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 (kobsd) have been measured for reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides (EtOM, M = Li, Na, and K). The plot of kobsd 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 kEtO and kEtOK, 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 constants exhibit highly scattered points, while the Yukawa–Tsuno plots result in an excellent linear correlation with ρ = 2.11 and r = 0.21 for kEtO -, and ρ = 1.62 and r = 0.26 for kEtOK, implying that the reaction proceeds through a concerted mechanism. The catalytic effect (i.e., the kEtOK/kEtO - 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.