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 2a-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 kEtO- 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-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 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 3.1×10 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). Concerted effort: The reactions of Y-substituted phenyl diphenylphosphinates (1a-g; see scheme) and diphenylphosphinothioates (2a-g) proceed through a concerted mechanism with TS 1 as a transition state. The nature of X and M in TS 1 determines whether metal-ion catalysis or inhibition occurs. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.