Origin of the α-Effect in Nucleophilic Substitution Reactions of Y-Substituted Phenyl Benzoates with Butane-2,3-dione Monoximate and Z-Substituted Phenoxides: Ground-State Destabilization vs. Transition-State Stabilization

Second-order rate constants (k(Nu)) have been measured for nucleophilic substitution reactions or Y-substituted phenyl benzoates (1a-i) with butane-2,3-dione monoximate (Ox, an alpha-nucleophile) and Z-substituted phenoxides in 80 mol% H2O/20 mol% DMSO at 25.0 +/- 0.1 degrees C. Hammett plots correlated with sigma degrees and sigma constants for reactions of 1a-h with Ox exhibit many scattered points. In contrast, the Yukawa-Tsuno plot results in a good linear correlation with rho(Y)=2.20 and r =0.45, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS). A stepwise mechanism with expulsion of the leaving-group being the RDS has been excluded, since Y-substituted phenoxides are less basic and better nucleofuges than Ox(-). Thus, the reactions have been concluded to proceed through a concerted mechanism. Ox is over 102 times more reactive than its reference nucleophile, 4-chlorophenoxide (4-CIPhO-). One might suggest that stabilization of the transition-state (TS) through intramolecular general acid/base catalysis is responsible for the alpha-effect since such general acid/base catalysis is not possible for the corresponding reactions with 4-CIPhO-. However, destabilization of the ground-state (GS) of Ox(-) has been concluded to be mainly responsible for the alpha-effect found in this study on the basis of the fact that the magnitude of the alpha-effect is independent of the nature of the substituent Y.