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

A kinetic study of the nucleophilic substitution reaction of Y-substituted phenyl diphenylphosphinothioates 2a-g with alkali-metal ethoxides (MOEt; M=Li, Na, K) in anhydrous ethanol at (25.0+/-0.1)degrees 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 kobsd 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 2b (Y=4-NO2) 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 1b, that is, LiOEt>NaOEt>KOEt>EtO-. The current study based on Yukawa-Tsuno analysis has revealed that the reactions of 2a-g (P=S) and Y-substituted phenyl diphenylphosphinates 1a-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 1a-g are approximately 80-fold more reactive than the P=S compounds 2a-g toward the dissociated EtO- (regardless of the electronic nature of substituent Y) but are up to 3.1x10(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 (Eisenmans theory).