Boron, Aluminum, and Gallium Fluorides as Catalysts for the Defluorofunctionalization of Electron-Deficient Arenes: The Role of NaBArF4 Promoters

A series of boron, aluminum, and gallium difluoride complexes [{(ArNCMe)2CH}­MF2] (M = B, Al, Ga) are reported as catalysts for the defluorofunctionalization of electron-deficient arenes. Thiodefluorination reactions between TMS–SPh and poly­(fluorinated aromatics) proceed under forcing conditions. Evidence is presented for the fluoride entering the catalytic cycle through a metathesis reaction with TMS–SPh to form metal thiolate intermediates, e.g., [{(ArNCMe)2CH}­MF­(SPh)], which are then nucleophiles for addition to the aromatic substrate, likely through a concerted SNAr mechanism. Attempts to expand the scope of reactivity to include the hydrodefluorination of electron-deficient arenes met with limited success. Activity could, however, be recovered through the addition of NaBArF4 as a catalytic additive (ArF = 3,5-C6H3(CF3)2). NMR titrations suggest that NaBArF4 is capable of coordinating with aluminum and gallium fluoride complexes, most likely through weak M–F---Na interactions (M = Al, Ga), and can play a role in lowering the barrier of metathesis between [{(ArNCMe)2CH}­MF2] and Et3SiH to form the group 13 hydrido fluoride [{(ArNCMe)2CH}­M­(H)­F], facilitating catalytic turnover. DFT calculations indicate that this weak interaction leads to a polarization of the M–F bond. The discovery of this additive effect has potentially broad implications in developing new reactivity and applications of thermodynamically stable metal fluorides.