Dual photoreodox/nickel catalyzed C–N cross-couplings are an attractive alternative to the palladium catalyzed Buchwald-Hartwig reaction, [1] but are limited to aryl halides containing electron-withdrawing groups. Here we show that the formation of catalytically inactive nickel-black is responsible for this limitation and causes severe reproducibility issues. We demonstrate that catalyst deactivation can be avoided by the combination of nickel catalysis and a carbon nitride semiconductor. [2] The broad absorption range of the organic, heterogeneous photocatalyst enables a wavelength dependent reactivity control to prevent nickel-black formation. A second approach, that is applicable to a broader set of substrates, is to run the reactions at high concentrations to increase the formation of nickel-amine complexes that reduce nickel-black formation. This allows reproducible, highly selective C–N cross-couplings of electron rich aryl bromides and enables efficient reactions of aryl chlorides. By combining an oscillatory pump with a microstructured plug flow photoreactor this semi-heterogeneous dual photoreodox/nickel catalyzed C–N cross-coupling was demonstrated in a multi-gram scale. [3]

References:
[1] J. Twilton, C. Le, P. Zhang, M. H. Shaw, R. W. Evans, D. W. C. MacMillan, Nat.Rev. Chem. 2017, 1, 0052. doi.org/10.1038/s41570-017-0052
[2] S. Gisbertz, S. Reischauer, B.Pieber, Nature Catal. 2020, 3, 611-620. doi.org/10.1038/s41929-020-0473-6
[3] C.Rosso, S.Gisbertz, J. D. Williams, H. P. L. Gemoets, W. Debrouwer, B. Pieber, C. O. Kappe, React. Chem. Eng. 2020, 5, 597-604. doi.org/10.1039/D0RE00036A