Diverse copper(iii) trifluoromethyl complexes with mono-, bi- and tridentate ligands and their versatile reactivity

Abstract

Cu(III) trifluoromethyl complexes are proposed as essential intermediates for many copper-promoted trifluoromethylation reactions, but remain elusive and scarcely explored. We report herein the isolation and spectroscopic and X-ray crystallographic characterization of diverse Cu^III–CF₃ complexes (6–10) with varied coordinating motifs (mono-, bi- or tridentate) and formal charges (neutral or monoanionic) for the ligand L (L is pyridine, 2,4,6-trimethylpyridine, quinol-8-yloxide or 2,6-bis(2′-pyridyl)pyridine). Square planar complexes 6, 7 and 9 show rapid dynamic behavior in solution possibly due to the weak Cu–CF₃ bonds that can quickly dissociate and recombine with the CF₃ radical. In contrast, 10, with octahedral geometry, is more rigid. Reactivity studies show that except for ion-pair complex 8, they are highly reactive for the trifluoromethylation of arylboronic acids and the C–H bond of terminal alkynes. Furthermore, syn-fluoro- and -oxy-trifluoromethylation across the triple bond of alkynes can be achieved using 6 and CsF or NaOPh. These results greatly expand the scope of isolated and well-characterized Cu(III)–CF₃ complexes, and establish their versatile reactivity that may be widely involved in copper-mediated trifluoromethylation reactions, thus substantiating the essential yet undeveloped aspects of trifluoromethylation chemistry.