【文章名】Effects on the reactivity by changing the electrophilic center from C=O to C=S: contrasting reactivity of hydroxide, p-chlorophenoxide, and butan-2,3-dione monoximate in DMSO/H2O mixtures.
Effects on the reactivity by changing the electrophilic center from C=O to C=S: contrasting reactivity of hydroxide, p-chlorophenoxide, and butan-2,3-dione monoximate in DMSO/H2O mixtures.
[摘要]：Second-order rate consts. have been measured spectrophotometrically for the reactions of O-p-nitrophenyl thionobenzoate (1, PNPTB) with HO-, butan-2,3-dione monoximate (Ox-, a-nucleophile), and p-chlorophenoxide (p-ClPhO-, normal nucleophile) in DMSO/H2O of varying mixts. at (25.0 ?0.1)癈. Reactivity of these nucleophiles significantly increases with increasing DMSO content. HO- is less reactive than p-ClPhO- toward 1 up to 70 mol% DMSO although HO- is over six pKa units more basic in these media. Ox- is more reactive than p-ClPhO- in all media studied, indicating that the a-effect is in effect. The magnitude of the a-effect (i.e., kOx-/kp-ClPhO-) increases with the DMSO content up to 50 mol % DMSO and decreases beyond that point. However, the dependency of the a-effect profile on the solvent for reactions of 1 contrasts to that reported previously for the corresponding reactions of p-nitrophenyl benzoate (2, PNPB); reactions of 1 result in much smaller a-effects than those of 2. Breakdown of the a-effect into ground-state (GS) and transition-state (TS) effects shows that the GS effect is not responsible for the a-effect across the solvent mixts. The role of the solvent has been discussed on the basis of the bell-shaped a-effect profiles found in the current study as well as in our previous studies, i.e., a GS effect in the H2O-rich region through H-bonding interactions and a TS effect in the DMSO-rich media through mutual polarizability interactions.
