Unprecedented Copper/Iron-Cocatalyzed Homocoupling of Propargyl Ethers to Conjugated Benzofurans.

A dual-catalytic strategy has been developed to access conjugated enyne benzofurans via an unprecedented unsymmetrical homocoupling of ortho-iodophenyl propargyl ethers. An earth-abundant Cu-(II)/Fe-(III) dual-catalyst system enables high chemoselectivity and efficiency. Detailed mechanistic investigations, including control experiments and comprehensive density functional theory (DFT) studies, reveal that the reaction proceeds through in situ generation of Cu-(I) species, followed by an annulation event. The transformation culminates in a unique three-carbon homologation, involving a second propargyl unit and facilitated by the Fe-(III) cocatalyst. A broad array of enyne benzofuran derivatives bearing diverse functional groups was synthesized in up to 90% isolated yield. The synthetic practicality and wide applicability of this methodology are further demonstrated through gram-scale synthesis and postfunctionalization of the enyne moiety.