Carbanions Stevens rearrangement

Simple 1,2-shifts of alkyl, from carbon to carbon, that are carbanionic in character are essentially unknown. Examples are known, however, in which alkyl is involved in a 1,2-shift from other atoms such as N and S to a carbanion atom—the Stevens rearrangement ... 

STEVENS REARRANGEMENT. Migration of an alkyl group from a quaternary ammonium salt to an adjacent carbanionic center on treatment with strong base. The product is a rearranged tertiary amine, sulfonium, or sulfide. 

The onium carbanion formed under phase-transfer conditions is unstable depending on the anion source, and in the absence of an electrophilic reaction partner, degradation of the accumulated onium carbanion in the organic phase may be observed. This is known to proceed via Hoffman elimination, nucleophilic substitution and/or Stevens rearrangement (Scheme 1.4) [4f,6,7]. The direct decomposition of onium salt, as influenced by the strong inorganic base at the interface, may be also operative. 

The ochotensine-type base (173 X = H2) has been obtained by Stevens rearrangement of the non-phenolic AT-methyltetrahydroprotoberberinium salt (172) with methyl sulphinyl carbanion (Scheme 19). Details have appeared of the benzocyc-lobutenyl route to (173 X = CH2) (c/. Vol. 4). The mass-spectral fragmentation pattern of various fumariline type alkaloids has been studied. ... 

A carbanionic mechanism similar to that proposed by Wittig and Krauss (33) involving a Steven s rearrangement can reasonably follow (Equation 5), i.e., an intramolecular attack on ylide (III) methyl forming dimethylethylamine. Protonation of dimethylethylamine followed by Hoffmann elimination (Equation 5a) would yield ethylene. 

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