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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 7
BKCSDE 32(7)
July 20, 2011 

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
Jun-Sung An, Gil Namkoong, Ji-Sun Kang, Ik-Hwan Um*
Alkali metal ion, Catalysis, Inhibition, Ion pair, HSAB principle
Pseudo-first-order rate constants (kobsd) 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 oC. The plot of kobsd 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 kobsd into kEtO_ and kEtOM (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 kEtOLi < kEtONa < kEtO_ ≈ kEtOK < kEtOK/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., kEtOLi > kEtONa > kEtOK > kEtO_ ≈ kEtOK/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.
2423 - 2427
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