Carbanions Stevens

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. ... 

Recently Klein and Hauser 84> used the dimethylsulfinyl carbanion to convert 6 to 7 in 81—85% yield. This method is far more convenient for the synthesis of 7 than the sodium amide method, for the yield is comparable and the reaction can be carried out at room temperature. Lepley and Brodof 97> reported another method of preparing 7 in good yield. Their technique consisted of treating the ether soluble p-(tert-butyl)-phenoxide salt of 6 with butyllithium at low temperature (0—15 °C). An 88—92% yield of 7 was obtained under these conditions. At higher temperatures, however, the amount of 8 increases with decreasing 7. In this later work it was suggested 97> that two different paths lead to 7 and 8. The benzyl ylid is the precursor to the Stevens product 8 as suggested earlier. However, an ion pair intermediate is the precursor to 7. 

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. 

The base-promoted migration of a side-chain of an onium compound to what is now recognised as an adjacent carbanionic site was discovered by Stevens in 1928 on attempted Hofinann degradation of 50 (Ar = Ar = Ph) (reaction 42) although a closely related process (reaction 43) had been recorded earher . Subsequent... 

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