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. File list (2)download file view on ChemRxiv manuscript.pdf (1.56 MiB) download file view on ChemRxiv SI.pdf (8.70 MiB)

Visible light photocatalysis relies mainly on expensive noble metal complexes and organic dyes that are not recyclable. Heterogeneous semiconductors, which are mainly applied for artificial photosynthesis and wastewater treatment, are a promising sustainable alternative and gain increasing attention. Inorganic and organic semiconductors with suitable bandgaps are among the most widely studied heterogeneous photocatalysts due to their high stability and recyclability. More recently, microporous materials, such as conjugated organic polymers, covalent organic frameworks, and metal organic frameworks that can be tuned and designed on a molecular level showed promising results. This review provides an overview of the most common heterogeneous photocatalysts with a focus on their applicability in organic synthesis.

Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201902785. Scheme 1. Homogeneous versus semi-heterogeneous dual Ni/photocatalysis. Angewandte Chemie Communications Angewandte Chemie Communications 9577 Angew.C hem. Int.E d. 2019, 58,9 575 -9580