Mechanisms of elimination reactions. 40. Attempted study of stereochemistry of elimination from 2-(p-nitrophenyl)ethyltrimethylammonium ion. Base-promoted cis–trans isomerization of p-nitrostyrene-β-d

Stereospecifically deuterated ArCHDCHDNMe3+ I − and ArCHDCHDNMe2O have been prepared, where Ar=C6H5 and p-NO2C6H4. When Ar=C6H5, the elimination reaction of the quaternary salt with ethoxide in ethanol goes with >98% anti stereochemistry, and the Cope elimination of the amine oxide with >98% syn stereochemistry. When Ar=p-No2C6H4, however, both reactions lead to apparent 50:50 anti/syn product. Subjection of (E)-p-nitrostyrene-β-d to the conditions of both the ethoxide-promoted and Cope eliminations results in complete cis–trans equilibration. No loss of deuterium from p-nitrosryrene-α-d occurs under either set of conditions, excluding isomerization via an α-arylvinyl carbanion. The most likely mechanism for isomerization is reversible addition of ethoxide under E2 conditions and ArCHDCHDNMe2O under Cope conditions to the β-carbon of p-nitrostyrene. The cis–trans isomerization of the p-nitrosryrene is sufficiently rapid to preclude determination of the stereochemistry of base-promoted eliminations leading to it.