Carbanion 11,2 -migration

The most popular method for generation of a-thio-carbanion (migration terminus) is direct lithiation (deprotonation) with alkyllithium or lithium amide. These deprotonation methods are widely applicable to various substrates, not only benzyl or allyl sulfides , but also dithioacetals 142 which form 143 (equation 83), and a phosphonate substituted system 144 which gives 145 (equation 84). ... 

Note that a concerted [l,2]-carbanion migration is symmetry forbidden [35]. 

An aza-oxy-carbanion relay via tandem Michael addition/ring opening of cyclopropane and recyclization/carbanion migration/electrophile trapping has been developed by the... 

A recent paper [44] shows that the treatment of silyl thioketones 68 with lithium diethylphosphite proceeds via a thiophiUc attack followed by a thio-phosphate mercaptophosphonate (69 70) carbanionic rearrangement and the migration of the silyl group from the carbon to the sulfur atom leading to the S-silylated sulfanylphosphonate carbanion 71. The last step represents the first example of the thia-Brook rearrangement (Scheme 18). 

Just as in its carbonium ion twin, the carbanion rearrangement takes place with retention of configuration of the migrating group. This proves at once that the reaction is intramolecular and that the displacement takes place at the front side.428... 

Pyrroline-A-oxide (258) is isomerized into y-lactam (259) in the presence of lithium diisopropylamine (LDA) (470) and sodium trityl (471). In these reactions, deprotonation at C3 occurs, leading to carbanion (260). Then oxygen migration from Ni to C2 takes place via intermediate formation of oxaziridine... 

It is to be noted that after the formation of the carbanion the migrating group goes to the carbanion carbon without a electron pair. This is in sharp contrast to re-arrangements in carbocations where the migrating group moves with the electron pairs. The above rearranged carbanion may combine with a proton to complete the reaction. 

Crossover experiments have been used to establish that the novel N to C acyl migration reaction of acyclic imides (69), to give o -amino ketones (70), proceeds by intramolecular reaction of the base-generated carbanion. ... 

The formation of tluorinated Q -hydroxy-jS-imino esters (180) by treatment of fluorinated imino ethers (179) with lithium 2,2,6,6-tetramethylpiperidide has been reported. A possible explanation for this interesting intramolecular rearrangement is proposed in Scheme 64. Acyclic imides derived from primary benzylic amines and amino acid esters have been found to undergo a novel nitrogen to carbon acyl migration via a base-generated carbanion to yield the corresponding a-amino... 

Rearrangement of a-silyl oxyanions to a-silyloxy carbanions via a reversible process involving a pentacoordinate silicon intermediate is known as the [l,2]-Brook rearrangement, or [l,2]-silyl migration. 

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