From Prof. Jingsong You's group at Sichuan University, Chengdu, P.R. China
Another new method of Copper-catalyzed Ullman-type coupling of N-heterocycles with aryl and heteroaryl halides. The methodology employed proline-derived ligands in the reaction. Ligands were synthesized according to the follwing scheme.
The catalyst CuI and ligands were then screened for the best conditions using 5a and 6a as test coupling partners.
The most optimal conditions for coupling reaction of 5a and 6a were found to be the catalyst system generated in situ from 5 mol % of CuI and 10 mol % of 4a (see ligand synthesis above) in the presence of 2 equiv of Cs2CO3 in DMF at 110 °C under N2. These conditions were used to explore the scope of the reaction in coupling of imidazole with various aryl and heteroaryl halides (see table below).
(a) Reaction conditions: 5 (1.2 mmol), 6a (1.0 mmol), 2.0 mmol of Cs2CO3 in the presence of 10 mol % of ligand 4a and 5 mol % of CuI in 2.0 mL of DMF at 110 °C under N2 atmosphere for 24 h. (b) Isolated yields (average of two runs) based on 6a. (c) 48 h at 110 °C.
The scope was further investigated in the coupling reactions of bromobenzene (5a) and iodobenzene (5v) with various heteroaryl partners as shown below.
A very extensive paper.
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