A general arene C-H functionalization strategy via electron donor-acceptor complex photoactivation.

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. However, when it comes to aryl radical formation, this strategy remains hamstrung by the electronic properties of the aromatic radical precursors, and electron-deficient aryl halide acceptors are required. This has prevented the implementation of a general synthetic platform for aryl radical formation. Our study introduces triarylsulfonium salts as acceptors in photoactive electron donor-acceptor complexes, used in combination with catalytic amounts of newly designed amine donors. The sulfonium salt label renders inconsequential the electronic features of the aryl radical precursor and, more importantly, it is installed regioselectively in native aromatic compounds by C-H sulfenylation. Using this general, site-selective aromatic C-H functionalization approach, we developed metal-free protocols for the alkylation and cyanation of arenes, and showcased their application in both the synthesis and the late-stage modification of pharmaceuticals and agrochemicals.

Advances in allylic and benzylic C-H bond functionalization enabled by metallaphotoredox catalysis.

Metallaphoto-catalysis has been established as a robust platform for efficient construction of a range of chemical bonds. Moreover, transformation of native functionalities such as C(sp3)-H bonds to produce functional molecules represents one of the most attractive strategies in organic synthesis. Merging two powerful methodologies, metallaphoto-catalyzed benzylic and allylic C(sp3)-H bond functionalizations provide a series of general and mild approaches for diversification of alkylbenzenes and alkenes.

Chemoselective Hydrogenation of Nitroarenes Using an Air-Stable Base-Metal Catalyst.

The reduction of nitroarenes to anilines as well as azobenzenes to hydrazobenzenes using a single base-metal catalyst is reported. The hydrogenation reactions are performed with an air-and moisture-stable manganese catalyst and proceed under relatively mild reaction conditions. The transformation tolerates a broad range of functional groups, affording aniline derivatives and hydrazobenzenes in high yields. Mechanistic studies suggest that the reaction proceeds via a bifunctional activation involving metal-ligand cooperative catalysis.

Synthesis of unsymmetrical ketones by applying visible-light benzophenone/nickel dual catalysis for direct benzylic acylation.

Herein, we report a dual catalytic system for the direct benzylic C-H acylation reaction furnishing a variety of unsymmetrical ketones. A benzophenone-derived photosensitizer combined with a nickel catalyst has been established as the catalytic system. Both acid chlorides and anhydrides are able to acylate the benzylic position of toluene and other methylbenzenes. The method offers a valuable alternative to late transition metal catalyzed C-H acylation reactions.

Photoredox/Nickel Dual-Catalyzed Reductive Cross Coupling of Aryl Halides Using an Organic Reducing Agent.

A successful protocol for the reductive aryl-aryl cross-coupling of polyfluorinated arenes with a broad range of aryl halides has been developed. Sequential carbon-fluorine bond cleavage and carbon-carbon bond formation are two of the important features of the reaction. Addition of an aryl radical anion to a nickel intermediate was achieved for the first time using polyfluoroarenes as radical precursors. This, in combination with the excellent para selectivity, paves the way for the synthesis of various new multifluorinated biaryl compounds.

The Dual Role of Benzophenone in Visible-Light/Nickel Photoredox-Catalyzed C-H Arylations: Hydrogen-Atom Transfer and Energy Transfer.

A dual catalytic protocol for the direct arylation of non-activated C(sp3 )-H bonds has been developed. Upon photochemical excitation, the excited triplet state of a diaryl ketone photosensitizer abstracts a hydrogen atom from an aliphatic C-H bond. This inherent reactivity was exploited for the generation of benzylic radicals which subsequently enter a nickel catalytic cycle, accomplishing the benzylic arylation.

Radical Hydrodehalogenation of Aryl Bromides and Chlorides with Sodium Hydride and 1,4-Dioxane.

A practical method for radical chain reduction of various aryl bromides and chlorides is introduced. The thermal process uses NaH and 1,4-dioxane as reagents and 1,10-phenanthroline as an initiator. Hydrodehalogenation can be combined with typical cyclization reactions, proving the nature of the radical mechanism. These chain reactions proceed by electron catalysis. DFT calculations and mechanistic studies support the suggested mechanism.

Radical Hydrodeiodination of Aryl, Alkenyl, Alkynyl, and Alkyl Iodides with an Alcoholate as Organic Chain Reductant through Electron Catalysis.

A simple and efficient method for radical hydrodeiodination is reported. The novel approach uses electron catalysis. In situ generated Na-alcoholates are introduced as radical chain reducing reagents and reactions work with O2 as cheap initiator. Hydrodeiodination works on aryl, alkenyl, alkynyl iodides and a tert-alkyl iodide also gets reduced applying the method. Albeit less general, the method is also applicable to the reduction of aryl bromides. The novel reagent is successfully used to conduct typical reductive radical cyclization reactions and mechanistic studies are reported.

C-F Activation in Perfluorinated Arenes with Isonitriles under UV-Light Irradiation.

Due to the great value of fluorinated arenes in agrochemistry, medicinal chemistry and materials science, development of methods for preparation of fluorinated arenes is of high importance. They can be either accessed by arene fluorination or by partial arene defluorination. However, the carbon-fluorine bond belongs to the strongest σ-bonds, which renders C-F activation highly challenging. Here it is shown that aryl and alkyl isonitriles efficiently activate the strong C-F bond in perfluoroarenes by simple UV irradiation under mild conditions. Reactions proceed by formal direct insertion of the isonitrile into the C-F bond without any transition metal. Activation occurs at arene C-F bonds whereas aliphatic C-F bonds remain unreacted. For selected perfluoroarenes C-F activation occurs with high regioselectivity and resulting imidoyl fluorides are transformed into other valuable compounds. Theoretical studies give insights into the reaction mechanism.

Phenyl hydrazine as initiator for direct arene C-H arylation via base promoted homolytic aromatic substitution.

A simple and efficient direct radical arylation of unactivated arenes is described which uses cheap and commercially available phenyl hydrazine as an initiator. The reaction occurs through a base promoted homolytic aromatic substitution (BHAS) mechanism involving aryl radicals and aryl radical anions as intermediates and offers a practical approach for preparation of an array of substituted biaryls.