Organocopper(III) Spiro Complexes: Synthesis, Structural Characterization, and Redox Transformation

Abstract

Reductive elimination of Cu(III) intermediates is often proposed as a key step in many copper-catalyzed or -mediated formation of C-C or C-heteroatom bonds. However, there still lacks concrete evidence on this key step, mainly because Cu(III) complexes are usually too unstable to be isolated and structurally characterized. In this work, novel organocopper(III) spiro complexes and their analogous organocopper(I) spiro complexes were synthesized and structurally characterized. Single-crystal X-ray structural analysis revealed that the spiro Cu(III) atom adopted a distorted square-planar geometry while its corresponding spiro Cu(I) atom was tetrahedrally coordinated. A redox transformation between these spiro Cu(I) and Cu(III) complexes was experimentally observed by reacting with reductants or oxidants, respectively. As concrete evidence, the organocopper(III) spiro compounds were found to form C-C bonds intramolecularly via reductive elimination.

Csp3–Csp3 Bond-Forming Reductive Elimination from Well-Defined Copper(III) Complexes

“…[13][14][15][16][17][18][19][20][21] However, Cu III complexes with well-defined structures remain rather limited and most reported examples were stabilized by rigid macrocyclic chelating ligands or perfluorinated groups, 18,[22][23][24][25][26] few of which provide experimental evidence for reductive elimination of Cu III species to form C-C or C-heteroatom bonds. [27][28][29] Seminal work by Stahl and Ribas have shown that a series of Cu III -mono-aryl species stabilized by an electrondonating macrocyclic ligand can undergo C-heteroatom bondforming reductive elimination reactions. The Xi group has recently reported a novel organocopper(III) spiro complex that can undergo intramolecular C-C bond-forming reductive elimination.…”

Synthesis of 9‐Gallafluorenes via Group 12 Metal Biphenylenes

“…Later on it was shown, that direct salt elimination from 2,2′‐dilithiobiphenyl and boron halides resulted in 9‐halo‐9‐borafluorenes and derivatives . This method allowed synthesis of transition metal derivatives, too . Wagner successfully used silafluorenes as precursors for borafluorenes …”

C,C- and C,N-Chelated Organocopper Compounds

“…Although neutral N,N-chelated spiro copper complexes are relatively common, the anionic C,N- or C,C-cheated spiro counterparts are sporadic. As mentioned above, Xi’s group found that the C,C-bidentate ligand is an excellent platform to build up diversified coordination complexes across the periodic table [ 32 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. The chelating effect increases the thermal stability of these organometallic species, facilitating isolation and following characterization.…”

“…Starting from the 1,4-dilithio 1,3-butadienes 1 (dilithio reagent for short) and 0.5 equiv. of copper(I) salts, Xi’s group isolated the first series of spiro organocopper(I) compounds 2–4 ( Scheme 2 ) [ 59 , 60 ]. It seems that the terminal alkenyl carbon atom is favorable for LiBr coordination, probably because it is more electron-rich than the phenyl ones.…”