Zinc catalysed electrophilic C–H borylation of heteroarenes

Cationic zinc Lewis acids catalyse the C–H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat). An electrophile derived from [IDippZnEt][B(C(6)F(5))(4)] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) combined with N,N-dimethyl-p-toluidine (DMT) proved the most active in terms of C–H borylation scope and yield. Using this combination weakly activated heteroarenes, such as thiophene, were amenable to catalytic C–H borylation using HBCat. Competition reactions show these IDipp–zinc cations are highly oxophilic but less hydridophilic (relative to B(C(6)F(5))(3)), and that borylation proceeds via activation of the hydroborane (and not the heteroarene) by a zinc electrophile. Based on DFT calculations this activation is proposed to proceed by coordination of a hydroborane oxygen to the zinc centre to generate a boron electrophile that effects C–H borylation. Thus, Lewis acid binding to oxygen sites of hydroboranes represents an under-developed route to access reactive borenium-type electrophiles for C–H borylation.