Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis
| Autor: | Robert F. Higgins, Niels H. Damrauer, Eric M. Ferreira, Steven M. Fatur, Samuel G. Shepard, Matthew P. Shores, David J. Boston, Susan M. Stevenson, Anthony K. Rappé |
|---|---|
| Rok vydání: | 2016 |
| Předmět: |
010405 organic chemistry
Superoxide Singlet oxygen Photoredox catalysis chemistry.chemical_element General Chemistry 010402 general chemistry Electrochemistry Photochemistry 01 natural sciences Biochemistry Oxygen Catalysis 0104 chemical sciences chemistry.chemical_compound Colloid and Surface Chemistry chemistry Radical ion Excited state |
| Zdroj: | Journal of the American Chemical Society. 138:5451-5464 |
| ISSN: | 1520-5126 0002-7863 |
| Popis: | A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels-Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3](3+), where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr(3+)* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 μs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels-Alder cycloadduct radical cation to the final product and reforms oxygen. We compare the results of these studies with those from cycloadditions mediated by related Ru(II)-containing complexes and find that the distinct reaction pathways are likely part of a unified mechanistic framework where the photophysical and photochemical properties of the catalyst species lead to oxygen-mediated photocatalysis for the Cr-containing complex but radical chain initiation for the Ru congener. These results provide insight into how oxygen can participate as a sustainable reagent in photocatalysis. |
| Databáze: | OpenAIRE |
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