Migrating groups

G IS a migrating group it may be either a hydrogen or an alkyl group) Section 5 14 See Table 5 2... 

The preferred alignment of orbitals for a 1,2-hydride or 1,2-alkyl shift involves coplanarity of the p orbital at the carbocation ion center and the a orbital of the migrating group. 

The reaction is strictly intramolecular the migrating group R is never completely released from the substrate. The driving force is the formation of the more stable rearranged carbenium ion 4, that is stabilized by the hydroxy substituent. The... 

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier... 

In a rearrangement reaction, a group moves from one atom to another in the same molecule. Most are migrations from an atom to an adjacent one (called 1,2 shifts), but some are over longer distances. The migrating group (Y) may move with its... 

This process is sometimes called the Whitmore 1,2 shift Since the migrating group carries the electron pair with it, the migration terminus B must be an atom with only six electrons in its outer shell (an open sextet). The first step therefore is creation of a system with an open sextet. Such a system can arise in various ways, but two of these are the most important ... 

The question as to whether 9 is an intermediate or a transition state has been much debated. When R is aryl or vinyl, then 9 is probably an intermediate and the migrating group lends anchimeric assistance (see p. 414 for resonance stabilization of this intermediate when R is aryl). When R is alkyl, 9 is a... 

There has been much discussion of H as a migrating group. There is no conclusive evidence for the viewpoint that 9 in this case is or is not a true intermediate, though both positions have been argued (see p. 419). 

In summary then, 1,2 free-radical migrations are much less prevalent than the analogous carbocation processes, and are important only for aryl, vinylic, acetoxy, and halogen migrating groups. The direction of migration is normally toward the... 

The mechanism is generally regarded as involving formation of a carbene. It is the divalent carbon that has the open sextet and to which the migrating group brings its electron pair ... 

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