Chiral acid-catalysed enantioselective C-H functionalization of toluene and its derivatives driven by visible light

Catalytic asymmetric photocycloaddition reactions mediated by enantioselective radical approaches

The use of olefins in the construction of cyclic compounds represents a powerful strategy for advancing the pharmaceutical industry. Photocycloaddition has attracted significant interest from chemists due to its ability to exploit simple and readily available olefins along with their reaction patterns under mild conditions. Moreover, the sustainable and versatile pathways for generating highly reactive intermediates can greatly enrich both substrate diversity and reaction patterns. As a result, numerous photocycloaddition reactions have been successfully developed, particularly asymmetric [2+2], [3+2], and [4+2] photocycloadditions mediated by enantioselective radical approaches, achieving remarkable enantioselectivities. This review offers a comprehensive overview of this rapidly evolving field, organizing the discussion into three distinct reaction types that facilitate the construction of enantioenriched derivatives of cyclobutanes, cyclopentanes, and cyclohexanes. Emphasis is placed on analyzing and summarizing established strategies aimed at circumventing the challenges posed by racemic background transformations. Additionally, the exploration of asymmetric [3+2] and [4+2] photocycloaddition reactions will be interwoven with a detailed discussion of the various substrate types involved. This systematic framework seeks to enhance understanding of the strategies employed to manage the high reactivity of radicals while achieving high enantioselectivity. Importantly, it aims to guide readers in identifying uncharted radical-based cycloaddition pathways, which possess significant potential to broaden the diversity of complex cyclic molecules.

Visible-Light Photocatalyzed Skeletal Rearrangement Enables the Synthesis of Highly Functionalized Xanthenes with Antitumor Activity

Highly functionalized xanthenes possess an impressive range of bioactivities and daunting synthetic challenge due to their unique ring systems and stereocenters. Here, we report an unprecedented ketyl radicals-induced skeletal rearrangement reaction of spirodihydrobenzofurans, enabled by zero-valent iron as reducing agents via photoredox catalysis, facilitating the facile preparation of various highly functionalized xanthenes. The features of this protocol include high chemo- and regioselectivity, exceptionally mild conditions, a broad substrate scope, scalability to gram-scale quantities, and consistent delivery of good to excellent yields. Mechanistic studies rationalize the function of this zero-valent iron-based reactivity in radical generation. Notably, this reaction was applied to the first asymmetric total synthesis of the complex polycyclic xanthene ent-myrtucomvalones E-F. Moreover, this work led to the discovery of an agent with highly effective antiosteosarcoma activity in vitro and in vivo, potentially paving the way for the development of new xanthene-based candidates for osteosarcoma treatment.

Enantiopure Turbo Chirality Targets in Tri-Propeller Blades: Design, Asymmetric Synthesis, and Computational Analysis

Enantiopure turbo chirality in small organic molecules, without other chiral elements, is a fascinating topic that has garnered significant interest within the chemical and materials science community. However, further research into and application of this concept have been severely limited by the lack of effective asymmetric tools. To date, only a few enantiomers of turbo chiral targets have been isolated, and these were obtained through physical separation using chiral HPLC, typically on milligram scales. In this work, we report the first asymmetric approach to enantiopure turbo chirality in the absence of other chiral elements such as central and axial chirality. This is demonstrated by assembling aromatic phosphine oxides, where three propeller-like groups are anchored to a P(O) center via three axes. Asymmetric induction was successfully carried out using a chiral sulfonimine auxiliary, with absolute configurations and conformations unambiguously determined by X-ray diffraction analysis. The resulting turbo frameworks exhibit three propellers arranged in either a clockwise (P,P,P) or counterclockwise (M,M,M) configuration. In these arrangements, the bulkier sides of the aromatic rings are oriented toward the oxygen atom of the P=O bond rather than in the opposite direction. Additionally, the orientational configuration is controlled by the sulfonimine auxiliary as well, showing that one of the Naph rings is pushed away from the auxiliary group (-CH2-NHSO2-tBu) of the phenyl ring. Computational studies were conducted on relative energies for the rotational barriers of a turbo target along the P=O axis and the transition pathway between two enantiomers, meeting our expectations. This work is expected to have a significant impact on the fields of chemistry, biomedicine, and materials science in the future.

Photoredox Catalytic Deracemization Enabled Enantioselective and Modular Access to Axially Chiral N-Arylquinazolinones

Visible light-driven photocatalytic deracemization is highly esteemed as an ideal tool for organic synthesis due to its exceptional atom economy and synthetic efficiency. Consequently, successful instances of deracemization of allenes have been established, where the activated energy of photosensitizer should surpass that of the substrates, representing an intrinsic requirement. Accordingly, this method is not applicable for axially chiral molecules with significantly high triplet energies. In this study, we present a photoredox catalytic deracemization approach that enables the efficient synthesis of valuable yet challenging-to-access axially chiral 2-azaarene-functionalized quinazolinones. The substrate scope is extensive, allowing for both 3-axis and unmet 1-axis assembly through facile oxidation of diverse central chiral 2,3-dihydroquinazolin-4(1H)-ones that can be easily prepared and achieve enantiomer enrichment via deracemization. Mechanistic studies reveal the importance of photosensitizer selection in attaining excellent chemoselectivity and highlight the indispensability of a chiral Brønsted acid in enabling highly enantioselective protonation to accomplish efficient deracemization.

Visible-light-driven asymmetric aldol reaction of ketones and glycinates via synergistic Lewis acid/photoredox catalysis

Visible-light-driven direct asymmetric α-C(sp3)-H bond functionalization of glycinate provides a direct and efficient route for the synthesis of diverse optically enriched α-amino acid derivatives. However, asymmetric coupling between glycinate radical species and ketones faces significant challenges, including competitive pathways, mutable intermediates, as well as congested stereogenic centers. Herein, we disclose the first example for the asymmetric photocatalytic synthesis of a diverse array of β-diaryl-β-hydroxy-α-amino acetate derivatives from glycinates and heteroaryl ketones through the synergistic catalysis of achiral iridium photoredox catalyst and chiral lanthanide Lewis acid catalysts. The enantioselective radical addition pathway is supported by spectroscopic experiments, control experiments and DFT calculations.

Enantioselective Relay Coupling of Perfluoroalkyl and Vinylogous Ketyl Radicals

β-Chiral carboxylic acids and their derivatives are highly valuable structural motifs in the fields of asymmetric synthesis and medicinal chemistry. However, the introduction of a sterically demanding sidechain to the β-carbon, such as an all-carbon quaternary center, remains a significant challenge in classical polar processes. Recently, N-heterocyclic carbene (NHC) mediated coupling reactions involving persistent ketyl radicals have emerged as a promising strategy to assemble highly crowded carbon-carbon bonds. Nevertheless, achieving enantioselectivity in these reactions remains highly challenging. In this work, we report our recent progress in controlling enantioselectivity for relay coupling of perfluoroalkyl and persistent vinylogous ketyl radicals. We developed a chiral bifunctional NHC-squaramide catalyst that achieves high facial selectivity in a critical bond-forming event involving the coupling of a congested tertiary carbon radical and vinylogous ketyl radical. Chiral carboxylates bearing an all-carbon quaternary center at the β-position can be prepared in good yield and excellent enantiomeric excess. Results from density functional theory (DFT) calculations and nuclear Overhauser effect (NOE) experiments indicate that the N,N'-diaryl squaramide motif adopts an unusual syn-syn conformation, enabling hydrogen bonding interactions with the enolate oxygen, thereby rigidifying the overall conformation of the transition state.

Enantioselective Hydroaminoalkylation of Azaaryl Ketones through Asymmetric Photoredox Catalysis

An enantioselective hydroaminoalkylation of azaaryl ketones under a transition-metal-free asymmetric photoredox catalysis platform is reported. A series of valuable azaarene-functionalized 1,2-amino alcohols featuring attractive quaternary carbon stereocenters have been synthesized in high yields with good to excellent enantioselectivities. The viability of readily accessible N-aryl glycines as reaction partners facilitates the conjugate modification of these products into important derivatives, thereby enhancing the synthetic utility of the current approach.

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.

Electrochemical and photochemical reaction of isatins: a decade update

This review presents the latest progress in photochemical and electrochemical reactions involving isatins. Isatin and its functionalized scaffolds e.g., oxindoles, spirooxindoles, and quinolines are privileged heterocycles as they are largely present in several agrochemical, natural products, and pharmaceuticals. Thus, the functionalization of isatins using sustainable approaches, i.e., electro- and photochemical methods is of recent research interest worldwide. In this review, we have discussed most of the important reactions of isatins based on types of bond formation involved under electro- and photochemical conditions over the last decade. The reaction mechanism for each reaction has been discussed in detail to offer an inclusive guide to readers. Lastly, a summary of current challenges and the future outlook toward the development of effective electrochemical and photochemical methods for the reaction of isatins is also presented.

Photoredox-catalyzed radical-radical cross coupling of ketyl radicals with unstabilized primary alkyl radicals

Herein, a novel protocol dealing with the preparation of sterically hindered alcohols has been successfully developed via radical-radical coupling reactions enabled by mild and redox-neutral photocatalysis. With alkylsilicates as the radical precursors, a range of primary alkyl radicals bearing various functional groups could couple with a range of phthalimides and activated ketones.

Photoredox and NHC Enabled Deoxygenative Alcohol Homologation via Formal 1,2-Addition

A highly efficient photoinduced iron-catalyzed method has been developed for the direct use of alcohols as surrogates for organometallic reagents in the synthesis of tertiary alcohols. This method can be applied to both primary and secondary alcohols with diverse structures, enabling their reaction with aryl ketones under mild conditions. A variety of functional groups, including those that are typically reactive under conventional tertiary alcohol synthesis conditions, are compatible. Mechanistically, this reaction proceeds through the direct addition of the radical to the carbonyl pathway.

Electroorganic synthesis in aqueous solution via generation of strongly oxidizing and reducing intermediates

Water is the ideal green solvent for organic electrosynthesis. However, a majority of electroorganic processes require potentials that lie beyond the electrochemical window for water. In general, water oxidation and reduction lead to poor synthetic yields and selectivity or altogether prohibit carrying out a desired reaction. Herein, we report several electroorganic reactions in water using synthetic strategies referred to as reductive oxidation and oxidative reduction. Reductive oxidation involves the homogeneous reduction of peroxydisulfate (S2O82-) via electrogenerated Ru(NH3)62+ at potential of -0.2 V vs. Ag/AgCl (3.5 M KCl) to form the highly oxidizing sulfate radical anion (E0' (SO4˙-/SO42-) = 2.21 V vs. Ag/AgCl), which is capable of oxidizing species beyond the water oxidation potential. Electrochemically generated SO4˙- then efficiently abstracts a hydrogen atom from a variety of organic compounds such as benzyl alcohol and toluene to yield product in water. The reverse analogue of reductive oxidation is oxidative reduction. In this case, the homogeneous oxidation of oxalate (C2O42-) by electrochemically generated Ru(bpy)33+ produces the strongly reducing carbon dioxide radical anion (E0' (CO2˙-/CO2) = -2.1 V vs. Ag/AgCl), which can reduce species at potential beyond the water or proton reduction potential. In preliminary studies, the CO2˙- has been used to homogeneously reduce the C-Br moiety belonging to benzyl bromide at an oxidizing potential in aqueous solution.

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.

Enantioselective Dearomatization of Indoles via SmI2-Mediated Intermolecular Reductive Coupling with Ketones

Samarium diiodide (SmI₂) mediated reductive coupling reactions are powerful methods for the construction of carbon-carbon bond in organic synthesis. Despite the extensive development in recent decades, successful examples of the corresponding asymmetric reactions remained scarce, probably due to the involvement of highly reactive radical intermediates. In this Article, we report an enantioselective dearomatization of indoles via SmI₂-mediated intermolecular reductive coupling with ketones. The utilization of samarium reductant supported by chiral tridentate aminodiol ligands allows the facile synthesis of indoline molecules bearing two contiguous stereogenic centers in high yields (up to 99%) and stereoselectivity (up to 99:1 er and >20:1 dr). Combined experimental and computational investigations suggested that parallel single-electron transfer to each substrate from the chiral samarium reductant allows the radical-radical recombination in an enantioselective manner, which is a unique mechanistic scenario in SmI₂-mediated reductive coupling reactions.

Photoinduced Vinylogous Dearomatization

The first vinylogous dearomatization is reported. Under a photoinduced platform, various benzothiophenes functionalized by ketones at the 3-position could react with 3-methylenechroman-4-ones efficiently, leading to a variety of valuable products that contain the pharmaceutically significant chromones and 2,3-dihydrobenzo[b]thiophenes concurrently. The transformations were revealed to experience hydrogen-atom transfer, dearomatization, olefin migration, and radical cross coupling.

LiHMDS-Mediated Deprotonative Coupling of Toluenes with Ketones

We demonstrate that lithium hexamethyldisilazide (LiHMDS) acts as an effective base for deprotonative coupling reactions of toluenes with ketones to afford stilbenes. Various functionalities (halogen, OCF₃ , amide, Me, aryl, alkenyl, alkynyl, SMe, and SPh) are allowed on the toluenes. Notably, this system proved successful with low-reactive toluenes bearing a large pKa value compared to that of the conjugate acid of LiHMDS (hexamethyldisilazane, 25.8, THF), as demonstrated by 4-phenyltoluene (38.57, THF) and toluene itself (∼43, DMSO).

Modified Chemoselective Reduction and Consecutive Regioselective Deuteration Reaction Catalyzed by B(C6F5)3

An efficient chemoselective reduction of isatin derivatives using catalyst B(C₆F₅)₃, benzyldimethylsilane, and H₂O is described. Notably, a small amount of water is shown to be a highly effective reaction promoter that decreases the reaction time and temperature for the synthesis of indolin-3-ones. Moreover, using method, excellent deuterium incorporation is achieved via the catalytic α-deuteration of indolin-3-ones using B(C₆F₅)₃ and D₂O.

Efficient Aerobic Oxidative Coupling of Methyl Heteroarenes with Indoles

Direct oxidative coupling of inert C(sp³ )-H bond has been a great challenge. Herein, an environmentally friendly aerobic oxidative coupling of α-methyl substituted N-heteroarenes with indoles is reported. A variety of diheteroaryl ketones were prepared in good yields (up to 72 %). This protocol features simple operation and broad substrates scope (26 examples). Significantly, a plausible mechanism about catalytic cycle was proposed, and two key intermediates were confirmed by high resolution mass spectrometry.

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.

Visible Light-Mediated Enantioselective Addition of α-Aminoalkyl Radicals to Ketones Catalyzed by Chiral Oxazaborolidinium Ion

The development of a visible light-mediated synthetic method for chiral 1,2-amino tertiary alcohols is described. In the presence of a chiral oxazaborolidinium ion catalyst and photosensitizer, the enantioselective addition of an α-aminoalkyl radical to aryl methyl ketones under visible light provides chiral 1,2-amino tertiary alcohol derivatives in high yields (up to 88%) with excellent enantioselectivities (up to 98% ee). With mechanistic studies such as radical trapping analysis, radical clock analysis, and the measurement of quantum yield, a plausible catalytic cycle is proposed.

Direct visible-light-induced synthesis of P-stereogenic phosphine oxides under air conditions

Over the past two decades, visible-light-induced transformations have been regarded as being among the most environmentally benign and powerful strategies for constructing complex molecules and diverse synthetic building blocks in organic synthesis. However, the development of efficient photochemical processes for assembling enantiomerically pure molecules remains a significant challenge. Herein, we describe a simple and efficient visible-light-induced C-P bond forming reaction for the synthesis of P-chiral heteroaryl phosphine oxides in moderate to high yields with excellent ee values (97-99% ee). Even in the absence of transition metal or photoredox catalysts, a variety of P-chiral heteroaryl phosphine oxides, including chiral diphosphine oxide 41, have been directly obtained under air conditions. Density functional theory (DFT) calculations have shown that the reaction involves intersystem crossing and single electron transfer to give a diradical intermediate under visible light irradiation.

A Catalytic Alkylation of Ketones via sp3 C-H Bond Activation

We identified a ternary hybrid catalyst system composed of an acridinium photoredox catalyst, a thiophosphoric imide (TPI) catalyst, and a titanium complex catalyst that promoted an intermolecular addition reaction of organic molecules with various ketones through sp³ C-H bond activation. The thiyl radical generated via single-electron oxidation of TPI by the excited photoredox catalyst abstracted a hydrogen atom from organic molecules such as toluene, benzyl alcohol, alkenes, aldehydes, and THF. The thus-generated carbon-centered radical species underwent addition to ketones and aldehydes. This intrinsically unfavorable step was promoted by single-electron reduction of the intermediate alkoxy radical by catalytically generated titanium(III) species. This reaction provided an efficient and straightforward route to a broad range of tertiary alcohols and was successfully applied to late-stage functionalization of drugs or their derivatives. The proposed mechanism was supported by both experimental and theoretical studies.

Asymmetric Formal Nucleophilic o-Cresolylation with Morita-Baylis-Hillman Carbonates of 2-Cyclohexenones via Palladium Catalysis

Here we report an asymmetric formal nucleophilic o-cresolylation reaction with the Morita-Baylis-Hillman (MBH) carbonates from 2-cyclohexanones and diverse aldehydes under palladium catalysis, by in situ generation of electron-neutral and HOMO-raised η²-Pd(0)-dienone complexes via an oxidative insertion/π-σ-isomerization/β-H elimination activation sequence. The subsequent umpolung vinylogous addition to a variety of imines is realized upon Pd(0)-mediated π-Lewis base catalysis, finally furnishing o-cresolylated products followed by another cascade of a π-σ-isomerization/β-H elimination/aromatization process. Moderate to excellent diastereo- and enantioselectivity are achieved for substantial substrate assemblies by employing a newly designed bulky chiral phosphonamidite ligand, and the resultant multifunctional products can be facilely elaborated to access diverse enantioenriched architectures. In addition, the catalytic reaction pathway is finely illuminated by control experiments.

Photoinduced Chemo-, Site- and Stereoselective α-C(sp3 )-H Functionalization of Sulfides

The ubiquity of sulfur-containing molecules in biologically active natural products and pharmaceuticals has long attracted synthetic chemists to develop efficient strategies towards their synthesis. The strategy of direct α-C(sp³ )-H modification of sulfides provides a streamlining access to complex sulfur-containing molecules. Herein, we report a photoinduced chemo-, site- and stereoselective α-C(sp³ )-H functionalization of sulfides using isatins as the photoredox reagent and coupling partner catalyzed by a chiral gallium(III)-N,N'-dioxide complex. The reaction proceeds through a verified single-electron transfer (SET) mechanism with high efficiency, excellent functional group tolerance, as well as a broad substrate scope. Importantly, this cross-coupling protocol is highly selective for the direct late-stage functionalization of methionine-related peptides, regardless of the inherent structural similarity and complexity of diverse residues.

From Center-to-Multilayer Chirality: Asymmetric Synthesis of Multilayer Targets with Electron-Rich Bridges

Asymmetric synthesis of new atropisomerically multilayered chiral targets has been achieved by taking advantage of the strategy of center-to-multilayer chirality and double Suzuki-Miyaura couplings. Diastereomers were readily separated via flash column chromatography and well characterized. Absolute configuration assignment was determined by X-ray structural analysis. Five enantiomerically pure isomers possessing multilayer chirality were assembled utilizing anchors involving electron-rich aromatic connections. An overall yield of 0.69% of the final target with hydroxyl attachment was achieved over 11 steps from commercially available starting materials.

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.

Tandem Photoredox-Chiral Phosphoric Acid Catalyzed Radical-Radical Cross-Coupling for Enantioselective Synthesis of 3-Hydroxyoxindoles

A photochemical protocol that couples diarylamines and α-ketoesters to afford the chiral 3-hydroxyoxindoles through tandem photoredox and chiral phosphoric acid catalysis is developed. The reaction involves an enantioselective photochemical radical-radical cross-coupling process. The chiral phosphoric acid is discovered to play crucial roles by decreasing the reductive potentials of α-ketoesters and stereocontrolling the downstream asymmetric radical-radical cross-coupling via the formation of pentacoordinate complex.

Easy access to drug building-blocks through benzylic C-H functionalization of phenolic ethers by photoredox catalysis

A visible light-mediated photocatalyzed C-C-bond forming method for the benzylic C-H functionalization of phenolether containing synthetic building blocks based on a radical-cation/deprotonation strategy is reported. This method allows the mild, selective generation of benzyl radicals in phenolic complex molecules and drug-like compounds, providing new entries in synthetic and medicinal chemistry.

Asymmetric Catalytic Approach to Multilayer 3D Chirality

The first asymmetric catalytic approach to multilayer 3D chirality has been achieved by using Suzuki-Miyaura cross-couplings. New chiral catalysts were designed and screened under various catalytic systems that proved chiral amide-phosphines to be more efficient ligands than other candidates. The multilayer 3D framework was unambiguously determined by X-ray structural analysis showing a parallel pattern of three layers consisting of top, middle and bottom aromatic rings. The X-ray structure of a catalyst complex, dichloride complex of Pd-phosphine amide, was obtained revealing an interesting asymmetric environment nearby the Pd metal center. Three rings of multilayer 3D products can be readily changed by varying aromatic ring-anchored starting materials. The resulting multilayer products displayed strong luminescence under UV irradiation and strong aggregation-induced emission (AIE). In the future, this work would benefit not only the field of asymmetric synthesis but also materials science, in particular polarized organic electronics, optoelectronics and photovoltaics.

Photocatalytic three-component asymmetric sulfonylation via direct C(sp3)-H functionalization

The direct and selective C(sp³)-H functionalization of cycloalkanes and alkanes is a highly useful process in organic synthesis owing to the low-cost starting materials, the high step and atom economy. Its application to asymmetric catalysis, however, has been scarcely explored. Herein, we disclose our effort toward this goal by incorporation of dual asymmetric photocatalysis by a chiral nickel catalyst and a commercially available organophotocatalyst with a radical relay strategy through sulfur dioxide insertion. Such design leads to the development of three-component asymmetric sulfonylation involving direct functionalization of cycloalkanes, alkanes, toluene derivatives or ethers. The photochemical reaction of a C(sp³)-H precursor, a SO₂ surrogate and a common α,β-unsaturated carbonyl compound proceeds smoothly under mild conditions, delivering a wide range of biologically interesting α-C chiral sulfones with high regio- and enantioselectivity (>50 examples, up to >50:1 rr and 95% ee). This method is applicable to late-stage functionalization of bioactive molecules, and provides an appealing access to enantioenriched compounds starting from the abundant hydrocarbon compounds.

Asymmetric C(sp3)-H functionalization of unactivated alkylarenes such as toluene enabled by chiral Brønsted base catalysts

Benzylic functionalisation of unactivated alkylarenes remains as a significant challenge in asymmetric catalysis due to their less reactive nature. Here, we show development of catalytic asymmetric C(sp³)-H functionalization of unactivated alkylarenes such as toluene with imines. The reactions proceeded smoothly under proton-transfer conditions using a chiral, strong Brønsted base catalyst system. A chiral Brønsted base prepared from an alkylpotassium and a chiral amine ligand was found to effectively form a promising asymmetric environment around a benzyl anion. Optimization of the reaction conditions revealed that the use of the alkaline metal amide, potassium hexamethyldisilazide (KHMDS), as an additive was most effective, and enantioselective and atom economical carbon-carbon bond-forming reactions at the benzylic positions of unactivated alkylarenes was achieved without using any transition-metal catalyst.

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

Recently, with the boosted development of radical chemistry, enantioselective functionalization of C(sp3)-H bonds via a radical pathway has witnessed a renaissance. In principle, two distinct catalytic modes, distinguished by the steps in which the stereochemistry is determined (the radical formation step or the radical functionalization step), can be devised. This Perspective discusses the state-of-the-art in the area of catalytic enantioselective C(sp3)-H functionalization involving radical intermediates as well as future challenges and opportunities.

Radical addition reaction between chromenols and toluene derivatives initiated by Brønsted acid catalyst under light irradiation

A Brønsted acid-catalyzed radical addition reaction between chromenols and toluene derivatives was accomplished under light irradiation, affording benzylated chromene derivatives through benzylic C(sp³)–H functionalization of toluene derivatives.