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

Add time:07/27/2019    Source:sciencedirect.com

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 SN2 attack mechanism was ruled out and a mechanism for a side-on attack to the CF3-S bond may possibly be applicable.

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