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Overcoming Limitations in Dual Photoredox/Nickel catalyzed C–N Cross-Couplings due to Catalyst Deactivation
preprintsubmitted on 13.11.2019, 13:49 and posted on 25.11.2019, 12:26 by Sebastian Gisbertz, Susanne Reischauer, Bartholomäus Pieber
Dual photoreodox/nickel catalyzed C–N cross-couplings are an attractive alternative to the palladium catalyzed Buchwald-Hartwig reaction, but are limited to aryl halides containing electron-withdrawing groups. We show that the formation of catalytically inactive nickel-black is responsible for this limitation. Deposition of nickel-black further deactivates heterogeneous photocatalysts restricting their recyclability. We demonstrate that catalyst deactivation can be avoided by the combination of nickel catalysis and a carbon nitride semiconductor. The broad absorption range of the organic, heterogeneous photocatalyst enables a wavelength dependent reactivity control to prevent nickel-black formation. A second approach is to run the reactions at high concentrations to increase the formation of nickel-amine complexes that reduce nickel-black formation. This allows reproducible, selective C–N cross-couplings of electron-rich aryl bromides.