Recyclable Silica-Supported Iridium Catalysts for Selective Reductive Transformation of Quinolines with Formic Acid in Water

Mesoporous silica (SBA-15)-supported iridium catalysts are prepared by grafting of the newly developed cyclometalated iridium 2-aryl benzoxazole complexes with 3-(triethoxysilyl)-propylisocyanate (TEPIC) as a linker. The supported catalysts are characterized by solid-state NMR spectroscopy, small-angle X-ray scattering (SAXS), nitrogen physisorption, FTIR spectroscopy, and electron microscopy analysis, which indicate that the coordination environment of the iridium center as well as the two-dimensional hexagonal pore structure of SBA-15 is maintained after the immobilization. These supported catalysts have been successfully applied to the selective transfer hydrogenation of quinoline derivatives to 1,2,3,4-tetrahydroquinolines (THQs) with formic acid in pure water. Moreover, by simply adapting the amount of formic acid used, N-formyltetrahydroquinolines (FTHQs) could be selectively synthesized from quinolines in two-step, one-pot transfer hydrogenation/N-formylation reactions with these supported catalysts. Notably, these supported catalysts showed better catalytic reactivity than their homogeneous analogs, and they can be easily reused 12 times in the transformation of quinoline to N-formyltetrahydroquinoline (FTHQ) without significant loss of activity.