The Role of the Counterion and of Silicon Chelation in the Selective Mono(trifluoromethylation) of Tartaric Acid Derived 1,4-Diketones

The addition of trifluoromethyl(trimethyl)silane (TFMTMS) to tartaric acid derived aromatic 1,4‐diketones was reported to be highly stereoselective but also chemoselective towards a monoaddition product. This chemoselectivity remained unexplained. Complementary experiments and monitoring of the reaction by electrospray ionization mass spectrometry (ESI‐MS) show that: i) the countercation (tetrabutylammonium (TBA+) or Cs+) associated to the initiator and the charged intermediates in the chain reaction plays a crucial role and ii) in the case of a tetrabutylammonium salt initiator, the second addition on the preformed monoadduct does occur but the resulting bis(trifluoromethyl)carbinolate is unable to evolve through the chain‐transfer process and it exhibits an intramolecular SiO interaction. One addition or two? The chemoselectivity (mono‐ or bisaddition) of the trifluoromethylation of tartaric acid derived aromatic 1,4‐diketones (see scheme) was studied by using chemical and ESI‐MS tools. Besides the crucial role of the counterion, ESI‐MS allowed the discrimination of an Si‐chelated intermediate, which collapsed to the monoadduct in the case of tetrabutylammonium fluoride (TBAF) or tetrabutylammonium difluorotriphenylsilicate (DFTPSi) initiation.