132803-38-4Relevant articles and documents
Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers.1 4. Mechanism of the reaction of aryl benzenesulfonates with alkali-metal ethoxides: Catalysis and inhibition by alkali-metal ions
Pregel, Marko J.,Dunn, Edward J.,Buncel, Erwin
, p. 3545 - 3550 (2007/10/02)
The rates of the nucleophilic displacement reactions of aryl benzenesulfonates (1a-f) with alkali-metal ethoxides (LiOEt, KOEt, and KOEt in the presence of completing agents) in anhydrous ethanol at 25 °C have been studied by spectrophotometric techniques. For all esters studied, the order of reactivity is LiOEt - +) and inhibition (Li+) are proposed to occur via reactive alkali-metal ethoxide ion pairs. Second-order rate constants for free ethoxide and metal-ethoxide ion pairs are calculated. Hammett treatment of leaving-group effects results in correlation of rate data with σ0 substituent constants and large ρ values of 3.0 (KOEt), 3.1 (LiOEt), and 3.4 (EtO-). A rate-determining transition state having well-advanced EtO-S bond formation but little S-OAr bond breakage is proposed. The similarity of the ρ values for KOEt, LiOEt, and EtO- implies that alkali-metal ions do not significantly alter the extent of S-OAr bond breakage in the transition state. However, metal ions do stabilize the transition state to differing degrees. Equilibrium constants for association of K+ and Li+ with the transition state are calculated, and it is concluded from the relative magnitudes of these values (Li+ +) that solvated metal ions interact with the transition state, rather than bare metal ions. Hammett plots of the free energy of association of metal ions with the transition state indicate that leaving-group substituent effects on metal ion binding in the transition state are small (ρ = -0.39 (K+) and -0.23 (Li+)) and lead to the conclusion that more electron-rich transition states bind metal ions more strongly.
