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

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 Bronsted beta=0.52-0.87, \beta(lg)\=0.37-0.65, and rho(H)=1.9-4.1. The beta and rho(H) values decrease as the leaving group becomes less basic and the \beta(lg)\ values decrease with a stronger base. The results are interpreted in terms of an E1cb-like transition state with extensive C-beta-H bond cleavage, significant negative charge development at the beta-carbon, and limited C-alpha-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.