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At a Glance
Researchers at Colorado State University have developed dimethyl-dihydroacridines, a new family of organic photoredox catalysts (PCs), which enable controlled polymerization of challenging acrylate monomers via Organocatalyzed Atom Transfer Radical Polymerization (O-ATRP). Structure-property relationships of these PCs demonstrate tunable photochemical and electrochemical properties. In application to O-ATRP, a combination of PC choice, implementation of continuous-flow reactors, and promotion of deactivation produce well-defined acrylate polymers. The utility of this approach has been demonstrated through the application of this novel system to diverse acrylate monomers as well as the synthesis of well-defined di-and triblock copolymers.
The ability of photoredox catalysis to manipulate electron or energy transfer reactivity has revolutionized small molecule and macromolecular chemistry, presenting opportunities to develop new chemical transformations under mild and energy efficient reaction conditions. Recently, photoredox catalysis has been applied in controlled radical polymerization (CRP) approaches for the light-regulated synthesis of well-defined polymers, most commonly in atom transfer radical polymerization (ATRP) and reversible addition-fragmentation transfer (RAFT).
Organocatalyzed atom transfer radical polymerization (O-ATRP) is a metal-free variant of photoredox-catalyzed ATRP which eliminates the concern of trace metal contamination in the polymer product and is advantageous in electronic and biomedical applications, while also enabling opportunities for “greener” reaction design in polymer synthesis. Induced by light, O-ATRP relies on a strongly-reducing organic PC to mediate an oxidative quenching catalytic cycle. O-ATRP processes following a reductive quenching pathway have also been reported but rely on the presence of stoichiometric quantities of sacrificial electron donors, which can also induce undesirable side reactions.
Photoredox catalysis offers broad benefits in chemical transformations which stem from the ability to use UV and visible light to support benign reaction conditions and novel synthetic transformations. This is accomplished through the use of a photocatalyst (PC) which can absorb light to mediate these transformations through single electron transfer events. The PC must possess specific light absorption, oxidation and reduction potentials, and electronic charge-transfer characteristics to be suitable for the intended transformation.
Access of diverse PC characteristics is necessary in order to be suitable for different chemical transformations that follow varying mechanistic pathways with a broad scope of reactivity. This innovation highlights development of novel photoredox PCs belonging to acridine and carbazole structural families which possess a wide variety of absorption, redox, and charge-transfer characteristics. This is accomplished through computationally directed discovery, followed by the synthesis and characterization of PCs that are predicted to show the desired properties. These PCs can be used to enable small molecule carbon-nitrogen bond forming reactions, as well as in the polymerization of traditionally-challenging butyl acrylate in organocatalyzed atom transfer radical polymerization (O-ATRP). For both families, a well-understood 1 or 3 step synthesis is used to synthesize the PCs to high reaction yields. Using DFT computational calculations, the properties of many PCs with a variety of neutral, electron donating, and electron withdrawing substitutents were modeled.
Development of these new PCs have broad applications in chemical synthesis, particularly the pharmaceutical industry and in drug discovery research. Within the same PC families with similar structural features, a variety of key properties can be accessed, which allows for rational PC tuning based on the desired chemical transformation.
- Utilization of sustainable materials (unlike current photoredox catalysts, e.g., iridium or ruthenium)
- Strong oxidizing potential applicable to a wide scope of potential substrates
- Variety of key properties can be accessed
- Allows rational PC tuning based on the desired chemical transformation
- Broad applications in chemical synthesis
- Pharmaceutical industry
- Drug discovery research
D. A. Corbin & G. M. Miyake. (2022) Photoinduced Organocatalyzed Atom Transfer Radical Polymerization (O-ATRP): Precision Polymer Synthesis Using Organic Photoredox Catalysis. Chemical Reviews. 122 (2), 1830-1874. DOI: 10.1021/acs.chemrev.1c00603
B. Buss, et al. (2020) Cover Picture & Graphical Abstract: Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers (Angew. Chem. Int. Ed. 8/2020) https://doi.org/10.1002/anie.202080811
B. Buss, et al. (2019). Dimethyl Dihydroacridines as Photocatalysts in Organocatalyzed Atom Transfer Radical Polymerization of Acrylate Monomers. Angew. Chem. Int. Ed. 59, 3209. https://doi.org/10.1002/ange.201910828
Last updated: October 2022