Nucleophilic Substitution Reactions of Phenyl Y-Substituted-Phenyl Carbonates with Butane-2,3-dione Monoximate and 4-ChIorophenoxide: Origin of the a-Effect

摘要

Second-order rate constants have been measured spectrophotometrically for the reactions of phenyl Y-substituted-phenyl carbonates 7a-g with butane-2,3-dione monoximate (Ox~-) in 80 mol % H2O/26 mol % DMSO at 25.0 ± 0.1 °C. The a-nucleophile Ox~- is 53-95 times more reactive than the corresponding normal-nucleophile 4-ClPhO" toward 7a-g, indicating that the a-effect is operative. The magnitude of the a-effect (e.g., the k_(Ox)-/k_(4-CIPhO-ratio)) is independent of the electronic nature of the substituent Y. The cause of the a-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 Bronsted-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 a-effect exerted by Ox".