Alkali‐Metal‐Ion Catalysis and Inhibition in the Nucleophilic Displacement Reaction of Y‐Substituted Phenyl Diphenylphosphinates and Diphenylphosphinothioates with Alkali‐Metal Ethoxides: Effect of Changing the Electrophilic Center from PO to PS

Abstract

A kinetic study of the nucleophilic substitution reaction of Y-substituted phenyl diphenylphosphinothioates 2 a-g with alkali-metal ethoxides (MOEt; M = Li, Na, K) in anhydrous ethanol at (25.0±0.1) °C is reported. Plots of pseudo-first-order rate constants (k(obsd)) versus [MOEt], the alkali ethoxide concentration, show distinct upward (KOEt) and downward (LiOEt) curvatures, respectively, pointing to the importance of ion-pairing phenomena and a differential reactivity of dissociated EtO(-) and ion-paired MOEt. Based on ion-pairing treatment of the kinetic data, the k(obsd) values were dissected into k EtO - and k(MOEt), the second-order rate constants for the reaction with the dissociated EtO(-) and ion-paired MOEt, respectively. The reactivity of MOEt toward 2 b (Y = 4-NO(2)) increases in the order LiOEt<EtO(-)<NaOEt<KOEt<[18]crown-6-complexed KOEt, which differs to the reactivity order reported previously for the reaction of 4-nitrophenyl diphenylphosphinate 1 b, that is, LiOEt>NaOEt>KOEt>EtO(-). The current study based on Yukawa-Tsuno analysis has revealed that the reactions of 2 a-g (P=S) and Y-substituted phenyl diphenylphosphinates 1 a-g (P=O) with MOEt proceed through the same concerted mechanism, which indicates that the contrasting selectivity patterns are not due to a difference in reaction mechanism. The P=O compounds 1 a-g are approximately 80-fold more reactive than the P=S compounds 2 a-g toward the dissociated EtO(-) (regardless of the electronic nature of substituent Y) but are up to 3.1×10(3)-fold more reactive toward ion-paired LiOEt. The origin of the contrasting selectivity patterns is further discussed on the basis of competing electrostatic effects and solvational requirements as a function of anionic electric field strength and cation size (Eisenman's theory).

Kinetic Study on Nucleophilic Substitution Reactions of 4-Nitrophenyl X-Substituted-Benzoates with Potassium Ethoxide: Reaction Mechanism and Role of K+Ion

“…In contrast, the plots of k obsd vs. [EtOK] for the reactions of substrates bearing a strong electron-withdrawing Figure 1 is typical for alkaline ethanolysis of esters in which alkali-metal ions behave as a Lewis acid catalyst. [8][9][10][11][12] Thus, one can suggest that K + ion catalyzes the reactions of 7a-i with EtOK, although the catalytic effect becomes insignificant for the reactions of substrates possessing a strong electron-withdrawing substituent in the benzoyl moiety (e.g., 7h and 7i).…”

“…8 In contrast, we have shown that Li + ion inhibits the corresponding reaction of 4-nitrophenyl diphenylphosphinothioate (1b), while K + and 18-crown-ether (18C6) complex K + ions catalyze the reaction. 9 Similarly contrasting results have been reported for the reactions of 4-nitrophenyl diethyl phosphate (2a, paraoxon) and phosphorothioate (2b, parathion) with EtOM. 10 Thus, a cyclic transition state (i.e., TS I ) has been suggested to catalyze the reaction by increasing the electrophilicity of the reaction center on the basis of the contrasting M + ion effects found for the reactions of the P=O and P=S centered electrophiles.…”

“…10 Thus, a cyclic transition state (i.e., TS I ) has been suggested to catalyze the reaction by increasing the electrophilicity of the reaction center on the basis of the contrasting M + ion effects found for the reactions of the P=O and P=S centered electrophiles. [8][9][10] Effects of alkali-metal ions on sulfonyl-transfer reactions of 4-nitrophenyl benzenesulfonate (3) with EtOM have also been investigated. 11,12 Buncel et al have reported that M + ions catalyze the reaction and the catalytic effect increases as the size of M + ions increases, i.e., Li + < Na + < K + .…”

Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Benzoates with Potassium Ethoxide in Anhydrous Ethanol: Reaction Mechanism and Role of K+Ion

“…5a In contrast, we have shown that Li + ion inhibits the corresponding reaction of 4-nitrophenyl diphenylphosphinothioate (1b) while K + and 18C6-complexed K + ions catalyze the reaction. 8 A similar result has been reported for the reactions of 4-nitrophenyl diethyl phosphate (2a, paraoxon) and 4-nitrophenyl diethyl phosphorothioate (2b, parathion) with EtOM (M = Li, Na, K), e.g., M + ions catalyze the reactions of 2a in the order K + < Na + < Li + while the reaction of 2b is strongly inhibited by Li + ion but is catalyzed by K + and 18C6-complexed K + ions. 9 These demonstrate convincingly that the effect of M + ions is dependent on the nature of the electrophilic centers (e.g., P=O vs. P=S).…”

“…[1][2][3][4][5][6][7][8][9][10][11] Since Lewis acidity increases as the charge density increases, most studies have focused on reactions involving multivalent metal ions (e.g., Mg …”