Reactions of aryl phenylacetates with secondary amines in MeCN. Structure-reactivity relationship in the ketene-forming eliminations and concurrent E2 and E1cb mechanisms

Elimination reactions of aryl esters of arylacetic acids 1 and 2 promoted by R(2)NH in MeCN have been investigated kinetically. The reactions are second-order and exhibit beta = 0.44-0.84, \beta(lg)\ = 0.41-0.50, and rho(H) 2.0-3.6. Bronsted beta and \beta(lg)\ decrease with the electron-withdrawing ability of the beta-aryl substituent. Hammett rho(H) values remain nearly the same, but the \beta(lg)\ value increases as the base strength becomes weaker. Both rho(H) and beta decrease with the change of the leaving group from 4-nitrophenoxide to 2,4-dinitrophenoxide. The results are consistent with an E2 mechanism and a reaction coordinate with a large horizontal component corresponding to proton transfer. When the base-solvent system is changed from R(2)NH-MeCN to R(2)NH/R(2)NH(2)(+)-70 mol % MeCN(aq), the Bronsted beta, rho(H), and \beta(lg)\ decrease. Finally, the ketene-forming elimination reactions from p-nitrophenyl p-nitrophenylacetate promoted by R(2)NH/R(2)NH(2)(+) buffers in 70 mol % MeCN(aq) have been shown to proceed by concurrent E2 and Elcb mechanisms.