C-substitution reactions of c,n-diaryl nitrones

C,N-Diaryl nitrones react rapidly with N-bromosuccinimide in an aprotic solvent to yield among other products, the E and Z isomers of the C-succinimidyl substituted nitrones and a small amount of the corresponding hydroxamic acid. The stereochemistry of the C-succinimidyl nitrones was confirmed by an X-ray structure determination of the C,N-(dimethoxyphenyl) derivative 8e. In the presence of the base DABCO the formation of hydroxamic acids is suppressed and the Z C-succinimidyl nitrone forms more rapidly than the more stable E isomer. Several mechanistic rationalisations of this observed stereochemistry are discussed.