The α-Effect in Nucleophilic Substitution Reactions of Y-Substituted-Phenyl X-Substituted-Cinnamates with Butane-2,3-dione Monoximate

Abstract

Second-order rate constants (<TEX>$k_{Ox^-}$</TEX>) have been measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl X-substituted-cinnamates (7a-7e) and Y-substituted-phenyl cinnamates (8a-8e) with butane-2,3-dione monoximate (<TEX>$Ox^-$</TEX>) in 80 mol % <TEX>$H_2O$</TEX>/20 mol % DMSO at <TEX>$25.0{\pm}0.1^{\circ}C$</TEX>. The Hammett plot for the reactions of 7a-7e consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linearity with <TEX>${\rho}_X$</TEX>=0.85 and r=0.58, indicating that the nonlinear Hammett plot is not due to a change in the rate-determining step but is caused by resonance stabilization of the ground state (GS) of the substrate possessing an electron-donating group (EDG). The Br<TEX>${\o}$</TEX>nsted-type plot for the reactions of Y-substituted-phenyl cinnamates (8a-8e) is linear with <TEX>${\beta}_{lg}$</TEX> = -0.64, which is typical of reactions reported previously to proceed through a concerted mechanism. The <TEX>${\alpha}$</TEX>-nucleophile (<TEX>$Ox^-$</TEX>) is more reactive than the reference normal-nucleophile (<TEX>$4-ClPhO^-$</TEX>). The magnitude of the <TEX>${\alpha}$</TEX>-effect (i.e., the <TEX>$k_{Ox^-}/k_{4-ClPhO^-}$</TEX> ratio) is independent of the electronic nature of the substituent X in the nonleaving group but increases linearly as the substituent Y in the leaving group becomes a weaker electron-withdrawing group (EWG). It has been concluded that the difference in solvation energy between <TEX>$Ox^-$</TEX> and <TEX>$4-ClPhO^-$</TEX> (i.e., GS effect) is not solely responsible for the <TEX>${\alpha}$</TEX>-effect but stabilization of transition state (TS) through a cyclic TS structure contributes also to the Y-dependent <TEX>${\alpha}$</TEX>-effect trend (i.e., TS effect).