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

Abstract

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) [a] Previously, we have reported a discrete Cu 2+ phen complex, namely [Cu(phen) 2 OTf ]OTf (1), [14] which shows a good reactivity performing C-and N-atom transfer reactions. [15] However, the phen ligand can be easily oxidized to functionalize the central ring of this ligand system. The C=C bond between C5 and C6 positions can be oxidized to form the 5,6-epoxyphen (L2) [16] and 5,6-phendione (L3). [17] L3 can be further reacted to form the 4,5-diazafluorenone (L4) by the loss of CO. [18] These ligands Full Paper

Synthesis and Characterization of a Tetradentate, N-Heterocyclic Carbene Copper(II) Complex and Its Use as a Chan–Evans–Lam Coupling Catalyst

“…This trend continues when we moved on to coupling imidazole and benzimidazole to phenylboronic acid. These reactions produced surprisingly low yields, compared to the same coupling reactions catalyzed by copper­(II)-phen based systems . Once again, it seems that steric interactions may be playing a significant role in how 4 turns over during the CEL process.…”

“…These reactions produced surprisingly low yields, compared to the same coupling reactions catalyzed by copper(II)-phen based systems. 15 Once again, it seems that steric interactions may be playing a significant role in how 4 turns over during the CEL process. Finally, methylbenzylamine and benzamide were also coupled to phenylboronic acid to generate C−N bonds with 43% and 12% yields, respectively.…”

“…10,12−14 Previously, we reported a series of bidentate ligands including diazafluorenone, which stabilized the Cu + state more efficiently than phenanthroline, and this shift in potential was linked to increased catalytic yields for the CEL process. 15 Building on these results, we set out to develop copper complexes that could more efficiently stabilize the Cu 2+ (or Cu 3+ ) oxidation states using a novel N-heterocyclic carbene (NHC) based ligand system, which has inspired us to explore these strong σ donor ligands and their propensity to support CEL reactions.…”

“…The mechanistic role of each of these species is still actively debated. − Another issue with adopting the CEL coupling is that many using this reaction still employ a large excess of aryl boronic acids and 1–2 equiv of a copper “catalyst,” even though, under these conditions, copper should be considered more of a reagent than a catalyst. Several groups have made dedicated progress in exploring the ligands that support CEL reactivity, but overall, the scope of these reports is limited. ,− Previously, we reported a series of bidentate ligands including diazafluorenone, which stabilized the Cu + state more efficiently than phenanthroline, and this shift in potential was linked to increased catalytic yields for the CEL process . Building on these results, we set out to develop copper complexes that could more efficiently stabilize the Cu 2+ (or Cu 3+ ) oxidation states using a novel N -heterocyclic carbene (NHC) based ligand system, which has inspired us to explore these strong σ donor ligands and their propensity to support CEL reactions.…”

Novel Copper Complexes as Visible Light Photoinitiators for the Synthesis of Interpenetrating Polymer Networks (IPNs)

“…On the other hand, organometallic compounds are not really used in industry; in other words, manufacturers avoid incorporating metallic compounds in their synthetic formulations due to their potential toxicity and price [ 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ]. With their photochemical properties, such as high-absorption properties in the near-UV and visible range [ 65 , 66 , 67 , 68 ], long-lived excited states [ 69 , 70 , 71 , 72 , 73 , 74 ], suitable redox potentials [ 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 ], copper complexes can be used as photoinitiators (PIs)/photoredox catalysts able to produce active species, according to a catalytic cycle [ 90 , 91 ]. Therefore, it is very important to develop new metal-free photoinitiators or low-cost organometallic-based complexes [ 92 , 93 , 94 , 95 ].…”