Homocoupling-free iron-catalysed twofold C–H activation/cross-couplings of aromatics via transient connection of reactants

Twofold C–H activation/cross-coupling of stoichiometric amounts of organic molecules, R1–H and R2–H, to form an R1–R2 product free of homocoupling products is a goal in the activation of unreactive C–H bonds, as it will dramatically simplify organic synthesis. No reliable strategy to eliminate the homocoupling side products effectively without recourse to the use of an excess of one reactant over another is known. We report herein that a transient connection of two reactants by an anionic group appended to one reactant achieves this goal under mildly oxidative iron-catalysed conditions, through the formation of a productive heteroleptic R1–M–R2 intermediate. We utilized an N-(quinolin-8-yl)amide anion for the temporary connection and cross-coupled a stoichiometric mixture of aromatics in high yield without any trace of homocoupling products. A short-step synthesis of several donor/acceptor thiophene compounds and carbon/sulfur-bridged flat conjugated systems illustrates the utility of this method to streamline organic synthesis.