Ruthenium-Catalysed Dehydrogenative C–H Borylation of Arenes with Pinacolborane

A ruthenium complex prepared in situ from [RuCl2(p-cymene)]2 and TpMe2K [TpMe2 = hydrotris(3,5-dimethylpyrazolyl)borate] is efficient for aromatic C–H borylation with pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane). Arenes were borylated at more electron-rich positions. DFT calculations and kinetic isotope effect experiments suggest that the catalytic cycle should involve an electrophilic aromatic substitution with a borenium cation.