Migration origin

After the migration has taken place, the atom at the migration origin (A) must necessarily have an open sextet. In the third step, this atom acquires an octet. In the case of carbocations, the most common third steps are combinations with a nucleophile (rearrangement with substitution) and loss of H (rearrangement with elimination). 

The stereochemistry at the migration origin A is less often involved, since in most cases it does not end up as a tetrahedral atom but when there is inversion here, there is an Sn2 type process at the beginning of the migration. This may or may not be accompanied by an Sn2 process at the migration terminus B ... 

On the other two points, the evidence supports predominant inversion of configuration at both migration origin (a) and terminus... 

The KIE also increased with increasing temperature for cyclobutylfluorocar-bene (17-F), where ku/kD was measured for a 1,2-H(D) shift from a tertiary migration origin Eq. 18.72... 

The bystander substituent (Y) exerts a direct influence on the migrant group (M) at the migration origin. In contrast, a spectator substituent (X in Eq. 22)... 

The effectiveness of equatorial vs. axial bystanders at promoting the 1,2-H shift (MeO > Me > Ph) may be related to their ability to stabilize the partial positive charge that arises at the migration origin during the 1,2-H shift. In this scenario, the lone pairs of the MeO group are superior to the hyperconjugative and inductive properties of Me, whereas the conformationally dependent h-electron release of Ph is the least effective. 

A fluorine bystander is more effective at promoting the 1,2-H shift than a chlorine bystander (15 to 65-F vs. 15 to 65-C1) the F for H bystander substitution lowers the a by 2.5 kcal/mol. The advantage of fluorine is due to more effective donation of its 2p lone pair electrons toward the developing positive chaige at the migration origin during the 1,2-H (hydride) shift.80... 

Fig. 21 Bond-angle changes at the migration origin in four oxime structures [105] (see text) as a function of the lengthening of the N-OX bond. Reprinted with permission from Edwards et al. (1987).

This follows from the principle that bonds are formed only by overlap of orbitals of the same sign. Since this is a concerted reaction, the hydrogen orbital in the transition slate must overlap simultaneously with one lobe from the migration origin and one from the terminus. It is obvious that both of these lobes must have the same sign. 

A second related, but distinct, question is whether there is an energy minimum on the reaction path when the migrating group is bonded to both migration origin and terminus—that is, whether there is a bridged intermediate (8). 

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