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

Abstract

Second-order rate constants (kNu-) have been measured for nucleophilic substitution reactions of Y-substituted phenyl benzoates (1a-i) with butane-2,3-dione monoximate (Ox-, an α-nucleophile) and Z-substituted phenoxides in 80 mol% H2O/20 mol% DMSO at 25.0 ± 0.1 °C. Hammett plots correlated with σ° and σ- 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 ρY = 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-ClPhO-). One might suggest that stabilization of the transition-state (TS) through intramolecular general acid/base catalysis is responsible for the α-effect since such general acid/base catalysis is not possible for the corresponding reactions with 4-ClPhO-. However, destabilization of the ground-state (GS) of Ox- has been concluded to be mainly responsible for the α-effect found in this study on the basis of the fact that the magnitude of the α-effect is independent of the nature of the substituent Y.