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Potent Reductants via Electron-Primed Photoredox Catalysis: Unlocking Aryl Chlorides for Radical Coupling

Nicholas G. W. Cowper, Colleen P. Chernowsky, Oliver P. Williams and Zachary K. Wickens*

*Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States, Email: wickenswisc.edu

N. G. W. Cowper, C. P. Chernowsky, O. P. Williams, Z. K. Wickens, J. Am. Chem. Soc., 2020, 142, 2093-2099.

DOI: 10.1021/jacs.9b12328



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Abstract

The energetic limitations of visible light can be circumvented by electrochemically priming a photocatalyst prior to excitation. This method enables the use of aryl chlorides with reduction potentials hundreds of millivolts beyond the potential of Na0 in carbon-carbon and carbon-heteroatom bond-forming reactions.

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General Procedure for Phosphorylation

Add NpMI (7.1 mg, 0.020 mmol, 5 mol %) to the cathodic chamber of the divided cell. If a solid, add the aryl chloride (0.40 mmol, 1.0 equiv) to the cathodic chamber. Add Bu4N•PF6 (155 mg, 0.40 mmol) to both the anodic and cathodic chambers. Add a stir bar to each chamber. Affix the electrode assembly and seal the cell (see H-cells and electrode assembly fabrication). Purge cells with nitrogen for 10 minutes; inlet needle into cathodic chamber, outlet needle into anodic chamber. If a liquid, add aryl chloride (0.40 mmol, 1.0 equiv) to the cathodic chamber. Add triethylphosphite (340 µL, 2.0 mmol, 5.0 equiv), and MeCN (4.0 mL) to the cathode. Add triethylamine (110 µL, 0.80 mmol, 2.0 equiv), and MeCN (4.0 mL) to the anode. Connect steel electrodes to a low-current power supply. With fan cooling, electrolyze at 0.8 mA and irradiate with a blue Kessil lamp for 14h (1 F/mol) or 8h (0.6 F/mol). Crude cathode and anode solution extracted from NaHCO3 (50 mL) with EtOAc (3×25mL). Organic layer washed with brine (50 mL) and dried over MgSO4. Concentrate crude material and dilute with 1:1 EtOAc/Hexanes (10 mL) to precipitate excess electrolyte, filter off solid and wash precipitate with 1:1 EtOAc/Hexanes. Concentrate crude product and purify by flash chromatography with silica.


Key Words

phosphorylation, electrochemistry, photochemistry, organocatalysis


ID: J48-Y2020