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.
By the modification on benzo[d]oxazolyl-NHC-ligated cyclometalated Ir complexes, the N-methylation of ortho-substituted aromatic amines with methanol was remarkably improved.
Herein,
an unprecedented bimetallic bis-NHC Cp*Ir complex 1 bearing
2-arylbenzo[d]oxazolyl and NHC
ligands is reported. A significant increase in activity was observed
for N-methylation of amines and reduction of aldehydes
with MeOH catalyzed by 1 compared to the monometallic
analogues (2–11). Under the optimal
conditions, it showed to be highly effective in N-methylation of nitroarenes with MeOH as both C1 and H2 source. Substrates, including aromatic amines, ketones, and nitro
compounds with various functional groups, can be well-tolerated. Mechanistic
studies and DFT calculation highlight the significance of bimetallic
centers cooperativity.
N-Heterocyclic carbene (NHC), generated in situ from easily available N-heterocyclic imidazolium salt with air as terminal oxidant, has successfully been utilized as a cheap and efficient catalyst for one-pot aerobic oxidative synthesis of 2-arylbenzo[d]oxazoles, 2-substituted benzo[d]thiazoles, and 1,2-disubstituted benzo[d]imidazoles.
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