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

Eliminations from (E)-2,4-Dinitrobenzaldehyde O-Aryloximes Promoted by R₃N 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 Br$\ddot{o}$nsted ${\beta}$ = 0.83-1.0 and ${\mid}{\beta}_{lg}{\mid}$ = 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 $Et_3N$-MeCN for a given substrate (1a-e).