Power-variable electrophilic trifluoromethylating agents. S-, Se-, and Te-(trifluoromethyl)dibenzothio-, -seleno-, and -tellurophenium salt system

Article abstract of DOI:10.1021/ja00059a009

S-, Se-, and Te-trifluoromethylated dibenzoheterocyclic onium salts, their derivatives, and related salts were synthesized by the direct fluorination of a mixture of 2-[(trifluoromethyl)thio- or seleno]biphenyls and triflic acid (TfOH) or HBF₄ etherate, by treatment of the corresponding sulfoxides and selenoxides with Tf₂O, by a new type of tellurium activation of 2-[(trifluoromethyl)telluro]biphenyl with Tf₂O and (CH₃)₂SO, or by derivation from the onium salts obtained. Examination of reactivity indicated the trifluoromethyl heterocyclic salts to be greatly reactive compared to nonheterocyclic salts and indicated that this heterocyclic salt system serves as a source of widely applicable trifluoromethylating agents. Their capacity to function as such varied remarkably and increased in the order of Te < Se < S and 2,8-dialkyl < 3,7-dialkyl < H < 3-NO₂ < 3,7-di-NO₂. For mixed heterocyclic salts, the orders differed, apparently being determined by the electron deficiency of the CF₃ group due to the electron-withdrawing or -donating effects of chalcogens and ring substituents, rather than the inherent nature of the chalcogens. Because of this variation, it was possible to trifluoromethylate a wide range of nucleophilic substrates differing in reactivity: carbanions, activated aromatics, heteroaromatics, enol silyl ethers, enamines, phosphines, thiolate anions, and iodide anions. The reaction mechanism is discussed, and a bimolecular ionic substitution mechanism competing with a free CF₃ radical chain mechanism is proposed. Thus, a new field, electrophilic trifluoromethylation, has been established by the present study.