Visible-light photoredox catalysis offers a distinct activation mode complementary to thermal transition metal catalyzed reactions. The vast majority of photoredox processes capitalizes on precious metal ruthenium(II) or iridium(III) complexes that serve as single-electron reductants or oxidants in their photoexcited states. As a low-cost alternative, organic dyes are also frequently used but in general suffer from lower photostability. Copper-based photocatalysts are rapidly emerging, offering not only economic and ecological advantages but also otherwise inaccessible inner-sphere mechanisms, which have been successfully applied to challenging transformations. Moreover, the combination of conventional photocatalysts with copper(I) or copper(II) salts has emerged as an efficient dual catalytic system for cross-coupling reactions.
A visible-light-mediated photocatalyzed protocol utilizing copper−phenanthroline-based catalysts has been developed that can convert a large number of olefins into their chlorosulfonylated products. Besides the Cu(I) complex [Cu(dap) 2 ]Cl, now well-established in photo-ATRA processes, the corresponding Cu(II) complex [Cu(dap)Cl 2 ] proved to be often even more efficient in the title reaction, being advantageous from an economic point of view but also opening up new avenues for photoredox catalysis. Moreover, the copper complexes outperformed commonly used ruthenium, iridium, or organic dye based photocatalysts, owing to their ability to stabilize or interact with transient radicals by inner sphere mechanisms. The use of stoichiometric Na 2 CO 3 in combination with the copper photocatalysts was found to be essential to convert unactivated olefins to the desired products, in contrast to activated olefins for which no additive was required. As suggested by appropriate control experiments, the role of Na 2 CO 3 is attributed to prevention of poisoning of the catalyst.
The visible-light-accelerated oxo-azidation of vinyl arenes with trimethylsilylazide and molecular oxygen as stoichiometric oxidant was achieved. In contrast to photocatalysts based on iridium, ruthenium, or organic dyes, [Cu(dap) ]Cl or [Cu(dap)Cl ] were found to be unique for this transformation, which is attributed to their ability to interact with the substrates through ligand exchange and rebound mechanisms. Cu is proposed as the catalytically active species, which upon coordinating azide will undergo light-accelerated homolysis to form Cu and azide radicals. This activation principle (Cu -X→Cu +X ) opens up new avenues for copper-based photocatalysis.
An efficient method for the synthesis of substituted cyclobutanes from cinnamates, chalcones, and styrenes has been developed utilizing a visible-light triplet sensitisation mode. This reaction provides a diverse range of substituted cyclobutanes in high yields under mild conditions without the need of external additives. Good regioselectivity is obtained due to strong π-π-stacking of arene moieties, whereas diastereoselectivity relies on the electronic effects or ortho-substitution of the arene substrate. The utility of this transformation is demonstrated by the formal synthesis of the lignane natural product (±)-Tanegool.
A visible-light-induced photocatalytic method for the arylsulfonylation of heterocycles has been developed. The synthetic utility of this reaction is reflected by the direct use of commercially available sulfonyl chlorides and heterocycles under room temperature conditions. Complementarily, the photocatalytic arylation of heterocycles by sulfonyl chlorides via extrusion of SO is feasible at elevated temperature conditions, allowing switching between arylation or arylsulfonylation with excellent chemoselectivity.
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