Reductive Arylation of Arylidene Malonates Using Photoredox Catalysis

Photocatalytic Proton-Coupled Electron Transfer Enabled Radical Cyclization for Isoquinoline-1,3-diones Synthesis

Radical cyclization has been demonstrated to be an efficient method to access functionalized heterocycles from easily accessible raw materials. Described herein is the development of a photocatalytic proton-coupled electron transfer (PCET) strategy for the synthesis of isoquinoline-1,3-diones using readily prepared naphthalimide (NI)-based organic photocatalysts. The process features free metal-complex photocatalysts, acids, and mild reaction conditions. This mild radical cyclization protocol has a broad substrate scope and can be effectively applied to a variety of medicinally relevant substrates. Furthermore, control experiments were conducted to elucidate the mechanism of this visible light-induced methodology.

Photoinduced Radical Desulfurative C(sp3)-C(sp2) Coupling via Electron Donor-Acceptor Complexes

Herein, we disclose a radical desulfurative C-C coupling protocol for the synthesis of 4-alkylpyridines. A variety of substituents on both benzyl thiols and 4-cyanopyridines are tolerated. The reaction is carried out under mild and photocatalyst- and transition-metal-free conditions. Preliminary mechanistic studies show that an electron donor-acceptor complex is formed between benzyl thiols and 4-cyanopyridines under alkaline conditions. Then, a variety of 1°, 2°, and 3° C(sp3)-centered radicals was formed by cleavage of the C-S bond, and the 4-alkylpyridines were achieved through a radical-radical coupling with the pyridyl radical anion.

Scandium-Catalyzed Electrochemical Synthesis of α-Pyridinyl Tertiary Amino Acids and Esters

α-Pyridyl tertiary amino acids have potential pharmaceutical applications because of their structural features. However, their synthesis is still highly limited. Herein, we report a straightforward approach for the electrochemical synthesis of tertiary α-substituted amino acid derivatives via three-component reductive coupling. Using gaseous ammonia as both the N and H source, the α-keto ester reacts directly with 4-CN-pyridine. The application of scandium catalysis is the key for achieving chemoselectivity among various side reaction pathways.

Photocatalytic Charge-Transfer Complex Enables Hydroarylation of Alkenes for Heterocycle Synthesis

Here, we report a photocatalytic charge-transfer complex (CTC) strategy for one electron reduction of alkenes using thiolate as a catalytic electron donor. This catalytic CTC system could engage hydroarylation of both activated and unactivated alkenes for the synthesis of various heterocycles. The reactions do not require any photocatalysts or acids and are easy to perform. Mechanistic studies revealed the formation of a CTC between catalytic thiolate and alkene.

Redox-neutral and metal-free synthesis of 3-(arylmethyl)chroman-4-ones via visible-light-driven alkene acylarylation

A metal- and aldehyde-free visible-light-driven photoredox-neutral alkene acylarylation with readily available cyanoarenes is described. A variety of 3-(arylmethyl)chroman-4-ones (i.e., homoisoflavonoids) and analogs are efficiently synthesized with good functional group tolerance. This mild protocol relies on a phosphoranyl radical-mediated acyl radical-initiated cyclization and selective radical-radical coupling sequence, and is also further highlighted by subsequent derivatization to chromone and 2H-chromene as well as its application in the three-component alkene acylarylation.

Palladium-catalyzed asymmetric allylic 4-pyridinylation via electroreductive substitution reaction

The enantioselective pyridinylation is important for providing chiral compounds bearing heterocycles of pharmaceutical interests. 4-CN-pyrinde is extensively applied in the radical pyridinylation reaction, however, its’ enantioselective application is highly challenging. To achieve this goal, we propose an electrochemical catalytic activation of 4-CN-pyridine with a chiral transition metal complex instead of direct cathodic reduction. The chiral catalyst acts as the electron mediator and the transition metal catalysis in turn. The radical species from 4-CN-pyridine is captured via radical rebound by chiral catalyst, and undergoes enantioselective pyridinylation reaction. Here, we show the first method for catalytic asymmetric allylic 4-pyridinylation reactions using 4-CN-pyridine under electrochemical conditions.

Controlling the enantioselectivity of radical reactions is a persistent challenge in organic synthesis. Here, the authors report a method to form asymmetric pyridine derivatives via the combination of chiral palladium catalysis and electrochemistry.

Visible Light-Driven Reductive Azaarylation of Coumarin-3-carboxylic Acids

In the manuscript, reductive and decarboxylative azaarylation of coumarin-3-carboxylic acids is described. It utilizes the photocatalytic activation of (cyano)azaarenes in the presence of fac-Ir(ppy)₃ as a photocatalyst. The methodology is versatile and provides access to biologically relevant 4-substituted-chroman-2-ones. Visible light, photoredox catalyst, base, anhydrous solvent, and inert atmosphere constitute key parameters for the success of the described strategy. The developed methodology involves a wide range of coumarin-3-carboxylic acids as well as (cyano)azaarenes.

Discovering and Evaluating the Reducing Abilities of Polar Alkanes and Related Family Members as Organic Reductants Using Thermodynamics

In this work, the pK_a values of 69 polar alkanes (YH₂) in acetonitrile were computed using the method developed by Luo and Zhang in 2020, and representative 69 thermodynamic network cards on 22 elementary steps of YH₂ and related polar alkenes (Y) releasing or accepting H₂ were naturally established. Potential electron reductants (YH^-), hydride reductants (YH^-), antioxidants (YH₂ and YH^-), and hydrogen molecule reductants (YH₂) are unexpectedly discovered according to thermodynamic network cards. It is also found that there are great differences between YH₂ and common hydrogen molecule reductants (XH₂), such as Hantzsch ester (HEH₂), benzothiazoline (BTH₂), and dihydro-phenanthridine (PH₂), releasing two hydrogen ions to unsaturated compounds. During the hydrogenation process, XH₂ release hydrides first, then the oxidation state XH⁺ release protons. However, in the case of YH₂, YH₂ release protons first, then YH^- release hydrides. It is the differences on acidic properties of YH₂ and XH₂ that result in the behavioral and thermodynamic differences on YH₂ and XH₂ releasing two hydrogen ions (H^--H⁺). The redox mechanisms and behaviors of Y, YH^-, and YH₂ as electron, hydrogen atom, hydride, and hydrogen molecule donors or acceptors in the chemical reaction are reasonably investigated and discussed in this paper using thermodynamics.

Catalytic Asymmetric Reductive Azaarylation of Olefins via Enantioselective Radical Coupling

Visible-light-driven photocatalytic reductive azaarylation has been widely used to construct the important imine-containing azaarene derivatives. In addition to the direct use of various commercially available cyanoazaarenes as feedstocks, the synthetic advantages include precise regioselectivity, high efficiency, mild reaction conditions, and good functional group tolerance. However, although many efficient reductive azaarylation methods have been established, the example of an enantioselective manner is still unmet, which most likely can be ascribed to the highly reactive radical coupling as the key step of forming stereocenters. Exploring the feasibility of enantiocontrol thus constitutes an attractive but highly challenging task. Here, we demonstrate that chiral hydrogen-bonding/photosensitizer catalysis is a viable platform as it enables the realization of the first enantioselective manifold. A variety of acyclic and cyclic enones as the reaction partners are compatible with the dual catalyst system, leading to a wide array of valuable enantioenriched azaarene variants with high yields and ees. Regulating the types of chiral catalysts represents one of the important manners to success, in which several readily accessible Cinchona alkaloid-derived bifunctional catalysts are introduced in asymmetric photochemical reactions.

Hydropyridylation of α,β-Unsaturated Esters through Electroreduction of 4-Cyanopyridine

A mild and highly efficient method for the hydropyridylation of α,β-unsaturated esters has been developed. This protocol provides the products smoothly with a wide substrate scope in an undivided cell under ambient conditions. Moreover, studies showed that the scope could be extended to other unsaturated compounds, including enones and aldehydes.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Electroreductive 4-pyridylation of unsaturated compounds using gaseous ammonia as a hydrogen source

By using ammonia as a hydrogen source, electrochemical pyridylation of unsaturated compounds is achieved with more than 50 examples. In particular, the β-keto ester could be converted to the corresponding tertiary β-hydroxyl ester for the first time.

Regioselective Umpolung Addition of Dicyanobenzene to α,β-Unsaturated Alkenes Enabled by Electrochemical Reduction

An umpolung addition of dicyanobenzene to α,β-unsaturated alkenes has been developed with an electroreductive strategy. This electrochemical protocol is well compatible with broad range of conventionally challenging substrates, including α,β-unsaturated...

Metal-Free Hydropyridylation of Thioester-Activated Alkenes via Electroreductive Radical Coupling

An electrochemical hydropyridylation of thioester-activated alkenes with 4-cyanopyridines has been developed. The reactions experience a tandem electroreduction of both substrates on the cathode surface, protonation, and radical cross-coupling process, resulting in a variety of valuable pyridine variants, which contain a tertiary and even a quaternary carbon at the α-position of pyridines, in high yields. The employment of thioesters to the conjugated alkenes enables no requirement of catalyst and high temperature, representing a highly sustainable synthetic method.

Ball-Milling-Enabled Reactivity of Manganese Metal*

Efforts to generate organomanganese reagents under ball-milling conditions have led to the serendipitous discovery that manganese metal can mediate the reductive dimerization of arylidene malonates. The newly uncovered process has been optimized and its mechanism explored using CV measurements, radical trapping experiments, EPR spectroscopy, and solution control reactions. This unique reactivity can also be translated to solution whereupon pre-milling of the manganese is required.

Computational Design of Radical Recognition Assay with the Possible Application of Cyclopropyl Vinyl Sulfides as Tunable Sensors

The processes involving the capture of free radicals were explored by performing DFT molecular dynamics simulations and modeling of reaction energy profiles. We describe the idea of a radical recognition assay, where not only the presence of a radical but also the nature/reactivity of a radical may be assessed. The idea is to utilize a set of radical-sensitive molecules as tunable sensors, followed by insight into the studied radical species based on the observed reactivity/selectivity. We utilize this approach for selective recognition of common radicals-alkyl, phenyl, and iodine. By matching quantum chemical calculations with experimental data, we show that components of a system react differently with the studied radicals. Possible radical generation processes were studied involving model reactions under UV light and metal-catalyzed conditions.

Visible light-induced alkylpyridylation of styrenes via a reductive radical three-component coupling

A visible light-induced and metal-free strategy for the intermolecular three-compoment alkylpyridylation of styrenes is reported. Hantzsch ester was found to be key to initiate the overall reductive radical coupling reaction. This radical process realized difunctionalization of styrenes, selectively yielding alkylated pyridines in good to excellent yields with a wide tolerance of functional groups, mild reaction conditions and simple operation. This new reaction complements existing visible light-induced variants of styrenes with NHP esters and expands the capabilities of radical-based cross-coupling reactions of pyridines.

Metal-free visible-light-induced photoredox-catalyzed intermolecular pyridylation/phosphinoylation of alkenes

A visible-light-induced and photoredox-catalyzed intermolecular pyridylation/phosphinoylation of alkenes using 4-cyanopyridine and diphenylphosphine oxide under mild metal-free conditions has been reported.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Electrochemical Iodine-Mediated Oxidation of Enamino-Esters to 2H-Azirine-2-Carboxylates Supported by Design of Experiments

An electrochemical iodine-mediated transformation of enamino-esters for the synthesis of 2H-azirine-2-carboxylates is presented. In addition, a thermic conversion of azirines to 4-carboxy-oxazoles in quantitative yield without purification was described. Both classes 2H-azirines-2-carboxylates and the 4-carboxy-oxazoles are substructures in natural products and therefore are of considerable interest for synthetic and pharmaceutical chemists. The optimization was not performed in a conventional manner with a one-factor-at-a-time process but with a Design of Experiments (DoE) approach. Beside a broad substrate scope the reaction was also employed to a robustness screen, a sensitivity assessment, and complemented with mechanistic considerations from cyclic voltammetry experiments.

Combined Photoredox and Carbene Catalysis for the Synthesis of Ketones from Carboxylic Acids

As a key element in the construction of complex organic scaffolds, the formation of C-C bonds remains a challenge in the field of synthetic organic chemistry. Recent advancements in single-electron chemistry have enabled new methods for the formation of various C-C bonds. Disclosed herein is the development of a novel single-electron reduction of acyl azoliums for the formation of ketones from carboxylic acids. Facile construction of the acyl azolium in situ followed by a radical-radical coupling was made possible merging N-heterocyclic carbene (NHC) and photoredox catalysis. The utility of this protocol in synthesis was showcased in the late-stage functionalization of a variety of pharmaceutical compounds. Preliminary investigations using chiral NHCs demonstrate that enantioselectivity can be achieved, showcasing the advantages of this protocol over alternative methodologies.

Radical coupling of β-ketoesters and amides promoted by Brønsted/Lewis acids

Recent advances in photocatalysis have enabled radical methods with complementary chemoselectivity to established two electron bond forming approaches. While this radical strategy has previously been limited to substrates with favorable redox potentials, Brønsted/Lewis acid activation has emerged as a means of facilitating otherwise difficult reductions. We report herein our investigations into the Lewis acid-promoted redox activation of β-ketocarbonyls in a model photocatalytic radical alkylation reaction. Rapid evaluation of substrates and reactions conditions was achieved by high throughput experimentation using 96-well plate photoreactors.

A Sequential Umpolung/Enzymatic Dynamic Kinetic Resolution Strategy for the Synthesis of γ-Lactones

Combining biological and small-molecule catalysts under a chemoenzymatic manifold presents a series of significant advantages to the synthetic community. We report herein the successful development of a two-step/single flask synthesis of γ-lactones through the merger of Umpolung catalysis with a ketoreductase-catalyzed dynamic kinetic resolution, reduction, and cyclization. This combined approach delivers highly enantio- and diastereoenriched heterocycles and demonstrates the feasibility of integrating NHC catalysis with enzymatic processes.

Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible-light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near-IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low-energy photons can address both challenges simultaneously, and whilst multi-photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi-photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.