A PEG/copper(i) halide cluster as an eco-friendly catalytic system for C-N bond formation

The catalytic activities of eight copper(i) halide clusters assembled from copper(i) halide and ferrocenyltelluroethers, 1-8, were investigated in C-N formation under various conditions. A catalytic procedure using poly(ethylene glycol) (PEG-400) as a greener alternative organic solvent has been developed. The PEG-400/5 system can achieve 99% targeted yield with a mild reaction temperature and short reaction time. After the isolation of the products by extraction with diethyl ether, this PEG-400/cluster system could be easily recycled. Spectroscopic studies elucidate a stepwise mechanism: firstly, proton-coupled electron transfer (PCET) involving the transfer of an electron from Cu⁺ and a proton from imidazole results in the formation of a labile penta-coordinated Cu²⁺ and aryl radical; the following effective electron transfer from the ferrocene unit reduces Cu²⁺ and forms the target product; finally, the ferrocenium unit is reduced by the I^- anion. The merits of this eco-friendly synthesis are the efficient utilization of reagents and easy recyclability.