Alkali metal ion catalysis and inhibition in nucleophilic substitution reactions of 3,4-Dinitrophenyl diphenylphosphinothioate with alkali metal ethoxides in anhydrous ethanol: Effect of changing electrophilic center from P=O to P=S

Abstract

Pseudo-first-order rate constants (k obsd) have been measured spectrophotometrically for nucleophilic substitution reactions of 3,4-dinitrophenyl diphenylphosphinothioate 9 with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol at 25.0 ± 0.1 °C. The plot of k obsd vs. [EtOM] is linear for the reaction of 9 with EtOK. However, the plot curves downwardly for those with EtOLi and EtONa while it curves upwardly for the one with EtOK in the presence of 18-crown-6-ether (18C6). Dissection of k obsd into k EtO- and k EtOM (i.e., the second-order rate constant for the reaction with dissociated EtO - and ion-paired EtOM, respectively) has revealed that the reactivity increases in the order k EtOLi < k EtONa < k EtO- ≈ k EtOK < k EtOK/18C6, indicating that the reaction is inhibited by Li + and Na + ions but is catalyzed by 18C6-crowned K + ion. The reactivity order found for the reactions of 9 contrasts to that reported previously for the corresponding reactions of 1, i.e., k EtOLi > k EtONa < k EtOK > k EtO- ≈ k EtOK/18C6, indicating that the effect of changing the electrophilic center from P=O to P=S on the role of M + ions is significant. A four-membered cyclic transition-state has been proposed to account for the M + ion effects found in this study, e.g., the polarizable sulfur atom of the P=S bond in 9 interacts strongly with the soft 18C6-crowned K + ion while it interacts weakly with the hard Li + and Na + ions.