Recent Advances in First-Row Transition Metal-Catalyzed Reductive Coupling Reactions for π-Bond Functionalization and C-Glycosylation

Noncovalent interactions: An emerging focal point in stereoselective catalytic carbohydrate synthesis

The incorporation of frontier synthetic concepts into stereoselective carbohydrate synthesis is gaining significant traction. In the last five years, there are increasing reports documenting that the consideration of weak non-covalent interactions (NCIs) constitutes a vital factor in steering the anomeric and site-selectivity, as well as in activating difficult glycosylations. In light of blossoming developments on this front, we present a brief overview of recent case studies that involve the harnessing of hydrogen bonding (HB), halogen bonding (XB), chalcogen bonding (ChB) and π-interactions. These NCIs represent a considerable palette of classical/non-classical weak interactions that is of current interest to the broad synthesis community. Significantly, a close mechanistic analysis often revealed that NCIs were instrumental in dictating the final stereoselectivity outcome of many glycosylation pathways. We are optimistic that by expanding the focal point from purely glycosyl substrate modifications towards tweaking catalytic NCIs at the supramolecular level of chemical glycosylations, this emerging concept offers new levers of stereoselectivity control beyond classical stereoelectronic and steric considerations.

Photoinduced Diastereoselective and Stereodivergent Synthesis of C-Styryl Glycosides

A novel and efficient method for the stereoselective and stereodivergent synthesis of both unprotected and protected styryl C-glycosides through the reaction of sodium glycosyl sulfinate and hypervalent styryl iodine reagents is described. Using 2-bromo-9-fluorenone as a photocatalyst, C-styryl glycosides with an E configuration were selectively obtained. Furthermore, the introduction of a newly designed iridium-based catalyst enabled the selective formation of glycosides predominantly in the Z configuration. Mechanistic studies reveal the glycosyl radical is involved in the transformation.

Mechanism and Origin of Nickel-Catalyzed Decarbonylative Construction of C(sp2)-C(sp3) Bonds from Carboxylic Acids and Their Derivatives

Nickel-catalyzed arylation of carboxylic acids provides a ligand-controlled chemoselectivity-switchable method for the construction of C(sp2)-C(sp3) bonds. Here, we employed density functional theory to provide a detailed understanding of the mechanism and origin of nickel-catalyzed ligand-controlled carbonyl transformation. This reaction generates decarbonylation products through oxidative addition, activation of C-C bonds, decarbonylation, binding of alkyl radicals with Ni(III) complexes, and final reduction elimination step. The activation of C-C bonds in aromatic carboxylate esters is more favorable than C-O bond activation because of the interaction between the nickel catalyst and the π orbitals of the substrate's aromatic moiety during C-C bond activation. The induction effect of the ligand and the carbonyl group together determines the transfer tendency of the carbonyl group.

Facile, general allylation of unactivated alkyl halides via electrochemically enabled radical-polar crossover

Electrochemically driven carbon-carbon formation is receiving considerable interest in organic synthesis. In this study, we present an electrochemically driven method for the formation of C(sp3)-C(sp3) bonds using readily available allylic carbonates, as well as primary, secondary, and tertiary alkyl bromides as electrophiles. This approach offers a highly selective route for synthesizing a broad range of allylic products with excellent functional group tolerance, all without the need for transition metal catalysts. Remarkably, this method also enables the smooth late-stage functionalization of various natural product- and drug-derived substrates, yielding the corresponding complex allylalkanes.

Recent Advances in Organic Photocatalyst-Promoted Carbohydrate Synthesis and Modification under Light Irradiation

Photoredox catalysis has been developed as a sustainable and eco-friendly catalytic strategy, which might provide innovative solutions to solve the current synthetic challenges and barriers in carbohydrate chemistry. During the last few decades, the study of organic photocatalyst-promoted carbohydrate synthesis and modification has received significant attention, which provides an excellent and inexpensive metal-free alternative to photoredox catalysis as well as introduces a new fastest-growing era to access complex carbohydrates simply. In this review, we aim to provide an overview of organic photocatalyst-promoted carbohydrate synthesis and modification under light irradiation, which is expected to provide new directions for further investigation.

Radical Strategy Towards N-glycosides: Current Advances and Future Prospects

N-glycosides exhibit diverse biological and pharmacological activities, making their efficient synthesis crucial for both biological research and drug development. Traditional acid-promoted N-glycosylation methods, which rely on the formation of oxocarbenium intermediates, often face significant challenges. These methods are water-sensitive and typically require neighboring group participation to achieve high selectivity. Furthermore, they depend on acid activation, rendering them incompatible with alkyl amine. Additionally, low-nucleophilicity amides often need to be converted into their TMS-derivatives to enhance reactivity, limiting the direct use of such substrates. In contrast, radical-based strategies have emerged as a promising alternative, addressing many of these limitations and leading to notable advances in N-glycosylation. This review explores the unique properties of N-glycosides, the inherent challenges of traditional N-glycosylation techniques, and the transformative advantages offered by radical-based approaches. Specifically, it highlights recent advancements in radical-mediated N-glycosylation, including photoredox radical strategies, radical/ionic hybrid approaches, and metallaphotoredox catalysis, accompanied by a detailed discussion of the underlying mechanisms. Finally, the ongoing challenges and potential future directions of N-glycoside synthesis using radical strategies are presented.

Modular Access to C2'-Aryl/Alkenyl Nucleosides with Electrochemical Stereoselective Cross-Coupling

Chemically modified oligonucleotides have garnered significant attention in medicinal chemistry, chemical biology, and synthetic biology due to their enhanced stability in vivo compared to naturally occurring oligonucleotides. However, current methods for synthesizing modified nucleosides, particularly at the C2'-position, are limited in terms of efficiency, modularity, and selectivity. Herein, we report a new approach for the synthesis of highly functionalized C2'-α-aryl/alkenyl nucleosides via an electrochemical nickel-catalyzed cross-coupling of 2'-bromo nucleosides with a variety of (hetero)aryl and alkenyl iodides. This method affords a diverse array of C2'- α-aryl/alkenyl nucleosides with excellent stereoselectivity, broad substrate scope, and good functional group compatibility. We further synthesized oligonucleotides incorporating C2'-aryl-modified thymidine moieties and demonstrated that their annealed double-stranded DNAs exhibit decreased melting temperatures (Tm). Additionally, oligonucleotides with C2'-aryl modifications at the 3' end showed enhanced resistance to 3'-exonuclease degradation and C2'-aryl modifications did not impede the cellular uptake process, highlighting the potential of these modified oligonucleotides for therapeutic applications.

1-(2'-Hydroxy-2'-Methylpropionyl)Glycoside as a Versatile Glycosyl Donor for O-/C-Glycosylation

Herein, we developed a Lewis acid-mediated O-glycosylation and C-glycosylation protocol using stable glycosyl 2'-hydroxy-2'-methylpropionates as donors. These glycosylation reactions reached completion within 1 h at room temperature. The practicality of this protocol is characterized by their straightforward operation and efficient applicability to various substrates, including both disarmed and armed glycosyl donors, through the remote activation of easily accessible TMSOTf.

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.

Photocatalyst-free light-promoted carbohydrate synthesis and modification

Photoredox catalysis has recently emerged as a powerful approach for preparing oligosaccharides because it uses mild conditions, is compatible with partially or completely unprotected carbohydrate substrates, and exhibits impressive regio- and stereo-selectivity and high functional group tolerance. However, most catalytic photoredox reactions require an external photocatalyst (organic dye or expensive transition-metal complex) to deliver key glycosyl radicals. Several photocatalyst-free photocatalytic reactions that avoid the use of expensive metal salts or organic-dye additives have received significant attention. In this review, we highlight the most recent developments in photocatalyst-free light-promoted carbohydrate synthesis and modification, which is expected to inspire broad interest in further innovations in the green synthesis of saccharides.

C-OH Bond Activation for Stereoselective Radical C-Glycosylation of Native Saccharides

Radical C-glycosylation presents a flexible and efficient method for synthesizing C-glycosides. Existing methods always require multistep processes for generating anomeric radicals. In this study, we introduce a streamlined approach to produce anomeric radicals through direct C-OH bond homolysis of unmodified saccharides, eliminating the need for protection, deprotection, or activation steps. These anomeric radicals selectively couple with activated alkenes, yielding C-glycosylation products with high stereoselectivity (>20:1). This method is applicable to a variety of native monosaccharides, such as l-arabinose, d-arabinose, d-xylose, l-xylose, d-galactose, β-d-glucose, α-d-glucose, and l-ribose, as well as oligosaccharides including α-lactose, d-(+)-melibiose, and acarbose. We also extend this approach to C-glycosylation of amino acid and peptide derivatives, and demonstrate a streamlined synthesis of an anti-inflammatory agent.

Pd-Catalyzed Stereospecific Glycosyl Cross-Coupling of Reversed Anomeric Stannanes for Modular Synthesis of Nonclassical C-Glycosides

Nonclassical C-glycosides, distinguished by their unique glycosidic bond connection mode, represent a promising avenue for the development of carbohydrate-based drugs. However, the accessibility of nonclassical C-glycosides hinders broader investigations into their structural features and modes of action. Herein, we present the first example of Pd-catalyzed stereospecific glycosylation of nonclassical anomeric stannanes with aryl or vinyl halides. This method furnishes desired nonclassical aryl and vinyl C-glycosides in good to excellent yields, while allowing for exclusive control of nonclassical anomeric configuration. Of significant note is the demonstration of the generality and practicality of this nonclassical C-glycosylation approach across more than 50 examples, encompassing various protected and unprotected saccharides, deoxy sugars, oligopeptides, and complex molecules. Furthermore, biological evaluation indicates that nonclassical C-glycosylation modifications of drug molecules can positively impact their biological activity. Additionally, extensive computational studies are conducted to elucidate the rationale behind differences in reaction reactivity, unveiling a transmetalation transition state containing silver (Ag) within a six-membered ring. Given its remarkable controllability, predictability, and consistently high chemical selectivity and stereospecificity regarding nonclassical anomeric carbon and Z/E configuration, the method outlined in this study offers a unique solution to the longstanding challenge of accessing nonclassical C-glycosides with exclusive stereocontrol.

The Progress of Reductive Coupling Reaction by Iron Catalysis

The transition metal catalyzed coupling reaction has revolutionized the strategies for forging the carbon-carbon bonds. In contrast to traditional cross-coupling methods using pre-prepared nucleophilic organometallic reagents, reductive coupling reactions for the C-C bonds formation provide some advantages. Because both coupling partners are reduced in the final products using a stoichiometric amount of a reductant, this approach not only avoids the need to use sensitive organometallic species, but also provides an orthogonal and complementary access to classical coupling reaction. Notably, the reductive coupling reactions feature readily available fragments, promote good step economy, exhibit high functional group tolerance and unique chemoselectivity, which have propelled their increasingly popular in the organic synthesis. In recent years, due to the low price, minimal toxicity, and environmentally benign character, iron-catalyzed carbon-carbon coupling reactions have garnered significant attention from the organic synthetic chemists and pharmacologists, especially the iron-catalyzed reductive coupling. This review aims to provide an insightful overview of recent advances in iron-catalyzed reductive coupling reactions, and to illustrate their possible reaction mechanisms.

Visible-light-promoted direct desulfurization of glycosyl thiols to access C-glycosides

C-Glycosides are essential for the study of biological processes and the development of carbohydrate-based drugs. Despite the tremendous hurdles, glycochemists have often fantasized about the efficient, highly stereoselective synthesis of C-glycosides with the shortest steps under mild conditions. Herein, we report a desulfurative radical protocol to synthesize C-alkyl glycosides and coumarin C-glycosides under visible-light induced conditions without the need of an extra photocatalyst, in which stable and readily available glycosyl thiols that could be readily obtained from native sugars are activated in situ by pentafluoropyridine. The benefits of this procedure include high stereoselectivity, broad substrate scope, and easy handling. Mechanistic studies indicate that the in situ produced tetrafluoropyridyl S-glycosides form key electron donor-acceptor (EDA) complexes with Hantzsch ester (for C-alkyl glycosides) or Et3N (for coumarin C-glycosides), which, upon irradiation with visible light, trigger a cascade of glycosyl radical processes to access C-glycosides smoothly.

Synthesis of alpha-Gal C-disaccharides

The synthesis of C-disaccharides of α-d-galactopyranosyl-(1 → 3)-d-galactopyranose (α-Gal), potential tools for studying the biology of α-Gal glycans, is described. The synthetic strategy, centers on the reaction of two easily available precursors 1,2-O-isopropylidene-d-glyceraldehyde and an α-C-glactosyl-E-crotylboronate, which affords a mixture of two diastereomeric anti-crotylation products. The stereoselectivity of this reaction was controlled with (R)- and (S)-TRIP catalysts, and the appropriate diastereomer was transformed to C-linked disaccharides of α-Gal, in which the aglycone segment comprised O-, C- and S-glycoside entities that can enable glycoconjugate synthesis.

Radical C-Glycosylation Using Photoexcitable Unprotected Glycosyl Borate

We have developed radical C-glycosylation using photoexcitable unprotected glycosyl borate. The direct excitation of glycosyl borate under visible light irradiation enabled the generation of anomeric radical without any photoredox catalysts. The in situ generated anomeric radical was applicable to the radical addition such as Giese-type addition and Minisci-type reaction to introduce alkyl and heteroaryl groups at the anomeric position. In addition, the radical-radical coupling between the glycosyl borate and acyl imidazolide provided unprotected acyl C-glycosides.

Nickel-Catalyzed Desulfurative Cross-Coupling of Aryl Iodides with Heteroaromatic Thioethers via C-S Bond Cleavage

Herein, we present a Ni-catalyzed direct cross-coupling of heteroaromatic thioethers with aryl iodides via selective C(sp2)-S bond cleavage under reductive conditions, thereby providing various biaryl frameworks with high efficiency. Mechanistic studies suggested Mo(CO)6 played a crucial role in facilitating the activation of the C(sp2)-S bond. This protocol demonstrated a wide substrate scope, operational simplicity, and good functional group compatibility. Furthermore, the utility of this reaction was highlighted by facile scale-up and sequential modification of heteroaryl frameworks.

A Radical Activation Strategy for Versatile and Stereoselective N-Glycosylation

Previous N-glycosylation approaches have predominately involved acidic conditions, facing challenges of low stereoselectivity and limited scope. Herein, we introduce a radical activation strategy that enables versatile and stereoselective N-glycosylation using readily accessible glycosyl sulfinate donors under basic conditions and exhibits exceptional tolerance towards various N-aglycones containing alkyl, aryl, heteroaryl and nucleobase functionalities. Preliminary mechanistic studies indicate a pivotal role of iodide, which orchestrates the formation of a glycosyl radical from the glycosyl sulfinate and subsequent generation of the key intermediate, a configurationally well-defined glycosyl iodide, which is subsequently attacked by an N-aglycone in a stereospecific SN2 manner to give the desired N-glycosides. An alternative route involving the coupling of a glycosyl radical and a nitrogen-centered radical is also proposed, affording the exclusive 1,2-trans product. This novel approach promises to broaden the synthetic landscape of N-glycosides, offering a powerful tool for the construction of complex glycosidic structures under mild conditions.

Electrochemical Glycosylation via Halogen-Atom-Transfer for C-Glycoside Assembly

Glycosyl donor activation emerged as an enabling technology for anomeric functionalization, but aimed primarily at O-glycosylation. In contrast, we herein disclose mechanistically distinct electrochemical glycosyl bromide donor activations via halogen-atom transfer and anomeric C-glycosylation. The anomeric radical addition to alkenes led to C-alkyl glycoside synthesis under precious metal-free reaction conditions from readily available glycosyl bromides. The robustness of our e-XAT strategy was further mirrored by C-aryl and C-acyl glycosides assembly through nickela-electrocatalysis. Our approach provides an orthogonal strategy for glycosyl donor activation with expedient scope, hence representing a general method for direct C-glycosides assembly.

Organophotocatalytic access to C-glycosides: multicomponent coupling reactions using glycosyl bromides

Photochemical multi-component coupling reactions initiated by the activation of glycosyl bromides in the presence of 1,4-bis(diphenylamino)benzene (BDB) as an organic photocatalyst were developed. C-glycosides accompanied by olefin (di)functionalization were obtained. This method allows us to access various C-glycosides with alkene, carbonyl, alcohol, ether, and amide functionalities.

Polarity-Matched Initiation of Radical-Polar Crossover Reactions for the Synthesis of C-Allyl Glycosides with gem-Difluoroalkene Groups

Herein, we disclose a general method for the assembly of C-allyl glycosides containing gem-difluoroalkene groups via a radical-polar crossover strategy. Central to the success of this process is the polarity matching between the benzenesulfinate radical and the glycosyl donor, which facilitates the initiation of the glycosyl radical and the subsequent formation of gem-difluoroalkene sugar derivatives. This method demonstrated good compatibility with various glycosyl donors and functional groups. Furthermore, we showcase the utility of this method in modifying amino acids, potentially paving the way for analogous modifications to peptides.

Palladium-catalyzed Suzuki-Miyaura cross-couplings of stable glycal boronates for robust synthesis of C-1 glycals

C-1 Glycals serve as pivotal intermediates in synthesizing diverse C-glycosyl compounds and natural products, necessitating the development of concise, efficient and user-friendly methods to obtain C-1 glycosides is essential. The Suzuki-Miyaura cross-coupling of glycal boronates is notable for its reliability and non-toxic nature, but glycal donor stability remains a challenge. Herein, we achieve a significant breakthrough by developing stable glycal boronates, effectively overcoming the stability issue in glycal-based Suzuki-Miyaura coupling. Leveraging the balanced reactivity and stability of our glycal boronates, we establish a robust palladium-catalyzed glycal-based Suzuki-Miyaura reaction, facilitating the formation of various C(sp2)-C(sp), C(sp2)-C(sp2), and C(sp2)-C(sp3) bonds under mild conditions. Notably, we expand upon this achievement by developing the DNA-compatible glycal-based cross-coupling reaction to synthesize various glycal-DNA conjugates. With its excellent reaction reactivity, stability, generality, and ease of handling, the method holds promise for widespread appication in the preparation of C-glycosyl compounds and natural products.

From Quaternary Carbon to Tertiary C(sp3)-Si and C(sp3)-Ge Bonds: Decyanative Coupling of Malononitriles with Chlorosilanes and Chlorogermanes Enabled by Ni/Ti Dual Catalysis

Transition-metal-catalyzed C-Si/Ge cross-coupling offers promising avenues for the synthesis of organosilanes/organogermanes, yet it is fraught with long-standing challenges. A Ni/Ti-catalyzed strategy is reported here, allowing the use of disubstituted malononitriles as tertiary C(sp3) coupling partners to couple with chlorosilanes and chlorogermanes, respectively. This method enables the catalytic cleavage of the C(sp3)-CN bond of the quaternary carbon followed by the formation of C(sp3)-Si/C(sp3)-Ge bonds from ubiquitously available starting materials. The efficiency and generality are showcased by a broad scope for both of the coupling partners, therefore holding the potential to synthesize structurally diverse quaternary organosilanes and organogermanes that were difficult to access previously.

Nickel-Catalyzed Hydrofluorination in Unactivated Alkenes: Regio- and Enantioselective C-F Bond Formation

Catalytic formation of a regio- and enantioselective C-F bond chiral center from readily available alkenes is a crucial goal, yet it continues to pose significant challenges in organic synthesis. Here, we report the regioselective formation of C-F bonds facilitated by NiH catalysis and a coordination directing strategy that enables precise hydrofluorination of both terminal and internal alkenes. Notably, we have optimized this methodology to achieve high enantioselectivity in creating aliphatic C-F stereogenic centers especially with β,γ-alkenyl substrates, using a tailored chiral Bn-BOx ligand. Another pivotal finding in our research is the identification of the (+)-nonlinear effect under optimized conditions, allowing for high enantioselectivity even with moderately enantiomerically enriched chiral ligands. Given the significant role of fluorine in pharmaceuticals and synthetic materials, this research offers essential insights into the regioselective and enantioselective formation of C-F bond chiral centers, paving the way for the efficient production of valuable fluorinated compounds.