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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 30, Number 12
BKCSDE 30(12)
December 20, 2009 

Origin of the α-Effect in Nucleophilic Substitution Reactions of Y-Substituted Phenyl Benzoates with Butane-2,3-dione Monoximate and Z-Substituted Phenoxides: Ground-State Destabilization vs. Transition-State Stabilization
Mi Sun Kim, Se Won Min, Jin A Seo, Ik Hwan Um*
The α-effect, Concerted mechanism, Ground-state destabilization, Solvent effect, Transition-state stabilization
Second-order rate constants (kNu? ) have been measured for nucleophilic substitution reactions of Y-substituted phenyl benzoates (1a-i) with butane-2,3-dione monoximate (Ox?, an α-nucleophile) and Z-substituted phenoxides in 80 mol% H2O/20 mol% DMSO at 25.0 ± 0.1 oC. Hammett plots correlated with σo and σ? constants for reactions of 1a-h with Ox? exhibit many scattered points. In contrast, the Yukawa-Tsuno plot results in a good linear correlation with ρY = 2.20 and r = 0.45, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS). A stepwise mechanism with expulsion of the leaving-group being the RDS has been excluded, since Y-substituted phenoxides are less basic and better nucleofuges than Ox?. Thus, the reactions have been concluded to proceed through a concerted mechanism. Ox? is over 102 times more reactive than its reference nucleophile, 4-chlorophenoxide (4-ClPhO?). One might suggest that stabilization of the transition-state (TS) through intramolecular general acid/base catalysis is responsible for the α-effect since such general acid/base catalysis is not possible for the corresponding reactions with 4-ClPhO?. However, destabilization of the ground-state (GS) of Ox? has been concluded to be mainly responsible for the α-effect found in this study on the basis of the fact that the magnitude of the α-effect is independent of the nature of the substituent Y.
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