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

A series of molecular boron, aluminium, and gallium difluoride complexes [{(ArNCMe)2CH}MF2] (M = B, Al, Ga) are reported as catalysts for the defluorofunctionalisation of electron-deficient arenes. Thiodefluorination reactions between TMS–SPh and poly(fluorinated aromatics) proceed with group 13 catalysis under forcing conditions. Evidence is presented for the fluoride entering the catalytic cyclic through a metathesis reaction with TMS–SPh to form metal thiolate intermediates, e.g. [{(ArNCMe)2CH}MF(SPh)], that are then competent nucleophiles for addition to the aromatic substrate, likely through a cSNAr mechanism. Attempts to expand the scope of reactivity to include the hydrodefluorination of electron-deficient arenes with the same group 13 difluoride catalysts met with limited success. Activity could, however, be recovered through addition of NaBArF4 as a catalytic additive (ArF = 3,5-C6H3(CF3)2). NMR titrations suggest that NaBArF4 is capable of coordinating to aluminium 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 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.