Migration to Electron-Deficient Carbon

Pinacol rearrangement. Migration to electron-deficient carbon... 

Photorearrangement of 4-/>-cyanophenyl-4-phenylcyclohexene (69) took place mainly byp-cyanophenyl migration/67,70,74 The conclusion could then be drawn that the rearranging excited state is not electron deficient at the /3-carbon atom, since one would not expect a cyanophenyl group to migrate to a positive carbon. The/3-carbon was proposed to have odd electron character ... 

Many carbenes, like carbocations, rearrange to more stable structures by the migration of a neighboring group to the electron-deficient carbon. Thus phenylmethylcarbene rearranges to ethenylbenzene (styrene) ... 

In contrast to the results obtained in reactions of M—CF3 complexes with BF3 (see Section III,B,1), the reaction of the perfluoronickelacyclopentane complex 49 appears to proceed via fluoride abstraction, followed by phosphine migration to the electron-deficient carbon to afford the a-fluorophos-phonium ylid complex 50 and related derivatives (117). 

The common feature in such reactions is the attack of a hydride ion on a relatively electron-deficient carbon atom. The hydride may migrate from another carbon atom in the same molecule, from a carbon atom on another molecule, or even from a Lewis-type site (—Al—H ) on the catalyst. Whatever the source, the structural environment to which the hydride adds becomes more hydrogen-rich, and that from which it derives, more hydrogen-deficient. In Scheme 6, some of these reactions are tabulated. 

An electron-deficient carbon is most commonly generated by the departure of a leaving group which takes the bonding electrons with it. The migrating group is, of course, a nucleophile, and so a rearrangement of this sort amounts to intramolecular nucleophilic substitution. Now, as we have seen, nucleophilic substitution can be of two kinds, Sn2 and SnI. Exactly the same possibilities exist for a re-... 

This hypothesis was supported by the fact that 2-bromo-6,6-diphenylbicyclo[3.1.0]hexan-3-one on treatment with potassium terr-butoxide afforded the same bicyclic ketone 1 in 74% yield." In a study of the migratory aptitudes in the 5-(4-cyanophenyl)-6-phenyIbicyclo[3.1.0]hex-3-en-2-one system 3 it was found that only phenyl migration occurred, contrary to the cyclohex-2-enone systems (see Section 4.3.5.) where 4-cyanophenyl was shown to migrate preferentially. The migratory aptitudes observed here were highly suggestive of phenyl migration to the electron-deficient carbon centers of a zwitterionic intermediate." ... 

The diazomethane insertion into ketones and aldehydes to provide their homologs also involves migration of an alkyl group to an electron-deficient carbon center of a zwitterion through a pentacoordinate carbocation according to the mechanism depicted in Equation (6.29). 

A number of mechanistically related procedures involving the use of allylmetal reagents as the three-carbon unit in stereoselective [3 + 2] cycloadditions have been documented. In general, the additions to electron-deficient olefins occur in the presence of Lewis acids and appear to proceed via a carbocationic intermediate which on 1,2-migration of the carbon-metal bond and subsequent cyclization provides the corresponding functionalized cyclopentane ring. 

In the initial coordination, the 7r-electron cloud of the olefin overlaps with the outer bond orbital of the metal cation. This causes stretching and eventual rupture of the R-M (metal) bond. An intramolecular rearrangement follows with the migration of the caibanion (R to the most electron-deficient carbon atom of the double bond. A new covalent bond and a new caibanion are formed simultaneously ... 

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