David C. Fabry

Dual Catalysis: Combination of Photocatalytic Aerobic Oxidation and Metal Catalyzed Alkynylation Reactions—CC Bond Formation Using Visible Light

A new dual catalytic system for efficient C[BOND]H functionalization was developed. The appropriate choice of two metal catalysts allows the oxidative alkynylation of tertiary amines under mild and sustainable reaction conditions.

Light‐Mediated Heterogeneous Cross Dehydrogenative Coupling Reactions: Metal Oxides as Efficient, Recyclable, Photoredox Catalysts in CC Bond‐Forming Reactions

Let there be light: A heterogeneous photocatalytic system based on easily recyclable TiO(2) or ZnO allows cross dehydrogenative coupling reactions of tertiary amines. The newly developed protocols have successfully been applied to various C-C and C-P bond-forming reactions to provide nitro amines as well as amino ketones, nitriles and phosphonates.

Synthesis of Indoles Using Visible Light: Photoredox Catalysis for Palladium-Catalyzed CH Activation†

A combined palladium- and photoredox-catalyzed C-H olefination enables the synthesis of indoles. By using visible light, the direct C-H activation of aromatic enamines can be achieved and a variety of indole derivatives can be obtained in good yields under mild reaction conditions.

Combining Rhodium and Photoredox Catalysis for CH Functionalizations of Arenes: Oxidative Heck Reactions with Visible Light

Direct, oxidative metal-catalyzed C-H functionalizations of arenes are important in synthetic organic chemistry. Often, (over-)stoichoimetric amounts of organic or inorganic oxidants have to be used in these reactions. The combination of rhodium and photoredox catalysis with visible light allows the direct C-H olefination of arenes. Small amounts (1 mol%) of a photoredox catalyst resulted in the efficient C-H functionalization of a broad range of substrates under mild conditions.

Merging Visible Light Photoredox Catalysis with Metal Catalyzed C–H Activations: On the Role of Oxygen and Superoxide Ions as Oxidants

Conspectus The development of efficient catalytic systems for direct aromatic C–H bond functionalization is a long-desired goal of chemists, because these protocols provide environmental friendly and waste-reducing alternatives to classical methodologies for C–C and C–heteroatom bond formation. A key challenge for these transformations is the reoxidation of the in situ generated metal hydride or low-valent metal complexes of the primary catalytic bond forming cycle. To complete the catalytic cycle and to regenerate the C–H activation catalyst, (super)stoichiometric amounts of Cu(II) or Ag(I) salts have often been applied. Recently, “greener” approaches have been developed by applying molecular oxygen in combination with Cu(II) salts, internal oxidants that are cleaved during the reaction, or solvents or additives enabling the metal hydride reoxidation. All these approaches improved the environmental friendliness but have not overcome the obstacles associated with the overall limited functional group and substrate tolerance. Hence, catalytic processes that do not feature the unfavorable aspects described above and provide products in a streamlined as well as economically and ecologically advantageous manner would be desirable. In this context, we decided to examine visible light photoredox catalysis as a new alternative to conventionally applied regeneration/oxidation procedures. This Account summarizes our recent advances in this expanding area and will highlight the new concept of merging distinct redox catalytic processes for C–H functionalizations through the application of visible light photoredox catalysis. Photoredox catalysis can be considered as catalytic electron-donating or -accepting processes, making use of visible-light absorbing homogeneous and heterogeneous metal-based catalysts, as well as organic dye sensitizers or polymers. As a consequence, photoredox catalysis is, in principle, an ideal tool for the recycling of any given metal catalyst via a coupled electron transfer (ET) process. Here we describe our first successful endeavors to address the above challenges by combining visible light photoredox catalysis with different ruthenium, rhodium, or palladium catalyzed C–H activations. Since only small amounts of the oxidant are generated and are immediately consumed in these transformations, side reactions of substrates or products can be avoided. Thus, usually oxidant-sensible substrates can be used, which makes these methods highly suitable for complex molecular structure syntheses. Moreover, mechanistic studies shed light on new reaction pathways, intermediates, and in situ generated species. The successful development of our dual catalysis concept, consisting of combined visible light photoredox catalysis and metal catalyzed C–H functionalization, provides many new opportunities for further explorations in the field of C–H functionalization.

CH Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis: In Situ Generation of the Oxidant

A combination of ruthenium and photoredox catalysis allowed the ortho olefination of phenols. Using visible light, the direct C-H functionalization of o-(2-pyridyl)phenols occurred, and diverse phenol ethers were obtained in good yields. The regeneration of the ruthenium catalyst was accomplished by a photoredox-catalyzed oxidative process.

Online monitoring and analysis for autonomous continuous flow self-optimizing reactor systems

In this review the recent progress in the field of self-optimizing reactor systems for continuous flow chemistry is presented. Particular focus is directed to the implementation of monitoring tools and realization of computer-controlled reaction optimizations without human interaction.

Immobilization and Continuous Recycling of Photoredox Catalysts in Ionic Liquids for Applications in Batch Reactions and Flow Systems: Catalytic Alkene Isomerization by Using Visible Light

A catalytic (E)- to (Z)-isomerization of olefins using a photoredox catalyst under mild reaction conditions is presented. A variety of (Z)-alkenes can be prepared in the presence of visible light. A new reaction system allows an easy and efficient scale-up, as well as a continuous flow process in which the photocatalyst is immobilized in an ionic liquid and continuously recycled by simple phase separation.

Blue light mediated C–H arylation of heteroarenes using TiO2 as an immobilized photocatalyst in a continuous-flow microreactor

Titanium dioxide was applied as an immobilized photocatalyst in a microstructured falling film reactor for the continuous-flow C–H arylation of heteroarenes with aryldiazonium salts as the starting material. Detailed investigations of the catalyst and a successful long-term run proved its excellent usability for this process. Very good yields up to 99% were achieved with broad substrate scope and were compared with batch synthesis. The transfer to the continuous-flow mode revealed an impressive boost in reactor performance solely resulting from the improved irradiation and contact of the catalyst, substrate and light.