Access to Planar Chiral Ferrocenes via N-Heterocyclic Carbene-Catalyzed Enantioselective Desymmetrization Reactions

Enantioselective electrophilic activation of aldehydes, esters and imines via N-heterocyclic carbene catalysis

We summarize in this review the recent key progress in the NHC-catalyzed electrophilic activation of carbonyl compounds for the regioselective functionalization of saccharides, enantioselective construction of heterocycles, and asymmetric synthesis of axially and planar chiral molecules.

Asymmetric synthesis of metallocenes with planar and central chirality by rhodium-catalyzed desymmetrization reactions

Metallocenes with planar and central chirality have emerged as a privileged skeleton for chiral ligand design, and such ligands have exhibited tremendous success in various asymmetric catalysis protocols. Herein, we report a rhodium/chiral diene-catalyzed asymmetric desymmetrization of 1,2-diformylmetallocenes with aryl/alkenylboronic acids to give enantio-enriched formylmetallocenes, which are diastereoisomers of Ugi-type products. This catalytic system also enables the kinetic resolution of 2-substituted 1-formylferrocene with a selectivity factor (s) of up to 4331. Compared with traditional synthesis methods, our method has the following advantages: (1) opposite diastereoselectivity; (2) catalytic asymmetric synthesis; (3) single-step construction of planar and central chirality. The synthetic utility of the present method is demonstrated by the asymmetric synthesis of a series of chiral phosphine ligands, including Josiphos- and PPFA-type ligands.

N-Heterocyclic carbene (NHC) organocatalysis: from fundamentals to frontiers

N-Heterocyclic carbenes (NHCs) have been used as organocatalysts for a multitude of C-C and C-heteroatom bond-forming reactions. They enable diverse modalities of activating a wide range of structurally distinct substrate classes and allow access to electronically distinct intermediates. The easy tunability of the NHC scaffold contributes to its versatility. Recent years have witnessed a surge of interest in various organocatalytic reactions of NHCs, leading to the forays of NHC catalysis into the relatively newer domains such as reactions involving radical intermediates, atroposelective synthesis, umpolung of electrophiles other than aldehydes, and the use of NHCs as non-covalent templates for enantioinduction. This tutorial review provides an overview of various important structural features and reactivity modes of NHCs and delves deep into some frontiers of NHC-organocatalysis.

Visible-Light-Induced Synthesis of Esters via a Self-Propagating Radical Reaction

We herein disclose a visible-light-induced synthesis of O-aryl esters through the cross-dehydrogenative coupling of aldehydes with phenols using BrCCl3, in which phenolate functions as both a substrate and a photosensitizer. This transition-metal- and photocatalyst-free visible-light-induced esterification is suitable for a wide range of substrates and gives moderate to excellent yields (up to 95%). Mechanistic studies provided evidence of a self-propagating radical reaction involving homolytic cleavage of the aldehydic C-H bond and the formation of acyl bromides. BrCCl3 serves as an oxidant and a hydrogen atom transfer (HAT) agent.

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.

Synthesis and Biological Properties of Ferrocenyl and Organic Methotrexate Derivatives

Synthesis and biological activity of two series of modified side chain methotrexate (MTX) derivatives are presented, one with a ferrocenyl moiety inserted between the pteroyl and glutamate portions of the molecule and the other with glutamate substituted for short chain amino acids. Ferrocenyl derivatives of MTX turned out to be rather moderate inhibitors of dihydrofolate reductase (DHFR) although molecular modeling suggested more effective interactions between these compounds and the target enzyme. More interestingly, ferrocene-decorated MTX derivatives were able to impede the proliferation of four murine and human cell lines as well as their methotrexate-resistant counterparts, overcoming the multidrug resistance (MDR) barrier. They were also able to directly interact with Abcc1, an MDR protein. Of the amino acid pteroyl conjugates, the γ-aminobutyric acid derivative was an efficient inhibitor of DHFR but had no effect on cell proliferation in the concentration range studied while a taurine conjugate was a poor DHFR inhibitor but able to affect cell viability. We postulate that modification of the methotrexate side chain may be an efficient strategy to overcome efflux-dependent methotrexate resistance.

Catalytic asymmetric synthesis of planar-chiral dianthranilides via (Dynamic) kinetic resolution

Chirality constitutes an inherent attribute of nature. The catalytic asymmetric synthesis of molecules with central, axial, and helical chirality is a topic of intense interest and is becoming a mature field of research. However, due to the difficulty in synthesis and the lack of a prototype, less attention has been given to planar chirality arising from the destruction of symmetry on a single planar ring. Herein, we report the catalytic asymmetric synthesis of planar-chiral dianthranilides, a unique class of tub-shaped eight-membered cyclic dilactams. This protocol is enabled by cinchona alkaloid-catalyzed (dynamic) kinetic resolution. Under mild conditions, various C2- or C1-symmetric planar-chiral dianthranilides have been readily prepared in high yields with excellent enantioselectivity. These dianthranilides can serve as an addition to the family of planar-chiral molecules. Its synthetic value has been demonstrated by kinetic resolution of racemic amines via acyl transfer, enantiodivergent synthesis of the natural product eupolyphagin, and preliminary antitumor activity studies.

Synthesis of Planar Chiral Compounds Containing α-Amino Phosphonates for Antiplant Virus Applications against Potato Virus Y

An unprecedented study of the application of planar chiral compounds in antiviral pesticide development is reported. A class of multifunctional planar chiral ferrocene derivatives bearing α-amino phosphonate moieties was synthesized. These compounds, exhibiting superior optical purities, were subsequently subjected to antiviral evaluations against the notable plant pathogen potato virus Y (PVY). The influence of the absolute configurations of the planar chiral compounds on their antiviral bioactivities was significant. A number of these enantiomerically enriched planar chiral molecules demonstrated superior anti-PVY activities. Specifically, compound (Sp, R)-9n displayed extraordinary curative activities against PVY, with a 50% maximal effective concentration (EC50) of 216.11 μg/mL, surpassing the efficacy of ningnanmycin (NNM, 272.74 μg/mL). The protective activities of compound (Sp, R)-9n had an EC50 value of 152.78 μg/mL, which was better than that of NNM (413.22 μg/mL). The molecular docking and defense enzyme activity tests were carried out using the planar chiral molecules bearing different absolute configurations to investigate the mechanism of their antiviral activities against PVY. (Sp, R)-9n, (Sp, R)-9o, and NMM all showed stronger affinities to the PVY-CP than the (Rp, S)-9n. Investigations into the mechanisms revealed that the planar chiral configurations of the compounds played pivotal roles in the interactions between the PVY-CP molecules and could augment the activities of the defense enzymes. This study contributes substantial insights into the role of planar chirality in defending plants against viral infections.

Carbene-Catalyzed Atroposelective Construction of Chiral Diaryl Ethers

Atropoisomeric chemotypes of diaryl ethers-related scaffolds are prevalent in naturally active compounds. Nevertheless, there remains considerable research to be carried out on the catalytic asymmetric synthesis of these axially chiral molecules. In this instance, we disclose an N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. NHC desymmetrization produces axially chiral diaryl ether atropisomers with high yields and enantioselectivities in moderate circumstances. Chiral diaryl ether compounds may be precursors for highly functionalized diaryl ethers with bioactivity and chiral ligands for asymmetric catalysis.

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.

Organocatalytic desymmetrization provides access to planar chiral [2.2]paracyclophanes

Planar chiral [2.2]paracyclophanes consist of two functionalized benzene rings connected by two ethylene bridges. These organic compounds have a wide range of applications in asymmetric synthesis, as both ligands and catalysts, and in materials science, as polymers, energy materials and dyes. However, these molecules can only be accessed by enantiomer separation via (a) time-consuming chiral separations and (b) kinetic resolution approaches, often with a limited substrate scope, yielding both enantiomers. Here, we report a simple, efficient, metal-free protocol for organocatalytic desymmetrization of prochiral diformyl[2.2]paracyclophanes. Our detailed experimental mechanistic study highlights differences in the origin of enantiocontrol of pseudo-para and pseudo-gem diformyl derivatives in NHC catalyzed desymmetrizations based on whether a key Breslow intermediate is irreversibly or reversibly formed in this process. This gram-scale reaction enables a wide range of follow-up derivatizations of carbonyl groups, producing various enantiomerically pure planar chiral [2.2]paracyclophane derivatives, thereby underscoring the potential of this method.

Synthesis of axially chiral diaryl ethers via NHC-catalyzed atroposelective esterification

Axially chiral diaryl ethers bearing two potential axes find unique applications in bioactive molecules and catalysis. However, only very few catalytic methods have been developed to construct structurally diverse diaryl ethers. We herein describe an NHC-catalyzed atroposelective esterification of prochiral dialdehydes, leading to the construction of enantioenriched axially chiral diaryl ethers. Mechanistic studies indicate that the matched kinetic resolutions play an essential role in the challenging chiral induction of flexible dual-axial chirality by removing minor enantiomers via over-functionalization. This protocol features mild conditions, excellent enantioselectivity, broad substrate scope, and applicability to late-stage functionalization, and provides a modular platform for the synthesis of axially chiral diaryl ethers and their derivatives.

N-Heterocyclic carbene-catalyzed enantioselective synthesis of planar-chiral cyclophanes via dynamic kinetic resolution

Planar-chiral cyclophanes have gained considerable concerns for drug discovery due to their unique conformational strain and 3D structure. However, the enantioselective synthesis of planar-chiral cyclophanes is a long-standing challenge for the synthetic community. We herein describe an N-heterocyclic carbene (NHC)-catalyzed asymmetric construction of planar-chiral cyclophanes. This transformation occurs through a dynamic kinetic resolution (DKR) process to convert racemic substrates into planar-chiral macrocycle scaffolds in good to high yields with high to excellent enantioselectivities. The ansa chain length and aromatic ring substituent size is crucial to achieve the DKR approach. Controlled experiments and DFT calculations were performed to clarify the DKR process.

Atroposelective Synthesis of Axially Chiral Diaryl Ethers by N-Heterocyclic-Carbene-Catalyzed Sequentially Desymmetric/Kinetic Resolution Process

We describe herein an N-heterocyclic-carbene-catalyzed atroposelective synthesis of axially chiral diaryl ethers. Through a sequentially enantioselective desymmetric process and a kinetic resolution process, the products could be constructed in good yields with excellent enantiopurities. Both alcohols and phenols were compatible with this catalytic system. The axially chiral carboxylic acids derived from the esters were proven to be potential chiral ligands for asymmetric synthesis, for example, Rh(III)-catalyzed enantioselective C-H functionalization.

N-Heterocyclic Carbene-Catalyzed Highly Enantioselective Macrolactonization to Access Planar-Chiral Macrocycles

An N-heterocyclic carbene (NHC)-catalyzed atroposelective macrolactonization has been disclosed. This approach affords planar-chiral macrocycles in high yields with excellent enantioselectivities over a broad substrate scope. Controlled experiments suggest that the enantioselectivity might arise from the cation-n interaction between the acyl azolium and the electron-rich moiety in the substrate. This mechanism is supported by density functional theory calculations, which also suggest an important π-π interaction in stabilizing the transition state.

Enantioselective Synthesis of C-O Axially Chiral Diaryl Ethers by NHC-Catalyzed Atroposelective Desymmetrization

Axially chiral diaryl ethers, a distinguished class of atropisomers possessing unique dual C-O axis, hold immense potential for diverse research domains. In contrast to the catalytic enantioselective synthesis of conventional single axis bearing atropisomers, the atroposelective synthesis of axially chiral ethers containing flexible C-O axis remains a significant challenge. Herein, we demonstrate the first N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. Mechanistically, the reaction proceeds via NHC-catalyzed desymmetrization strategy to afford the corresponding axially chiral diaryl ether atropisomers in good yields and high enantioselectivities under mild conditions. The derivatization of the synthesized product expands the utility of present strategy via access to a library of C-O axially chiral compounds. The temperature dependency and preliminary investigations on the racemization barrier of C-O bonds are also presented.

Facile access to benzofuran derivatives through radical reactions with heteroatom-centered super-electron-donors

The development of suitable electron donors is critical to single-electron-transfer (SET) processes. The use of heteroatom-centered anions as super-electron-donors (SEDs) for direct SET reactions has rarely been studied. Here we show that heteroatom anions can be applied as SEDs to initiate radical reactions for facile synthesis of 3-substituted benzofurans. Phosphines, thiols and anilines bearing different substitution patterns work well in this inter-molecular radical coupling reaction and the 3-functionalized benzofuran products bearing heteroatomic functionalities are given in moderate to excellent yields. The reaction mechanism is elucidated via control experiments and computational methods. The afforded products show promising applications in both organic synthesis and pesticide development.

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.