Heteroleptic Copper(I)-Based Complexes for Photocatalysis: Combinatorial Assembly, Discovery, and Optimization

Tuning Co-Operative Energy Transfer in Copper(I) Complexes Using Two-Photon Absorbing Diimine-Based Ligand Sensitizers

Photocatalysis mediated by low energy light wavelengths has potential to enable safer, sustainable synthetic methods. A phenanthroline-derived ligand bathocupSani, with a large two-photon absorption (TPA) cross section was used to construct a heteroleptic complex [Cu(bathocupSani)(DPEPhos)]BF4 and a homoleptic complex [Cu(bathocupSani)2]BF4. The ligand and the respective homoleptic complex with copper exhibit two-photon upconversion with an anti-Stokes shift of 1.2 eV using red light. The complex [Cu(bathocupSani)2]BF4 promoted energy transfer photocatalysis enabling oxidative dimerization of benzylic amines, sulfide oxidation, phosphine oxidation, boronic acid oxidation and atom-transfer radical addition.

Copper(I) Photocatalyzed Bromonitroalkylation of Olefins: Evidence for Highly Efficient Inner-Sphere Pathways

We report the visible light-mediated copper-catalyzed vicinal difunctionalization of olefins utilizing bromonitroalkanes as ATRA reagents. This protocol is characterized by high yields and fast reaction times under environmentally benign reaction conditions with exceptional scope, allowing the rapid functionalization of both activated and unactivated olefins. Moreover, late-stage functionnalization of biologically active molecules and upscaling to gram quantities is demonstrated, which offers manifold possibilities for further transformations, e.g. access to nitro- and aminocyclopropanes. Besides the synthetic utility of the title transformation, this study undergirds the exclusive role of copper in photoredox catalysis showing its ability to stabilize and interact with radical intermediates in its coordination sphere. EPR studies suggest that such interactions can even outperform a highly favorable cyclization of transient to persistent radicals contrasting iridium-based photocatalysts.

Diquat Based Dyes: A New Class of Photoredox Catalysts and Their Use in Aerobic Thiocyanation

A series of new organic donor-π-acceptor dyes incorporating a diquat moiety as a novel electron-acceptor unit have been synthesized and characterized. The analytical data were supported by DFT calculations. These dyes were explored in the aerobic thiocyanation of indoles and pyrroles. Here they showed a high photocatalytic activity under visible light, giving isolated yields of up to 97 %. In addition, the photocatalytic activity of standalone diquat and methyl viologen through formation of an electron donor acceptor complex is presented.

Synthesis of Ketonylated Carbocycles via Excited-State Copper-Catalyzed Radical Carbo-Aroylation of Unactivated Alkenes

Carbocycles are core skeletons in natural and synthetic organic compounds possessing a wide diversity of important biological activities. Herein, we report the development of an excited-state copper-catalyzed radical carbo-aroylation of unactivated alkenes to synthesize ketonylated tetralins, di- and tetrahydrophenanthrenes, and cyclopentane derivatives. The reaction is operationally simple and features mild reaction conditions that tolerate a broad range of functional groups. Preliminary mechanistic studies suggest a reaction pathway beginning with photoexcitation of [CuI-BINAP]2 and followed by a single electron transfer (SET), radical aroylation of unactivated alkenes, radical cyclization, and re-aromatization, affording the desired ketonylated carbocycles.

Excited-State Copper Catalysis for the Synthesis of Heterocycles

Heterocycles are one of the largest groups of organic moieties with significant medicinal, chemical, and industrial applications. Herein, we report the discovery and development of visible-light-induced, synergistic excited-state copper catalysis using a combination of Cu(IPr)I as a catalyst and rac-BINAP as a ligand, which produces more than 10 distinct classes of heterocycles. The reaction tolerates a broad array of functional groups and complex molecular scaffolds, including derivatives of peptides, natural products, and marketed drugs. Preliminary mechanistic investigation suggests in situ generations of [Cu(BINAP)₂ ]⁺ and [Cu(IPr)₂ ]⁺ catalysts that work cooperatively under visible-light irradiation to facilitate catalytic carbo-aroylation of unactivated alkenes, affording a wide range of useful heterocycles.

Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis

The mainstream applications of visible-light photoredox catalysis predominately involve outer-sphere single-electron transfer (SET) or energy transfer (EnT) processes of precious metal RuII or IrIII complexes or of organic dyes with low photostability. Earth-abundant metal-based Mn Ln -type (M=metal, Ln =polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner-sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited-state lifetimes for use as effective photocatalysts. The generic process, termed visible-light-induced homolysis (VLIH), entails the formation of suitable light-absorbing ligated metal-substrate complexes (Mn Ln -Z; Z=substrate) that can undergo homolytic cleavage to generate Mn-1 Ln and Z. for further transformations.

Decarboxylative C-H Alkylation of Heteroarene N-Oxides by Visible Light/Copper Catalysis

This paper reports a highly site-selective alkylation of heteroarene N-oxides using hypervalent iodine(III) carboxylates to serve as an alkylating agent in the presence of a cheap copper catalyst under visible light conditions. This mild method proceeds at room temperature in an air atmosphere and can withstand various heteroarene N-oxides as well as various primary, secondary, and tertiary alkyl carboxylic acids. It also provides a practical method for enabling the rapid conversion of commercially available raw materials into medically relevant "drug-like" molecules.

New Photosensitizers Based on Heteroleptic CuI Complexes and CO2 Photocatalytic Reduction with [NiII (cyclam)]Cl2

Earth-abundant metal complexes have been attracting increasing attention in the field of photo(redox)catalysis. In this work, the synthesis and full characterisation of four new heteroleptic CuI complexes are reported, which can work as photosensitizers. The complexes bear a bulky diphosphine (DPEPhos=bis[(2-diphenylphosphino)phenyl] ether) and a diimine chelating ligand based on 1-benzyl-4-(quinol-2'yl)-1,2,3-triazole. Their absorption has a relative maximum in the visible-light region, up to 450 nm. Thus, their use in photocatalytic systems for the reduction of CO2 with blue light in combination with the known catalyst [NiII (cyclam)]Cl2 was tested. This system produced CO as the main product through visible light (λ=420 nm) with a TON up to 8 after 4 hours. This value is in line with other photocatalytic systems using the same catalyst. Nevertheless, this system is entirely noble-metal free.

Copper-Photocatalyzed Borylation of Organic Halides under Batch and Continuous-Flow Conditions

The copper-photocatalyzed borylation of aryl, heteroaryl, vinyl and alkyl halides (I and Br) was reported. The reaction proceeded using a new heteroleptic Cu complex under irradiation with blue LEDs, giving the corresponding boronic-acid esters in good to excellent yields. The reaction was extended to continuous-flow conditions to allow an easy scale-up. The mechanism of the reaction was studied and a mechanism based on a reductive quenching (Cu^I /Cu^I */Cu⁰ ) was suggested.