Ligand-to-metal ratio controls stereoselectivity: Highly functional-group-tolerant, iridium-based, (E)-selective alkyne transfer semihydrogenation

Herein, we present (E)-selective transfer semihydrogenation of alkynes based on an iridium complex in situ generated from [Ir(COD)Cl]2 and an unsymmetrical (bearing two different phosphorous centers), ferrocene-based phosphine ligand utilizing formic acid as a hydrogen donor. Interestingly, a ligand-to-metal ratio may be used to control the stereoselectivity of the semihydrogenation process: a ratio of 1:1 iridium to ligand led to the formation of (Z)-alkene as a major product, whereas a ratio of 1:2 gave exclusively (E)-alkene. The latter 1:2 catalytic system is distinguished by its unprecedented chemoselectivity and exceptional stereoselectivity, which is substantiated by the broad scope of tested substrates, including natural product derivatives. The intriguing difference in catalytic activity between unsymmetrical and symmetrical ferrocene-based ligands was attributed to divergent coordination and steric hindrance. The presented methodology constitutes a solution to the common limitations of the known catalytic systems for semihydrogenation.