title: 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).
authors.affiliation: Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea. ihum@ewha.ac.kr
authors.forename: Ik-Hwan
authors.initials: IH
authors.lastname: Um
authors.forename: Young-Hee
authors.initials: YH
authors.lastname: Shin
authors.forename: Jee-Eun
authors.initials: JE
authors.lastname: Park
authors.forename: Ji-Sun
authors.initials: JS
authors.lastname: Kang
authors.forename: Erwin
authors.initials: E
authors.lastname: Buncel
mesh_terms: D002384:Catalysis; D007700:Kinetics; D008672:Metals, Alkali / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D009943:Organophosphorus Compounds / Q000737:chemistry*; D013329:Structure-Activity Relationship
chemical_list: D008672:Metals, Alkali; D009943:Organophosphorus Compounds