Carbocations aryl group migration

The rearrangements that we have considered to date all have one feature in common the migration of an alkyl or aryl group, with its electron pair, to a carbon atom which, whether it be a carbocation or not, is electron-deficient. Another atom that can similarly become electron-deficient is nitrogen in, for example, R2N or RN (a nitrene, cf. carbenes above), and it might be expected that alkyl or aryl migration to such centres would take place, just as it did to R3C and R2C this is indeed found to be the case. 

The normal pathway for rearrangement of the substrates (1) is via loss of the heteroatom substituent with migration of an adjacent alkyl (or aryl) group, and concomitant ketone formation (Scheme 1, path a). An alternative migration (path b), is occasionally observed in protic media, i.e. protonation of the alcohol (1) and migration with loss of water generates the stabilized carbocation (2), which is then hydrolyzed. 

Examples Wagner-Meerwein rearrangement—a process in which a carbocation rearranges by a 1,2-migration of a hydrogen, alkyl or aryl group. 

The fundamental mechanism of the pinacol rearrangement is widely accepted and consists of protonation of an alcohol, departure of water to generate a carbocation, [1,2]-migration of an adjacent alkyl group, aryl group, or hydride, and deprotonation. 

The manner in which the extensively studied fenchyl carbocation rearranges is also highly sensitive to attached functionality as demonstrated in studies of the effects of a 2-aryl substituent at C(2) of the fenchyl system. In a study conducted by Starling, it was found that ort/ o-substituted aryl groups inhibited Wagner-Meerwein migration except when substituted with... 

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... 

This transformation has been shown (56JA2278) by 14C-labeIling to involve a 1,2-aroyl migration, rather than a 1,2-aryl shift which would necessitate a carbocation intermediate in which the charge is formally adjacent to the carbonyl group (Scheme 167). 

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