Quantification of the Nucleophilic Reactivities of Cyclic β-Keto Ester Anions
European Journal of Organic Chemistry, 2015, 34, 7594-7601
Kinetics of the reactions of enolate anions derived from acyclic and cyclic β-keto esters with benzhydrylium ions and quinone methides have been determined photometrically in dimethyl sulfoxide solution at 20 °C. The reactions follow second-order rate laws: first-order with respect to the electrophile and first-order with respect to the enolate. Reactions conducted in the presence of 18-crown-6 ether and in the presence of variable concentrations of K+ allowed the influence of ion-pairing on the nucleophilic reactivities of the various enolate ions to be determined quantitatively. Enolate ions derived from oxocycloalkanecarboxylic esters show reactivities similar to those of their acyclic analogue. Enolate ions derived from δ-lactones are less nucleophilic, which is explained by the enforced (E) conformation of the corresponding esters.