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 PO to PS
✍ Scribed by Prof. Ik-Hwan Um; Young-Hee Shin; Jee-Eun Park; Ji-Sun Kang; Prof. Erwin Buncel
- Publisher
- John Wiley and Sons
- Year
- 2011
- Tongue
- English
- Weight
- 398 KB
- Volume
- 18
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
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 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−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 (PS) and Y‐substituted phenyl diphenylphosphinates 1 a–g (PO) 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 PO compounds 1 a–g are approximately 80‐fold more reactive than the PS 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).