Electron-Poor Acridones and Acridiniums as Super Photooxidants in Molecular Photoelectrochemistry by Unusual Mechanisms

Electron-deficient acridones and in situ generated acridinium salts are reported as potent, closed-shell photooxidants that undergo surprising mechanisms. When bridging acyclic triarylamine catalysts with a carbonyl group (acridones), this completely diverts their behavior away from open-shell, radical cationic, 'beyond diffusion' photocatalysis to closed-shell, neutral, diffusion-controlled photocatalysis. Brønsted acid activation of acridones dramatically increases excited state oxidation power (by +0.8 V). Upon reduction of protonated acridones, they transform to electron-deficient acridinium salts as even more potent photooxidants (*E1/2 =+2.56-3.05 V vs SCE). These oxidize even electron-deficient arenes where conventional acridinium salt photooxidants have thusfar been limited to electron-rich arenes. Surprisingly, upon photoexcitation these electron-deficient acridinium salts appear to undergo two electron reductive quenching to form acridinide anions, spectroscopically-detected as their protonated forms. This new behaviour is partly enabled by a catalyst preassembly with the arene, and contrasts to conventional SET reductive quenching of acridinium salts. Critically, this study illustrates how redox active chromophoric molecules initially considered photocatalysts can transform during the reaction to catalytically active species with completely different redox and spectroscopic properties.

Electrophotochemical Metal-Catalyzed Enantioselective Decarboxylative Cyanation

In contrast to the rapid growth of electrophotocatalysis in recent years, enantioselective catalytic reactions powered by this unique methodology remain rare. In this work, we report an electrophotochemical metal-catalyzed protocol for direct asymmetric decarboxylative cyanation of aliphatic carboxylic acids. The synergistic merging of electrophotochemical cerium catalysis and asymmetric electrochemical copper catalysis permits mild reaction conditions for the formation and utilization of the key carbon centered radicals by combining the power of light and electrical energy. Electrophotochemical cerium catalysis enables radical decarboxylation to produce alkyl radicals, which could be effectively intercepted by asymmetric electrochemical copper catalysis for the construction of C-CN bonds in a highly stereoselective fashion. This environmentally benign method smoothly converts a diverse array of arylacetic acids into the corresponding alkyl nitriles in good yields and enantioselectivities without using chemical oxidants or pre-functionalization of the acid substrates and can be readily scaled up.

Electrophotocatalytic C-H Functionalization of N-Heteroarenes with Unactivated Alkanes under External Oxidant-Free Conditions

The Minisci alkylation of N-heteroarenes with unactivated alkanes under external oxidant-free conditions provides an economically attractive route to access alkylated N-heteroarenes but remains underdeveloped. Herein, a new electrophotocatalytic strategy to access alkyl radicals from strong C(sp³ )-H bonds was reported for the following Minisci alkylation reactions in the absence of chemical oxidants. This strategy realized the first example of cerium-catalyzed Minisci alkylation reaction directly from abundant unactivated alkanes with excellent atom economy. It is anticipated that the general design principle would enrich catalytic strategies to explore the functionalizations of strong C(sp³ )-H bonds under external oxidant-free conditions with H₂ evolution.

Electrochemical Activation of Diverse Conventional Photoredox Catalysts Induces Potent Photoreductant Activity*

Herein, we disclose that electrochemical stimulation induces new photocatalytic activity from a range of structurally diverse conventional photocatalysts. These studies uncover a new electron-primed photoredox catalyst capable of promoting the reductive cleavage of strong C(sp2 )-N and C(sp2 )-O bonds. We illustrate several examples of the synthetic utility of these deeply reducing but otherwise safe and mild catalytic conditions. Finally, we employ electrochemical current measurements to perform a reaction progress kinetic analysis. This technique reveals that the improved activity of this new system is a consequence of an enhanced catalyst stability profile.

Synthetic Molecular Photoelectrochemistry: New Frontiers in Synthetic Applications, Mechanistic Insights and Scalability

Synthetic photoelectrochemistry (PEC) is receiving increasing attention as a new frontier for the generation and handling of reactive intermediates. PEC permits selective single‐electron transfer (SET) reactions in a much greener way and broadens the redox window of possible transformations. Herein, the most recent contributions are reviewed, demonstrating exciting new opportunities, namely, the combination of PEC with other reactivity paradigms (hydrogen‐atom transfer, radical polar crossover, energy transfer sensitization), scalability up to multigram scale, novel selectivities in SET super‐oxidations/reductions and the importance of precomplexation to temporally enable excited radical ion catalysis.

Catalytic SN Ar Hydroxylation and Alkoxylation of Aryl Fluorides

Nucleophilic aromatic substitution (S_N Ar) is a powerful strategy for incorporating a heteroatom into an aromatic ring by displacement of a leaving group with a nucleophile, but this method is limited to electron-deficient arenes. We have now established a reliable method for accessing phenols and phenyl alkyl ethers via catalytic S_N Ar reactions. The method is applicable to a broad array of electron-rich and neutral aryl fluorides, which are inert under classical S_N Ar conditions. Although the mechanism of S_N Ar reactions involving metal arene complexes is hypothesized to involve a stepwise pathway (addition followed by elimination), experimental data that support this hypothesis is still under exploration. Mechanistic studies and DFT calculations suggest either a stepwise or stepwise-like energy profile. Notably, we isolated a rhodium η⁵ -cyclohexadienyl complex intermediate with an sp³ -hybridized carbon bearing both a nucleophile and a leaving group.