Alkali metal ion catalysis and inhibition in alkaline ethanolysis of O-Y-substituted-phenyl O-phenyl thionocarbonates: contrasting M+ ion effects upon changing electrophilic centre from C=O to C=S
Canadian journal of chemistry (Print)
https://mc06.manuscriptcentral.com/cjc-pubs Canadian Journal of Chemistry Pseudo-first-order rate constants (k obsd ) have been measured for nucleophilic substitution reactions of O-Y-substituted-phenyl O-phenyl thionocarbonates (4a-h) with alkali-metal ethoxides (EtOM; M = Li, Na, and K) in anhydrous ethanol at 25.0 ± 0.1 o C. Plots of k obsd vs. [EtOM] exhibit upward curvature for the reaction of O-4-nitrophenyl O-phenyl thionocarbonate (4a) with EtOK in the presence of 18-crown-6-ether
... crown-6-ether (18C6) but show downward curvature for the reaction with EtOLi, indicating that the reaction is catalyzed by 18C6-crowned-K + ion but is inhibited by Li + ion. The k obsd values have been dissected into k EtO − and k EtOM , the second-order rate constant for the reaction with dissociated EtOand ionpaired EtOM, respectively. The reactivity of EtOM toward 4a increases in the order EtOLi < EtONa < EtO -< EtOK < EtOK/18C6, which is in contrast to that reported previously for the corresponding reaction of 4-nitrophenyl phenyl carbonate (a C=O analogue of 4a), e.g., EtO -≈ EtOK/18C6 < EtOLi < EtONa < EtOK. The reaction mechanism including the transitionstate model, and the origin of the contrasting reactivity patterns found for the reactions of the C=O and C=S compounds are discussed.