Elimination Reactions of (E)-2,4,6-Trinitrobenzaldehyde O-Aryloximes Promoted by R3N/R3NH+in 70 mol% MeCN(aq). Effect of β-Aryl Group the Nitrile-Forming Transition-State

Abstract

Nitrile-forming eliminations from <TEX>$(E)-2,4,6-(NO_2)_3C_6H_2CH=NOC_6H_4-2-X-4-NO_2$</TEX> (1) promoted by <TEX>$R_3NH/R_3NH^+$</TEX> in 70 mol % MeCN(aq) have been studied kinetically. When X = <TEX>$NO_2$</TEX>, the reactions exhibited second-order kinetics as well as Br<TEX>$\"{o}$</TEX>nsted <TEX>${\beta}$</TEX> = 0.63 and <TEX>${\mid}{\beta}_{lg}{\mid}$</TEX> = 0.34-0.46, and an E2 mechanism is evident. As the leaving group was made poorer (X = H, Cl, and <TEX>$CF_3$</TEX>), Br<TEX>$\"{o}$</TEX>nsted <TEX>${\beta}$</TEX> value increased from 0.63 to 0.85-0.89 without much change in the <TEX>${\mid}{\beta}_{lg}{\mid}$</TEX> value E2, indicating that structure of the transition state changed to an E1cb-like with extensive <TEX>$C_{\beta}-H$</TEX> bond cleavage, significant negative charge development at the <TEX>${\beta}$</TEX>-carbon, and limited <TEX>$C_{\alpha}$</TEX>-OAr bond cleavage.