Unprecedented Copper/Iron Co-Catalyzed Homocoupling of Propargyl Ethers to Conjugated Benzofurans

In this study, we explored chemical routes to synthesize conjugated enyne benzofurans from easily accessible ortho-iodophenyl propargyl ethers employing unsymmetrical homocoupling. To achieve this goal, we employed an inexpensive earth-abundant Cu(II)/Fe(III) based dual catalyst system. A detailed mechanistic investigation using several control experiments and a thorough DFT study suggests that the reaction proceeds via oxidative addition of the iodo-carbon moiety by a Cu(I) center, generated in situ, followed by a rare annulation reaction. The final steps involve an unprecedented 3-carbon homologation through the involvement of another propargyl moiety and the Fe(III) co-catalyst. An array of enyne-benzofuran derivatives consisting of variable functionalities was synthesized with up to 90% isolated yields. The synthetic practicality and immense applicability of this methodology were established by scale-up synthesis and derivatization of the corresponding enyne side chain.