A Redox Strategy for Light-Driven, Out-of-Equilibrium Isomerizations and Application to Catalytic C-C Bond Cleavage Reactions

Photoredox-Catalyzed Hydroalkylation of C(sp3)-H Acids

We present a detailed study on a photoredox catalysis platform that directly engages 1,3-dicarbonyl C(sp3)-H acids toward radical reactions. This platform enables redox-neutral hydroalkylation and cross-coupling, as well as oxidative transformations that demonstrably improve on the prior state of the art. Herein, we present interrogations of the underlying catalytic cycle and mechanism for this platform through kinetic, thermodynamic, and computational studies. The present investigations also demonstrate the key role of lithium trifluoroacetate under complementary Ce-containing and Ce-free photoredox conditions to enable ligand-to-metal charge transfer (LMCT) or multi-site proton-coupled electron transfer (MS-PCET) activations, respectively.

Dehomologative C-C Borylation of Aldehydes and Alcohols via a Rh-Catalyzed Dehydroformylation-Borylation Relay

The dehomologative conversion of linear or α-methyl aldehydes to vinyl boronates is achieved via a one-pot sequence of rhodium-catalyzed transfer dehydroformylation and transfer borylation of the resulting alkenes. Similarly, allylic or aliphatic alcohols are converted to vinyl boronates through a sequence involving, respectively, rhodium-catalyzed isomerization or transfer dehydrogenation to aldehyde intermediates, followed by dehydroformylation-borylation. The vinyl boronates can be further hydrogenated to alkyl boronates using the same rhodium precatalyst, enabling all five catalytic steps with a single catalyst system.

A Mind Map to Address the Next Generation of Artificial Photosynthesis Experiments

Artificial photosynthesis (APS) is using light for uphill chemical reactions that converts light energy into chemical energy. It follows the example of natural photosynthesis, but offers a broader choice of materials and components, which can enhance its performance it terms of application conditions, stability, efficiency, and uphill reactions to be carried out. This work presents here first the status of the field, just to focus afterward on the current problems seen at the forefront of the field, as well as discussing some general misunderstandings, which are often repeated in the primary literature. Finally, this perspective article is daring to define some grand challenges, which have to be tackled for the translation of APS into society.

Enantioselective Photoredox- and Cu-Catalyzed Cyanoalkylation of Styrenes via Deoxygenation of Alkoxyl Radicals with Organophosphorus Compounds(III)

The enantioselective cyanoalkylation of styrenes by a cooperative photoredox and copper catalysis system has been established, providing straightforward access to structurally diverse enantioenriched alkyl nitriles in good yields with excellent enantioselectivities under mild conditions via deoxygenation of alkoxyl radicals with organophosphorus compounds(III). In addition, the reaction features a wide substrate scope and good functional group tolerance, and the resultant alkyl nitriles could be easily converted into a series of chiral carboxylic acids, amides, esters, etc.

Environmental Fate of the Azole Fungicide Fluconazole and Its Persistent and Mobile Transformation Product 1,2,4-Triazole

Fluconazole is a persistent and mobile pharmaceutical azole fungicide observed in natural waters globally. It does not significantly degrade via traditional wastewater treatment, resulting in likely environmental and human exposure and environmental-origin azole fungicide resistance. Indirect photochemistry is known to degrade many recalcitrant contaminants in natural waters but has not been tested for fluconazole. We systematically measured rates and identified products of the indirect photodegradation of fluconazole in genuine and synthetic surface waters with varying nitrate, bicarbonate, and dissolved organic matter using high resolution mass spectrometry. Degradation half-lives of fluconazole ranged from 2 weeks to a year, indicating indirect photochemistry is slow but competitive with other loss processes. The transformation products 1,2,4-triazole and 1,2,4-triazole-1-acetic acid were produced in 30 to 100% yield during fluconazole degradation. These products are far more resistant to indirect photochemistry than fluconazole, with half-lives for 1,2,4-triazole in the environment of between 1 and 3 years when measurable with our methods. These "very persistent very mobile" contaminants are likely formed by most pharmaceutical and agrochemical azole fungicides, are regularly detected in the US and Denmark in monitoring programs and our exposure modeling demonstrates high potential for human exposure through drinking water with uncertain health implications.

Radical Brook rearrangement: past, present, and future

The Brook rearrangement has emerged as one of the most pivotal transformations in organic chemistry, with broad applications spanning organic synthesis, drug design, and materials science. Since its discovery in the 1950s, the anion-mediated Brook rearrangement has been extensively studied, laying the groundwork for the development of numerous innovative reactions. In contrast, the radical Brook rearrangement has garnered comparatively less attention, primarily due to the challenges associated with the controlled generation of alkoxyl radicals under mild conditions. However, recent advancements in visible-light catalysis and transition-metal catalysis have positioned the radical Brook rearrangement as a promising alternative synthetic strategy in organic synthesis. Despite these developments, significant limitations and challenges remain, warranting further investigation. This review provides an overview of the radical Brook rearrangement, tracing its development from past to present, and offers perspectives on future directions in the field to inspire the creation of novel synthetic tools based on this transformation.

Organic photoredox-catalyzed unimolecular PCET of benzylic alcohols

Proton-coupled electron transfer (PCET) is a crucial chemical process involving the simultaneous or sequential transfer of protons and electrons, playing a vital role in biological processes and energy conversion technologies. This study investigates the use of an organic photoredox catalyst to facilitate a unimolecular PCET process for the generation of alkyl radicals from benzylic alcohols, with a particular focus on alcohols containing electron-rich arene units. By employing a benzophenone derivative as the catalyst, the reaction proceeds efficiently under photoirradiation, achieving significant yields without the need for a Brønsted base. The findings highlight the potential of this unimolecular PCET mechanism to streamline radical generation in organic synthesis, offering a more efficient and flexible alternative to conventional methods.

Photoinduced Lignin Cα-Cβ Bond Cleavage and Chemodivergent Functionalization via Iron Catalysis

The photocatalytic conversion of lignin into value-added chemicals especially those functionalized molecules represent one of the most important strategies for sustainable and environmental-friendly development. Cleavage of C-C bonds in lignin under mild photocatalytic conditions for refining lignin into useful molecules is meaningful but challenging. Meanwhile, the assembly of diverse functional groups into active lignin fragments during the depolymerization is of great challenging. Herein, using cheap iron catalysts under visible light irradiation, the highly selective and efficient cleavage of Cα-Cβ bond in lignin is realized via ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) processes. The subsequent divergent functionalization of generated lignin fragment-based radical intermediates enables an efficient formation of diverse functionalized molecules. This method is also effective for cleavage of Cα-Cβ bond in native lignin, yielding two identified benzaldehyde monomers in a total yield of 8.7 wt %.

Organometallic Photocatalyst-Promoted Synthesis and Modification of Carbohydrates under Photoirradiation

Carbohydrates are natural, renewable, chemical compounds that play crucial roles in biological systems. Thus, efficient and stereoselective glycosylation is an urgent task for the preparation of pure and structurally well-defined carbohydrates. Photoredox catalysis has emerged as a powerful tool in carbohydrate chemistry, providing an alternative for addressing some of the challenges of glycochemistry. Over the last few decades, Ir- and Ru-based organometallic photocatalysts have attracted significant interest because of their high stability, high-energy triplet state, strong visible-light absorption, long luminescence lifetime, and amenability to ligand modification. This review highlights the recent progress in the organometallic photocatalyst-promoted synthesis and modification of carbohydrates under photoirradiation, as well as the related benefits and drawbacks.

Visible-Light-Enabled Catalytic Intramolecular Double Oxidation of Olefins to ortho-Hydroxylactones

We have effectively utilized cost-effective 2-bromoanthraquinone as a photocatalyst to develop an efficient and environmentally friendly method for producing o-hydroxy lactones under mild visible light irradiation. Importantly, this protocol only relies on oxygen as an oxidant, completely eliminating the need for additional chemical reagents and showcasing a sustainable approach to chemical transformation. Operating at room temperature, we utilized a mixed solvent system of DMF and CHCl3, which greatly facilitated the selective conversion of various 2-vinylbenzoic acids and carboxylic acids to functional o-hydroxyl lactones. The process also exhibited excellent diastereoselectivity. Moreover, this versatile strategy is compatible with a wide range of biologically active and complex molecules, offering new opportunities for late-stage structural modifications of these compounds.

Decarboxylative Cross-Coupling Enabled by Fe and Ni Metallaphotoredox Catalysis

Decarboxylative cross-coupling of carboxylic acids and aryl halides has become a key transformation in organic synthesis to form C(sp2)-C(sp3) bonds. In this report, a base metal pairing between Fe and Ni has been developed with complementary reactivity to the well-established Ir and Ni metallaphotoredox reactions. Utilizing an inexpensive FeCl3 cocatalyst along with a pyridine carboxamidine Ni catalyst, a range of aryl iodides can be preferentially coupled to carboxylic acids over boronic acid esters, triflates, chlorides, and even bromides in high yields. Additionally, carboxylic acid derivatives containing heterocycles, N-protected amino acids, and protic functionality can be coupled in 23-96% yield with a range of sterically hindered, electron-rich, and electron-deficient aryl iodides. Preliminary catalytic and stoichiometric reactions support a mechanism in which Fe is responsible for the activation of carboxylic acid upon irradiation with light and a NiI alkyl intermediate is responsible for activation of the aryl iodide coupling partner followed by reductive elimination to generate product.

Dual transition metal electrocatalysis enables selective C(sp3)-C(sp3) bond cleavage and arylation of cyclic alcohols

We report a dual transition metal electrocatalytic approach for C(sp3)-C(sp3) bond cleavage and arylation of cyclic alcohols, providing an efficient and sustainable method for site-specific arylation of ketones. The reaction involves electrophotochemical cerium-catalysed generation of alkoxyl radicals from readily accessible alcohols. Subsequently, homolytic cleavage of the β-C-C bond leads to the generation of carbon-centered radicals that could be effectively utilized by nickel catalysis powered by cathode reduction to deliver the remote arylated ketone products.

Photoinduced Deconstructive Alkylation Approach Enabled by Oxy-Radicals from Alcohols

Alcohols are the most commercially abundant, synthetically versatile and operationally convenient functional groups in organic chemistry. Therefore, a strategy that utilizes hydroxy-containing compounds to develop novel bond disconnection and formation process would achieve molecular diversity. Herein, a deconstructive strategy for the generation of quinoxalin-2(1H)-one derivatives has been developed from alcohol precursors via oxy-radical-induced β-fragmentation. Additionally, 1,5-HAT and deoxygenation by P(III) along with oxy-radical were demonstrated as alternative pathways for this transformation. Furthermore, with the deep-seated reorganization of a few terpenes carbon framework, a unique activity with inhibition against the growth of pathogenic fungi was observed.

Excited State Bond Homolysis of Vanadium(V) Photocatalysts for Alkoxy Radical Generation

Advancements in photocatalysis have transformed synthetic organic chemistry, using light as a powerful tool to drive selective chemical transformations. Recent approaches have focused on metal-halide ligand-to-metal charge transfer (LMCT) photoactivated bond homolysis reactions leveraged by earth-abundant elements to generate valuable synthons for radical-mediated cross-coupling reactions. Of recent utility, oxovanadium(V) LMCT photocatalysts exhibit selective alkoxy radical generation from aliphatic alcohols upon blue light (UVA) irradiation under mild conditions. The selective photochemical liberation of alkoxy radicals is valuable for applying late-stage fragmentation approaches in organic synthesis and depolymerization strategies for nonbiodegradable polymers. Steady-state and time-resolved spectroscopy were used to assign the electronic structure of three well-defined V(V) photocatalysts in their ground and excited states. We assign the excited state for this transformation at earth-abundant vanadium(V), interrogating the electronic structure using static UV-visible absorption, ultrafast transient absorption, and electron paramagnetic resonance spectroscopy coupled to computational approaches. These findings afford assignments of the short-lived excited state intermediates that dictate selective homolytic bond cleavage in metal alkoxides, illustrating the valuable insight gleaned from fundamental investigations of the molecular photochemistry responsible for light-escalated chemical transformations.

Iron-Catalyzed Electrophotochemical α-Functionalization of a Silylcyclobutanol

Four-membered ring structure is important in organic chemistry, and selective cleavage and functionalization of these strained rings are of great interest. However, direct α-functionalization of cyclobutanols is rarely reported because of the high O-H bond dissociation energy and the occurrence of β-scission of C-C bonds in these alcohols. Recently, transition-metal catalysis has facilitated alkoxy radical generation. Herein, we report a method for electrophotochemical α-functionalization of a silylcyclobutanol via visible-light-induced LMCT reactions of M-alkoxy complexes. Introduction of the silyl group into the cyclobutanol structure favored fast [1,2]-silyl transfer over ring opening, thus allowing the generation of α-functionalized products.

Modifying Proton Relay into Bioinspired Dye-Based Coordination Polymer for Photocatalytic Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) imparts an energetic advantage over single electron transfer in activating inert substances. Natural PCET enzyme catalysis generally requires tripartite preorganization of proton relay, substrate-bound active center, and redox mediator, making the processes efficient and precluding side reactions. Inspired by this, a heterogeneous photocatalytic PCET system was established to achieve higher PCET driving forces by modifying proton relays into anthraquinone-based anionic coordination polymers. The proximally separated proton relays and photoredox-mediating anthraquinone moiety allowed pre-assembly of inert substrate between them, merging proton and electron into unsaturated bonds by photoreductive PCET, which enhanced reaction kinetics compared with the counter catalyst without proton relay. This photocatalytic PCET method was applied to a broad-scoped reduction of aryl ketones, unsaturated carbonyls, and aromatic compounds. The distinctive regioselectivities for the reduction of isoquinoline derivatives were found to occur on the carbon-ring sides. PCET-generated radical intermediate of quinoline could be trapped by alkene for proton relay-assisted Minisci addition, forming the pharmaceutical aza-acenaphthene scaffold within one step. When using heteroatom(X)-H/C-H compounds as proton-electron donors, this protocol could activate these inert bonds through photooxidative PCET to afford radicals and trap them by electron-deficient unsaturated compounds, furnishing the direct X-H/C-H functionalization.

The sugar cube: Network control and emergence in stereoediting reactions

Stereochemical editing strategies have recently enabled the transformation of readily accessible substrates into rare and valuable products. Typically, site selectivity is achieved by minimizing kinetic complexity by using protecting groups to suppress reactivity at undesired sites (substrate control) or by using catalysts with tailored shapes to drive reactivity at the desired site (catalyst control). We propose "network control," a contrasting paradigm that exploits hidden interactions between rate constants to greatly amplify modest intrinsic biases and enable precise multisite editing. When network control is applied to the photochemical isomerization of hexoses, six of the eight possible diastereomers can be selectively obtained. The amplification effect can be viewed as a mesoscale phenomenon between the limiting regimes of kinetic control in simple chemical systems and metabolic regulation in complex biological systems.

Accessing Alkoxy Radicals via Frustrated Radical Pairs: Diverse Oxidative Functionalizations of Tertiary Alcohols

Alkoxy radicals are versatile reactive intermediates in organic synthesis. Here, we leverage the principle of frustrated radical pair to provide convenient access to these highly reactive species directly from tertiary alcohols via oxoammonium-mediated oxidation of the corresponding alkoxides. This approach enabled various synthetically useful transformations including β-scission, radical cyclization, and remote C-H functionalization, giving rise to versatile alkoxyamines that can be further elaborated to various functionalities.

Organic Synthesis Away from Equilibrium: Contrathermodynamic Transformations Enabled by Excited-State Electron Transfer

ConspectusChemists have long been inspired by biological photosynthesis, wherein a series of excited-state electron transfer (ET) events facilitate the conversion of low energy starting materials such as H2O and CO2 into higher energy products in the form of carbohydrates and O2. While this model for utilizing light-driven charge transfer to drive catalytic reactions thermodynamically "uphill" has been extensively adapted for small molecule activation, molecular machines, photoswitches, and solar fuel chemistry, its application in organic synthesis has been less systematically developed. However, the potential benefits of these approaches are significant, both in enabling transformations that cannot be readily achieved using conventional thermal chemistry and in accessing distinct selectivity regimes that are uniquely enabled by excited-state mechanisms. In this Account, we present work from our group that highlights the ability of visible light photoredox catalysis to drive useful organic transformations away from their equilibrium positions, addressing a number of long-standing synthetic challenges.We first discuss how excited-state ET enabled the first general methods for the catalytic anti-Markovnikov hydroamination of unactivated alkenes with alkyl amines. In these reactions, an excited-state iridium(III) photocatalyst reversibly oxidizes secondary amine substrates to their corresponding aminium radical cations (ARCs). These electrophilic N-centered radicals can then react with olefins to furnish valuable tertiary amine products with complete anti-Markovnikov regioselectivity. Notably, some of these products are less thermodynamically stable than their corresponding amine and alkene starting materials. We next present a strategy for light-driven C-C bond cleavage within various aliphatic alcohols mediated by homolytic activation of alcohol O-H bonds by excited-state proton-coupled electron transfer (PCET). The resulting alkoxy radical intermediates then undergo C-C β-scission to ultimately provide isomeric linear carbonyl products that are often higher in energy than their cyclic alcohol precursors. Applications of this chemistry for the light-driven depolymerization of lignin biomass, commercial phenoxy resin, hydroxylated polyolefin derivatives, and thermoset polymers are presented as well. We then describe a method for the contrathermodynamic positional isomerization of highly substituted olefins by means of cooperative photoredox and chromium(II) catalysis. In this work, generation of an allylchromium(III) species that can undergo highly regioselective in situ protodemetalation enables access to a less substituted and thermodynamically less stable positional isomer. Product selectivity in this reaction is determined by the large differential in oxidation potentials between differently substituted olefin isomers. Lastly, we discuss a light-driven deracemization reaction developed in collaboration with the Miller group, wherein a racemic urea substrate undergoes spontaneous optical enrichment upon visible light irradiation in the presence of an iridium(III) chromophore, a chiral Brønsted base, and a chiral peptide thiol. Excellent levels of enantioselectivity are achieved via sequential and synergistic proton transfer (PT) and H atom transfer (HAT) steps. Taken together, these examples highlight the ability of excited-state ET events to enable access to nonequilibrium product distributions across a wide range of catalytic, redox-neutral transformations in which photons are the only stoichiometric reagents.

Organophotocatalytic Remote Thiocyanation Reaction via Ring-Opening Functionalization of Cycloalkanols

The remote C(sp3)-SCN bond formation via ring-opening functionalization of cycloalkanols with N-thiocyanatosaccharin as the precursor of SCN radicals and pyrylium salt as the organic photocatalyst under visible light has been developed. Thus, various terminal keto thiocyanates were prepared without transition metals and oxidants in moderate to good yields. The simplicity, wide substrate scope and mild conditions feature its synthetic application capability.

Alkoxy Radical Generation Mediated by Sulfoxide Cation Radicals for Alcohol-Directed Aliphatic C-H Functionalization

The C-H functionalization of remote, unactivated C-H bonds offers a unique method of garnering structural complexity in a synthesis. The use of directing groups has provided a means of enacting C-H functionalization on these difficult-to-access bonds; however, the installation and removal of directing groups on a substrate require additional synthetic manipulations, detracting from both the efficiency and economic feasibility of a transformation. The use of alkoxy radicals as transient directing groups for the functionalization of remote C-H bonds allows access to the synthesis of complex molecules without the need for additional functionality. Herein, we report a method for alkoxy radical formation from unactivated alcohols and reactivity mediated by photoredox-generated sulfoxide cation radicals. This protocol leverages the unique reactivity of alkoxy radicals to implement different reaction manifolds: 1,5-hydrogen atom transfer (HAT), cyclization, and β-scission. Furthermore, it was discovered that this methodology could be utilized to impose radical group transfer reactions via the β-scission pathway. Stern-Volmer analysis supports the formation of an alkoxy radical via the intermediacy of a sulfurane radical rather than a proton-coupled electron transfer (PCET) mechanism.

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Redox-Neutral Multicatalytic Cerium Photoredox-Enabled Cleavage of O-H Bearing Substrates

The need for synthetic methodologies capable of rapidly altering molecular structure are in high demand. Most existing methods to modify scaffolds rely on net exothermicity to drive the desired transformation. We sought to develop a general strategy for the cleavage of C-C bonds β to hydroxyl groups independent of inherent substrate strain. To this end we have applied a multicatalytic cerium photoredox-based system capable of activating O-H bonds in lactols to deliver formate esters. The same system is also capable of effecting hydrodecarboxylation and hydrodecarbonylation reactions. Initial mechanistic probes demonstrate atomic chlorine (Cl⋅) is generated under the reaction conditions, but substrate activation through cerium-alkoxides or -carboxylates cannot be ruled out.

Light-driven polymer recycling to monomers and small molecules

Only a small proportion of global plastic waste is recycled, of which most is mechanically recycled into lower quality materials. The alternative, chemical recycling, enables renewed production of pristine materials, but generally comes at a high energy cost, particularly for processes like pyrolysis. This review focuses on light-driven approaches for chemically recycling and upcycling plastic waste, with emphasis on reduced energy consumption and selective transformations not achievable with heat-driven methods. We focus on challenging to recycle backbone structures composed of mainly C‒C bonds, which lack functional groups i.e., esters or amides, that facilitate chemical recycling e.g., by solvolysis. We discuss the use of light, either in conjunction with heat to drive depolymerization to monomers or via photocatalysis to transform polymers into valuable small molecules. The structural prerequisites for these approaches are outlined, highlighting their advantages as well as limitations. We conclude with an outlook, addressing key challenges, opportunities, and provide guidelines for future photocatalyst (PC) development.

Visible light-promoted difluoromethylthiolation of cycloalkanols by C-C bond cleavage

A mild and general methodology for the difluoromethylthiolation of cycloalkanols has been developed by employing N-difluoromethylthiophthalimide as the SCF2H radical source, in combination with an acridinium-derived organo-photosensitizer, under redox-neutral conditions. This reaction protocol demonstrates high efficiency, scalability, and mild reaction conditions, thus presenting a green approach for the rapid synthesis of distal difluoromethylthiolated alkyl ketones that are challenging to be synthesized through alternative means.

Construction of C-S and C-Se Bonds from Unstrained Ketone Precursors under Photoredox Catalysis

A mild photoredox catalyzed construction of sulfides, disulfides, selenides, sulfoxides and sulfones from unstrained ketone precursors is introduced. Combination of this deacylative process with SN 2 or coupling reactions provides novel and convenient modular strategies toward unsymmetrical or symmetric disulfides. Reactivity studies favor a bromine radical that initiates a HAT (Hydrogen Atom Transfer) from the aminal intermediate resulting in expulsion of a C-centered radical that is intercepted to make C-S and C-Se bonds. Gram scale reactions, broad substrate scope and tolerance towards various functional groups render this method appealing for future applications in the synthesis of organosulfur and selenium complexes.

Photocatalytic Conversion of Lignin Models into Functionalized Aromatic Molecules Initiated by the Proton-Coupled Electron Transfer Process

A mild and efficient method for lignin β-O-4 cleavage and functionalization was achieved via photocatalysis. This protocol exhibits a broad scope of lignin models and excellent compatibility of functionalization reagents, constructing a series of functionalized lignin-based aromatic compounds. Highly selective formation of alkyl radical species through a proton-coupled electron transfer and β-scission process provides the opportunity to form new C-C and C-N bonds by reaction with electrophilic reagents.

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.

Visible-Light-Enabled Catalytic Approach to N,O-Spirocycles through Amidyl Radical Addition/Cyclization

A rational combination of photoredox catalyst anthraquinone and hydrogen atom transfer (HAT) catalyst methyl thioglycolate allows for the rapid and straightforward conversion of a range of 2-amidated acetylenic alcohols to multifunctional N,O-spirocycles under visible light irradiation. With oxygen as the sole terminal oxidant, these reactions can be carried out efficiently at room temperature without the involvement of transition metals or strong oxidants. The successful application of this mild catalytic strategy in the late-stage functionalization of bioactive skeletons further highlights its practical value.

Ratcheting synthesis

Synthetic chemistry has traditionally relied on reactions between reactants of high chemical potential and transformations that proceed energetically downhill to either a global or local minimum (thermodynamic or kinetic control). Catalysts can be used to manipulate kinetic control, lowering activation energies to influence reaction outcomes. However, such chemistry is still constrained by the shape of one-dimensional reaction coordinates. Coupling synthesis to an orthogonal energy input can allow ratcheting of chemical reaction outcomes, reminiscent of the ways that molecular machines ratchet random thermal motion to bias conformational dynamics. This fundamentally distinct approach to synthesis allows multi-dimensional potential energy surfaces to be navigated, enabling reaction outcomes that cannot be achieved under conventional kinetic or thermodynamic control. In this Review, we discuss how ratcheted synthesis is ubiquitous throughout biology and consider how chemists might harness ratchet mechanisms to accelerate catalysis, drive chemical reactions uphill and programme complex reaction sequences.

Molecular mechanism of the transformation of oxidized lignin to N-substituted aromatics

The cleavage of C-C bonds in oxidized lignin model compounds is a highly effective methodology for achieving lignin depolymerization, as well the generation of N-substituted aromatics. Here, density functional theory calculations were performed to understand the mechanism of the transformation of an oxidized lignin model compound (ligninox) and hydroxylamine hydrochloride to N-substituted aromatics. The reaction was proposed to proceed via an energetically viable mechanism featuring the initial production of HOAc acting as proton bridge. According to our calculations, Z-type oxime is the major intermediate of the reaction, with an energy barrier of 22.9 kcal mol-1, owing to the weak interactions between methoxy and oximino groups being stronger than that of E-type oxime. Additionally, the hydroxy addition is the rate-determining step, with an energy barrier of 27.0 kcal mol-1. Moreover, the huge net energy change of Beckmann and abnormal Beckmann rearrangements is the main overall thermodynamic driving force for producing N-substituted aromatics from oximes. The theoretical results have provided a clear picture of how ligninox transforms into N-substituted aromatics and are expected to provide valuable theoretical guidance for lignin depolymerization.

Synthesis, anti-leukemia activity, and molecular docking of novel 3,16-androstenedione derivatives

In this study, we synthesized androsta-4,14-diene-3,16-dione, 12β-hydroxyandrosta-4,14-diene-3,16-dione, and other 3,16-androstenedione derivatives from commercially available dehydroepiandrosterone as a starting material in 9-13 steps with high yields. The bioactivity of the obtained compounds was evaluated. Compounds 14a and 23a were shown to have high antitumor activity against acute lymphoblastic leukemia cell lines Nalm-6 and BALL-1, respectively. Network pharmacology analysis showed that the anti-leukemia activity of compounds 14a and 23a might be related to the JAK2, ABL1 protein, and PI3K/Akt signaling pathways. The molecular docking of compounds 14a and 23a identified possible active sites, with the lowest docking scores for PTGS2 and MAPK14, respectively. In addition, the absorption, distribution, metabolism, and excretion prediction results revealed the drug-likeness of the two compounds. Therefore, compounds 14a and 23a should be considered anti-leukemia candidates in future studies.

Photocatalytic C-C Bond Cleavage and Fluorosulfonylation of Strained Cycloalkanols for Carbonyl-Containing Aliphatic Sulfonyl Fluorides

In this report, we present a photocatalytic ring-opening fluorosulfonylation of strained cycloalkanols with sulfur dioxide and NFSI under mild conditions for the synthesis of carbonyl-containing aliphatic sulfonyl fluorides. The synthetic potential of the carbonyl-containing aliphatic sulfonyl fluoride products has been examined by diverse transformations, including SuFEx reactions and Baeyer-Villiger oxidation reactions. Mechanistic studies demonstrate that the reaction operates through a radical C-C bond cleavage/SO2 insertion/fluorination cascade process.

Zinc Vacancy Promotes Photo-Reforming Lignin Model to H2 Evolution and Value-Added Chemicals Production

Lignin, rich in β-O-4 bonds and aromatic structure, is a renewable and potential resource for value-added chemicals and promoting H2 evolution. However, direct photo-reforming lignin remains a huge challenge due to its recalcitrant structure. Herein, a collaborative strategy is proposed by dispersing Pt on zinc-vacancy-riched ZnIn2 S4 (Pt/VZn -ZIS) for revealing the effect of lignin structure during photo-reforming process with lignin models. And a series of theoretical calculations and experimental results show that lignin model substances with more nucleophilic group structures will have a stronger tendency to occur the photo-reforming reactions. In addition, benefiting of Pt-S electronic channel is formed by occupying Pt atom onto zinc vacancies in ZnIn2 S4 , which can effectively reduce the energy barrier of H2 evolution and accompany the selective oxidation of lignin model from Cα-OH to Cα = O under simulated sunlight. The natural lignin is used to further demonstrate this selective oxidation mechanism. The presented work demonstrates the photo-reforming lignin model mechanism and the influence of lignin-structure during the process of photo-reforming.

Sensitizer-Free Photochemical Regeneration of Benzimidazoline Organohydride

Organohydrides are an important class of organic compounds that can provide hydride anions for chemical and biochemical reactions, as demonstrated by reduced nicotinamide adenine dinucleotides serving as important natural redox cofactors. The coupling of hydride transfer from the organohydride to the substrate and subsequent regeneration of the organohydride from its oxidized form can realize organohydride-catalyzed reduction reactions. Depending on the structure of the organohydride, its hydridicity and ease of regeneration vary. Benzimidazoline (BIH) is one of the strongest synthetic C-H hydride donors; however, its reductive regeneration requires highly reducing conditions. In this study, we synthesized various oxidized and reduced forms of BIH derivatives with aryl groups at the 2-position and investigated their photophysical and electrochemical properties. 4-(Dimethylamino)phenyl-substituted BIH exhibited salient red-shifted absorption compared with other synthesized BIH derivatives, and visible-light-driven regeneration without using an external photosensitizer was achieved. This knowledge has significant implications for the future development of solar-energy-based catalytic photoreduction technologies that utilize organohydride regeneration strategies.

Chemical Recycling of Thiol Epoxy Thermosets via Light-Driven C-C Bond Cleavage

Epoxy thermosets are high-volume materials that play a central role in a wide range of engineering applications; however, technologies to recycle these polymers remain rare. Here, we present a catalytic, light-driven method that enables chemical recycling of industrially relevant thiol epoxy thermosets to their original monomer at ambient temperature. This strategy relies on the proton-coupled electron transfer (PCET) activation of hydroxy groups within the polymer network to generate key alkoxy radicals that promote the fragmentation of the polymer through C-C bond β-scission. The method fully depolymerizes insoluble thiol epoxy thermosets into well-defined mixtures of small-molecule products, which can collectively be converted into the original monomer via a one-step dealkylation process. Notably, this process is selective and efficient even in the presence of other commodity plastics and additives commonly found in commercial applications. These results constitute an important step toward making epoxy thermosets recyclable and more generally exemplify the potential of PCET to offer a more sustainable end-of-life for a diverse array of commercial plastics.

Photoredox-Catalyzed Multicomponent Synthesis of Functionalized γ-Amino Butyric Acids via Reductive Radical Polar Crossover

Multicomponent radical polar crossover (RPC) reactions are useful for leveraging both radical and polar bond-forming steps to rapidly build molecular complexity in a single transformation. However, multicomponent RPC reactions that utilize carbonyl π-bond electrophiles are underrepresented in the literature. Herein, we describe a mild, photoredox-catalyzed decarboxylative multicomponent RPC reaction that couples carboxylic acids, Michael acceptors, and carbonyl electrophiles for the formation of diversely functionalized γ-amino butyric acid derivatives. This transformation also facilitates the synthesis of complex and biologically relevant γ-lactam compounds.

Deacylative Thiolation by Redox-Neutral Aromatization-Driven C-C Fragmentation of Ketones

Herein we report the development of deacylative thiolation of diverse methyl ketones. The reaction is redox-neutral, and heavy-metal-free, which provides a new way to introduce thioether groups site-specifically to unactivated aliphatic positions. It also features excellent functional group tolerance and broad substrate scope, thus allowing late-stage derivatization. The process benefits from efficient condensation between the activation reagent and ketone substrates, which triggers aromatization-driven C-C fragmentation and rapid radical coupling with thiosulfonates. Experimental and computational mechanistic studies suggest the involvement of a radical chain pathway.

Skeletal diversification by C-C cleavage to access bicyclic frameworks from a common tricyclooctane intermediate

Herein, the diversification of tricyclo[3.2.1.03,6]octane scaffolds to afford diverse bicyclic scaffolds is described. The strained tricyclooctanes are prepared in two steps featuring a blue light-mediated [2+2] cycloaddition. Strategies for the cleavage of this scaffold were then explored resulting in the selective syntheses of the bicyclo[3.1.1]heptane, bicyclo[3.2.1]octane, and bicyclo[3.2.0]heptane cores. These findings may guide future studies of C-C cleavage reactions in strained carbon frameworks and their application in complex molecule synthesis.

Skeletal Transformation of Oxindoles into Quinolinones Enabled by Synergistic Copper/Iminium Catalysis

We describe a synergistic Cu/secondary amine catalysis for skeletal transformation of an oxindole core into a quinolinone skeleton, which generates several structurally new pyridine-fused quinolinones. The synergistic reactions allow expansion of a five-membered lactam ring by radical cation-triggered C-C bond cleavage and enable a further intramolecular cyclization with the aim to construct totally distinct core skeletons.

A Nitrogen Out-of-Plane (NOOP) Mechanism for Imine-Based Light-Driven Molecular Motors

Light-driven molecular motors have generated considerable interest due to their potential applications in material and biological systems. Recently, Greb and Lehn reported a new class of molecular motors, chiral N-alkyl imines, which undergo unidirectional rotation induced by light and heat. The mechanism of unidirectional motion in molecular motors containing a C═N group has been assumed to consist of photoinduced torsion about the double bond. In this work, we present a computational study of the photoisomerization dynamics of a chiral N-alkyl imine motor. We find that the location and energetics of minimal energy conical intersections (MECIs) alone are insufficient to understand the mechanism of the motor. Furthermore, a key part of the mechanism consists of out-of-plane distortions of the N atom (followed by isomerization about the double bond). Dynamic effects and out-of-plane distortions are critical to understand the observed (rather low) quantum yield for photoisomerization. Our results provide hints as to how the photoisomerization quantum yield might be increased, improving the efficiency of this class of molecular motors.

Deconstructive Functionalization of Unstrained Cycloalkanols via Electrochemically Generated Aromatic Radical Cations

Herein we report an electrochemical approach for the deconstructive functionalization of cycloalkanols, where various alcohols, carboxylic acids, and N-heterocycles are employed as nucleophiles. The method has been demonstrated across a broad range of cycloalkanol substrates, including various ring sizes and substituents, to access useful remotely functionalized ketone products (36 examples). The method was demonstrated on a gram scale via single-pass continuous flow, which exhibited increased productivity in relation to the batch process.

Lignin C-C bond cleavage induced by consecutive two-photon excitation of a metal-free photocatalyst

Photocatalytic lignin valorization has caught widespread attention; yet the reaction systems often employ noble metal complexes, hydrogen atom transfer (HAT) agents, and/or sacrificial electron donors/acceptors that do not comply with atom economy or environmental friendliness. Herein, we discovered that N-phenylphenothiazine (PTH) as a metal-free photocatalyst induced the cleavage of the lignin C_α-C_β bond under ambient conditions free of those additional agents with a high yield and selectivity toward benzoic acid. Transient spectroscopic investigations revealed that the energy-demanding C_α-C_β bond cleavage was induced by the potent oxidant, ²PTH˙⁺*, that was derived from consecutive two-photon excitation of PTH.

PCET-Mediated Ring-Opening Alkenylation of Cycloalkanols via Dual Photoredox and Cobalt Catalysis

The construction of molecular skeletons and modification of molecules using widely available and easily prepared alcohols as radical precursors for coupling reactions are significant and challenging subjects. We herein report a straightforward strategy for the dehydrogenative ring-opening alkenylation of cycloalkanols with alkenes by combining a proton-coupled electron transfer strategy and a dual photoredox and cobalt catalysis system. With this approach, a series of distally unsaturated ketones were obtained in 17-83% yields with high E selectivity.

Modern Photocatalytic Strategies in Natural Product Synthesis

Modern photocatalysis has proven its generality for the development and functionalization of native functionalities. To date, the field has found broad applications in diverse research areas, including the total synthesis of natural products. This contribution covers recent reports of total syntheses involving as a key step a photocatalytic reaction. Among the selected examples, the photocatalytic processes proceed in a highly chemo-, regio-, and stereoselective manner, thereby allowing the rapid access to structurally complex architectures under light-driven conditions.

Combined conversion of lignocellulosic biomass into high-value products with ultrasonic cavitation and photocatalytic produced reactive oxygen species - A review

The production of high-value products from lignocellulosic biomass is carried out through the selective scission of crosslinked CC/CO bonds. Nowadays, several techniques are applied to optimize biomass conversion into desired products with high yields. Photocatalytic technology has been proven to be a valuable tool for valorizing biomass at mild conditions. The photoproduced reactive oxygen species (ROSs) can initiate the scission of crosslinked bonds and form radical intermediates. However, the low mass transfer of the photocatalytic process could limit the production of a high yield of products. The incorporation of ultrasonic cavitation in the photocatalytic system provides an exceptional condition to boost the fragmentation and transformation of biomass into the desired products within a lesser reaction time. This review critically discusses the main factors governing the application of photocatalysis for biomass valorization and tricks to boost the selectivity for enhancing the yield of desired products. Synergistic effects obtained through the combination of sonolysis and photocatalysis were discussed in depth. Under ultrasonic vibration, hot spots could be produced on the surface of the photocatalysts, improving the mass transfer through the jet phenomenon. In addition, shock waves can assist the dissolution and mixing of biomass particles.

Synthesis of remote fluoroalkenyl ketones by photo-induced ring-opening addition of cyclic alkoxy radicals to fluorinated alkenes

Fluoroalkenyl moieties are often used as carbonyl mimics in medicine preparation, and thus the development of facile routes for the synthesis of such compounds is of great importance. In this work, we report a photocatalytic ring-opening addition of cyclic alcohols to α-(trifluoromethyl)styrenes, which underwent a proton-coupled electron transfer and β-scission process, delivering a great variety of remote gem-difluoroalkenyl ketone derivatives. This methodology can also be applied in the reaction of gem-difluorostyrenes and 1,1,2-trifluorostyrenes to access monofluoro- and 1,2-difluoroalkenyl ketones.

Using Catalysis to Drive Chemistry Away from Equilibrium: Relating Kinetic Asymmetry, Power Strokes, and the Curtin-Hammett Principle in Brownian Ratchets

Chemically fueled autonomous molecular machines are catalysis-driven systems governed by Brownian information ratchet mechanisms. One fundamental principle behind their operation is kinetic asymmetry, which quantifies the directionality of molecular motors. However, it is difficult for synthetic chemists to apply this concept to molecular design because kinetic asymmetry is usually introduced in abstract mathematical terms involving experimentally inaccessible parameters. Furthermore, two seemingly contradictory mechanisms have been proposed for chemically driven autonomous molecular machines: Brownian ratchet and power stroke mechanisms. This Perspective addresses both these issues, providing accessible and experimentally useful design principles for catalysis-driven molecular machinery. We relate kinetic asymmetry to the Curtin-Hammett principle using a synthetic rotary motor and a kinesin walker as illustrative examples. Our approach describes these molecular motors in terms of the Brownian ratchet mechanism but pinpoints both chemical gating and power strokes as tunable design elements that can affect kinetic asymmetry. We explain why this approach to kinetic asymmetry is consistent with previous ones and outline conditions where power strokes can be useful design elements. Finally, we discuss the role of information, a concept used with different meanings in the literature. We hope that this Perspective will be accessible to a broad range of chemists, clarifying the parameters that can be usefully controlled in the design and synthesis of molecular machines and related systems. It may also aid a more comprehensive and interdisciplinary understanding of biomolecular machinery.