Ketene-forming eliminations from aryl phenylacetates promoted by R2NH in MeCN. Origin of the curved Hammett plot

Abstract

Elimination reactions of YC₆H₄CH₂C(O)OC₆H₃-2-X-4-NO₂ promoted by R₂NH in MeCN have been studied kinetically. The reactions are second-order and exhibit Brønsted β = 0.52–0.87, |β_lg| = 0.37–0.65, and ρ_H = 1.9–4.1. The β and ρ_H values decrease as the leaving group becomes less basic and the |β_lg| values decrease with a stronger base. The results are interpreted in terms of an E1cb-like transition state with extensive C_β–H bond cleavage, significant negative charge development at the β-carbon, and limited C_α–OAr bond cleavage. For all reactions, the Hammett plots are curvilinear and show downward curvature. The curvature is more pronounced as the leaving group becomes poorer. The results have been attributed to the highly carbanionic transition state, which is moved towards the reactant on the horizontal reaction coordinate by a stronger electron-withdrawing substituent.