Regioselective Arene C−H Alkylation Enabled by Organic Photoredox Catalysis

Chemoselective Decarboxylative Protonation Enabled by Cooperative Earth-Abundant Element Catalysis

Decarboxylative protonation is a general deletion tactic to replace polar carboxylic acid groups with hydrogen or its isotope. Current methods rely on the pre-activation of acids, non-sustainable hydrogen sources, and/or expensive/highly oxidizing photocatalysts, presenting challenges to their wide adoption. Here we show that a cooperative iron/thiol catalyst system can readily achieve this transformation, hydrodecarboxylating a wide range of activated and unactivated carboxylic acids and overcoming scope limitations in previous direct methods. The reaction is readily scaled in batch configuration and can be directly performed in deuterated solvent to afford high yields of d-incorporated products with excellent isotope incorporation efficiency; characteristics not attainable in previous photocatalyzed approaches. Preliminary mechanistic studies indicate a radical mechanism and kinetic results of unactivated acids (KIE=1) are consistent with a light-limited reaction.

Photochemical C(sp2 )-H Pyridination via Arene-Pyridinium Electron Donor-Acceptor Complexes

This report describes the development of a photochemical method for C(sp² )-H pyridination that leverages the photoexcitation of electron donor-acceptor (EDA) complexes. Experimental and DFT studies show that black light (λ_max ≈350 nm) irradiation of solutions of protonated pyridines (acceptors) and aromatic C-H substrates (donors) results in single electron transfer to form aryl radical cation intermediates that can be trapped with pyridine nucleophiles under aerobic conditions. With some modification of the reaction conditions, this EDA activation mode is also effective for promoting the oxidatively triggered S_N Ar pyridination of aryl halides. Overall, this report represents an inexpensive and atom-economical approach to photochemical pyridination reactions that eliminates the requirement of an exogenous photocatalyst.

Dearomative (3+2) Cycloadditions between Indoles and Vinyldiazo Species Enabled by a Red-Shifted Chromium Photocatalyst

A direct dearomative photocatalyzed (3+2) cycloaddition between indoles and vinyldiazo reagents is described. The transformation is enabled by the development of a novel oxidizing Cr^III photocatalyst, its specific reactivity attributed to increased absorptive properties over earlier Cr analogs and greater stability than Ru counterparts. A variety of fused indoline compounds are synthesized using this method, including densely functionalized ring systems that are feasible due to base-free conditions. Experimental insights corroborate a cycloaddition initiated by nucleophilic attack at C3 of the indole radical cation by the vinyldiazo species.

Organophotocatalytic Regioselective C-H Alkylation of Electron-Rich Arenes Using Activated and Unactivated Alkenes

Direct alkylation of the C-H bond arenes in a selective manner is a long-standing challenge. Herein, a metal-free photocatalytic regioselective C-H alkylation method for electron-rich arenes with both activated and unactivated alkenes was developed. The reaction tolerates a wide range of aromatic rings with diverse substitution patterns, as well as terminal and internal alkenes, providing a general and straightforward metal-free method for C-C bond formation from inert C-H bonds. Moreover, alkynes are also compatible to give the C-H vinylation of electron-rich arenes.

Acridinium Salts and Cyanoarenes as Powerful Photocatalysts: Opportunities in Organic Synthesis

The use of organic photocatalysts has revolutionized the field of photoredox catalysis, as it allows access to reactivities that were traditionally restricted to transition-metal photocatalysts. This Minireview reports recent developments in the use of acridinium ions and cyanoarene derivatives in organic synthesis. The activation of inert chemical bonds as well as the late-stage functionalization of biorelevant molecules are discussed, with a special focus on their mechanistic aspects.

Assembly of α-(Hetero)aryl Nitriles via Copper-Catalyzed Coupling Reactions with (Hetero)aryl Chlorides and Bromides

α-(Hetero)aryl nitriles are important structural motifs for pharmaceutical design. The known methods for direct synthesis of these compounds via coupling with (hetero)aryl halides suffer from narrow reaction scope. Herein, we report that the combination of copper salts and oxalic diamides enables the coupling of a variety of (hetero)aryl halides (Cl, Br) and ethyl cyanoacetate under mild conditions, affording α-(hetero)arylacetonitriles via one-pot decarboxylation. Additionally, the CuBr/oxalic diamide catalyzed coupling of (hetero)aryl bromides with α-alkyl-substituted ethyl cyanoacetates proceeds smoothly at 60 °C, leading to the formation of α-alkyl (hetero)arylacetonitriles after decarboxylation. The method features a general substrate scope and is compatible with various functionalities and heteroaryls.

Across the Board: Yang Li on Visible-Light Photoredox Catalysis

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. Y. Li, who discusses visible-light photoredox catalysis by acridinium photocatalysts, which show both powerful oxidative and reductive abilities. She also highlights the mechanism for generation of a neutral acridine radical and its application in the reduction of aryl halides.