Ketone-Olefin Coupling of Aliphatic and Aromatic Carbonyls Catalyzed by Excited-State Acridine Radicals

Photochemically Enabled Total Syntheses of Stemoamide Alkaloids

Photochemical transformations continue to serve as powerful synthetic tools for rapid chemical synthesis and diversification. Recent developments in photoredox and photochemical reactivity have captured the attention of researchers in a wide array of disciplines, where many new applications of these reactions have been reported. We disclose the use of photochemical synthetic strategies as a modern approach to natural product synthesis that leverages the inherent reactivity of radicals as a platform for constructing complex scaffolds. We demonstrate this in an iterative photochemical synthesis, offering novel synthetic tactics, mild conditions, and operationally simple synthetic procedures to construct three stemoamide alkaloids in the shortest sequences to date. The key disconnection involves the use of both the oxidative and reductive capabilities of an acridinium photoredox catalyst to forge the densely functionalized tetrahydrofuran ring via a polar radical crossover cycloaddition. The resultant butyrolactone serves as a handle for a radical polar crossover cycloaddition to construct a unique oxaspirocyclic butenolide. Finally, a late-stage heteroarene transmutation provides a linchpin intermediate used to access three stemoamide alkaloids. The efficiency of these syntheses exemplifies the power of this approach while also demonstrating a departure from traditional disconnections and shedding light on a new type of synthetic art.

Excited Organic Radicals in Photoredox Catalysis

Many important synthetic-oriented works have proposed excited organic radicals as photoactive species, yet mechanistic studies raised doubts about whether they can truly function as photocatalysts. This skepticism originates from the formation of (photo)redox-active degradation products and the picosecond decay of electronically excited radicals, which is considered too short for diffusion-based photoinduced electron transfer reactions. From this perspective, we analyze important synthetic transformations where organic radicals have been proposed as photocatalysts, comparing their theoretical maximum excited state potentials with the potentials required for the observed photocatalytic reactivity. We summarize mechanistic studies of structurally similar photocatalysts indicating different reaction pathways for some catalytic systems, addressing cases where the proposed radical photocatalysts exceed their theoretical maximum reactivity. Additionally, we perform a kinetic analysis to explain the photoinduced electron transfer observed in excited radicals on subpicosecond time scales. We further rationalize the potential anti-Kasha reactivity from higher excited states with femtosecond lifetimes, highlighting how future photocatalysis advancements could unlock new photochemical pathways.

Light-Driven Stepwise Reduction of Aliphatic Carboxylic Esters to Aldehydes and Alcohols

The reduction of carboxylic esters to aldehydes and alcohols is a fundamental functional group transformation in chemistry. However, the inertness of carbonyl group and the instability of ketyl radical anion intermediate impede the reduction of carboxylic esters via photochemical strategy. Herein, we described the reduction of aliphatic carboxylic esters with synergistic dual photocatalysis via phenolate-catalyzed single electron transfer process and thiol-catalyzed hydrogen atom transfer process. The competitive back electron transfer process was effectively inhibited by protonation of the ketyl-type radical anion. This protocol enabled the efficient reduction of carboxylic esters to alcohols under mild conditions. By interruption of the reduction with prolinol, the step-controlled reduction of carboxylic esters to aldehydes was accomplished. The developed process was also successfully applied to the preparation of deuterated alcohols and aldehydes from esters with D2O as the deuterium source.

Metallaphotoredox-catalyzed alkynylcarboxylation of alkenes with CO2 and alkynes for expedient access to β-alkynyl acids

Carboxylation with CO2 offers an attractive and sustainable access to valuable carboxylic acids. Among these methods, direct C-H carboxylation of terminal alkynes with CO2 has attracted much attention for one-carbon homologation of alkynes, enabling rapid synthesis of propiolic acids. In contrast, the multi-carbons homologation of alkynes with CO2 to construct important non-conjugated alkynyl-containing acids has not been reported. Herein, we present alkynylcarboxylation of alkenes with CO2 via photoredox and copper dual catalysis. This protocol provides a direct and practical method to form valuable non-conjugated alkynyl acids from readily available alkynes, alkenes and CO2. Additionally, this approach also features mild (room temperature, 1 atm of CO2) and redox-neutral conditions, high atom and step economy, good functional group tolerance, and high selectivities. Moreover, diverse transformations of the β-alkynyl acid products and the rapid synthesis of bioactive molecule (GPR40/FFA1 agonist) further illustrate the synthetic utility of this methodology.

Visible-Light-Mediated Nucleophilic Addition of Alkene with Aldehyde: Synthesis of Secondary Alcohols

Herein, a photocatalytic strategy for the synthesis of secondary alcohols by nucleophilic addition of an alkene with an aldehyde is described. This operationally simple methodology opens an approach for the synthesis of alcohols using commercially available reagents in moderate to excellent yields. Mechanistic studies indicate that the formation of the radical anion from alkene via single-electron transfer is the key step in this reaction.

Catalytic Aldehyde-Alkyne Couplings Triggered by Ketyl Radicals

A general and flexible platform for catalytic aldehyde-alkyne couplings triggered by ketyl radicals is described. This open-shell strategy necessitates only a catalytic quantity of a photoredox catalyst, along with Hünig's base (DIPEA) as a halogen atom transfer reagent. The reaction proceeds through sequential steps involving activation, halogen atom transfer, and radical addition. This carbonyl-alkyne coupling exhibits a wide substrate scope and functional group compatibility and has been successfully applied to the late-stage modification of complex architectures.

Dearomative hydroamination of heteroarenes catalyzed by the phenolate photocatalyst

Dearomative functionalization of heteroarenes offers an attractive and sustainable approach for the rapid construction of complex 3D heterocyclic scaffolds from planar structures. Despite progress in this field, dearomative amination of heteroarenes via a radical anion intermediate remains a challenge. Here, we report a photoredox-catalyzed dearomative hydroamination of heteroarenes with hydrazodiformates under mild and transition-metal-free reaction conditions. Various benzofurans and benzothiophenes can efficiently participate in this transformation. A series of mechanistic experiments revealed that heteroaryl radical anions are the crucial intermediates, generated through photo-induced electron transfer between the excited phenolate photocatalyst and heteroarenes.

Electroinduced Reductive and Dearomative Alkene-Aldehyde Coupling

The direct coupling of alkene feedstocks with aldehydes represents an expedient approach to the generation of new and structurally diverse C(sp3)-hybridized alcohols that are primed for elaboration into privileged architectures. Despite their abundance, current disconnection strategies enabling the direct coupling of carbon-carbon π-bonds and aldehydes remain challenging because contemporary methods are often limited by substrate or functional group tolerance and compatibility in complex molecular environments. Here, we report a coupling between simple alkenes, heteroarenes and unactivated aliphatic aldehydes via an electrochemically induced reductive activation of C-C π-bonds. The cornerstone of this approach is the discovery of rapid alternating polarity (rAP) electrolysis to access and direct highly reactive radical anion intermediates derived from conjugated alkenes and heterocyclic compounds. Our developed catalyst-free protocol enables direct access to new and structurally diverse C(sp3)-hybridized alcohol products. This is achieved by the controlled reduction of conjugated alkenes and the C2-C3 π-bond in heteroarenes via an unprecedented reductive dearomative functionalization for heterocyclic compounds. Experimental mechanistic studies demonstrate a kinetically biased single-electron reduction of C-C π-bonds over aldehydes. Application of rAP enables chemoselective generation of olefinic radical anion intermediates and avoids undesired saturative overreduction. Overall, this technology provides a versatile approach to the reductive coupling of olefin and heterocycle feedstocks with aliphatic aldehydes, offering straightforward access to diverse C(sp3)-rich oxygenated scaffolds.

Aldehyde-Olefin Couplings Via Sulfoxylate-Mediated Oxidative Generation of Ketyl Radical Anions

Ketyl radicals are valuable reactive intermediates because they allow carbonyl chemistry to be extended beyond traditional electrophilic reactivity through simple single-electron reduction to a nucleophilic radical. However, this pathway is challenging due to the large negative reduction potentials of carbonyls, thus requiring highly reducing conditions. Herein, we describe the development of an alternative strategy to access ketyl radicals from aldehydes, which avoids the reduction pathway by instead proceeding via single-electron oxidation and desulfination of α-hydroxy sulfinates. These redox-active aldehyde adducts are generated in situ through the addition of sulfoxylate (SO22-) to aldehydes and possess low oxidation potentials, thereby facilitating ketyl radical formation and circumventing the need for strongly reducing conditions. We demonstrate the application of this sulfoxylate-mediated ketyl radical formation in ketyl-olefin coupling reactions.

Pioneering Metal-Free Late-Stage C-H Functionalization Using Acridinium Salt Photocatalysis

Using organic dyes as photocatalysts is an innovative approach to photocatalytic organic transformations. These dyes offer advantages such as widespread availability, adaptable absorption properties, and diverse chemical structures. Recent progress has led to the development of organic photocatalysts that can utilize visible light to modify chemically inert C-H bonds. These catalysts are sustainable, selective, and versatile, enabling mild reactions, late-stage functionalization, and various transformations in line with green chemistry principles. As catalysts in photoredox chemistry, they contribute to the development of efficient and environmentally friendly synthetic pathways. Acridinium-based organic photocatalysts have proved valuable in late-stage C-H functionalization, enabling transformative reactions under mild conditions. This review emphasizes their innovative features, such as organic frameworks, efficient light absorption properties, and their applications in modifying complex molecules. It provides an overview of recent advancements in the use of acridinium-based organic photocatalysts for late-stage C-H bond functionalization without the need for transition metals, showcasing their potential to expedite the development of new molecules and igniting excitement about the prospects of this research in the field.

Harnessing the potential of acyl triazoles in bifunctional cobalt-catalyzed radical cross-coupling reactions

Persistent radicals facilitate numerous selective radical coupling reactions. Here, we have identified acyl triazole as a new and versatile moiety for generating persistent radical intermediates through single-electron transfer processes. The efficient generation of these persistent radicals is facilitated by the formation of substrate-coordinated cobalt complexes, which subsequently engage in radical cross-coupling reactions. Remarkably, triazole-coordinated cobalt complexes exhibit metal-hydride hydrogen atom transfer (MHAT) capabilities with alkenes, enabling the efficient synthesis of diverse ketone products without the need for external ligands. By leveraging the persistent radical effect, this catalytic approach also allows for the development of other radical cross-coupling reactions with two representative radical precursors. The discovery of acyl triazoles as effective substrates for generating persistent radicals and as ligands for cobalt catalysis, combined with the bifunctional nature of the cobalt catalytic system, opens up new avenues for the design and development of efficient and sustainable organic transformations.

Mild Photochemical Reduction of Alkenes and Heterocycles via Thiol-Mediated Formate Activation

The reduction of alkenes to their respective alkanes is one of the most important transformations in organic chemistry, given the abundance of natural and commercial olefins. Metal-catalyzed hydrogenation is the most common way to reduce alkenes; however, the use of H2 gas in combination with the precious metals required for these conditions can be impractical, dangerous, and expensive. More complex substrates often require extremely high pressures of H2, further emphasizing the safety concerns associated with these hydrogenation reactions. Here we report a safe, cheap, and practical photochemical alkene reduction using a readily available organophotocatalyst, catalytic thiol, and formate. These conditions reduce a variety of di-, tri-, and tetra-substituted alkenes in good yield as well as dearomatize pharmaceutically relevant heterocycles to generate sp3-rich isosteres of benzofurans and indoles. These formal-hydrogenation conditions tolerate a broad range of functionalities that would otherwise be sensitive to typical hydrogenations and are likely to be important for industry applications.

Study on Pyridine-Boryl Radical-Promoted, Ketyl Radical-Mediated Carbon-Carbon Bond-Forming Reactions

Ketyl radicals are synthetically versatile reactive species, but their applications have been hampered by harsh generation conditions employing highly reducing metals. Recently, the pyridine-boryl radical received wide attention as a promising organic reductant because of its mildness as well as convenience in handling. While probing the utility of the pyridine-boryl radical, our group observed facile pinacol coupling reactivity that had not been known at that time. This serendipitous finding was successfully rendered into a practical synthesis of tetraaryl-1,2-diols in up to 99% yield within 1 h. Subsequently, upon examinations of various reaction manifolds, a diastereoselective ketyl-olefin cyclization was accomplished to produce cycloalkanols such as trans-2-alkyl-1-indanols. Compared to the previous methods, the stereocontrolling ability was considerably enhanced by taking advantage of the structurally modifiable boryl group that would be present near the bond-forming site. In this full account, our synthetic efforts with the O-boryl ketyl radicals are disclosed in detail, covering the discovery, optimization, scope expansion, and mechanistic analysis, including density functional theory (DFT) calculations.

Palladium-Catalyzed Allylation of Endocyclic 1-Azaallyl Anions

Endocyclic 1-azaallyl anions engage allyl acetates in a palladium-catalyzed allylation followed by reduction to give unprotected 2-(hetero)aryl-3-allylpiperidines and 2-allyl-3-arylmorpholines, products not easily accessible by other means. The allyl group is then readily transformed into a variety of functional groups. Preliminary studies on the asymmetric variant of the reaction using an enantiomerically pure BI-DIME-type ligand provide the product with moderate enantioselectivity. Computational studies suggest that energy barriers of inner-sphere reductive elimination and outer-sphere nucleophilic substitution are almost the same, which makes both of them possible reaction pathways. In addition, the inner-sphere mechanism displays an enantiodiscriminating C-C bond forming step, while the outer-sphere mechanism is much less selective, which combined to give the asymmetric variant of the reaction moderate enantioselectivity.

Alkyl Cyclopropyl Ketones in Catalytic Formal [3 + 2] Cycloadditions: The Role of SmI2 Catalyst Stabilization

Alkyl cyclopropyl ketones are introduced as versatile substrates for catalytic formal [3 + 2] cycloadditions with alkenes and alkynes and previously unexplored enyne partners, efficiently delivering complex, sp3-rich products. The key to effectively engaging this relatively unreactive new substrate class is the use of SmI2 as a catalyst in combination with substoichiometric amounts of Sm0; the latter likely acting to prevent catalyst deactivation by returning SmIII to the catalytic cycle. In the absence of Sm0, background degradation of the SmI2 catalyst can outrun product formation. For the most recalcitrant alkyl cyclopropyl ketones, catalysis is "switched-on" using these new robust conditions, and otherwise unattainable products are delivered. Combined experimental and computational studies have been used to identify and probe reactivity trends among alkyl cyclopropyl ketones, including more complex bicyclic alkyl cyclopropyl ketones, which react quickly with various partners to give complex products. In addition to establishing alkyl cyclopropyl ketones as a new substrate class in a burgeoning field of catalysis, our study provides vital mechanistic insight and robust, practical approaches for the nascent field of catalysis with SmI2.

Cu(I)-Photosensitizer-Catalyzed Olefin-α-Amino Radical Metathesis/Demethylenative Cyclization of 1,7-Enynes

A demethylenative En-Yne radical cyclization of 1,7-enynes has been successfully developed to chemoselectively afford 3,4-dihyroquinolin-2-ones or quinolin-2-ones under the catalysis of Cu(I) photosensitizers PS3 and PS6 with different redox potentials. The preliminary mechanistic experiments revealed that the reaction underwent an unprecedented olefin-α-amino radical metathesis-type process. A reasonable mechanism was proposed to illustrate the catalyst-controlled chemoselectivity of the reaction based on preliminary mechanistic experiments and DFT calculations.

Enantioselective Cyanofunctionalization of Aromatic Alkenes via Radical Anions

Alkene radical ions constitute an integral and unique class of reactive intermediates for the synthesis of valuable compounds because they have both unpaired spins and charge. However, relatively few synthetic applications of alkene radical anions have emerged due to a dearth of generally applicable and mild radical anion generation approaches. Precise control over the chemo- and stereoselectivity in alkene radical anion-mediated processes represents another long-standing challenge due to their high reactivity. To overcome these issues, here, we develop a new redox-neutral strategy that seamlessly merges photoredox and copper catalysis to enable the controlled generation of alkene radical anions and their orthogonal enantioselective cyanofunctionalization via distonic-like species. This new strategy enables highly regio-, chemo-, and enantioselective hydrocyanation, deuterocyanation, and cyanocarboxylation of alkenes without stoichiometric reductants or oxidants under visible light irradiation. This protocol provides a new blueprint for the exploration of the transformation potential of alkene radical anions.

Photoredox and NHC Enabled Deoxygenative Alcohol Homologation via Formal 1,2-Addition

A highly efficient photoinduced iron-catalyzed method has been developed for the direct use of alcohols as surrogates for organometallic reagents in the synthesis of tertiary alcohols. This method can be applied to both primary and secondary alcohols with diverse structures, enabling their reaction with aryl ketones under mild conditions. A variety of functional groups, including those that are typically reactive under conventional tertiary alcohol synthesis conditions, are compatible. Mechanistically, this reaction proceeds through the direct addition of the radical to the carbonyl pathway.

Ruthenium-Catalyzed 1,6-Hydroalkylation of para-Quinone Methides with Ketones via the in Situ Activation of C(sp3)-H Bonds

A simple and efficient method for the ruthenium-catalyzed 1,6-hydroalkylation of para-quinone methides (p-QMs) with ketones via the in situ activation of C(sp3)-H bonds has been disclosed. Without the need for preactivation of the substrates and oxidant, a broad range of p-QMs and ketones are well tolerated, producing the expected 1,6-hydroalkylation products with moderate to good yields. Step-by-step control experiments and DFT calculation were conducted systematically to gain insights for the plausible reaction mechanism. This finding may have potential application in the selective diarylmethylation of ketones at the α-C position in organic synthesis.

Reductive Cross-Coupling of Olefins via a Radical Pathway

Olefins are widely available at low costs, which explains the usefulness of developing new methods for their functionalization. Here we report a simple protocol that uses a photoredox catalyst and an inexpensive thiol catalyst to stitch together two olefins, forming a new C-C bond. Specifically, an electron-poor olefin is reduced by the photoredox catalyst to generate, upon protonation, a carbon radical, which is then captured by a neutral olefin. This intermolecular cross-coupling process provides a tool for rapidly synthesizing sp3-dense molecules from olefins using an unconventional disconnection.

Stereochemical modulation of ketyl radical cyclization enabled by pyridine-boryl radicals: catalytic diastereoselective synthesis of trans-2-alkyl-1-indanols

Previously available ketyl radical cyclization conditions suffer from low and uncontrollable diastereoselectivity because of the absence of reagent-substrate interactions. In this report, stereochemical modulation was accomplished by taking advantage of the pyridine-boryl radical, which leaves the synthetically modifiable boronate moiety on the carbonyl oxygen near the reacting center during the stereo-determining cyclization step. In consequence, a catalytic diastereoselective synthesis of trans-2-substituted-1-indanols was achieved in the presence of a sterically congested six-membered diboronic ester and an efficient hydrogen atom donor.

Photo-Induced, Phenylhydrazine-Promoted Transition-Metal-Free Dehalogenation of Aryl Fluorides, Chlorides, Bromides, and Iodides

In this study, we present a straightforward and highly effective photo-triggered hydrogenation method for aryl halides, devoid of transition-metal catalysts. Through the synergistic utilization of light, PhNHNH2, and a base, we have successfully initiated the desired radical-mediated hydrogenation process. Remarkably, utilizing mild reaction conditions, a wide range of aryl halides, including fluorides, chlorides, bromides, and iodides, can be selectively transformed into their corresponding (hetero)arene counterparts, with exceptional yields. Additionally, this approach demonstrates a remarkable compatibility with diverse functional groups and heterocyclic compounds, highlighting its versatility and potential for use in various chemical transformations.

Ketyl Radical Coupling Enabled by Polycyclic Aromatic Hydrocarbon Electrophotocatalysts

Herein, we report a new class of electrophotocatalysts, polycyclic aromatic hydrocarbons, that promote the reduction of unactivated carbonyl compounds to generate versatile ketyl radical intermediates. This catalytic platform enables previously challenging intermolecular ketyl radical coupling reactions, including those that classic reductants (e.g., SmI2/HMPA) have failed to promote. More broadly, this study outlines an approach to fundamentally expand the array of reactive radical intermediates that can be generated via electrophotocatalysis by obviating the need for rapid mesolytic cleavage following substrate reduction.

Reductive photoredox transformations of carbonyl derivatives enabled by strongly reducing photosensitizers

Visible-light photoredox catalysis is well-established as a powerful and versatile organic synthesis strategy. However, some substrate classes, despite being attractive precursors, are recalcitrant to single-electron redox chemistry and thus not very amenable to photoredox approaches. Among these are carbonyl derivatives, e.g. ketones, aldehydes, and imines, which in most cases require Lewis or Brønsted acidic additives to activate via photoinduced electron transfer. In this work, we unveil a range of photoredox transformations on ketones and imines, enabled by strongly reducing photosensitizers and operating under simple, general conditions with a single sacrificial reductant and no additives. Specific reactions described here are umpolung C-C bond forming reactions between aromatic ketones or imines and electron-poor alkenes, imino-pinacol homocoupling reactions of challenging alkyl-aryl imine substrates, and γ-lactonization reactions of aromatic ketones with methyl acrylate. The reactions are all initiated by photoinduced electron transfer to form a ketyl or iminyl that is subsequently trapped.

Shining Fresh Light on Complex Photoredox Mechanisms through Isolation of Intermediate Radical Anions

Photoredox catalysis (PRC) has gained enormous and wide-ranging interest in recent years but has also been subject to significant mechanistic uncertainty, even controversy. To provide a method by which the missing understanding can begin to be filled in, we demonstrate herein that it is possible to isolate as authentic materials the one-electron reduction products of representative PRC catalysts (PCs). Specifically, KC₈ reduction of both 9,10-dicyanoanthracene and a naphthalene monoamide derivative in the presence of a cryptand provides convenient access to the corresponding [K(crypt)⁺][PC^·-] salts as clean materials that can be fully characterized by techniques including EPR and XRD. Because PC^·- states are key intermediates in PRC reactions, such isolation allows for highly controlled study of these anions' specific reactivity and hence their mechanistic roles. As a demonstration of this principle, we show that these salts can be used to conveniently interrogate the mechanisms of recent, high-profile "conPET" and "e-PRC" reactions, which are currently the subject of both significant interest and acute controversy. Using very simple experiments, we are able to provide striking insights into these reactions' underlying mechanisms and to observe surprising levels of hidden complexity that would otherwise have been very challenging to identify and that emphasize the care and control that are needed when interrogating and interpreting PRC mechanisms. These studies provide a foundation for the study of a far broader range of questions around conPET, e-PRC, and other PRC reaction mechanisms in the future, using the same strategy of PC^·- isolation.

Radical Hydrocarboxylation of Unactivated Alkenes via Photocatalytic Formate Activation

Herein we disclose a strategy to promote the hydrocarboxylation of unactivated alkenes using photochemical activation of formate salts. We illustrate that an alternative initiation mechanism circumvents the limitations of prior approaches and enables hydrocarboxylation of this challenging substrate class. Specifically, we found that accessing the requisite thiyl radical initiator without an exogenous chromophore eliminates major byproducts that have plagued attempts to exploit similar reactivity for unactivated alkene substrates. This redox-neutral method is technically simple to execute and effective across a broad range of alkene substrates. Feedstock alkenes, such as ethylene, are hydrocarboxylated at ambient temperature and pressure. A series of radical cyclization experiments indicate how the reactivity described in this report can be diverted by more complex radical processes.

In(OTf)3-catalyzed reorganization/cycloaddition of two imine units and subsequent modular assembly of acridinium photocatalysts

Herein, we disclose a novel reorganization/cycloaddition between two imine units catalyzed by In(OTf)3 Lewis acid that differs from the well-known [4 + 2] cycloaddition version via the Povarov reaction. By means of this unprecedented imine chemistry, a collection of synthetically useful dihydroacridines has been synthesized. Notably, the obtained products give rise to a series of structurally novel and fine-tuneable acridinium photocatalysts, offering a heuristic paradigm for synthesis and efficiently facilitating several encouraging dihydrogen coupling reactions.

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.

Regioselective and Diastereoselective Halofunctionalization of Alkenes Promoted by Organophotocatalytic Solar Catalysis

A visible-light metal-free photocatalytic regioselective and enantioselective alkene halofunctionalization reaction under mild conditions is reported. Various terminal and internal alkenes were transformed to their α-halogenated and α,β-dibrominated derivatives in good to excellent yields within reaction time as short as 5 min. Water can be used as the "green" nucleophile and solvent in the halohydroxylation and halo-oxidation reactions. Different types of products can be obtained by adjusting the reaction conditions. In addition, sunlight is proved to produce products with similar yields, representing a practical example of solar synthesis and providing an opportunity for solar energy utilization.

Triarylamine-Substituted Benzimidazoliums as Electron Donor-Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations

Triarylamine-substituted benzimidazoliums (BI⁺-PhNAr₂), new electron donor-acceptor dyad molecules, were synthesized. Their photocatalytic properties for reductive organic transformations were explored using absorption and fluorescence spectroscopy, redox potential determinations, density functional theory calculations, transient absorption spectroscopy, and reduction reactions of selected substrates. The results show that irradiation of BI⁺-PhNAr₂ promotes photoinduced intramolecular electron transfer to form a long-lived (∼300 μs) charge shifted state (BI^•-PhN^•+Ar₂). In the pathway for photocatalysis of reduction reactions of substrates, BI^•-PhN^•+Ar₂ is subsequently transformed to the neutral benzimidazolyl radical (BI^•-PhNAr₂) by single-electron transfer from the donor 1,3-dimethyl-2-phenylbenzimidazoline (BIH-Ph) serving as a cooperative agent. Among the benzimidazoliums explored, the bromo-substituted analogue BI⁺-PhN(C₆H₄Br-p)₂ in conjunction with BIH-Ph demonstrates the most consistent catalytic performance.

Visible-light acridinium-based organophotoredox catalysis in late-stage synthetic applications

The field of photoredox catalysis has been transformed by the use of organic photocatalysts, which give access to re-activities that were previously only possible with transition-metal photocatalysts. Recent advancements in the use of an acridinium photocatalyst in organic synthesis are covered in this review. Both the late-stage functionalization of biorelevant molecules and the activation of inert chemical bonds are explored, with an emphasis on their mechanistic features.

Four-Step Synthesis of (-)-4-epi-Presilphiperfolan-8α-ol by Intramolecular Iron Hydride Atom Transfer-Mediated Ketone-Alkene Coupling and Studies to Access trans-Hydrindanols with a Botryane Scaffold

From an (R)-(+)-pulegone-derived building block that incorporates the stereo-defined tertiary carbon bearing a methyl group, as found in the targeted sesquiterpenoid, a four-step synthesis of (-)-4-epi-presilphiperfolan-8-α-ol was achieved. The key processes involved are a ring-closing metathesis leading to a bridged alkene-tethered ketone and its subsequent Fe^III -mediated metal-hydride atom transfer (MHAT) transannular cyclization. This synthetic method, implying an irreversible addition of a carbon-centered radical upon a ketone by means of a hydrogen atom transfer upon the alkoxy radical intermediate, was also applied in the synthesis of trans-fused hydrindanols structurally related to botrydial compounds.

Accessing Unusual Reactivity through Chelation-Promoted Bond Weakening

Highly reducing Sm(II) reductants and protic ligands were used as a platform to ascertain the relationship between low-valent metal-protic ligand affinity and degree of ligand X-H bond weakening with the goal of forming potent proton-coupled electron transfer (PCET) reductants. Among the Sm(II)-protic ligand reductant systems investigated, the samarium dibromide N-methylethanolamine (SmBr₂-NMEA) reagent system displayed the best combination of metal-ligand affinity and stability against H₂ evolution. The use of SmBr₂-NMEA afforded the reduction of a range of substrates that are typically recalcitrant to single-electron reduction including alkynes, lactones, and arenes as stable as biphenyl. Moreover, the unique role of NMEA as a chelating ligand for Sm(II) was demonstrated by the reductive cyclization of unactivated esters bearing pendant olefins in contrast to the SmBr₂-water-amine system. Finally, the SmBr₂-NMEA reagent system was found to reduce substrates analogous to key intermediates in the nitrogen fixation process. These results reveal SmBr₂-NMEA to be a powerful reductant for a wide range of challenging substrates and demonstrate the potential for the rational design of PCET reagents with exceptionally weak X-H bonds.

Reductive Cleavage of C-X or N-S Bonds Catalyzed by Super Organoreductant CBZ6

The reductive cleavage of C(Ar)-X bonds is the key step for the cross coupling of Ar-X with other groups. In this work, under the irradiation of 407 nm LEDs using sodium formate as reductant and thiol as hydrogen atom transfer agent, a variety of (hetero)aryl chlorides, bromides, and iodides could be reduced to corresponding (hetero)arenes. The key intermediates, aryl radicals, could be trapped by either hydrogen, phosphite, or borates. The same reduction conditions can be extended to the deprotection of sulfonamides.

Chromium-Catalyzed Defluorinative Reductive Coupling of Aldehydes with gem-Difluoroalkenes

Herein, a mild and convenient defluorinative reductive cross coupling of gem-difluoroalkenes with aliphatic aldehydes has been developed to afford diverse silyl-protected β-fluorinated allylic alcohols. The reaction is operationally simple and shows good functional group tolerance with moderate to excellent yields. The utility of this method is demonstrated by converting the products into various bioactive fluorinated compounds, showing its potential applications in drug discovery and biochemistry.