Asymmetric reactions of N-heterocyclic carbene (NHC)-based chiral acyl azoliums and azolium enolates

Highly Enantioselective Synthesis of Polycyclic Dihydroisoquinolinones via NHC-Catalyzed [4 + 2] Annulations

The NHC-catalyzed enantioselective [4 + 2] annulation of 9H-fluorene-1-carbaldenydes with cyclic imines was successfully developed. A series of optically enriched polycyclic dihydroisoquinolinones were synthesized in moderate to excellent yields with good to excellent enantioselectivities. In addition, this efficient method could also be amenable to the synthesis of spirocyclic compounds by using isatin-derived ketimines as the electrophiles.

Direct Access to 3,4,6-Trisubstituted 2-Pyrones via Carbene Catalysis

An N-heterocyclic carbene-catalyzed [4 + 2] annulation of β,γ-unsaturated α-keto esters and phenylacetate esters was developed for the direct and efficient construction of 2-pyrones. This approach provides a practical synthesis pathway for various 3,4,6-trisubstituted 2-pyrones in moderate to good yields and features broad substrate scope and good functional group tolerance. Moreover, the products can also be readily transformed to naphthalene and acylamide.

Mechanochemical Synthesis of Corannulene Flanked N-heterocyclic Carbene (NHC) Precursors and Preparation of Their Metal Complexes

The synthesis of new compounds is an important pillar for the advancement of the field of chemistry and adjacent fields. In this regard, over the last decades huge efforts have been made to not only develop new molecular entities but also more efficient sustainable synthetic methodologies due to the increasing concerns over environmental sustainability. In this context, we have developed synthetic routes to novel corannulene flanked imidazolium bromide NHC precursors both in the solid-state and solution phases. Our work presents a comprehensive comparative study of mechanochemical routes and conventional solution-based methods. Green metrics and energy consumption comparison were performed for both routes revealing ball-milling generation of these compounds to be an environmentally greener technique to produce such precursors compared to conventional solvent-based methods. In addition, we have demonstrated proof-of-concept of the herein reported corannulene flanked NHCs to be robust ligands for transition metals and their ligand substitution reactions.

Design, synthesis and Anti-PVY activity of planar chiral thiourea derivatives incorporated with [2.2]Paracyclophane

BACKGROUND

Potato virus Y (PVY) is a prominent representative of plant viruses. It can inflict severe damage upon Solanaceae plants, leading to global dissemination and substantial economic losses. To discover new antiviral agents, a class of planar chiral thiourea molecules through the key step of N-heterocyclic carbene-catalyzed nitrile formation reaction was synthesized with excellent optical purities for antiviral evaluations against plant virus PVY.

RESULTS

The absolute configurations of the planar chiral compounds exhibited obvious distinctions in the anti-PVY activities. Notability, compound (S)-4u exhibited remarkable curative activities against PVY, with a half maximal effective concentration (EC50) of 349.3 μg mL-1, which was lower than that of the ningnanmycin (NNM) (EC50 = 400.8 μg mL-1). Additionally, The EC50 value for the protective effects of (S)-4u was 146.2 μg mL-1, which was superior to that of NNM (276.4 μg mL-1). Furthermore, the mechanism-of-action of enantiomers of planar chiral compound 4u was investigated through molecular docking, defensive enzyme activity tests and chlorophyll content tests.

CONCLUSION

Biological mechanism studies have demonstrated that the configuration of planar chiral target compounds plays a crucial role in the molecular interaction with PVY-CP, enhancing the activity of defense enzymes and affecting chlorophyll content. The current study has provided significant insights into the roles played by planar chiralities in plant protection against viruses. This paves the way for the development of novel green pesticides bearing planar chiralities with excellent optical purities. © 2024 Society of Chemical Industry.

Application of Asymmetric Catalysis in Chiral Pesticide Active Molecule Synthesis

The different configurations of chiral pesticides generally have significant influence on their biological activities. Chiral agrochemicals with high optical purities have become a prominent topic in the research field of new pesticides due to their advantages including lower toxicity, higher efficiency, and reduced residue levels. However, most commercially available pesticides that possess chiral elements are still used in their racemic forms. To date, asymmetric catalysis has emerged as a versatile tool for the enantioselective synthesis of various chiral agrochemicals and novel chiral pesticide active molecules. This perspective provides a comprehensive overview of the applications of diverse asymmetric catalytic approaches in the facile preparation of numerous novel pesticide active molecules, and our own outlook on the future development of this highly active research direction is also presented at the end of this review.

Recent Advances in NHC-Catalyzed Chemoselective Activation of Carbonyl Compounds

N-Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo- and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules.

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

Redox Active N-Heterocyclic Carbenes in Oxidative NHC Catalysis

An N-heterocyclic carbene (NHC) covalently linked to a quinone introduces a novel avenue for internal oxidations within oxidative NHC catalysis. The deployment of this hybrid NHC class promotes intramolecular electronic flow in the oxidation of the Breslow intermediate to acyl azolium. The use of the redox active NHC as a catalyst is facilitated by employing aerobic regeneration, yielding carboxylic esters with efficiencies of ≤99%, while generating water as the sole byproduct.

Discovery of Novel Chiral Indole Derivatives Containing the Oxazoline Moiety as Potential Antiviral Agents for Plants

Potato virus Y (PVY) is an important plant virus that has spread worldwide, causing significant economic losses. To search for novel structures as potent antiviral agents, a series of chiral indole derivatives containing oxazoline moieties were designed and synthesized and their anti-PVY activities were evaluated. Biological activity tests demonstrated that many chiral compounds exhibited promising anti-PVY activities and that their absolute configurations exhibited obvious distinctions in antiviral bioactivities. Notably, compound (S)-4v displayed excellent curative and protective efficacy against PVY, with EC50 values of 328.6 and 256.1 μg/mL, respectively, which were superior to those of commercial virucide ningnanmycin (NNM, 437.4 and 397.4 μg/mL, respectively). The preliminary antiviral mechanism was investigated to determine the difference in antiviral activity between the two enantiomers of 4v chiral compounds. Molecular docking indicated a stronger binding affinity between the coating proteins of PVY (PVY-CP) and (S)-4v (-6.5 kcal/mol) compared to (R)-4v (-6.2 kcal/mol). Additionally, compound (S)-4v can increase the chlorophyll content and defense-related enzyme activities more effectively than its enantiomer. Therefore, this study provides an important basis for the development of chiral indole derivatives containing oxazoline moieties as novel agricultural chemicals.

N-Heterocyclic Carbene-Catalyzed Facile Synthesis of Phthalidyl Sulfonohydrazones: Density Functional Theory Mechanistic Insights and Docking Interactions

N-heterocyclic carbene catalysis reaction protocol is disclosed for the synthesis of phthalidyl sulfonohydrazones. A broad range of N-tosyl hydrazones react effectively with phthalaldehyde derivatives under open-air conditions, enabling the formation of a new C-N bond via an oxidative path. The reaction proceeds under mild reaction conditions with broad substrate scopes, wide functional group tolerance, and good to excellent yields. The mechanistic pathway is studied successfully using control experiments, competitive reactions, ESI-MS spectral analyses of the reaction mixture, and computational study by density functional theory. The potential use of one of the phthalidyl sulfonohydrazone derivatives as the inhibitor of β-ketoacyl acyl carrier protein synthase I of Escherichia coli is investigated using molecular docking.

NHC-Catalyzed Chemo- and Enantioselective Reaction between Aldehydes and Enals for Access to Axially Chiral Arylaldehydes

A chiral carbene-catalyzed chemo- and enantioselective reaction with racemic biaryl aldehydes and α-bromoenals is developed for access to axially chiral 2-arylbenzaldehydes through atroposelective dynamic kinetic resolution (DKR) processes. This atroposelective DKR strategy can tolerate a broad scope of substrates with diverse functionalities. The axially chiral 2-aryl benzaldehyde products generally afford moderate to good yields and enantioselectivities. The axially chiral molecules afforded from the current approach are variable through simple transformations to afford axially chiral functional molecules with excellent optical purities.

Access to Axially Chiral Aryl Aldehydes via Carbene-Catalyzed Nitrile Formation and Desymmetrization Reaction

An approach utilizing N-heterocyclic carbene for nitrile formation and desymmetrization reaction is developed. The process involves kinetic resolution, with the axially chiral aryl monoaldehydes obtained in moderate yields with excellent optical purities. These axially chiral aryl monoaldehydes can be conveniently transformed into functionalized molecules, showing great potential as catalysts in organic chemistry.

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

N-Heterocyclic Carbene-Catalyzed [3 + 3] Annulation of 5-Aminopyrazoles with Enals: Enantioselective Synthesis of Pyrazolo[3,4-b]pyridones

An enantioselective construction of pyrazolo[3,4-b]pyridones was achieved via N-heterocyclic carbene-catalyzed [3 + 3] annulation of enals with 5-aminopyrazoles. This protocol not only offers a highly efficient synthetic approach for the preparation of various substituted pyrazolo[3,4-b]pyridones but also provides an effective method for the rapid synthesis of enantiopure spirooxindone derivatives.

Highly Enantioselective Construction of the Polycyclic Piperazin-2-ones via NHC-Catalyzed [12 + 2] Cycloadditions

NHC-catalyzed diastereoselective and enantioselective [12 + 2] higher-order cycloadditions of 5H-benzo[a]-pyrrolizine-3-carbaldehydes with cyclic sulfonic imines are described. Various optically pure polycyclic piperazin-2-ones were successfully constructed under mild reaction conditions with excellent diastereoselectivities and enantioselectivities. The generated chiral polycyclic piperazin-2-ones could be transformed to planar-chiral nine-membered cyclic amides via squaramide-catalyzed asymmetric ring-expansion reactions.

Atroposelective Access to Dihydropyridinones with C-N Axial and Point Chirality via NHC-Catalyzed [3 + 3] Annulation

An N-heterocyclic carbene-catalyzed atroposelective [3 + 3] annulation of enals with 2-aminomaleate derivatives is described. A series of substituted dihydropyridones bearing both C-N axis and point chirality were synthesized with good diastereo- and enantioselectivity under mild conditions. This efficient strategy successfully superpositions an extra point chiral element with an axial backbone, and the generated structurally interesting atropisomers may have potential application in drug discovery.

Carbene-Catalyzed Atroposelective Annulation for Quick Access to Axially Chiral Thiazine Derivatives

An N-heterocyclic carbene (NHC)-catalyzed atroposelective annulation reaction is disclosed for quick and efficient access to thiazine derivatives. A series of axially chiral thiazine derivatives bearing various substituents and substitution patterns were produced in moderate to high yields with moderate to excellent optical purities. Preliminary studies revealed that some of our products exhibit promising antibacterial activities against Xanthomonas oryzae pv. oryzae (Xoo) that causes rice bacterial blight.

Access to Spiropyrazolone-butenolides through NHC-Catalyzed [3 + 2]-Asymmetric Annulation of 3-Bromoenals and 1H-Pyrazol-4,5-diones

The stereoselective synthesis of spirocyclic pyrazolin-5-ones by N-heterocyclic carbene (NHC) organocatalysis has been less studied so far. For this reason and considering the interest of this class of compounds, here, we present the NHC-catalyzed [3 + 2]-asymmetric annulation of β-bromoenals and 1H-pyrazol-4,5-diones that achieves to produce chiral spiropyrazolone-butenolides. The synthesis is general for aryl and heteroaryl β-bromo-α,β-unsaturated aldehydes and 1,3-disubstituted pyrazolones. The spirobutenolides have been obtained in good yields (up to 88%) and enantioselectivities (up to 97:3 er). This constitutes the first described example using pyrazoldiones as the starting materials for this class of spiro compounds.

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

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

Modular and Selective Access to Functionalized Alkynes and Allenes via the Intermediacy of Propargylic Acetates

We report an efficient one-pot, two-step procedure for the modular synthesis of α-difunctionalized alkynes and trisubstituted allenes by sequential cross-coupling of benzal gem-diacetates with organozinc or -copper reagents in the absence of external transition metals. The intermediacy of propargylic acetates enables the divergent and selective synthesis of these valuable products. This method features its readily accessible substrates, relatively mild conditions, wide scope, and scalability in practical synthesis.

Facile Approach for the Oxidative Enolate Activation of Aliphatic Aldehydes

A proline/N-heterocyclic carbene relay catalytic strategy is developed for the oxidative enolate activation of aliphatic aldehydes. A broad scope of electrophiles including oxindole-derived pyrazolones, oxindole-derived α,β-unsaturated esters, and α,β-unsaturated imines are effective as the reactants in the asymmetric [2 + 4] cycloaddition reaction with the alkyl aldehydes bearing different substitution patterns. Structural complex multicyclic chiral products can be afforded in generally excellent yields and enantio- and diastereoselectivities through this approach under similar reaction conditions. Several of the optical pure products afforded from this protocol exhibit excellent antibacterial activities against plant pathogens and are promising in the development of novel pesticides for plant protection.

Stereoselective N-Heterocyclic-Carbene-Catalyzed Formal [4 + 2] Cycloaddition: Access to Chiral Heterocyclic Cyclohexenones

The present study reports an asymmetric NHC-catalyzed formal [4 + 2] cycloaddition of heterocyclic alkenes containing a polarized double bond with an azolium-dienolate intermediate generated from α-bromo-α,β-unsaturated aldehydes without external oxidation of the Breslow intermediate. Heterocyclic cyclohexenones were produced in good isolated yields (typically about 90%) with good stereochemical outcomes (in most cases, dr > 20/1, and ee = 70-99%). The synthetic utility of the protocol was exemplified by the scope of heterocyclic alkenes.

Enantioselective Synthesis of Pyrazolo[3,4-b]pyridone Derivatives with Antifungal Activities against Phytophthora capsici and Colletotrichum fructicola

A chiral NHC-catalyzed [3 + 3] cycloaddition reaction is developed for the efficient synthesis of pyrazolo[3,4-b]pyridones in generally excellent yields and optical purities. The R, S, and racemic forms of these molecules are systematically studied via in vitro tests that detect antifungal activity against Phytophthora capsici and Colletotrichum fructicola. Chiral compounds (R)-3i, (R)-3j, and (R)-3p are identified to have excellent inhibitory effects against P. capsici and C. fructicola.

A Review on Generation and Reactivity of the N-Heterocyclic Carbene-Bound Alkynyl Acyl Azolium Intermediates

N-heterocyclic carbene (NHC) has been widely used as an organocatalyst for both umpolung and non-umpolung chemistry. Previous works mainly focus on species including Breslow intermediate, azolium enolate intermediate, homoenolate intermediate, alkenyl acyl azolium intermediate, etc. Notably, the NHC-bound alkynyl acyl azolium has emerged as an effective intermediate to access functionalized cyclic molecular skeleton until very recently. In this review, we summarized the generation and reactivity of the NHC-bound alkynyl acyl azolium intermediates, which covers the efforts and advances in the synthesis of achiral and axially chiral cyclic scaffolds via the NHC-bound alkynyl acyl azolium intermediates. In particular, the mechanism related to this intermediate is discussed in detail.

Catalyst-Controlled Selectivity Switch in Three-Component Reaction: An NHC-Catalyzed Strategy for the Synthesis of δ-Lactone-Fused Spirobenzofuran-3-ones

An efficient, three-component reaction of aldehydes and benzofuran-3-ones was developed. This process provides a new approach for the preparation of synthetically and biologically important spirobenzofuran-3-one derivatives with moderate-to-good yields under mild conditions. A switch of intramolecular to intermolecular domino Michael-aldol-lactonization leading to differential product formation was achieved by different NHCs catalysis.

Asymmetric Synthesis of Structurally Sophisticated Spirocyclic Pyrano[2,3-c]pyrazole Derivatives Bearing a Chiral Quaternary Carbon Center

A carbene-catalyzed enantio- and diastereoselective [2 + 4] cycloaddition reaction is developed for quick and efficient access to structurally complex multicyclic pyrano[2,3-c]pyrazole molecules. The reaction tolerates a broad scope of substrates bearing various substitution patterns, with the multicyclic pyrano[2,3-c]pyrazole products afforded in generally good to excellent yields and optical purities. The chiral molecules obtained from this approach has found promising applications in the development of novel bacteriacides for plant protection.

N-Heterocyclic Carbene (NHC)-Catalyzed Transformations Involving Azolium Enolates

The recent advances in the N-heterocyclic carbene (NHC)-organocatalyzed generation of azolium enolate intermediates and their subsequent interception with electrophiles are highlighted. The NHC-bound azolium intermediates are generated by the addition of NHCs to suitably substituted aldehydes, acid derivatives or ketenes. A broad range of coupling partners can intercept the azolium enolates to form [2+n] cycloadducts (n=2,3,4) and various α-functionalized compounds. The enantioselective synthesis of the target compounds are achieved with the use of chiral NHCs. Herein, we summarized the development that occurred in this subclass of NHC catalysis.

An Unexpected Inactivation of N-Heterocyclic Carbene Organic Catalyst by 1-Methylcyclopropylcarbaldehyde and 2,2,2-Trifluoroacetophenone

An unprecedented inactivation process of the indanol-derived NHC catalysts bearing N-C₆F₅ groups is reported. An unexpected multi-cyclic complex product is obtained from the 3-component reaction with the 1-methylcyclopropyl-carbaldehyde, the 2,2,2-trifluoroacetophenone and the NHC catalyst. The absolute structure of the inactivation product is unambiguously assigned via X-ray analysis on its single crystals. The formation of the structurally complex product is rationalized through a multi-step cascade cyclization process.

The Role of the Fused Ring in Bicyclic Triazolium Organocatalysts: Kinetic, X-ray, and DFT Insights

Bicyclic triazolium scaffolds are widely employed in N-heterocyclic carbene (NHC) organocatalysis. While the incorporation of a fused ring was initially for synthetic utility in accessing chiral, modular triazolyl scaffolds, recent results highlight the potential for impact upon reaction outcome with the underpinning origins unclear. The common first step to all triazolium-catalyzed transformations is C(3)-H deprotonation to form the triazolylidene NHC. Herein, we report an analysis of the impact of size of the fused (5-, 6-, and 7-membered, n = 1, 2, and 3, respectively) ring on the C(3) proton transfer reactions of a series of bicyclic triazolium salts. Rate constants for the deuteroxide-catalyzed C(3)-H/D-exchange of triazolium salts, kDO, were significantly influenced by the size of the adjacent fused ring, with the kinetic acidity trend, or protofugalities, following the order kDO (n = 1) > kDO (n = 2) ≈ kDO (n = 3). Detailed analyses of X-ray diffraction (XRD) data for 20 triazolium salts (including 16 new structures) and of computational data for the corresponding triazolylidene NHCs provide insight on structural effects of alteration of fused ring size. In particular, changes in internal triazolyl NCN angle and positioning of the most proximal CH2 with variation in fused ring size are proposed to influence the experimental protofugality order.

Stereoselective synthesis of chiral sultam-fused dihydropyridinones via photopromoted NHC catalyzed [4 + 2] annulation

A photopromoted NHC catalyzed asymmetric [4+2] annulation of saccharine-derived azadienes and α-diazoketones was developed, affording the corresponding sultam-fused dihydropyridinones efficiently (up to 80% yield, 99% ee and >20 : 1 d.r.).

Silver-catalyzed stereoselective C-4 arylthiodifluoromethylation of coumarin-3-carboxylic acids via a double decarboxylative strategy

A facile silver-catalyzed dual decarboxylation of arylthio-difluoroacetic acid with coumarin-3-carboxylic acids/chromone-3-carboxylic acids was developed.

Rh2(II)-Catalyzed Enantioselective Intramolecular Büchner Reaction and Aromatic Substitution of Donor-Donor Carbenes

The chiral dirhodium(ii) tetracarboxylate-catalyzed enantioselective intramolecular Büchner reaction of donor/donor-carbenes was reported and a series of valuable chiral polycyclic products were synthesized. Both aryloxy enynones and diazo compounds were efficient carbene precursors for this reaction. Excellent yields (up to 99%) and outstanding enantioselectivities (up to >99% ee) were achieved under standard conditions. For furyl substituted chiral cyclohepta[b]benzofurans bearing a substituent at the C4 position on cycloheptatrienes, control reactions showed that the chiral Büchner products could slowly racemize either under dark or natural light conditions. A diradical-involved mechanism rather than a zwitterionic intermediate was proposed to explain the racemization. Furthermore, furyl substituted chiral fluorene derivatives were obtained via asymmetric aromatic substitution when biaryl enynones were employed as carbene precursors.

The chiral dirhodium(ii) tetracarboxylate-catalyzed enantioselective intramolecular Büchner reaction and aromatic substitution of donor/donor-carbenes were reported and a series of valuable chiral polycyclic products were synthesized.

NHC-catalysed [3 + 2]-asymmetric annulation between pyrazolin-4,5-diones and enals: synthesis of novel spirocyclic pyrazolone γ-butyrolactones and computational study of mechanism and stereoselectivity

Herein we present the first asymmetric synthesis of spiropyrazolone γ-butyrolactones from 1H-pyrazol-4,5-diones and enals by an NHC-catalysed [3 + 2] annulation. DFT calculations carried out predict the experimental configuration of final adducts.

Triazolium Salt Organocatalysis: Mechanistic Evaluation of Unusual Ortho-Substituent Effects on Deprotonation

Organocatalysis by N-heterocyclic carbenes is normally initiated by the deprotonation of precursor azolium ions to form active nucleophilic species. Substituent effects on deprotonation have an impact on catalytic efficiency and provide insight into general catalytic mechanisms by commonly used azolium systems. Using an NMR kinetic method for the analysis of C(3)-H/D exchange, we determined log kex–pD profiles for three ortho-substituted N-aryl triazolium salts, which enables a detailed analysis of ortho-substituent effects on deprotonation. This includes N-5-methoxypyrid-2-yl triazolium salt 7 and di-ortho-methoxy and di-ortho-isopropoxyphenyl triazolium salts 8 and 9, and we acquired additional kinetic data to supplement our previously published analysis of N-pyrid-2-yl triazolium salt 6. For 2-pyridyl triazoliums 6 and 7, novel acid catalysis of C(3)-H/D exchange is observed under acidic conditions. These kinetic data were supplemented by DFT analyses of the conformational preferences of 6 upon N-protonation. A C(3) deprotonation mechanism involving intramolecular general base deprotonation by the pyridyl nitrogen of the N(1)-deuterated dicationic triazolium salt is most consistent with the data. We also report kDO values (protofugalities) for deuteroxide-catalyzed exchange for 6–9. The protofugalities for 8 and 9 are the lowest values to date in the N-aryl triazolium series.