SPOs as Non-Innocent Ligands in Chiral-at-Iridium Catalyzed Asymmetric Hydrogenations

A series of bis-cyclometalated chiral-at-metal iridium(III) complexes containing a coordinated secondary phosphine oxide (SPO) have been synthesized and evaluated as catalysts in the asymmetric transfer hydrogenation (ATH) of acetophenone. The catalytic results show that SPO ligands have a non-innocent role in activating the catalytic process. Additionally, it has been observed that for the same chiral descriptor (Delta-at-Ir or Lambda-at-Ir), the major enantiomer formed depends on the nature of the cyclometalating ligand. These enantiodivergent results contravene the general assumption that the chiral-at-metal core's chirality dictates the sense of the chiral induction. A combined analysis of the main structural features of the catalysts deduced from XRD structures and in situ NMR spectroscopy allowed us to propose a simplified catalytic cycle and a working hypothesis to explain the observed enantioselectivities. Secondary phosphine oxides (SPOs) are efficient non-innocent ligands in the asymmetric transfer hydrogenation (ATH) of acetophenone catalyzed by chiral-at-metal iridium(III) complexes. The enantiodivergent results obtained contradict the general assumption that there is a direct relationship between the chiral descriptor of the catalyst and that of the obtained product. image