C–N bond formation using copper complexes
Development of C–N bond formation chemistry is a continuing field of study. Recently copper based Chan-Evans-Lam cross-coupling was discovered, and although our understanding of the mechanism has grown, there are still many unanswered questions to be explored. Herein, we set out to develop a series of copper complexes that can stabilize different oxidation states in an attempt to probe how this impacts the mechanism. Initially complexes of copper(II) with 1,10 phenanthroline (phen) and related ligands were generated for use with CEL coupling. The complexes were probed electrochemically and photophysically and found that the 4,5-diazafluorenone complex 4 had a Cu2+/Cu+ potential +1.1 V relative to phen. We observed that the copper complex 4, which stabilize the Cu+, gave the greatest yield of the desired product. Following this study, we aimed to synthesize novel N-heterocyclic carbene copper complexes, which could better stabilize the copper(II) and copper(III) states. Several ligands and derivatives were synthesized, and copper(II) complexes were made using Cu(OAc)2 as a direct metalation agent. This complex was characterized via several spectroscopic methods and it was found to be a copper(II) complex. Initially this complex was thought to be relatively air stable but it was found to slowly decomposed to a novel di-copper bis-imidazolium complex over the course of 96 hours. While the identity of some of the intermediates are unknown, crystals of a potential end point were grown. A series of preliminary C–N coupling reactions show that the NHC copper(II) complexes are possible catalysts in alcoholic solvents.