Tuning the Copper(II)/Copper(I) Redox Potential for More Robust Copper-Catalyzed C-N Bond Forming Reactions

摘要

Complexes of copper and 1,10-phenanthroline have been utilized for organic transformations over the last 50 years. In many cases these systems are impacted by reaction conditions and perform best under an inert atmosphere. Here we explore the role the 1,10-phenanthroline ligand plays on the electronic structure and redox properties of copper coordination complexes, and what benefit related ligands may provide to enhance copper-based coupling reactions. Copper(II) triflate complexes bearing 1,10-phenanthroline (phen), ([Cu(phen)(2)(OTf)]OTf, 1) and oxidized derivatives of phen including [Cu(edhp)(2)](OTf)(2) (2), [Cu(pdo)(2)](OTf)(2) (3), [Cu(dafo)(2)](OTf)(2) (4) were prepared and characterized. X-ray crystallographic data show these related ligands subtly impacted the coordination geometry of the copper(II) ion. Complexes 1-3 had only incremental changes to the redox properties of the copper ions, complex 4 showed a drastically different redox potential affording a remarkably air stable copper(I) complex. Complexes 1-4 were then used to catalyze the C-N bond forming cross coupling between imidazole and various boronic acid substrates, where the increased stability of the copper(I) species in complex 4 appears to better support Chan-Evans-Lam (CEL) cross couplings.