Kinetic study of trifluoromethylation with s-(trifluoromethyl) dibenzothiophenium salts

Abstract

The kinetic parameters were determined for C-trifluoromethylation of aniline with S-(trifluoromethyl)dibenzothiophenium triflate ( 1), its 3,7-dinitro derivative ( 2) and S-(trifluoromethyl)diphenylsulfonium triflate ( 3) in DMF- d 7. The higher reactivity of heterocyclic 1 compared with non-heterocyclic 3 could be explained on the basis of its greatly enhanced activation entropy (ΔS ≠: −11.2 cal mol −1 K −1 for 1; −47.1 for 3), but not its enhanced activation enthalpy (ΔH ≠: 21.2 kcal mol −1 for 1; 12.1 for 3). The aromatic delocalization of the heterocyclic ring may thus be only slightly disturbed by the S-trifluoromethyl substituent. The high reactivity of 2 was attributed to the great electron deficiency caused by two nitro groups in addition to the heterocyclic salt system (ΔH ≠ 17.0 kcal mol −1, ΔS ≠ −9.1 cal mol −1 K −1 for 2). The reaction mechanism is discussed; the conventional S N2 attack mechanism was ruled out and a mechanism for a side-on attack to the CF 3-S bond may possibly be applicable.