Electron-withdrawing group 3-Elimination

If alkyl groups are attached to the ylide carbon atom, cis-olefins are formed at low temperatures with stereoselectivity up to 98Vo. Sodium bis(trimethylsilyl)amide is a recommended base for this purpose. Electron withdrawing groups at the ylide carbon atom give rise to trans-stereoselectivity. If the carbon atom is connected with a polyene, mixtures of cis- and rrans-alkenes are formed. The trans-olefin is also stereoseiectively produced when phosphonate diester a-carbanions are used, because the elimination of a phosphate ester anion is slow (W.S. Wadsworth, 1977). 

This group is stable to TEA and morpholine in pyridine at 20°. It is cleaved by oxidation with MCPBA followed by elimination with TEA in Pyr, 10 min, 20°." The rate of cleavage is proportional to the strength of the electron-withdrawing group on the phenyl ring. ... 

This mechanism does not apply to unsubstituted or N,N-disubstituted aryl carbamates, which hydrolyze by the normal mechanisms. Carboxylic esters substituted in the a position by an electron-withdrawing group (e.g., CN or COOEt) can also hydrolyze by a similar mechanism involving a ketene intermediate. These elimination-addition mechanisms usually are referred to as ElcB mechanisms, because that is the name given to the elimination portion of the mechanism (p. 1308). 

The reactions of AN without electron-withdrawing groups at the j3-C atom proceed through the pathway 1. In this case, trimethylsilanol is eliminated from intermediate BENA to give silyl derivatives (566), which can easily be transformed into oximes (567) (504, 545). Evidently, the presence of bases would accelerate elimination of trimethylsilanol. 

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