A kinetic study on nucleophilic displacement reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides: Metal ion effect and reaction mechanism

Abstract

Pseudo-first-order rate constants (k obsd) have been measured for reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides (EtOM, M = Li, Na, and K). The plot of k obsd vs. [EtOM] curves upward for the reaction of diphenyl carbonate with EtOM but is linear for that with EtOK in the presence of 18-crown-6-ether (18C6), indicating that the reaction is catalyzed by M + ions and the catalytic effect disappears in the presence of 18C6. The kobsd values for the reactions with EtOK have been dissected into f EtO- and k EtOK, i.e., the second-order rate constants for the reactions with dissociated EtO - and ion-paired EtOK, respectively. The Hammett plots correlated with σ - and σ -0 constants exhibit highly scattered points, while the Yukawa-Tsuno plots result in an excellent linear correlation with p = 2.11 and r = 0.21 for k EtO-, and P = 1.62 and r = 0.26 for k EtOK, implying that the reaction proceeds through a concerted mechanism. The catalytic effect (i.e., the k EtOK/k EtOr ratio) is independent of the electronic nature of the substituent Y. Thus, it has been concluded that K + ion catalyzes the reaction by increasing the electrophilicity of the reaction center. © 2012 The Chemical Society of Japan.