Reactions of (E)-O-Arylbenzaldoximes with Secondary Amines in Acetonitrile. Effect of β-Aryl Substituents upon the Competition between E2 and S_NAr Reactions

Reactions of (E)-O-arylbenzaldoximes in which the O-aryl group is 2,4-dinitrophenyl (1a–d) and picryl (2a–d) with secondary amines in acetonitrile have been studied kinetically. The reactions proceeded via competing E2 and S_NAr mechanisms. For eliminations from la–d promoted by R₂NH in MeCN, the transition state was changed toward Elcb-like by a more electron-withdrawing β-aryl substituent and a stronger base. On the other hand, the transition states for eliminations from 2a–d and for the S_NAr reactions of all substrates were relatively insensitive to the variation of either β-aryl substituent or base strength. The yield of S_NAr product increased with base concentration, electron-withdrawing ability of the O-aryl group, and base strength. When the electron-withdrawing ability of the β-aryl substituent is increased, the yield of S_NAr product from 1a–d decreased, although no clear trend was observed for 2a–d. From these results, factors that influence the competition between E2 and S_NAr reaction pathways are assessed.