Electrophotocatalytic Decarboxylative C−H Functionalization of Heteroarenes

Visible-light-mediated direct C3 alkylation of quinoxalin-2(1H)-ones using alkanes

Due to the high C-H bond dissociation energy of alkanes, the utilization of alkanes as alkyl radical precursors for C-H functionalization of heteroarenes is synthetically captivating but practically challenging, especially under metal- and photocatalyst-free conditions. We report herein a mild and practical visible-light-mediated method for C-H alkylation of quinoxalin-2(1H)-ones using trifluoroacetic acid as a hydrogen atom transfer reagent and air as an oxidant. This mild protocol was performed under metal- and photocatalyst-free circumstances and presented good functional-group tolerance as well as a broad substrate scope.

para-Selective, Direct C(sp2)-H Alkylation of Electron-Deficient Arenes by the Electroreduction Process

Direct para-selective C(sp²)-H alkylation of electron-deficient arenes based on the electroreduction-enabled radical addition of alkyl bromides has been developed under mild conditions. In the absence of any metals and redox agents, the simple electrolysis system tolerates a variety of primary, secondary, and tertiary alkyl bromides and behaves as an important complement to the directed alkylation of the C(sp²)-H bond and the classic Friedel-Crafts alkylation. This electroreduction process provides a more straightforward, environmentally benign, and effective alkylation method for electron-deficient arenes.

PIDA-mediated Oxidative Decarboxylation of Oxamic Acids. The Role of Radical Acidity Enhancement

The PIDA-mediated oxidative decarboxylation of oxamic acids in the presence of alcohols is shown to afford the corresponding urethanes under thermal conditions. Computational and experimental mechanistic exploration allows to rationalize the different reactivity of PIDA as compared to related cyclic BI-OAc and highlights the importance of the enhanced acidity of the proton in the carbamoyl radical intermediate.

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.

(NH4)2S2O8-Mediated Metal-Free Decarboxylative Formylation/Acylation of α-Oxo/Ketoacids and Its Application to the Synthesis of Indole Alkaloids

A metal-free method for the formylation/acylation of indoles and β-carbolines with (NH₄)₂S₂O₈ via direct decarboxylative cross-coupling of α-oxo/ketoacids in moderate to good yields is described. The reaction occurs between ambient temperature and 40 °C under mild reaction conditions with commercially available starting materials. This methodology can be expanded to some biologically active indole alkaloids like pityriacitrins, eudistomins Y₁ and Y₃, and marinacarbolines A-D.

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.

Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis

The mainstream applications of visible-light photoredox catalysis predominately involve outer-sphere single-electron transfer (SET) or energy transfer (EnT) processes of precious metal RuII or IrIII complexes or of organic dyes with low photostability. Earth-abundant metal-based Mn Ln -type (M=metal, Ln =polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner-sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited-state lifetimes for use as effective photocatalysts. The generic process, termed visible-light-induced homolysis (VLIH), entails the formation of suitable light-absorbing ligated metal-substrate complexes (Mn Ln -Z; Z=substrate) that can undergo homolytic cleavage to generate Mn-1 Ln and Z. for further transformations.

Electro-mediated PhotoRedox Catalysis for Selective C(sp3 )-O Cleavages of Phosphinated Alcohols to Carbanions

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E-selective and can be made Z-selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch-type reductions.

The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

In the field of medicinal chemistry, the precise installation of a trideuteromethyl group is gaining ever-increasing attention. Site-selective incorporation of the deuterated "magic methyl" group can provide profound pharmacological benefits and can be considered an important tool for drug optimization and development. This review provides a structured overview, according to trideuteromethylation reagent, of currently established methods for site-selective trideuteromethylation of carbon atoms. In addition to CD3 , the selective introduction of CD2 H and CDH2 groups is also considered. For all methods, the corresponding mechanism and scope are discussed whenever reported. As such, this review can be a starting point for synthetic chemists to further advance trideuteromethylation methodologies. At the same time, this review aims to be a guide for medicinal chemists, offering them the available C-CD3 formation strategies for the preparation of new or modified drugs.

Access to Diarylmethanol Skeletons via a Samarium/Copper-Mediated Sequential Three-Component C-H Functionalization Reaction

A novel three-component reaction was developed via a one-pot strategy for the construction of diarylmethanol esters by using a halobenzene and an ester in N,N-dimethylformamide (DMF) under mild conditions. The reaction involves the direct functionalization of halobenzene under the Sm-CuI catalyst system. It was proved that 10% (mol) of CuI is sufficient to realize the reductive coupling reaction. Influences of substituents were illustrated from both electronic and steric effects. The reaction mechanism was also discussed.

Electrophotocatalytic Acetoxyhydroxylation of Aryl Olefins

A method for the acetoxyhydroxylation of olefins with syn stereoselectivity under electrophotocatalytic conditions is described. The procedure uses a trisaminocyclopropenium (TAC) ion catalyst with visible light irradiation under a controlled electrochemical potential to convert aryl olefins to the corresponding glycol monoesters with high chemo- and diastereoselectivity. This reaction can be performed in batch or in flow, enabling multigram synthesis of the monoester products.

Electrophotocatalytic C-H Heterofunctionalization of Arenes

The electrophotocatalytic heterofunctionalization of arenes is described. Using 2,3-dichloro-5,6-dicyanoquinone (DDQ) under a mild electrochemical potential with visible-light irradiation, arenes undergo oxidant-free hydroxylation, alkoxylation, and amination with high chemoselectivity. In addition to batch reactions, an electrophotocatalytic recirculating flow process is demonstrated, enabling the conversion of benzene to phenol on a gram scale.

Light-triggered elimination of CO2 and absorption of O2 (artificial breathing reaction) in photolysis of 2-(4-nitrophenyl)-1H-indole derivatives

A new chromophore, 2-(4-nitrophenyl)-1H-indole (NPI), was synthesized as a potential photolabile protecting group. Caged benzoic acids featuring the NPI chromophore were synthesized as model compounds. Benzoic acid was released in moderate yields (~ 40-60%) upon photolysis of the caged benzoic acids without any additional chemical reagents. Interestingly, an aldehyde, 1-(5-(1-formyl-1H-indol-2-yl)-2-nitrophenyl)ethyl benzoate, was isolated in ≈ 20% together with benzoic acid (≈ 40%) in photolysis of a caged benzoic acid, 2-(2-(3-(1-(benzoyloxy)ethyl)-4-nitrophenyl)-1H-indol-1-yl)acetic acid. The functional group, CH₂COOH, at the indole nitrogen was transformed into the aldehyde group, CHO, under photolysis conditions in air. The similar photochemical transformation was observed in the photolysis of 2-(2-(4-nitrophenyl)-1H-indol-1-yl)acetic acid, in which the benzoate group is not attached at the nitrophenyl ring. Products analysis, transient absorption spectroscopy, and computational study suggested that intramolecular electron transfer is key for the elimination of CO₂ and absorption of O₂ for the formation of the aldehyde. The artificial breathing-type reaction can apply to transition metal-free oxidation of amino acids under mild conditions.

Photocatalytic Aerobic Oxidative Ring Expansion of Cyclic Ketones to Macrolactones by Cerium and Cyanoanthracene Catalysis

We describe a cerium-catalyzed aerobic oxidative ring expansion for the expedient construction of synthetically challenging macrolactones under visible-light conditions. Cyanoanthracene has been employed as co-catalyst to accelerate the turnover of the cerium cycle leading to a fast conversion within 20 min of irradiation. Taking advantage of the high efficiency and operationally simple conditions, a collection of over 100 macrolactones equipped with ring systems ranging from 9- to 19-membered macrocycles have been prepared from simple building blocks. Moreover, the enabling potential of this strategy to simplify the generation of molecular complexity has been demonstrated through the concise synthesis of sonnerlactone.