Iron-catalysed regioselective thienyl C–H/C–H coupling

Regioselective thienyl–thienyl coupling is arguably one of the most important transformations for organic electronic materials. A prototype of ideal organic synthesis to couple two thienyl groups by cutting two C–H bonds requires formal removal of two hydrogen atoms with an oxidant, which often limits the synthetic efficiency and versatility for oxidation-sensitive substrates (for example, donor and hole-transporting materials). Here, we found that diethyl oxalate, used together with AlMe3, acts as a two-electron acceptor in an iron-catalysed C–H activation. We describe the regioselective thienyl C–H/C–H coupling with an iron(III) catalyst, a trisphosphine ligand, AlMe3 and diethyl oxalate under mild conditions. The efficient catalytic system accelerated by ligand optimization polymerizes thiophene-containing monomers into homo- and copolymers bearing a variety of electron-donative π motifs. The findings suggest the versatility of iron catalysis for the synthesis of functional polymers, for which the potential of this ubiquitous metal has so far not been fully appreciated. The coupling of substituted thiophenes is central to the synthesis of conducting polymers but relies on costly Pd-catalysed cross-couplings. Here, Earth-abundant iron and aluminium are shown to catalyse a regioselective thienyl C–H/C–H dimerization, enabling the synthesis of an array of π-conjugated polythiophenes.