Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. 4. Mechanism of the reaction of aryl benzenesulfonates with alkali-metal ethoxides: catalysis and inhibition by alkali-metal ions

Abstract

The rates of the nucleophilic displacement reactions of aryl benzenesulfonates with alkali-metal ethoxides (LiOEt, KOEt, and KOEt in the presence of complexing agents) in anhydrous ethanol at 25 o C have been studied by spectrophotometric techniques. For all esters studied, the order of reactivity if LiOEt<EtO − <KOEt. Metal ion catalysis (K + ) and inhibition (Li + ) are proposed to ocur 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 σ 0 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