Migration origin and terminus

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). 

For analysis of the photochemical reaction, the interaction of the hydrogen Is orbital with -tt3 of the allyl system is used. The interaction is bonding at both the migration origin and terminus, so the [ 1,3] sigmatropic rearrangement is photochemically allowed. 

The acid-catalyzed cyclodehydration of ( )- and meso-2,5-hexanediol has been shown to proceed with inversion of configuration in most cases, affording cis- and rra/if-2,S-dimethyltetrahydrofuran, respectively. However, various extents of racemization are observed in the actions of certain Lewis acids and H2S04. The stereospecific cyclization of diols to tetrahydrofurans occurs if phenylthio participation is involved. The reactions of 2-phenylthio-l,4-diols such as (118) with dimethyl sulfate afford (119) with net retention of configuration, whereas acid-catalyzed cyclizations of 4-phenylthio-l,3-diols such as (120) take place with PhS migration and hence inversion at both the migration origin and terminus (Scheme 50). °°... 

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. 

The inherent migratory aptitude of a group depends mainly on three factors (i) the intrinsic migratory aptitude, (ii) the bystander substituent on the migration origin, and (iii) the spectator substituent on the migration terminus. For reviews, see (a) Ref. 66a (b) Ref. 66d (see pp. 80-91)... 

Unlike hydrogen, which can migrate in only one way because of its spherical s orbital, carbon has two ways to migrate because it has a two-lobed p orbital. Carbon can simultaneously interact with the migration origin and the migration terminus using one of its lobes. 

A typical migration involved in a carbocation rearrangement is called a 1,2-shift because the origin and terminus of the migration are (typically, but not invariably) adjacent to each other. We will encounter a few examples 1,2-shifts involving main-group elements. A common reaction pattern is the following ... 

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

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