Eliminations from (E)-2,4-Dinitrobenzaldehyde O-Aryloximes Promoted by R3N in MeCN. Effects of beta-Aryl Group and Base-Solvent on the Nitrile-Forming Transition-State

Abstract

Nitrile-forming eliminations from (E)-2,4-(NO2)(2)C6H2CH=NOC6H4-2-X-4-NO2 (1a-e) promoted by R3N in MeCN have been studied kinetically. The reactions are second-order and exhibit Bronsted beta = 0.83-1.0 and vertical bar beta(1g)vertical bar = 0.41-0.46. The results have been interpreted in terms of highly E1cb-like transition state with extensive C-beta-H bond cleavage and limited N-alpha-OAr bond cleavage. Comparison with existing data reveals that the structure of the transition state changes from E2-central to highly E1cb-like either by the change of the beta-aryl group from Ph to 2,4-dinitrophenyl under the same condition or by the base-solvent system variation from EtO--EtOH to Et3N-MeCN for a given substrate (1a-e).