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

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 (Delta S-not equal: - 11.2 cal mol(-1) K-1 for 1; -47.1 for 3), but not its enhanced activation enthalpy (Delta H-not equal: 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 (Delta H-not equal 17.0 kcal mol(-1), Delta S-not equal -9.1 cal mol(-1) K-1 for 2), The reaction mechanism is discussed; the conventional S(N)2 attack mechanism was ruled out and a mechanism for a side-on attack to the CF3-S bond may possibly be applicable.