Borylation of fluorinated arenes using the boron-centred nucleophile B(CN)₃^2- - a unique entry to aryltricyanoborates

The potassium salt of the boron-centred nucleophile B(CN)₃^2- (1) readily reacts with perfluorinated arenes, such as hexafluorobenzene, decafluorobiphenyl, octafluoronaphthalene and pentafluoropyridine, which results in KF and the K⁺ salts of the respective borate anions with one {B(CN)₃} unit bonded to the (hetero)arene. An excess of K₂1 leads to the successive reaction of two or, in the case of perfluoropyridine, even three C-F moieties and the formation of di- and trianions, respectively. Moreover, all of the 11 partially fluorinated benzene derivatives, C₆F_6- H (n = 1-5), generally react with K₂1 to give new tricyano(phenyl)borate anions with high chemo- and regioselectivity. A decreasing number of fluorine substituents on benzene results in a decrease in the reaction rate. In the cases of partially fluorinated benzenes, the addition of LiCl is advantageous or even necessary to facilitate the reaction. Also, pentafluorobenzenes R-C₆F₅ (R = -CN, -OMe, -Me, or -CF₃) react via C-F/C-B exchange that mostly occurs in the para position and to a lesser extent in the meta or ortho positions. Most of the reactions proceed via an S_NAr mechanism. The reaction of 1,4-F₂C₆H₄ with K₂1 shows that an aryne mechanism has to be considered in some cases as well. In summary, a wealth of new stable tricyano(aryl)borates have been synthesised and fully characterized using multi-NMR spectroscopy and most of them were characterised using single-crystal X-ray diffraction.