Organic Photoredox Catalysis
Dyes and Light Lead to New Reactivity
Historically, organic dyes were first used ~12,000 years ago to provide color to textiles. These colorful charged organic molecules come in a plethora of structures, colors and can fluoresce or phosphoresce. Light-absorbing organic dyes have been researched by chemists for decades and comprise the vast majority of fluorescent labeling agents used in a range of applications today. Our group is particularly interested in the excited state reactivity of these molecules in their role as single electron oxidants and reductants. Most organic dyes are commercially available or are easily synthesized - our laboratory even created an undergraduate laboratory to make pyrylium dyes at UNC!
For our review, see: Romero, N. A.; Nicewicz, D. A. “Organic Photoredox Catalysis.” Chem. Rev. 2016, 116, 10075-10166
Undergraduate laboratory on pyrylium synthesis: Cruz, C. L.; Holmberg-Douglas, N.; Onuska, N. P. R.; McManus, J. B.; MacKenzie, I. A.; Hutson, B. L.; Eskew, N. A.; Nicewicz, D. A. “Development of a Large Enrollment Course-Based Research Experiment in an Undergraduate Organic Chemistry Laboratory: Structure-Function Relationships in Pyrylium Photooxidants.” J. Chem. Ed., 2020, 6, 1572-1578.
Acridinium Photooxidants
We have been researching the use of organic photoredox catalysts for their ability to facilitate one electron processes in a range of organic tranformations. The Fukuzumi acridinium salt, 9-mesitylacridinium, has provided a wealth of new chemical reactivity and represents a truly unique photoredox catalyst scaffold. We’ve studied acridiniums for so long that by now, our lab members should all have tattoos of this structure (that is, if some of us haven’t done it already and not admitted it…). The excited state of these catalysts, a twisted intramolecular charge transfer state, is a potent single electron oxidant, with reduction potentials >+2.0 V vs SCE, capable of arene and alkene oxidation to give reactive cation radicals intermediates.
Alkene cation radical reactions: Margrey, K. A.; Nicewicz, D. A. “A General Approach to Catalytic Alkene Anti-Markovnikov Hydrofunctionalization Reactions via Acridinium Photoredox Catalysis.” Acc. Chem. Res. 2016, 49, 1997-2006.
This arene radical reaction started it all for us: Romero, N. A.; Margrey, K. A.; Tay, N. E.; Nicewicz, D. A. “Site-Selective Arene C–H Amination via Photoredox Catalysis.” Science,2015, 349, 1326-1330
Acridine Radical Reductants
Photophysical studies also led our group to propose that the reduced form of the catalyst, an acridyl radical, can absorb a second photon and achieve a new twisted intramolecular charge transfer state that is highly reducing (-3.36 V vs SCE); this state is more reducing than elemental lithium. This now allows the potent acridinium photooxidant to be employed as a catalytic super reductant in the presence of a sacrificial electron source. This discovery has led to a host of reactivity including challenging haloarene reduction and desulfonylation of sulfonamides.
For the our first report on acridine radical reductants: MacKenzie, I. A.; Wang, L.; Onuska, N. P. R.; Williams, O. F.; Begam, K.; Moran, A. M.; Dunietz, B. D.; Nicewicz, D. A. “Discovery and Characterization of Acridine Radical Photoreductants.” Nature, 2020, 580, 76-80
Alkene cation radical reactions: Margrey, K. A.; Nicewicz, D. A. “A General Approach to Catalytic Alkene Anti-Markovnikov Hydrofunctionalization Reactions via Acridinium Photoredox Catalysis.” Acc. Chem. Res. 2016, 49, 1997-2006.
This arene radical reaction started it all for us: Romero, N. A.; Margrey, K. A.; Tay, N. E.; Nicewicz, D. A. “Site-Selective Arene C–H Amination via Photoredox Catalysis.” Science,2015, 349, 1326-1330