RESEARCH PAPER Single electron transfer in S_NAr process: reactions of 2,4,6-tris(trifluoromethanesulfonyl)anisole with anilines in dimethyl sulfoxide

摘要

Second-order rate constants (k_1) have been measured spectrophotometrically for reactions of 2,4,6-tris(trifluoromethanesulfonyl)anisole 1 with 4-X-substituted anilines (X=OH, OCH_3, CH_3, H, I and C1) 2a-f in dimethyl sulfoxide (DMSO) at 25 °C. The Hammett plot is nonlinear, i.e. the anilines having an electron-donating substituent exhibit positive deviations from the log k_1 versus correlation. The Hammett plot consists of two intersecting straight lines, i.e. p=-1.40 for electron-withdrawing substituents (X=I and C1) while p=-3.26 for electron-donating substituents (X=OH, OCH_3 and CH_3). The Br?nsted-type plot is also nonlinear with a change in slope from a small value (β_(nuc) = 0.50) to a large value (β_(nuc)= 1.29 at pK_a=4.58). This change could be interpreted in terms of a change in mechanism from a polar S_NAr process to a single electron transfer. On the other hand, the experimental data show that the rate constants (k_1) values exhibit good correlation with the oxidation potentials (E°) of the aniline nucleophiles and are consistent with the suggested mechanism.