Nucleophilic substitution reactions of phenyl y-substituted-phenyl carbonates with butane-2,3-dione monoximate and 4-chlorophenoxide: Origin of the α-effect

Abstract

Second-order rate constants have been measured spectrophotometrically for the reactions of phenyl Ysubstituted- phenyl carbonates 7a-g with butane-2,3-dione monoximate (Ox-) in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. The α-nucleophile Ox- is 53-95 times more reactive than the corresponding normalnucleophile 4-ClPhO- toward 7a-g, indicating that the α-effect is operative. The magnitude of the α-effect (e.g., the kOx/k4-ClPhO ratio) is independent of the electronic nature of the substituent Y. The cause of the α- effect for the reactions of 7a-g has been suggested to be ground-state (GS) effect rather than transition-state (TS) stabilization through a six-membered cyclic TS, in which Ox- behaves a general acid/base catalyst. This idea is further supported by the result that OH- exhibits negative deviation from the linear Brønsted-type plot composed of a series of aryloxides, while Ox- deviates positively from the linearity. Differential solvation of the GS of Ox- and 4-ClPhO- has been suggested to be responsible for the α-effect exerted by Ox-.