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

Abstract

Mesoporous silica (SBA‐15)‐supported iridium catalysts are prepared by grafting of the newly developed cyclometalated iridium 2‐aryl benzoxazole complexes with 3‐(triethoxysilyl)‐propyl‐isocyanate (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.

Recent advances in heterogeneous catalytic hydrogenation and dehydrogenation of N-heterocycles

“…Immobilization of an active homogeneous catalyst onto a solid support can combine the advantages of homogeneous and heterogeneous catalysis, which not only expose the active sites for catalytic transformations, but also allows the catalyst to be employed in successive reactions [33]. Recently, Ir complexes immobilized on SBA-15 (Ir/Si) have been fabricated, which show a good performance in the transfer hydrogenation of quinolines [35]. Notably, the catalyst is also reusable due to its immobilization.…”

“…In terms of the selective hydrogenation of quinoline bearing methyl or methoxy substituents, precious metal-based heterogeneous catalysts such as Pd [19][20][21][22][23][24][25], Pt [26,27], Ru [28][29][30][31][32], Ir [33], and Rh [34] have received a large amount of interest due to their excellent activity and selectivity. However, the selectivity is not satisfactory for functionalized quinoline compounds bearing easily reducible groups, such as alkenes, ketones, nitriles, aldehydes, amines, and halogens [35,36]. By careful selection of the support, modifying the properties of the support or the fabrication unique catalyst structures, Pd [24,37], Pt [38], Ru [39,40], Ir [41], and Rh-based [42] catalysts have been transformed into chemoselective catalysts.…”

Silica‐Supported Molecular Catalysts for Tandem Reactions

“…Very recently, Hou groups reported an interesting SBA‐15‐supported organoiridium catalyst. In this case, the homogeneous organoiridium complexes (Cp*Ir‐Bozp) obtained by the coordination of 4‐(benzo[ d ]oxazol‐2‐yl)phenol (Bozp) and [Cp*IrCl 2 ] 2 were postgrafted into mesoporous SBA‐15 through the use of 3‐(triethoxysilyl)‐propyl‐isocyanate as a linker, forming SBA‐15‐supported Cp*Ir‐Bozp/SBA‐15 catalyst (Figure ).…”

Cobalt Nanoparticles Apically Encapsulated by Nitrogen‐doped Carbon Nanotubes for Oxidative Dehydrogenation and Transfer Hydrogenation of N‐Heterocycles

“…Due to the high atomic efficiency, the hydrogenation of unsaturated N‐heterocycles has possibility to produce saturated N‐heterocycles derivatives . Traditional hydrogenation routes generally require a high pressure of hydrogen, probably forming the overreduction products . So CTH may be a fascinating alternative to solve this problem.…”

“…[4] Traditional hydrogenation routes generally require a high pressure of hydrogen, probably forming the overreduction products. [5] So CTH may be a fascinating alternative to solve this problem. Formic acid (HCOOH) always serves as a hydrogen source for CTH reaction via the following dehydro-genation reaction: HCOOH!H 2 + CO 2 .…”