C–H Borylation Catalysis of Heteroaromatics by a Rhenium Boryl Polyhydride

Transition metal complexes bearing metal–boron bonds are of particular relevance to catalytic C–H borylation reactions, with iridium polyboryl and polyhydrido-boryl complexes the current benchmark catalysts for these transformations. Herein, we demonstrate that polyhydride boryl phosphine rhenium complexes are accessible and catalyze the C–H borylation of heteroaromatic substrates. Reaction of [K­(DME)­(18-c-6)]­[ReH4(Bpin)­(η2-HBpin)­(κ2-H2Bpin)] 1 with 1,3-bis­(diphenylphosphino)­propane (dppp) produced [K­(18-c-6)]­[ReH4(η2-HBpin)­(dppp)] 2 through substitution of two equivalents of HBpin, and protonation of 2 formed the neutral complex [ReH6(Bpin)­(dppp)] 3. Combined X-ray crystallographic and DFT studies show that 2 is best described as a σ-borane complex, whereas 3 is a boryl complex. Significantly, the boryl complex 3 acted as a catalyst for the C­(sp2)–H borylation of a variety of heteroarenes (14 examples including furan, thiophene, pyrrole and indole derivatives) and displayed similar reactivity to the iridium analogues.