Cu-catalysed enantioselective radical heteroatomic S-O cross-coupling

Desymmetrization strategy in natural product total synthesis

Desymmetrization strategies have emerged as practical methods for the efficient construction of desired molecules from readily accessible symmetrical precursors. This approach has found widespread application in natural product total synthesis as it simplifies the materials preparation, enables the precise control of chirality formation, and inspires innovative retrosynthetic analysis. Here, we summarize recent advances (2014-2024) of the applications of desymmetrization strategies in natural product total synthesis, and showcase their advantages in designing new reactions and synthetic strategies, with the aim to foster their wider application in total synthesis.

Cu(I)-catalysed chemo-, regio-, and stereoselective radical 1,2-carboalkynylation with two different terminal alkynes

Transition-metal-catalysed asymmetric multicomponent reactions with two similar substrates often suffer from the lack of strategies to control the chemo-, regio-, and stereoselectivity of these substrates due to the close similarity in the chemical structures and properties of each reagent. Here, we describe a Cu(I)-catalysed asymmetric radical 1,2-carboalkynylation of two different terminal alkynes and alkyl halides with high chemo-, regio-, and stereoselectivity by using sterically bulky chiral tridentate anionic N,N,P-ligands and modulating alkynes with different electronic properties to circumvent above-mentioned challenges. This method features good substrate scope, high functional group tolerance of two different terminal alkynes, and diverse alkyl halides, providing universal access to a series of useful axially chiral 1,3-enyne building blocks.

Pd(II)/Cu(I) Co-catalyzed Dual C-H Functionalization of Indoles at C2 and C3 Positions Using Bifunctional Arylsulfonyl Reagents

A Pd(II)/Cu(I) co-catalyzed dual C-H functionalization strategy enables the one-pot synthesis of C2/C3-difunctionalized indoles using arylsulfonyl reagents. In this method, Cu(I) reduces S(VI) reagents to generate arylsulfonyl radicals, which react with Pd(II) to form electrophilic Ar-Pd(III) intermediates. These intermediates regioselectively arylate indoles at the C2 position, followed by Cu(II)-mediated C-H functionalization at the C3 position, to deliver the final products. This synergistic method enhances efficiency, reduces waste, and streamlines complex indole functionalization.

Cobalt-Catalyzed Diester Formation with CO at Atmospheric Pressure via Three-Component Reactions

We propose a cobalt-catalyzed three-component reaction conducted under mild conditions. Following the carbonyl insertion at atmospheric pressure, the reaction proceeds via a nucleophilic attack by alcohols to form diesters. Notably, the scope of nucleophilic reagents can be extended to include a range of anilines. Furthermore, the products of this transformation serve as crucial synthetic building blocks for diverse organic synthesis processes, with subsequent derivatizations yielding promising results.

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.

Palladium-Catalyzed Dual Csp2─Csp3 Bond Formation: A Versatile Platform for the Synthesis of Benzo-Fused Heterocycles

Transition-metal-catalyzed transformations offer a powerful approach to rapidly synthesize complex benzo-fused heterocycles, crucial for drug and material development. However, existing synthetic strategies face challenges such as limited functional group compatibility, reliance on complex ligands, and difficulties in controlling chemoselectivity with prefunctionalized substrates. Herein, a ligand-free Pd(II)/Cu(I) catalytic system is presented that facilitates reactions between arylsulfonyl chlorides and unactivated olefins under mild conditions, enabling the efficient synthesis of saturated benzo-fused six-membered heterocycles. This streamlined strategy employs dual Csp2─Csp3 bond formation, producing diverse N/O-polyheterocycles and allowing late-stage functionalization of bioactive molecules with excellent yields and high chemoselectivity. The key to the success of this reaction is the formation of high-valent Ar-Pd(III) intermediate, which drives the reaction through 1,2-Pd migration and electrophilic C─H arylation. This unique reactivity pathway facilitates the formation of benzo-fused heterocycles while effectively avoiding the β-H elimination typically associated with Heck-type reactions.

Enantioconvergent and Site-Selective Etherification of Carbohydrate Polyols through Chiral Copper Radical Catalysis

Going beyond currently reported two electron transformations that formed the core backdrop of asymmetric catalytic site-selective carbohydrate polyol functionalizations, we herein report a seminal demonstration of an enantioconvergent copper catalyzed site-selective etherification of minimally protected saccharides through a single-electron radical pathway. Further, this strategy paves a rare strategy, through which a carboxamide scaffold that is present in some glycomimetics of pharmacological relevance, can be selectively introduced. In light of the burgeoning interest in chiral radical catalysis, and the virtual absence of such stereocontrol broadly in carbohydrate synthesis, our strategy showcased the unknown capability of chiral radical copper catalysis as a contemporary tool to address the formidable site-selectivity challenge on a remarkable palette of naturally occurring saccharides. When reducing sugars were employed, a further dynamic kinetic resolution type glycosylation can be activated by the catalytic system to selectively generate the challenging β-O-glycosides.

Traceless Nucleophile Strategy for C5-Selective C-H Sulfonylation of Pyridines

The functionalization of pyridines is crucial for the rapid construction and derivatization of agrochemicals, pharmaceuticals, and materials. Conventional functionalization approaches have primarily focused on the ortho- and para-positions, while achieving precise meta-selective functionalization, particularly at the C5 position in substituted pyridines, remains a formidable challenge due to the intrinsic electronic properties of pyridines. Herein, we present a new strategy for meta- and C5-selective C-H sulfonylation of N-amidopyridinium salts, which employs a transient enamine-type intermediate generated through a nucleophilic addition to N-amidopyridinium salts. This process harnesses the power of electron donor-acceptor complexes, enabling high selectivity and broad applicability, including the construction of complex pyridines bearing valuable sulfonyl functionalities under mild conditions without the need for an external photocatalyst. The remarkable C5 selectivity, combined with the broad applicability to late-stage functionalization, significantly expands the toolbox for pyridine functionalization, unlocking access to previously unattainable meta-sulfonylated pyridines.

Halogen-Atom Transfer Enabled Catalytic Enantioselective Coupling to Chiral Trifluoromethylated Alkynes via Dual Nickel and Photocatalysis

With halogen-atom transfer as an effective tool, a novel catalytic enantioselective protocol to generate chiral trifluoromethylated alkynes has been established by a cooperative photoredox and nickel catalysis system, providing a straightforward and modular route to access this type of product in good yields and enantioselectivities. The halogen-atom transfer process is essential for the reaction and this novel strategy offers another promising way to utilize alkyl halides with highly negative reduction potentials. It firstly expands nickel-catalyzed asymmetric reductive cross-couplings of organohalides from the traditional single-electron transfer to halogen-atom transfer.

Metalloradical Catalysis: General Approach for Controlling Reactivity and Selectivity of Homolytic Radical Reactions

Since Friedrich Wöhler's groundbreaking synthesis of urea in 1828, organic synthesis over the past two centuries has predominantly relied on the exploration and utilization of chemical reactions rooted in two-electron heterolytic ionic chemistry. While one-electron homolytic radical chemistry is both rich in fundamental reactivities and attractive with practical advantages, the synthetic application of radical reactions has been long hampered by the formidable challenges associated with the control over reactivity and selectivity of high-energy radical intermediates. To fully harness the untapped potential of radical chemistry for organic synthesis, there is a pressing need to formulate radically different concepts and broadly applicable strategies to address these outstanding issues. In pursuit of this objective, researchers have been actively developing metalloradical catalysis (MRC) as a comprehensive framework to guide the design of general approaches for controlling over reactivity and stereoselectivity of homolytic radical reactions. Essentially, MRC exploits the metal-centered radicals present in open-shell metal complexes as one-electron catalysts for homolytic activation of substrates to generate metal-entangled organic radicals as the key intermediates to govern the reaction pathway and stereochemical course of subsequent catalytic radical processes. Different from the conventional two-electron catalysis by transition metal complexes, MRC operates through one-electron chemistry utilizing stepwise radical mechanisms.

Silver Surface-Assisted Dehydrobrominative Cross-Coupling between Identical Aryl Bromides

Cross-coupling reactions represent an indispensable tool in chemical synthesis. An intriguing challenge in this field is to achieve selective cross-coupling between two precursors with similar reactivity or, to the limit, the identical molecules. Here we report an unexpected dehydrobrominative cross-coupling between 1,3,5-tris(2-bromophenyl)benzene molecules on silver surfaces. Using scanning tunneling microscopy, we examine the reaction process at the single-molecular level, quantify the selectivity of the dehydrobrominative cross-coupling, and reveal the modulation of selectivity by substrate lattice-related catalytic activity or molecular assembly effect. Theoretical calculations indicate that the dehydrobrominative cross-coupling proceeds via regioselective C-H bond activation of debrominated TBPB and subsequent highly selective C-C coupling of the radical-based intermediates. The reaction kinetics plays an important role in the selectivity for the cross-coupling. This work not only expands the toolbox for chemical synthesis but also provides important mechanistic insights into the selectivity of coupling reactions on the surface.

Brønsted-Acid-Catalyzed Enantioselective Desymmetrization of 1,3-Diols: Access to Chiral β-Amino Alcohol Derivatives

Herein, we describe a Brønsted-acid-catalyzed enantioselective desymmetrization of 1,3-diols with alkynes through a hydroalkoxylation/hydrolysis process. The reaction leads to the atom-economical synthesis of valuable chiral β-amino alcohols under mild reaction conditions. Further synthetic transformations based on the β-amino alcohol moiety provide divergent approaches toward chiral N-containing heterocycles.

Copper-Catalyzed Aerobic Oxidative/Decarboxylative Phosphorylation of Aryl Acrylic Acids with P(III)-Nucleophiles

A copper-catalyzed aerobic oxidative/decarboxylative phosphorylation of aryl acrylic acids with P(III)-nucleophiles via the Michaelis-Arbuzov rearrangement for the synthesis of β-ketophosphine oxides, β-ketophosphinates, and β-ketophosphonates is reported. The present reaction could be conducted effectively without the use of a ligand and a base. Various kinds of aryl acrylic acids and P(III)-nucleophiles are tolerated in the transformation, generating the desired β-keto-organophosphorus compounds as a valuable class of phosphorus-containing intermediates with good to excellent yields. In addition, the possible mechanism and kinetic studies for the reaction have been explored by step-by-step control experiments and competitive experiments, and the results proved that this transformation may follow second-order chemical kinetics as well as involve a radical process.

Photoredox-Catalyzed Radical-Radical Cross-Coupling of Sulfonyl Chlorides with Trifluoroborate Salts

Sulfones are widely found in natural products and drug molecules. Here, we disclose a strategy for direct synthesis of sulfone compounds with diverse structures by visible-light-catalyzed radical-radical cross-coupling of sulfonyl chlorides and trifluoroborate salts. Allyl, benzyl, vinyl, and aryl trifluoroborates can be successfully cross-coupled with (hetero)aryl and alkyl sulfonyl chlorides, respectively. This strategy features redox neutrality, good substrate generality, simple operation, and benign reaction conditions.

N-Heterocyclic carbene catalyzed desymmetrization of diols: access to enantioenriched oxindoles having a C3-quaternary stereocenter

Herein, we describe an effective strategy for enantioselective synthesis of oxindoles having a C3-quaternary stereocenter via N-heterocyclic carbene (NHC) catalyzed desymmetrization of diols. The process is based on the catalytic asymmetric transfer acylation of primary alcohols using readily available aldehydes as an acylation agent. The reaction enables easy access to diversely functionalized C3-quaternary oxindoles with excellent enantioselectivity. The synthetic potential of the process is further demonstrated via the preparation of the key intermediate for (-)-esermethole and (-)-physostigmine.

Cu(I)-Catalyzed Chemo- and Enantioselective Desymmetrizing C-O Bond Coupling of Acyl Radicals

Transition-metal-catalyzed enantioselective functionalization of acyl radicals has so far not been realized, probably due to their relatively high reactivity, which renders the chemo- and stereocontrol challenging. Herein, we describe Cu(I)-catalyzed enantioselective desymmetrizing C-O bond coupling of acyl radicals. This reaction is compatible with (hetero)aryl and alkyl aldehydes and, more importantly, displays a very broad scope of challenging alcohol substrates, such as 2,2-disubstituted 1,3-diols, 2-substituted-2-chloro-1,3-diols, 2-substituted 1,2,3-triols, 2-substituted serinols, and meso primary 1,4-diols, providing enantioenriched esters characterized by challenging acyclic tetrasubstituted carbon stereocenters. Partnered by one- or two-step follow-up transformations, this reaction provides a convenient and practical strategy for the rapid preparation of chiral C3 building blocks from readily available alcohols, particularly the industrially relevant glycerol. Mechanistic studies supported the proposed C-O bond coupling of acyl radicals.

Metal-free sulfonylation of quinoxalinones to access 2-sulfonyl-oxylated quinoxalines via oxidative O-S cross coupling

The C2 sulfonylation of quinoxalinones via a metal-free oxidative S-O cross-coupling strategy for synthesizing 2-sulfonyloxylated quinoxalines is established. It effectively solved the long-standing problems in the C2 transformation of quinoxalinones via a metal-free oxidative O-S coupling strategy. Compared with the traditional C2 transformed quinoxalinones-C2 chlorination method, this protocol is mild, facile, and environmentally friendly and exhibits good atomic economy and excellent functional group tolerance. Moreover, the utility of this methodology and the sulfonyloxyl handles was demonstrated through the synthesis of 2-substituted quinoxaline-based bioactive molecules.