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

Article abstract of DOI:10.1016/S0022-1139(96)03392-1

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-d₇. 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 CF₃-S bond may possibly be applicable.