Eliminations from (E)-2,4-dinitrobenzaldehyde O-aryloximes promoted by R ₃N in MeCN. Effects of β-aryl group and base-solvent on the nitrile-forming transition-state

Nitrile-forming eliminations from (E)-2,4-(NO ₂) ₂C ₆H ₂CH=NOC ₆H ₄-2-X-4-NO ₂ (1a-e) promoted by R ₃N in MeCN have been studied kinetically. The reactions are second-order and exhibit Brönsted β = 0.83-1.0 and |βlg| = 0.41-0.46. The results have been interpreted in terms of highly E1cb-like transition state with extensive Cβ- H bond cleavage and limited Nα-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 β-aryl group from Ph to 2,4-dinitrophenyl under the same condition or by the base-solvent system variation from EtO ^--EtOH to Et ₃N-MeCN for a given substrate (1a-e).