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Alkali metal ion catalysis and inhibition in nucleophilic displacement reactions at phosphorus centers: Ethyl and methyl paraoxon and ethyl and methyl parathion

Title
Alkali metal ion catalysis and inhibition in nucleophilic displacement reactions at phosphorus centers: Ethyl and methyl paraoxon and ethyl and methyl parathion
Authors
Um I.-H.Shin Y.-H.Lee S.-E.Yang K.Buncel E.
Ewha Authors
엄익환
SCOPUS Author ID
엄익환scopusscopus
Issue Date
2008
Journal Title
Journal of Organic Chemistry
ISSN
0022-3263JCR Link
Citation
vol. 73, no. 3, pp. 923 - 930
Indexed
SCI; SCIE; SCOPUS WOS scopus
Abstract
(Chemical Equation Presented) We report on the ethanolysis of the P=O and P=S compounds ethyl and methyl paraoxon (1a and 1b) and ethyl and methyl parathion (2a and 2b). Plots of spectrophotometrically measured rate constants, k obsd versus [MOEt], the alkali ethoxide concentration, show distinct upward and downward curvatures, pointing to the importance of ion-pairing phenomena and a differential reactivity of free ions and ion pairs. Three types of reactivity and selectivity patterns have been discerned: (1) For the P=O compounds 1a and 1b, LiOEt > NaOEt > KOEt > EtO -; (2) for the P=S compound 2a, KOEt > EtO - > NaOEt > LiOEt; (3) for P=S, 2b, 18C6-crown-complexed KOEt > KOEt = EtO - > NaOEt > LiOEt. These selectivity patterns are characteristic of both catalysis and inhibition by alkali-metal cations depending on the nature of the electrophilic center, P=O vs P=S, and the metal cation. Ground-state (GS) vs transition-state (TS) stabilization energies shed light on the catalytic and inhibitory tendencies. The unprecedented catalytic behavior of crowned-K + for the reaction of 2b is noteworthy. Modeling reveals an extreme steric interaction for the reaction of 2a with crowned-K +, which is responsible for the absence of catalysis in this system. Overall, P=O exhibits greater reactivity than P=S, increasing from 50- to 60-fold with free EtO - and up to 2000-fold with LiOEt, reflecting an intrinsic P=O vs P=S reactivity difference (thio effect). The origin of reactivity and selectivity differences in these systems is discussed on the basis of competing electrostatic effects and solvational requirements as function of anionic electric field strength and cation size (Eisenman's theory). © 2008 American Chemical Society.
DOI
10.1021/jo702138h
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자연과학대학 > 화학·나노과학전공 > Journal papers
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