interacts with the anionic system. As the energy barrier is raised with respect to the anion, where it was only 2 kcal mol(-1) high, the transition structure becomes later, in a geometrical sense. Concurrently, the exoergicity (Delta E = -24 kcal mol(-1)) is reduced to some extent with respect to the free anion (Delta E = -30 kcal mol(-1)). A dissociation/reassociation pathway is then explored, which leads, via O-Ge bond cleavage, to a rather stable complex between the two H(2)CO and LiGeH(3) moieties (-10 kcal mol(-1)). However, similarly 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 migration, which is as a consequence the preferred pathway. Also in the free anion the dissociation pathway was required to overcome an energy barrier higher than that for direct [1,2] shift by 8 kcal mol(-1). A similar situation had been found for silicon too. This result contrasts the description recently obtained for the Wittig carbon re arrangement that shows a sharp preference for a dissociation/reassociation mechanism.
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.
ANTONIOTTI, Paola;TONACHINI, Glauco
1999-01-01
Abstract
The mechanism of the [1,2] germyl rearrangement in the (H(2)COGeH(3))Li model system is studied and compared with the results previously obtained for (1) the corresponding free anion model and (2) the two formally similar silyl and methyl migrations. The mechanism of the germanium rearrangement is found to diverge from both of these. The nondissociative rearrangement pathway is 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 found for Si, is located ca. 17 kcal mol(-1) above the initial lithiated carbanion. The energy shoulder previously found in the free anion Ge system, in correspondence of such a geometry, completely disappears when Lif interacts with the anionic system. As the energy barrier is raised with respect to the anion, where it was only 2 kcal mol(-1) high, the transition structure becomes later, in a geometrical sense. Concurrently, the exoergicity (Delta E = -24 kcal mol(-1)) is reduced to some extent with respect to the free anion (Delta E = -30 kcal mol(-1)). A dissociation/reassociation pathway is then explored, which leads, via O-Ge bond cleavage, to a rather stable complex between the two H(2)CO and LiGeH(3) moieties (-10 kcal mol(-1)). However, similarly 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 migration, which is as a consequence the preferred pathway. Also in the free anion the dissociation pathway was required to overcome an energy barrier higher than that for direct [1,2] shift by 8 kcal mol(-1). A similar situation had been found for silicon too. This result contrasts the description recently obtained for the Wittig carbon re arrangement that shows a sharp preference for a dissociation/reassociation mechanism.
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