Enantioselective functionalization of radical intermediates in redox catalysis: copper-catalyzed asymmetric oxytrifluoromethylation of alkenes

Three-Component Radical Cross-Coupling: Asymmetric Vicinal Sulfonyl-Esterification of Alkenes Involving Sulfur Dioxide

A novel catalytic system for radical cross-coupling reactions based on copper and chiral Pyridyl-bis(imidazole) (PyBim) ligands is described. It overcomes the challenges of chemoselectivity and enantioselectivity, achieving a highly enantioselective vicinal sulfonyl-esterification reaction of alkenes involving sulfur dioxide. This strategy involves the use of earth-abundant metal catalyst, mild reaction conditions, a broad range of substrates (84 examples), high yields (up to 97% yield), and exceptional control over enantioselectivity. The reaction system is compatible with different types of radical precursors, including O-acylhydroxylamines, cycloketone oxime esters, aryldiazonium salts, and drug molecules. Chiral ligand PyBim is identified as particularly effective in achieving the desired high enantioselectivity. Mechanistic studies reveal that copper/PyBim system plays a vital role in C─O coupling, employing an outer-sphere model. In addition, the side arm effect of ligand is observed.

Atom Transfer Radical Coupling Enables Highly Enantioselective Carbo-Oxygenation of Alkenes with Hydrocarbons

The selective functionalization of C(sp3)-H bonds has emerged as a transformative approach for streamlining synthetic routes, offering remarkable efficiency in the preparation and modification of complex organic molecules. However, the direct enantioselective transformation of hydrocarbons to medicinally valuable chiral molecules remains a significant challenge that has yet to be addressed. In this study, we adopt an atom transfer radical coupling (ATRC) strategy to achieve the asymmetric functionalization of C(sp3)-H bonds in hydrocarbons. This approach involves intermolecular H atom transfer (HAT) between a hydrocarbon and an alkoxy radical, leading to the formation of a carbon-centered radical. The resulting radical adds to alkenes, generating a new radical species that is intercepted by a chiral copper-mediated C-O bond coupling. By employing this method, we can directly access valuable chiral lactones bearing a quaternary stereocenter with high efficiency and excellent enantioselectivity. Importantly, ATRC exhibits great potential as a versatile platform for achieving stereoselective transformations of hydrocarbons.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Emerging Trends in Copper-Promoted Radical-Involved C-O Bond Formations

The C-O bond is ubiquitous in biologically active molecules, pharmaceutical agents, and functional materials, thereby making it an important functional group. Consequently, the development of C-O bond-forming reactions using catalytic strategies has become an increasingly important research topic in organic synthesis because more conventional methods involving strong base and acid have many limitations. In contrast to the ionic-pathway-based methods, copper-promoted radical-mediated C-O bond formation is experiencing a surge in research interest owing to a renaissance in free-radical chemistry and photoredox catalysis. This Perspective highlights and appraises state-of-the-art techniques in this burgeoning research field. The contents are organized according to the different reaction types and working models.

Recent updates and future perspectives in aziridine synthesis and reactivity

In this review, selected recent advances in the preparation and reactivity of aziridines using modern synthetic approaches are highlighted, while comparing these new strategies with more classical approaches. This critical analysis is designed to help identify current gaps in the field and is showcasing new and exciting opportunities to move the chemistry of aziridines forward in the future.

Regiodivergent synthesis of sulfone-tethered lactam-lactones bearing four contiguous stereocenters

Sulfone-tethered lactones/amides/amines display a diverse spectrum of biological activities, including anti-psychotic and anti-hypertensive. Sulfones are also widely present in functional materials and fragrances. We therefore reasoned that a regiodivergent and stereocontrolled strategy that merges the sulfone, lactone, and lactam motifs would likely lead to the discovery of new pharmacophores and functional materials. Here, we report mild conditions for the sulfonyllactonization of γ-lactam-tethered 5-aryl-4(E)-pentenoic acids. The annulation is highly modular, chemoselective, and diastereoselective. With respect to regioselectivity, trisubstituted alkenoic acids display a preference for 5-exo-trig cyclization whereas disubstituted alkenoic acids undergo exclusive 6-endo-trig cyclization. The lactam-fused sulfonyllactones bear angular quaternary as well as four contiguous stereocenters. The products are post-modifiable, especially through a newly developed Co-catalyzed reductive cross-coupling protocol.

Enantioselective Copper-Catalyzed Fukuyama Indole Synthesis from 2-Vinylphenyl Isocyanides

Enantioenriched chiral indoles are of high interest for the pharmaceutical and agrochemical industries. Herein, we present an asymmetric Fukuyama indole synthesis through a mild and efficient radical cascade reaction to access 2-fluoroalkylated 3-(α-cyanobenzylated) indoles by stereochemical control with a chiral copper-bisoxazoline complex using 2-vinylphenyl arylisocyanides as radical acceptors and fluoroalkyl iodides as C-radical precursors. Radical addition to the isonitrile moiety, 5-exo-trig cyclization, and Cu-catalyzed stereoselective cyanation provide the targeted indoles with excellent enantioselectivity and good yields. Due to the similar electronic and steric properties of the two aryl substituents to be differentiated, the enantioselective construction of the cyano diaryl methane stereocenter is highly challenging. Mechanistic studies reveal a negative nonlinear effect which allows proposing a model to explain the stereochemical outcome. Scalability and potential utility of the enantioenriched 3-(α-cyanobenzylated) indoles as hubs for chiral tryptamines, indole-3-acetic acid derivatives, and triarylmethanes are demonstrated, and a formal synthesis of a natural product analogue is disclosed.

Organophotoredox-Catalyzed Sulfurization of Alkenes and Alkynes: Selective and Controlled Synthesis of Sulfoxides, β-Hydroxysulfoxides, and β-Keto Sulfides

A visible-light organophotoredox-catalyzed sulfurization of alkenes and alkynes with aromatic and heteroaromatic thiols using eco-friendly air (O₂) as the sole oxidant has been demonstrated. The established method delivers a novel, benign, and metal-free strategy for the difunctionalization of alkenes and alkynes using organophotoredox catalyst Eosin Y. The selective and controlled synthetic approach shows good substrate generality to afford sulfoxides, β-hydroxysulfoxides, and β-keto sulfides in high yield and excellent regioselectivity.

Ligand Development for Copper-Catalyzed Enantioconvergent Radical Cross-Coupling of Racemic Alkyl Halides

The enantioconvergent cross-coupling of racemic alkyl halides represents a powerful tool for the synthesis of enantioenriched molecules. In this regard, the first-row transition metal catalysis provides a suitable mechanism for stereoconvergence by converting racemic alkyl halides to prochiral radical intermediates owing to their good single-electron transfer ability. In contrast to the noble development of chiral nickel catalyst, copper-catalyzed enantioconvergent radical cross-coupling of alkyl halides is less studied. Besides the enantiocontrol issue, the major challenge arises from the weak reducing capability of copper that slows the reaction initiation. Recently, significant efforts have been dedicated to basic research aimed at developing chiral ligands for copper-catalyzed enantioconvergent radical cross-coupling of racemic alkyl halides. This perspective will discuss the advances in this burgeoning area with particular emphasis on the strategic chiral anionic ligand design to tune the reducing capability of copper for the reaction initiation under thermal conditions from our research group.

Aryl iodine-catalysed divergent synthesis of isobenzofuranones and isocoumarins via oxidative 1,2-aryl migration/elimination

The divergent synthesis of isobenzofuranones and isocoumarins was realized from the reaction of 2-alkenyl benzoic acids and mCPBA in the presence of catalytic aryl iodine and (±)-10-camphorsulfonic acid (CSA). The organocatalytic oxidative reaction is assumed to undergo a cascade process involving lactonization, 1,2-aryl migration and elimination enabled by a modified Koser reagent generated in situ.

Copper-Catalyzed Radical Enantioselective Carbo-Esterification of Styrenes Enabled by a Perfluoroalkylated-PyBox Ligand

Chiral lactones are found in many natural products. The reaction of simple alkenes with iodoacetic acid is a powerful method to build lactones, but the enantioselective version of this reaction has not been implemented to date. Herein, we report the efficient catalytic radical enantioselective carbo-esterification of styrenes enabled by a newly developed Cu^I -perfluoroalkylated PyBox system. Simple styrenes have been converted to useful chiral lactones, whose synthetic applications are showcased. Mechanistic studies reveal that this reaction is a rare example of an efficient ligand-decelerated system, in which the ligand decelerates the reaction, but the reaction is still efficient with reduced amounts of ligand. This uncommon catalytic system may inspire further consideration of the effect of ligands in asymmetric catalysis.

Radical cyclization/bis(pentafluoroethylation) of 1,6-dienes using HCF2CF3-derived CuCF2CF3

A domino radical cyclization/bis(pentafluoroethylation) of 1,6-dienes is described. This method provides access to N-heterocycles/carbocycles containing two C2F5 groups in one step.

Metal-free photocatalytic intermolecular trifluoromethylation-gem-difluoroallylation of unactivated alkenes

An efficient, transition-metal-free, photocatalytic three-component intermolecular trifluoromethylation-gem-difluoroallylation of unactivated alkenes has been achieved.

Copper-Catalyzed 1,2-Trifluoromethylation Carbonylation of Unactivated Alkenes: Efficient Access to β-Trifluoromethylated Aliphatic Carboxylic Acid Derivatives

A novel copper-catalyzed carbonylative trifluoromethylation of unactivated alkenes has been developed. A broad range of β-trifluoromethylated carboxylic acid derivatives were prepared in moderate to excellent yields from simple alkenes with excellent regioselectivity. It is noteworthy that ethylene gas, as the simplest olefin, can also be applied directly to obtain β-trifluoromethylated amides and ester. This transformation presents the first example on carbonylative trifluoromethylation of alkenes.

Asymmetric radical carboesterification of dienes

The straightforward strategy of building a chiral C-O bond directly on a general carbon radical center is challenging and stereocontrol of the reactions of open-chain hydrocarbon radicals remains a largely unsolved problem. Advance in this elementary step will spur the development of asymmetric radical C-O bond construction. Herein, we report a copper-catalyzed regioselective and enantioselective carboesterification of substituted dienes using alkyl diacyl peroxides as the source of both the carbon and oxygen substituents. The participation of external acids in this reaction substantially extends its applicability and leads to structurally diverse allylic ester products. This work represents the advance in the key elementary reaction of intermolecular enantioselective construction of C-O bond on open-chain hydrocarbon radicals and may lead to the discovery of other asymmetric radical reactions.

Stereocontrol of C–O bond formation from a carbon-based radical is very difficult due to the rapid inversion of the carbon radical. Here the authors present a method to form chiral esters from conjugated dienes with copper and chiral PyBox ligands, likely proceeding via an allylic radical.

Photoinduced Copper-Catalyzed Asymmetric Three-Component Coupling of 1,3-Dienes: An Alternative to Kharasch-Sosnovsky Reaction

Kharasch-Sosnovsky reaction is one of the most powerful methods for allylic oxidation of alkenes. However, the inherent radical mechanism and use of peroxides as both oxidants and oxygen nucleophiles render dearth of universal catalytic systems for highly enantioselective variants and limited scope. Herein, an alternative to the asymmetric Kharasch-Sosnovsky reaction that utilized a chiral copper catalyst and purple-LED irradiation to enable the three-component coupling of 1,3-dienes, oxime esters, and carboxylic acids is reported. This protocol features mild conditions, remarkable scope and functional group tolerance as evidenced by >80 examples and utility in the late-stage modification of pharmaceuticals and natural products. Detailed mechanistic studies provide evidences for the radical-based reaction pathway.

Photoinduced Copper-Catalyzed Asymmetric C-O Cross-Coupling

The construction of carbon-heteroatom bonds is one of the most active areas of research in organic chemistry because the function of organic molecules is often derived from the presence of heteroatoms. Although considerable advances have recently been achieved in radical-involved catalytic asymmetric C-N bond formation, there has been little progress in the corresponding C-O bond-forming processes. Here, we describe a photoinduced copper-catalyzed cross-coupling of readily available oxime esters and 1,3-dienes to generate diversely substituted allylic esters with high regio- and enantioselectivity (>75 examples; up to 95% ee). The reaction proceeds at room temperature under excitation by purple light-emitting diodes (LEDs) and features the use of a single, earth-abundant copper-based chiral catalyst as both the photoredox catalyst for radical generation and the source of asymmetric induction in C-O coupling. Combined experimental and density functional theory (DFT) computational studies suggest the formation of π-allylcopper complexes from redox-active oxime esters as bifunctional reagents and 1,3-dienes through a radical-polar crossover process.

Lactonization with concomitant 1,2-aryl migration and alkoxylation mediated by dialkoxyphenyl iodides generated in situ

A series of alkoxylated isobenzofuranones were conveniently synthesized from the reaction of 2-(1-arylvinyl)benzoic acids with PhI(OR)2, generated in situ from the reaction of iodosobenzene (PhIO) with alkyl alcohols. This hypervalent iodine mediated one-pot transformation is postulated to undergo a cascade reaction involving lactonization, 1,2-aryl migration and alkoxylation processes. The organocatalytic and chiral organoiodine-catalyzed asymmetric reactions of the current transformation were also probed.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Cyanoalkylation/alkynylation of allylic alcohol through intramolecular radical 1,2-alkynyl migration

A di-tert-butyl peroxide (DTBP)-promoted difunctionalization of α-aryl α-alkynyl allylic alcohols with alkyl nitriles was developed, affording a series of α-alkynyl γ-cyano functionalized ketones in moderate yields. This procedure involved C(sp³)-H bond cleavage of alkyl nitriles and radical 3-exo-dig cyclization. After this, radical 1,2-alkynyl migration is preferred rather than 1,2-aryl migration.

Beyond osmium: progress in 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes

Olefin 1,2-difunctionalization has emerged as a popular strategy within modern synthetic chemistry for the synthesis of vicinal amino alcohols and derivatives. The advantage of this approach is the single-step simplicity for rapid diversification, feedstock nature of the olefin starting materials, and the possible modularity of the components. Although there is a vast number of possible iterations of 1,2-olefin difunctionalization, 1,2-amino oxygenation is of particular interest due to the prevalence of both oxygen and nitrogen within pharmaceuticals, natural products, agrochemicals, and synthetic ligands. The Sharpless amino hydroxylation provided seminal results in this field and displayed the value in achieving methods of this nature. However, a vast number of new and novel methods have emerged in recent decades. This review provides a comprehensive review of modern advances in accomplishing 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes that move beyond osmium to a range of other transition metals and more modern strategies such as electrochemical, photochemical, and biochemical reactivity.

Enantioselective Alkylamination of Unactivated Alkenes under Copper Catalysis

An enantioselective addition reaction of various alkyl groups to unactivated internal alkenes under Cu catalysis has been developed. The reaction uses amide-linked aminoquinoline as the directing group, 4-alkyl Hantzsch esters as the donor of alkyl radicals, and rarely used biaryl diphosphine oxide as a chiral ligand. β-lactams featuring two contiguous stereocenters at Cβ and the β substituent can be obtained in good yield with excellent enantioselectivity. Mechanistic studies indicate that a nucleophilic addition of the alkyl radical to Cu^II-coordinated alkene is the enantio-determining step.

Copper-Catalyzed Enantioselective Radical 1,4-Difunctionalization of 1,3-Enynes

Chiral allenes are important structural motifs frequently found in natural products, pharmaceuticals, and other organic compounds. Asymmetric 1,4-difunctionalization of 1,3-enynes is a promising strategy to construct axial chirality and produce substituted chiral allenes from achiral substrates. However, the previous state of the art in 1,4-difunctionalization of 1,3-enynes focused on the allenyl anion pathway. Because of this, only electrophiles can be introduced into the allene backbones in the second functionalization step, consequently limiting the reaction and allene product types. The development of asymmetric 1,4-difunctionalization of 1,3-enynes via a radical pathway would complement previous methods and support expansion of the toolbox for the synthesis of asymmetric allenes. Herein, we report the first radical enantioselective allene formation via a group transfer pathway in the context of copper-catalyzed radical 1,4-difunctionalization of 1,3-enynes. This method addresses a longstanding unsolved problem in asymmetric radical chemistry, provides an important strategy for stereocontrol with free allenyl radicals, and offers a novel approach to the valuable, but previously inaccessible, chiral allenes. This work should shed light on asymmetric radical reactions and may lead to other enantioselective group transfer reactions.

Synthesis of Oxygen-Containing Heterocyclic Compounds by Iron-Catalyzed Alkylative Cyclization of Unsaturated Carboxylic Acids and Alcohols

Iron-catalyzed alkylative cyclization of alkenes bearing oxygen nucleophiles with secondary and tertiary alkyl bromides through carbon-carbon and carbon-oxygen bond formations has been developed. A broad substrate scope is an attractive feature of this synthetic method, providing a variety of potentially bioactive five- and six-membered oxygen-containing heterocycles. The reaction pathway is proposed to involve a radical addition of the in situ-formed alkyl radical to an alkene followed by carbon-oxygen bond-forming intramolecular cyclization.

Enantioselective Copper-Catalyzed Alkynylation of Benzylic C-H Bonds via Radical Relay

The first enantioselective alkynylation of benzylic C-H bonds via copper-catalyzed radical relay has been established herein, which provides an easy access to structurally diverse benzylic alkynes in good yields with excellent enantioselectivities. A key step for the asymmetric copper-catalyzed radical relay process is the enantioselective capture of a benzylic radical with chiral (Box)Cu^II-alkynyl species. In addition, the reaction displays good functional group tolerance, broad substrate scope, and mild conditions. The enantioenriched alkynylation products can be readily transformed into highly valuable synthons, such as chiral terminal alkynes, allenes, alkenes, and carboxylic acids. More importantly, our methodology can be applied to the synthesis of bioactive molecule AMG 837.

Fluorofunctionalizations of C-C Multiple Bonds and C-H Bonds

In spite of only a few naturally occurring products having one or more fluorine atoms, organofluorine compounds have been widely utilized in pharmaceutical, agrochemical, and functional material science fields due to the characteristic properties of the fluorine atom. Therefore, the development of new methods for the introduction of fluorine-containing functional groups has been a long-standing research topic. This article discusses our contributions to this area. The first topic is on the trifluoromethylations of C-C multiple bonds using Togni reagent based on our working hypothesis that hypervalent iodine could be activated by coordination of the carbonyl moiety to the Lewis acid catalyst. The second topic relates to asymmetric fluorofunctionalization of alkenes. A newly designed phase-transfer catalyst consisting of a carboxylate anion functioning as a phase-transfer agent and a primary hydroxyl group as a site that captures the anionic substrate was revealed to be an effective catalyst for asymmetric fluorolactonization. Inspired by the mechanistic studies of fluorolactonization, we produced a linked binaphthyl dicarboxylate catalyst, which catalyzes the 6-endo-fluorocyclization and the deprotonative fluorination of allylic amides in a highly enantioselective manner. The third topic is on C-H fluorofunctionalizations using either catalysis or photoactivation. Benzylic trifluoromethylation, which is still a rare reaction, using Togni reagent and aromatic C-H trifluoromethylation using Umemoto reagent under simple photoirradiation conditions were achieved. In addition, the Csp³-H fluorination of alkyl phthalimide derivatives is demonstrated.