Catalytic enantioselective C(sp3)-H functionalization involving radical intermediates

Radical C(sp3)-H functionalization and cross-coupling reactions

C─H functionalization reactions are playing an increasing role in the preparation and modification of complex organic molecules, including pharmaceuticals, agrochemicals, and polymer precursors. Radical C─H functionalization reactions, initiated by hydrogen-atom transfer (HAT) and proceeding via open-shell radical intermediates, have been expanding rapidly in recent years. These methods introduce strategic opportunities to functionalize C(sp3)─H bonds. Examples include synthetically useful advances in radical-chain reactivity and biomimetic radical-rebound reactions. A growing number of reactions, however, proceed via "radical relay" whereby HAT generates a diffusible radical that is functionalized by a separate reagent or catalyst. The latter methods provide the basis for versatile C─H cross-coupling methods with diverse partners. In the present review, highlights of recent radical-chain and radical-rebound methods provide context for a survey of emerging radical-relay methods, which greatly expand the scope and utility of intermolecular C(sp3)─H functionalization and cross coupling.

Directed evolution of nonheme iron enzymes to access abiological radical-relay C(sp3)-H azidation

We report the reprogramming of nonheme iron enzymes to catalyze an abiological C(sp³)‒H azidation reaction through iron-catalyzed radical relay. This biocatalytic transformation uses amidyl radicals as hydrogen atom abstractors and Fe(III)‒N₃ intermediates as radical trapping agents. We established a high-throughput screening platform based on click chemistry for rapid evolution of the catalytic performance of identified enzymes. The final optimized variants deliver a range of azidation products with up to 10,600 total turnovers and 93% enantiomeric excess. Given the prevalence of radical relay reactions in organic synthesis and the diversity of nonheme iron enzymes, we envision that this discovery will stimulate future development of metalloenzyme catalysts for synthetically useful transformations unexplored by natural evolution.

Modular Access to Chiral α-(Hetero)aryl Amines via Ni/Photoredox-Catalyzed Enantioselective Cross-Coupling

Chiral α-aryl N-heterocycles are commonly found in natural products, pharmaceutical agents, and chiral catalysts but remain challenging to access via asymmetric catalysis. Herein, we report a general and modular approach for the direct enantioselective α-arylation of saturated azacycles and acyclic N-alkyl benzamides via nickel/photoredox dual catalysis. This process exploits the hydrogen atom transfer ability of photoeliminated chlorine radicals to convert azacycles to the corresponding α-amino alkyl radicals that then are coupled with ubiquitous and inexpensive (hetero)aryl chlorides. These coupling reactions require no oxidants or organometallic reagents, feature feedstock starting materials, a broad substrate scope, and high enantioselectivities, and are applicable to late-stage diversification of medicinally relevant complex molecules. Mechanistic studies suggest that the nickel catalyst uncommonly plays multiple roles, accomplishing chlorine radical generation, α-amino radical capture, cross-coupling, and asymmetric induction.

Electronic control over site-selectivity in hydrogen atom transfer (HAT) based C(sp3)-H functionalization promoted by electrophilic reagents

The direct functionalization of C(sp³)-H bonds represents one of the most investigated approaches to develop new synthetic methodology. Among the available strategies for intermolecular C-H bond functionalization, increasing attention has been devoted to hydrogen atom transfer (HAT) based procedures promoted by radical or radical-like reagents, that offer the opportunity to introduce a large variety of atoms and groups in place of hydrogen under mild conditions. Because of the large number of aliphatic C-H bonds displayed by organic molecules, in these processes control over site-selectivity represents a crucial issue, and the associated factors have been discussed. In this review article, attention will be devoted to the role of electronic effects on C(sp³)-H bond functionalization site-selectivity. Through an analysis of the recent literature, a detailed description of the HAT reagents employed in these processes, the associated mechanistic features and the selectivity patterns observed in the functionalization of substrates of increasing structural complexity will be provided.

Radical differentiation of two ester groups in unsymmetrical diazomalonates for highly asymmetric olefin cyclopropanation

Diazomalonates have been demonstrated as effective metalloradicophiles for asymmetric radical olefin cyclopropanation via Co(II)-metalloradical catalysis (MRC). Supported by D ₂-symmetric chiral amidoporphyrin ligand, Co(II)-based metalloradical system can efficiently activate unsymmetrical methyl phenyl diazomalonate (MPDM) with effective differentiation of the two ester groups for asymmetric cyclopropanation, enabling stereoselective construction of 1,1-cyclopropanediesters bearing two contiguous chiral centers, including all-carbon quaternary stereogenic center. The Co(II)-catalyzed asymmetric cyclopropanation, which operates at room temperature without slow addition of the diazo compound, is generally applicable to broad-ranging olefins and tolerates various functionalities, providing a streamlined synthesis of chiral 1,1-cyclopropanediesters in high yields with both high diastereoselectivity and enantioselectivity. Combined computational and experimental studies support the underlying stepwise radical mechanism for Co(II)-catalyzed cyclopropanation. In addition to functioning as 1,3-dipoles for forming five-membered structures, enantioenriched (E)-1,1-cyclopropanediesters serve as useful building blocks for stereoselective synthesis of different cyclopropane derivatives. In addition, the enantioenriched (E)-1,1-cyclopropanediesters can be stereoselectively converted to (Z)-diastereomers.

Copper-catalyzed radical relay in C(sp3)-H functionalization

Radical-involved transition metal (TM) catalysis has greatly enabled new reactivities in recent decades. Copper-catalyzed radical relay offers enormous potential in C(sp³)-H functionalization which combines the unique regioselectivity of hydrogen atom transfer (HAT) and the versatility of copper-catalyzed cross-coupling. More importantly, significant progress has been achieved in asymmetric C-H functionalization through judicious ligand design. This tutorial review will highlight the recent advances in this rapidly growing area, and we hope this survey will inspire future strategic developments for selective C(sp³)-H functionalization.

Metalloradical Activation of In Situ-Generated α-Alkynyldiazomethanes for Asymmetric Radical Cyclopropanation of Alkenes

α-Alkynyldiazomethanes, generated in situ from the corresponding sulfonyl hydrazones in the presence of a base, can serve as effective metalloradicophiles in Co(II)-based metalloradical catalysis (MRC) for asymmetric cyclopropanation of alkenes. With D₂-symmetric chiral amidoporphyrin 2,6-DiMeO-QingPhyrin as the optimal supporting ligand, the Co(II)-based metalloradical system can efficiently activate different α-alkynyldiazomethanes at room temperature for highly asymmetric cyclopropanation of a broad range of alkenes. This catalytic radical process provides a general synthetic tool for stereoselective construction of alkynyl cyclopropanes in high yields with high both diastereoselectivity and enantioselectivity. Combined computational and experimental studies offer several lines of evidence in support of the underlying stepwise radical mechanism for the Co(II)-catalyzed olefin cyclopropanation involving a unique α-metalloradical intermediate that is associated with two resonance forms of α-Co(III)-propargyl radical and γ-Co(III)-allenyl radical. The resulting enantioenriched alkynyl cyclopropanes, as showcased with several stereospecific transformations, may serve as valuable chiral building blocks for stereoselective organic synthesis.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Asymmetric synthesis of chiral imidazolidines by merging copper and visible light-induced photoredox catalysis

A visible light induced copper catalyzed synthesis of decarboxylative radical coupling/cyclization reaction for the synthesis of chiral imidazolidines in high yields and enantioselectivities was reported.

Radical addition to the CC bond meets (1,n)-HAT: recent advances in the remote C(sp3)–H or C(sp2)–H functionalization of alkenes

This review summarizes the recent development of remote C(sp3)–H bond or aldehydic C(sp2)–H functionalizations enabled by intermolecular radical addition to CC bond/(1,n)-HAT tandem sequences.

Copper-Catalyzed Fluoroamide-Directed Remote Benzylic C-H Olefination: Facile Access to Internal Alkenes

A general, site-selective copper-catalyzed fluoroamide-directed remote benzylic C-H olefination of N-fluoroamides with terminal alkenes for producing internal alkenes is disclosed. This protocol proceeds via a hybrid Cu-radical mechanism, which synergistically...

Iridium-Catalyzed Regioselective Hydroalkynylation of Internal Alkenes Directed by an Oxime

We report here an iridium-catalyzed hydroalkynylation of allylic alcohols protected by an oxime group. Catalytic alkynylation occurs exclusively at the distal position of the alkene. This method generates γ-alkynyl alcohol oximes directly from internal alkenes and terminal alkynes. The oxime group can be readily removed to afford a free alcohol, thus providing an indirect route for the catalytic hydroalkynylation of allylic alcohols.

Copper-Catalyzed Intermolecular Enantioselective Radical Oxidative C(sp3 )-H/C(sp)-H Cross-Coupling with Rationally Designed Oxazoline-Derived N,N,P(O)-Ligands

The intermolecular asymmetric radical oxidative C(sp³ )-C(sp) cross-coupling of C(sp³ )-H bonds with readily available terminal alkynes is a promising method to forge chiral C(sp³ )-C(sp) bonds because of the high atom and step economy, but remains underexplored. Here, we report a copper-catalyzed asymmetric C(sp³ )-C(sp) cross-coupling of (hetero)benzylic and (cyclic)allylic C-H bonds with terminal alkynes that occurs with high to excellent enantioselectivity. Critical to the success is the rational design of chiral oxazoline-derived N,N,P(O)-ligands that not only tolerate the strong oxidative conditions which are requisite for intermolecular hydrogen atom abstraction (HAA) processes but also induce the challenging enantiocontrol. Direct access to a range of synthetically useful chiral benzylic alkynes and 1,4-enynes, high site-selectivity among similar C(sp³ )-H bonds, and facile synthesis of enantioenriched medicinally relevant compounds make this approach very attractive.

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.

Enantioconvergent Cu-Catalyzed Radical C-N Coupling of Racemic Secondary Alkyl Halides to Access α-Chiral Primary Amines

α-Chiral alkyl primary amines are virtually universal synthetic precursors for all other α-chiral N-containing compounds ubiquitous in biological, pharmaceutical, and material sciences. The enantioselective amination of common alkyl halides with ammonia is appealing for potential rapid access to α-chiral primary amines, but has hitherto remained rare due to the multifaceted difficulties in using ammonia and the underdeveloped C(sp³)-N coupling. Here we demonstrate sulfoximines as excellent ammonia surrogates for enantioconvergent radical C-N coupling with diverse racemic secondary alkyl halides (>60 examples) by copper catalysis under mild thermal conditions. The reaction efficiently provides highly enantioenriched N-alkyl sulfoximines (up to 99% yield and >99% ee) featuring secondary benzyl, propargyl, α-carbonyl alkyl, and α-cyano alkyl stereocenters. In addition, we have converted the masked α-chiral primary amines thus obtained to various synthetic building blocks, ligands, and drugs possessing α-chiral N-functionalities, such as carbamate, carboxylamide, secondary and tertiary amine, and oxazoline, with commonly seen α-substitution patterns. These results shine light on the potential of enantioconvergent radical cross-coupling as a general chiral carbon-heteroatom formation strategy.

Copper-Catalyzed Cross-Coupling of Benzylic C-H Bonds and Azoles with Controlled N-Site Selectivity

Azoles are important motifs in medicinal chemistry, and elaboration of their structures via direct N-H/C-H coupling could have broad utility in drug discovery. The ambident reactivity of many azoles, however, presents significant selectivity challenges. Here, we report a copper-catalyzed method that achieves site-selective cross-coupling of pyrazoles and other N-H heterocycles with substrates bearing (hetero)benzylic C-H bonds. Excellent N-site selectivity is achieved, with the preferred site controlled by the identity of co-catalytic additives. This cross-coupling strategy features broad scope for both the N-H heterocycle and benzylic C-H coupling partners, enabling application of this method to complex molecule synthesis and medicinal chemistry.

New Catalytic Radical Process Involving 1,4-Hydrogen Atom Abstraction: Asymmetric Construction of Cyclobutanones

While alkyl radicals have been well demonstrated to undergo both 1,5- and 1,6-hydrogen atom abstraction (HAA) reactions, 1,4-HAA is typically a challenging process both entropically and enthalpically. Consequently, chemical transformations based on 1,4-HAA have been scarcely developed. Guided by the general mechanistic principles of metalloradical catalysis (MRC), 1,4-HAA has been successfully incorporated as a key step, followed by 4-exo-tet radical substitution (RS), for the development of a new catalytic radical process that enables asymmetric 1,4-C-H alkylation of diazoketones for stereoselective construction of cyclobutanone structures. The key to success is the optimization of the Co(II)-based metalloradical catalyst through judicious modulation of D2-symmetric chiral amidoporphyrin ligand to adopt proper steric, electronic, and chiral environments that can utilize a network of noncovalent attractive interactions for effective activation of the substrate and subsequent radical intermediates. Supported by an optimal chiral ligand, the Co(II)-based metalloradical system, which operates under mild conditions, is capable of 1,4-C-H alkylation of α-aryldiazoketones with varied electronic and steric properties to construct chiral α,β-disubstituted cyclobutanones in good to high yields with high diastereoselectivities and enantioselectivities, generating dinitrogen as the only byproduct. Combined computational and experimental studies have shed light on the mechanistic details of the new catalytic radical process, including the revelation of facile 1,4-HAA and 4-exo-tet-RS steps. The resulting enantioenriched α,β-disubstituted cyclobutanones, as showcased with several enantiospecific transformations to other types of cyclic structures, may find useful applications in stereoselective organic synthesis.

Rh(III)-Catalyzed C-H Activation/[3 + 2] Annulation of N-Phenoxyacetamides via Carbooxygenation of 1,3-Dienes

A unique Rh(III)-catalyzed C-H activation/[3 + 2] annulation of N-phenoxyacetamides has been developed for the construction of dihydrobenzofurans via carbooxygenation of 1,3-dienes. This transformation features a redox-neutral process with specific chemoselectivity, good substrate/functional group compatibility, and profound synthetic potentials. A preliminary exploration to realize their asymmetric synthesis have been also successfully demonstrated, which further strengthens the practicality of this approach.

Asymmetric Photocatalytic C(sp3)-H Bond Addition to α-Substituted Acrylates

Asymmetric functionalization of inert C(sp³)-H bonds is a straightforward approach to realize versatile bond-forming events, allowing the precise assembly of molecular complexity with minimal functional manipulations. Here, we describe an asymmetric photocatalytic C(sp³)-H bond addition to α-substituted acrylates by using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer (HAT) photocatalyst and chiral phosphoric acid as a chiral proton-transfer shuttle. This protocol is supposed to occur via a radical/ionic relay process, including a TBADT-mediated HAT to cleave the inert C(sp³)-H bond, a 1,4-radical addition, a back hydrogen abstraction, and an enantioselective protonation. A variety of inert C-H bond patterns and α-substituted acrylates are well tolerated to enable the rapid synthesis of enantioenriched α-stereogenic esters from simple raw materials.