Isolable diboryl radicals acting as highly efficient reaction intermediates under mild conditions

Activation of the B–B bond in diborane B2(OR)4 with dimesitylpyridylboranes 1 and 2 afforded the stable diboryl radicals 3 and 4 in moderate yields, respectively, which were studied by single crystal X-ray crystallography, EPR and UV/Vis spectroscopy, in conjunction with theoretical calculations. The diboryl radicals could react with the substrates p-benzoquinone, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and its derivative to form useful boron-containing reagents. Moreover, much higher yield was achieved in the catalytic reaction of p-benzoquinone and diborane with 1 or 2 in comparison to 4-cyanopyridine as the catalyst, in which the diboryl radicals act as reaction intermediates, highlighting the importance of the stability of the boryl radicals in improving the reaction efficiency.