Enantioselective Radical Hydroacylation of α,β-Unsaturated Carbonyl Compounds with Aldehydes by Triplet Excited Anthraquinone

Unlocking Reactivity of Unprotected Oximes via Green-Light-Driven Dual Copper/Organophotoredox Catalysis

Oximes are widely used precursors in synthetic chemistry due to their broad availability and versatile chemical properties, in which N─O bond fragmentation represents a key reactivity mode. However, these transformations typically require the use of oxygen-protected oximes, and a general strategy to directly utilize free oximes remains challenging due to their vulnerability to side reaction pathways, rendering low tendency towards N─OH bond cleavage. Here a unified platform is reported to achieve direct cyclization of unprotected oximes with enals, as well as other coupling partners through dual copper/organophotoredox catalysis under green light irradiation. This protocol enables concurrent activation of both N─OH and α-C(sp3)─H bonds of free oximes to form multisubstituted pyridines with exceeding structural diversity and functional group tolerance. In this process, Rose Bengal serves as a hydrogen atom transfer agent to generate radical intermediates. In the meanwhile, copper catalyst activates of free oximes via single-electron reduction-induced N─O bond fragmentation and controls the selectivity for intermediate trapping. The synthetic utility of this approach is further demonstrated by its successful applications in late-stage modification of biologically active compounds and rapid assembly of solvatochromic fluorescent materials.

Photocatalyst-free formate-mediated C-O cleavage by the EDA complex and SCS strategy for the synthesis of diaryl 1,4-diketone in air

Under mild visible light conditions, formates facilitate C-O cleavage via the EDA complex and SCS strategy, yielding α-carbonyl alkyl radicals. These radicals then react with olefins under air conditions, leading to the synthesis of diaryl 1,4-dicarbonyl compounds. Mechanistic studies reveal that α-formyloxy ketone is generated in situ by the reaction between α-brominated acetophenone and formates, followed by the formation of the EDA complex. Additionally, formates also serve as a single-electron reducing reagent in the reaction.

Synthesis of 1,4-Diketones from Esters Enabled by a Tetraborylethane Reagent

A modular synthesis method for 1,4-diketones has been developed. Utilizing inexpensive carboxylic acid esters as carbonyl sources and tetraborylethane (TBE) as a nucleophilic reagent, a one-pot strategy for constructing two C-C bonds was established. Notably, this reaction proceeds without the involvement of transition metals and exhibits excellent functional group compatibility. A diverse array of α-substituted 1,4-diketones were synthesized using various electrophiles for capture.

Asymmetric Three-Component Radical Cascade Reactions Enabled by Synergistic Photoredox/Brønsted Acid Catalysis: Access to α-Amino Acid Derivatives

Multicomponent reactions (MCRs), highly sought-after methods to produce atom-, step-, and energy-economic organic syntheses, have been developed extensively. However, catalytic asymmetric MCRs, especially those involving radical species, remain largely unexplored owing to the difficulty in stereoselectively regulating the extraordinarily high reactivity of open-shell radical species. Herein, we report a conceptually novel catalytic asymmetric three-component radical cascade reaction of readily accessible glycine esters, α-bromo carbonyl compounds and 2-vinylcyclopropyl ketones via synergistic photoredox/Brønsted acid catalysis, in which three sequential C-C (σ/π/σ) bond-forming events occurred through a radical addition/ring-opening/radical-radical coupling protocol, affording an array of valuable enantioenriched unnatural α-amino acid derivatives bearing two contiguous stereogenic centers and an alkene moiety in moderate to good yield with high diastereoselectivity, excellent enantioselectivity and good E-dominated geometry under mild reaction conditions. The radical relay cascade process, especially a unique proton-coupled electron transfer (PCET)-promoted radical-radical coupling, is supported by mechanistic investigations and quantum mechanics calculations and should garner broad interest and further inspire the development of asymmetric multicomponent radical reactions.

Photoinduced Enantioselective Triplet Radical Reaction on Metal: Copper-Catalyzed Conjugate Addition of Acylsilanes to α,β-Unsaturated Ketones and Aldehydes

A photoinduced copper-catalyzed enantioselective conjugate addition of acylsilanes has been developed. The conjugate acylation of α,β-unsaturated ketones and aldehydes was promoted by a copper(I)/chiral NHC catalyst under visible-light irradiation for synthesizing various 2-substituted 1,4-dicarbonyl compounds in enantioenriched forms. Mechanistic studies combining experiments and quantum chemical calculations indicated a reaction mechanism involving copper-to-acyl charge transfer (i. e., metal-to-ligand charge transfer (MLCT)) excitation of an alkene-bound acylcopper complex. The MLCT excitation is followed by an electronical and geometrical change to generate a triplet β-radical-C-enolate-Cu(II)-acyl complex with an acyl radical character, which undergoes facile excited state C-C bond formation in the copper coordination sphere, affording the 1,4-conjugate addition product.

Catalytic Asymmetric Hydrogen Atom Transfer Based on a Chiral Hydrogen Atom Donor Generated from TBADT and Chiral BINOL

Enantioselective radical reactions mediated by TBADT have seldom been seen due to the inherent challenges. Herein, we disclose a new chiral hydrogen atom transfer (HAT) reagent that was generated easily from 8H-BINOL, potassium carbonate, and TBADT under irradiation. The new complex 8H-BINOL/DTs could be used as a chiral H donor. A series of azaarenes could be converted into the corresponding chiral compounds via radical addition followed by enantioselective HAT.

Visible-Light-Promoted Enantioselective Acylation and Alkylation of Aldimines Enabled by 9-Fluorenone Electron-Shuttle Catalysis

Chiral acyclic α-tertiary amino ketones are widely present in various natural products and pharmaceuticals; however, the direct synthesis of this pharmacophore through a robust strategy still presents significant challenges. The emerging photocatalysis provides a powerful approach to construct chemical bonds that are difficult to form via a traditional two-electron pathway. Herein, we developed visible-light-induced chiral Lewis acid-catalyzed highly enantioselective acylation/alkylation of aldimines enabled by cooperative FLN (9-fluorenone) electron-shuttle catalysis via radical addition. An array of α-tertiary amino ketones, β-amino alcohols, and chiral amines were achieved with high yields and good to excellent stereocontrol (87 examples, up to 84% yield, 96% ee). These products can be easily transformed into valuable and bioactive skeletons. Extensive control experiments, detailed mechanism studies, and density functional theory calculations elucidated the reaction process and highlighted the crucial role played by FLN.

A Type of Chiral C2-Symmetric Arylthiol Catalyst for Highly Enantioselective Anti-Markovnikov Hydroamination

The development of chiral hydrogen donor catalysts is fundamental in the expansion and innovation of asymmetric organocatalyzed reactions via an enantioselective hydrogen atom transfer (HAT) process. Herein, an unprecedented type of chiral C2-symmetric arylthiol catalysts derived from readily available enantiomeric lactate ester was developed. With these catalysts, an asymmetric anti-Markovnikov alkene hydroamination-cyclization reaction was established, affording a variety of pharmaceutically interesting 3-substituted piperidines with moderate to high enantioselectivity. Results of the designed control experiments and theoretical computation rationalized the origin of stereocontrol and disclosed the spatial effect of the moiety of chiral thiols on the enantioselectivity. We believed the facile synthesis, flexible tunability, and effective enantioselectivity-controlling capability of these catalysts would shed light on the development of versatile chiral HAT catalysts and related asymmetric reactions.

Photoredox-catalyzed C(sp3)─H radical functionalization to enable asymmetric synthesis of α-chiral alkyl phosphine

α-Chiral alkyl phosphines are privileged structural motifs with a wide application in organic and medical synthesis. It is highly desirable to develop stereoselective methods to prepare these enantioenriched molecules. The incorporation of C(sp3)─H functionalization and chiral phosphine chemistry is much less explored, probably because of the weak reactivity of C(sp3)─H bonds and/or the challenging site- and stereoselectivity issues. Herein, we disclose a synergistic catalysis system to enable an enantioselective radical addition process of α-substituted vinylphosphine oxides. An array of diverse α-chiral alkyl phosphors compounds is smoothly accessed by using the readily available chemicals as the inert C(sp3)─H bond reagent, such as sulfides, amines, alkenes, and toluene derivatives, exerting remarkable chemo-, site-, and enantioselectivity. On the basis of the mechanistic studies, both the C(sp3)─H bond activation and the stereochemistry-determining step are proposed to involve a single-electron transfer/proton transfer process.

Light-Driven Photocatalyst-Free Synthesis of β, δ-Functionalized Ketones from Aldehydes

The synthesis of ketones has been a long focus of chemistry research, on account of its unique reactivity. Herein, we report a simple light-driven photocatalyst-free synthesis of β, δ-functionalized ketones from aldehydes, using inexpensive and commercially abundant feedstock chemicals. This reaction is enabled by the direct acyl radical generation via hydrogen atom transfer and the subsequent radical addition process, avoiding the need for prefunctionalized substrates and organometallic reagent.

Single-Electron Oxidation-Initiated Enantioselective Hydrosulfonylation of Olefins Enabled by Photoenzymatic Catalysis

Controlling the enantioselectivity of hydrogen atom transfer (HAT) reactions has been a long-standing synthetic challenge. While recent advances on photoenzymatic catalysis have demonstrated the great potential of non-natural photoenzymes, all of the transformations are initiated by single-electron reduction of the substrate, with only one notable exception. Herein, we report an oxidation-initiated photoenzymatic enantioselective hydrosulfonylation of olefins using a novel mutant of gluconobacter ene-reductase (GluER-W100F-W342F). Compared to known photoenzymatic systems, our approach does not rely on the formation of an electron donor-acceptor complex between the substrates and enzyme cofactor and simplifies the reaction system by obviating the addition of a cofactor regeneration mixture. More importantly, the GluER variant exhibits high reactivity and enantioselectivity and a broad substrate scope. Mechanistic studies support the proposed oxidation-initiated mechanism and reveal that a tyrosine-mediated HAT process is involved.

Harnessing Dpp-Imine as a Powerful Achiral Cocatalyst to Dramatically Increase the Efficiency and Stereoselectivity in a Magnesium-Mediated Oxa-Michael Reaction

Dpp-imines are classic model substrates for synthetic method studies. Here, we disclose their powerful use as achiral coligands in metal-catalyzed reactions. It is highly interesting to find that the Dpp-imine can not only act as powerful ligand to create excellent chiral pockets with magnesium complexes but also, more importantly, this coligand can dramatically enhance the catalytic ability of the metal catalyst. The underlying reaction mechanism was extensively explored by conducting a series of experiments, including 31P NMR studies of the coordination complex between the Dpp-imine coligand and magnesium complexes, ESI capture results, multiple control experiments, studies and comparison of different coligands, 1H NMR studies on the relationship between the substrate and Dpp-imine coligand, as well as the relationship between the substrate and the full complexes. Furthermore, DFT calculation provided valuable insights in the role of the imine additive and demonstrated that adding the Dpp-imine coligand in the magnesium catalyst can switch the deprotonation/nucleophilic addition steps from a stepwise mechanism to a concerted process during the oxa-cyclization reaction. The crucial factors responsible for the excellent enantioselectivity and enhanced reaction efficiency brought by Dpp-imine have been extracted from the calculation model. These mechanistic experiments and DFT calculation data clearly disclose and prove the powerful and interesting functions of the Dpp-imine coligand, which also direct a novel application of this type of active imine as useful ligands in metal-catalyzed asymmetric reactions.

Amide-Directed, Rhodium-Catalyzed Regio- and Enantioselective Hydroacylation of Internal Alkenes with Unfunctionalized Aldehydes

Despite the current progress achieved in asymmetric hydroacylations, highly enantioselective catalytic addition of unfunctionalized aldehydes to internal alkenes remains an unsolved challenge. Here, using a coordination-assisted strategy, we developed a rhodium-catalyzed regio- and enantioselective addition of unfunctionalized aldehydes to internal alkenes such as enamides and β,γ-unsaturated amides. Valuable α-amino ketones and 1,4-dicarbonyl compounds were directly obtained with high enantioselectivity from readily available materials.

Photocatalytic Redox-Neutral Alkoxyacylation of Alkenes

β-Alkoxyketones are important building blocks in organic synthesis. By utilizing CBZ6, with an oxidative potential of -2.16 V (vs the saturated calomel electrode), as a redox-neutral photocatalyst, alkoxyacylation of olefins was accomplished under the irradiation of visible light via a cationic intermediate. It involves the addition of an acyl radical to olefin to form a radical intermediate and the following oxidation of the radical intermediate to the benzyl cationic intermediate that is captured by alkoxy anions. This process provides concise and practical access to the β-functionalized ketones.

Substituent-Determined Intramolecular Hydrogen Transfer for Photopromoted Intermolecular Cycloaddition of Anthraquinones with Aryl Olefins

The formation of intramolecular hydrogen bonds in anthraquinones makes them inert to photoinduced reactions; therefore, it is a great challenge to phototransform these compounds. Herein, we reported a formal visible-light-induced [4 + 2] cycloaddition of both 1-hydroxyanthraquinones and 1-aminoanthraquinones with olefins under external photocatalyst-free conditions with high regioselectivity. More than 60 substrates are disclosed, demonstrating the reliability of this protocol to construct diverse functionalized anthraquinone derivatives.

Palladium-catalyzed coupling of amides and cyclopropanols for the synthesis of γ-diketones

A palladium catalytic method has been developed for the coupling of amides and cyclopropanols to γ-diketones, through simultaneous C-N and C-C activation. Heteroatom ligand exchange and heteroatom-to-carbon ligation mode switching enable the achievement of molecular cross-coupling in an amide N-atom structural context-dependent manner, avoiding any stoichiometric organometallic reagent or base.

A Quadruple Catalysis Enabling Intermolecular Branch-Selective Hydroacylation of Styrenes

A quadruple N-heterocyclic carbene/cobalt/photoredox/Brønsted base catalysis to realize branch-selective hydroacylation of styrenes with aromatic and aliphatic aldehydes is demonstrated. This protocol allows access to branched ketones from readily available materials in an atom-economical manner. The quadruple catalysis can transfer a formyl hydrogen of aldehydes as a hydrogen radical equivalent onto the terminal carbon of an alkene by controlled electron and proton transfers.

Sequential Modifications of Metal-Organic Layer Nodes for Highly Efficient Photocatalyzed Hydrogen Atom Transfer

Herein, we report the synthesis of a bifunctional photocatalyst, Zr-OTf-EY, through sequential modifications of metal cluster nodes in a metal-organic layer (MOL). With eosin Y and strong Lewis acids on the nodes, Zr-OTf-EY catalyzes cross-coupling reactions between various C-H compounds and electron-deficient alkenes or azodicarboxylate to afford C-C and C-N coupling products, with turnover numbers of up to 1980. In Zr-OTf-EY-catalyzed reactions, Lewis acid sites bind the alkenes or azodicarboxylate to increase their local concentrations and electron deficiency for enhanced radical additions, while EY is stabilized by site isolation on the MOL to afford a long-lived catalyst for hydrogen atom transfer. The proximity between photostable EY sites and Lewis acids on the nodes of Zr-OTf-EY enhances the catalytic efficiency by approximately 400 times over the homogeneous counterpart in the cross-coupling reactions.

Enantioselective anti-Dihalogenation of Electron-Deficient Olefin: A Triplet Halo-Radical Pylon Intermediate

The textbook alkene halogenation reaction establishes straightforward access to vicinal dihaloalkanes. However, a robust catalytic method for dihalogenizing electron-deficient olefins in an enantioselective manner is still under development, and its mechanism remains controversial. Herein, we disclose efficient regio-, anti-diastereo-, and enantioselective dibromination, bromochlorination, and dichlorination reactions of enones catalyzed by a chiral N,N'-dioxide/Yb(OTf)₃ complex. With the combination of electrophilic halogen and halide salts as halogenating agents, an array of homo- and heterodihalogenated derivatives is achieved in moderate to good enantioselectivities. Moreover, DFT calculations reveal that a novel triplet halo-radical pylon intermediate is probable in accounting for the exclusive regio- and anti-diastereoselectivity.

Catalytic photochemical enantioselective α-alkylation with pyridinium salts

We have developed a chiral amine catalyzed enantioselective α-alkylation of aldehydes with amino acid derived pyridinium salts as alkylating reagents. The reaction proceeds in the presence of visible light and in the absence of a photocatalyst via a light activated charge-transfer complex. We apply this photochemical stereoconvergent process to the total synthesis of the lignan natural products (-)-enterolactone and (-)-enterodiol. Mechanistic studies support the ground-state complexation of the reactive components followed by divergent charge-transfer processes involving catalyst-controlled radical chain and in-cage radical combination steps.

Enantioselective Radical Addition to Ketones through Lewis Acid-Enabled Photoredox Catalysis

Photocatalysis opens up a new window for carbonyl chemistry. Despite a multitude of photochemical reactions of carbonyl compounds, visible light-induced catalytic asymmetric transformations remain elusive and pose a formidable challenge. Accordingly, the development of simple, efficient, and economic catalytic systems is the ideal pursuit for chemists. Herein, we report an enantioselective radical photoaddition to ketones through a Lewis acid-enabled photoredox catalysis wherein the in situ formed chiral N,N'-dioxide/Sc(III)-ketone complex serves as a temporary photocatalyst to trigger single-electron transfer oxidation of silanes for the generation of nucleophilic radical species, including primary, secondary, and tertiary alkyl radicals, giving various enantioenriched aza-heterocycle-based tertiary alcohols in good to excellent yields and enantioselectivities. The results of electron paramagnetic resonance (EPR) and high-resolution mass spectrum (HRMS) measurements provided favorable evidence for the stereocontrolled radical addition process involved in this reaction.