Photoredox Catalysts for Small Molecule and Macromolecular Transformations
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.
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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).
ATRP, the most widely studied CRP methodology, is used to access polymers with controlled properties, higher-order architectures, and consequently diverse applications. Traditionally, ATRP is operated through activation of a Cu(I) catalyst by heat to promote an inner-sphere electron transfer to generate a propagating radical species. However, in recent advances, new light-driven ATRP processes have been reported using photoredox catalysts (PC) derived from copper, ruthenium, or iridium.
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.
- 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
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: Sept 2021