Catalytic Activity of Mono‐ and Bimetallic Rhodium(I) and Iridium(I) Complexes Bearing Carbon‐Ether‐Carbon (COC) Ligand for the Intramolecular Cyclization of 4‐Pentynoic Acid

The catalytic performance of a series of mono‐ and bimetallic Ir(I) and Rh(I) complexes bearing the (COC) bis‐triazolylidene ligand, coordinated in a bridging or chelating fashion, was evaluated in the cyclization of 4‐pentynoic acid. The chelated mononuclear cationic complex [Ir(cod)(COC)]PF6 (cod=1,5‐cyclooctadiene; COC=biscarbene ether) displayed the highest reaction rate for 4‐pentynoic acid to form γ‐methyl‐ene‐γ‐butyrolactone (5‐methylenedihydrofuran‐2(3H)‐one). Density Functional Theory calculations were carried out to rationalize the reaction mechanism which proceeds in three steps, namely coordination of the alkyne to the active catalyst followed by intramolecular cyclization and final reductive elimination. Its catalytic performance was further assessed against different substituted 4‐pentynoic acid derivatives. The catalytic activity of a set of mono‐ and bimetallic Rh(I) and Ir(I) complexes featuring carbene‐linker‐carbene (COC) bis‐triazolylidene ligands coordinated in a bridging or chelating fashion was assessed in the cyclization of 4‐pentynoic acid.