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

Abstract

Pseudo-first-order rate constants (kobsd) were measured for nucleophilic substitution reactions of O-Y-substituted-phenyl O-phenyl thionocarbonates (4a–4h) with alkali metal ethoxides (EtOM, M = Li, Na, or K) in anhydrous ethanol at 25.0 ± 0.1 °C. Plots of kobsd vs. [EtOM] exhibited upward curvature for the reaction of O-4-nitrophenyl O-phenyl thionocarbonate (4a) with EtOK in the presence of 18-crown-6-ether (18C6), but showed downward curvature for the reaction with EtOLi, indicating that the reaction is catalyzed by the 18C6-crowned K+ ion, but is inhibited by Li+ ion. The kobsd values were dissected into kEtO− and kEtOM, the second-order rate constant for the reaction with dissociated EtO− and ion-paired 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 transition-state model and the origin of the contrasting reactivity patterns found for the reactions of the C=O and C=S compounds, are discussed.

Revealing the Hydrolysis Mechanism of a Hg2+-Reactive Fluorescein Probe: Novel Insights on Thionocarbonated Dyes

“…Indeed, the coordination of mercury to sulfur increases the electrophilicity of the carbon atom of the CS bond, thus making the site more reactive toward hydrolysis and accelerating the process. This catalytic activity is supported by a study showing that alkali metal ions, thanks to their Lewis acidity, may catalyze the ethanolysis of substituted o -phenyl thionocarbonates . Moreover, the efficiency of the assisted hydrolysis follows Pearson’s qualitative HSAB concept, with soft acids allowing to coordinate a soft CS site to catalyze the nucleophilic attack.…”

“…This catalytic activity is supported by a study showing that alkali metal ions, thanks to their Lewis acidity, may catalyze the ethanolysis of substituted o-phenyl thionocarbonates. 38 Moreover, the efficiency of the assisted hydrolysis follows Pearson's qualitative HSAB concept, 39 with soft acids allowing to coordinate a soft CS site to catalyze the nucleophilic attack. The importance of the nature of the electrophilic center was confirmed by synthesizing the analogous dicarbonated fluorescein ester (compound 3, Scheme 5) and assessing its Hg 2+ -assisted hydrolytic reactivity.…”