Semibenzylic pathway

The various mechanisms that have been proposed for the rearrangement have been discussed in full, and only a brief summary will be given here. Two main reaction mechanisms have been established (i) a symmetrical mechanism, also known as the cyclopropanone pathway and (ii) an unsymmetrical mechanism or semibenzylic pathway (Schemes 4 and 5). 

In the semibenzylic pathway there is a nucleophilic attack by the base at the carbonyl carbon, followed by a concerted 1,2-migration of a carbanionic moiety with concomitant expulsion of halide. Implicit in this mechanism is the requirement for an antiparallel arrangement of the C—C bond that is broken and the carbon-halogen bond. This further requires an inversion of configuration at the carbon center initially bonded to the halogen atom, and an elegant demonstration of this feature has been provided by Charpen-tier and coworkers (Scheme 9). ... 

In cyclic systems where ring strain may be large for the cyclopropanone pathway, the semibenzylic pathway is preferred, and a good example of this is shown in Scheme 10. Even when the ring strain is less severe, the semibenzylic pathway will be favored if there is a pseudo-equatorial halogen, thus allowing the new C—C bond to form from the pseudo-axial direction. 

The semibenzylic pathway was also used to prepare an intermediate in the synthesis of the core of tricycloclavulone. The rearrangement took place only on warming and was not successful when vinyllithium was used as the nucleophile. A similar approach was used in a synthesis of... 

For substrates that lack an enolizable a-proton or for which a cyclopropanone intermediate would be highly strained, Favorskii products can be formed instead through a semibenzylic (also known as quasi-Favorskii) mechanistic pathway. This pathway involves addition of the base to the carbonyl followed by migration of an alkyl or aryl group with concomitant displacement of the halide. 

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