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 promo ted by R(2)NH in MeCN have been investigated kinetically. The reaction s are second-order and exhibit beta = 0.44-0.84, \beta(lg)\ = 0.41-0.5 0, 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 wit h the change of the leaving group from 4-nitrophenoxide to 2,4-dinitro phenoxide. The results are consistent with an E2 mechanism and a react ion coordinate with a large horizontal component corresponding to prot on 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), a nd \beta(lg)\ decrease. Finally, the ketene-forming elimination reacti ons 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 concu rrent E2 and Elcb mechanisms.