Using highly reductive and colored photo-activated homoleptic copper(I) complexes in organic photochemistry – CRAC

The photo-induced transformation of organic substrates is a relevant method to synthesize commodity chemicals in a renewable way. The key compound in photochemistry is the photosensitizer PS which absorbs light and converts it into redox power.
For example, PS* (PS in its excited state) can oxidize a donor D, yielding the reduced photosensitizer PS(red) and the oxidized donor Dox: PS* + D -> PS(red) + D(ox) (equation 1). This photochemical mechanism is called reductive quenching of the excited state of PS. PS(red) can be a strong reductant, able to activate challenging electron acceptors A into their reduced radical form A(red). But PS are often based on expensive and toxic heavy metals. In addition, sacrificial reagents are often required, leading to chemical wastes.
In order to achieve photocatalytic reactions in a fully sustainable way, we propose a project dedicated to the reductive quenching process, where PS is based on cheap, Earth abundant species, and where all the molecular species involved in the photo-induced electron-transfers are valorized (no waste). This implies a careful choice of PS, D and A.
As PS, we chose homoleptic Cu(I) complexes CuL2+ where L is a diimine ligand (e.g. phenanthroline) because they are cheap, safe and versatile (varying L). In addition, their excited state (CuL2+)* is sufficiently long-lived to allow easy bimolecular reactions with appropriate substrates, and they are strongly colored in the visible. Finally, CuL2 (reduced CuL2+) is a strong reductant meaning that the reductive quenching step involving (CuL2+)* as PS* would lead to a strong reductive power at low cost and toxicity.
Importantly, the reductive quenching of the excited state of a homoleptic Cu(I) complex CuL2+ is difficult to implement because (CuL2+)* is a weak oxidant, unable to react with most known electron donors. This implies new donors D must be developed: we chose amine boranes and pro-aromatic compounds because they are versatile compounds (tunable redox properties) and well-known reductants.
As regards A, species which are challenging to reduce will be selected (alkyl and aryl halides) because they constitute a vast pool of readily available commodity chemicals which can be further functionalized to reach relevant target molecules.
Our main objectives are therefore (1) to implement cheap Cu complexes in a photocatalytic cycle based on reductive quenching; (2) to value products formed during the reduction of the excited photocatalyst (Dox) and its regeneration (Ared); (3) to use low-cost substrates such as organic halides as A or amine boranes as D and get highly valued molecules such as functionalized organoboranes; (4) to use photoflow for better kinetics, and easier upscaling.
To achieve these objectives, we split the overall work between 2 partners (P1: CEISAM, Cu(I) complexes chemistry and photochemistry. P2: ISM, borane chemistry). In WP1, we will optimize the reductive quenching steps by preparing new donors D (e.g. ligand boranes and pro-aromatics, with tuned redox properties) and new homoleptic Cu(I) complexes with stronger reductive power. In WP2, we will test those new complexes as PS and those new donors D with well-chosen challenging acceptors A and perform relevant photochemical coupling reactions (C-B, C-C and C-P). Mechanistic studies, especially in operando NMR spectroscopy, will help understand and improve the parameters of the photochemical reactions. In WP3, we will finally use all acquired knowledge from WP1 and 2 to implement industrially relevant photochemical reactions. Two additional WP are dedicated to the management of the project and the dissemination of the results.