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

Second-order rate constants (kOx−) 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 (Ox–) in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. 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 ρX = 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 Brønsted-type plot for the reactions of Y-substituted-phenyl cinnamates (8a-8e) is linear with βlg = – 0.64, which is typical of reactions reported previously to proceed through a concerted mechanism. The α-nucleophile (Ox–) is more reactive than the reference normalnucleophile (4-ClPhO–). The magnitude of the α-effect (i.e., the kOx−/k4-ClPhO− 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 Ox– and 4-ClPhO– (i.e., GS effect) is not solely responsible for the α-effect but stabilization of transition state (TS) through a cyclic TS structure contributes also to the Y-dependent α-effect trend (i.e., TS effect). KCI Citation Count: 3