Visible-Light Copper Nanocluster Catalysis for the C-N Coupling of Aryl Chlorides at Room Temperature
Arunachalam Sagadevan, Atanu Ghosh, Partha Maity, Omar F. Mohammed, Osman M. Bakr* and Magnus Rueping*
*KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia, Email: osman.bakrkaust.edu.sa, magnus.ruepingkaust.edu.sa
A Sagadevan, A. Ghosh, P. Maity, O. F. Mohammed, O. M. Bakr, M. Rueping, J. Am. Chem. Soc., 2022, 144, 12052-12061.
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Ultrasmall nanoclusters offer a high surface area and unsaturated active sites. A copper nanocluster-based catalyst enables C-N bond-forming reactions of aryl chlorides under visible-light irradiation at room temperature. A range of N-heterocyclic nucleophiles and electronically and sterically diverse aryl/hetero chlorides react to provide C-N coupling products in good yields.
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Synthesis and characterizations of [Cu61(StBu)26S6Cl6H14]:
(Cu61) cluster: The cluster was synthesised following previous literature. Initially 320 mg of the copper complex, [Cu(CH3CN)4]BF4, was taken in 22 mL glass vial and 15mL of the acetonitrile solvent was added to it. The solution was kept for stirring. After 5 min, 180 µL of the tert-butyl thiol (StBu) was added to the above solution under stirring condition. The solution was turned to yellow from colorless due to the dropwise addition of 1 mL of the trimethylamine. After 3h of stirring, the solvent was removed from the above solution using a rotary evaporator and 15 mL of the chloroform was added to the obtained yellow residue. Subsequent resting at room temperature turned the copper(I)-thiolate complex as a clear yellow solution (15mL) in approximately 20 minutes. The solution was transferred to a 50 mL conical flask and chloroform solution (5mL) of 140 mg of the borane tert-butylamine complex (reducing agent) was added to it. The reaction was continued for 4 h under stirring condition. After that, the solution was centrifuged and equally divided into 5 parts (4mL each). 3.5 mL of ethanol was added to each solution and kept for crystallization at room temperature (23 °C). Black crystals were observed after 12-14 days. The crystals were collected and washed 3-4 times with acetonitrile to remove the unwanted materials. The formation of the Cu61 cluster was confirmed using UV-vis and ESI MS measurements.
General C-N coupling procedure:
A 10 mL microwave vial was charged with a magnetic stirrer bar, Cu61NC, (0.0003 mmol, 0.1 mol%, 1.95 mg), K3PO4 (2.0 equiv, 0.6 mmol, 127 mg) and arylamines (carbazole or indoles) (0.3 mmol, 1.0 equiv), aryl halides (X=Br/Cl) (0.45 mmol, 1.5 equiv) (if aryl bromide/chloride is solid or higher boiling liquid then added at this point before sealing the vial). The vial was then sealed and evacuated under vacuum for 10-15 minutes and backfilled with nitrogen five times. Acetonitrile (4 mL), aryl halides (X=Br/Cl) (0.45 mmol, 1.5 equiv) (if aryl bromide/chloride is lower boiling liquid then added last via syringe) were added via syringe through the septum. The reaction mixture was then irradiated with blue LEDs (40 W blue LED lamps (Kessil A160WE Tuna Blue; 5-7 cm away, with cooling fans to maintain the reaction at room temperature, 32-36 °C) under N2 (1 atm) for 24 hrs (until completion of the reaction, monitored by TLC or GC analysis). The mixture was filtered through celite and silica gel pads and washed with ethyl acetate. The filtrate was concentrated, and the residue was purified by column chromatography on silica gel to collect the expected C-N coupling products.