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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 () in 80 mol % /20 mol % DMSO at . The -nucleophile is 53-95 times more reactive than the corresponding normal-nucleophile 4- toward 7a-g, indicating that the -effect is operative. The magnitude of the -effect (e.g., the ratio) is independent of the electronic nature of the substituent Y. The cause of the -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 behaves a general acid/base catalyst. This idea is further supported by the result that exhibits negative deviation from the linear Brnsted-type plot composed of a series of aryloxides, while deviates positively from the linearity. Differential solvation of the GS of and 4- has been suggested to be responsible for the -effect exerted by .

bulletin of the korean chemical society

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