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 |
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Authors: | Um Ik-Hwan Shin Young-Hee Park Jee-Eun Kang Ji-Sun Buncel Erwin |
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Institution: | Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea. ihum@ewha.ac.kr |
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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 (kobsd) 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 and kMOEt, 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‐NO2) increases in the order 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 3.1×103‐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). |
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Keywords: | alkali metals homogeneous catalysis kinetics nucleophilic substitution reaction mechanisms |
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