Cinchona Alkaloids as Organocatalysts in Enantioselective Halofunctionalization of Alkenes and Alkynes

Organocatalytic Enantioselective Chloroiminocyclization for the Synthesis of Imidazoline

Imidazoline is an important scaffold in organic synthesis and a pharmacophore in medicinal chemistry. We apply basic imines as nucleophiles for the catalytic asymmetric chloroiminocyclization to furnish tetrasubstituted stereocenter-containing imidazolines in excellent yields and enantioselectivities. The reaction can be conducted in the polar solvent acetonitrile under concentrated reaction conditions.

Controlling the regioselectivity of the bromolactonization reaction in HFIP

The halolactonization reaction provides rapid access to densely functionalized lactones from unsaturated carboxylic acids. The endo/exo regioselectivity of this cyclization reaction is primarily determined by the electronic stabilization of alkene substituents, thus making it inherently dependent on substrate structures. Therefore this method often affords one type of halolactone regioisomer only. Herein, we introduce a simple and efficient method for regioselectivity-switchable bromolactonization reactions mediated by HFIP solvent. Two sets of reaction conditions were developed, each forming endo-products or exo-products in excellent regioselectivity. A combination of computational and experimental mechanistic studies not only confirmed the crucial role of HFIP, but also revealed the formation of endo-products under kinetic control and exo-products under thermodynamic control. This study paves the way for future work on the use of perfluorinated solvents to dictate reaction outcomes in organic synthesis.

Regioselective and Diastereoselective Halofunctionalization of Alkenes Promoted by Organophotocatalytic Solar Catalysis

A visible-light metal-free photocatalytic regioselective and enantioselective alkene halofunctionalization reaction under mild conditions is reported. Various terminal and internal alkenes were transformed to their α-halogenated and α,β-dibrominated derivatives in good to excellent yields within reaction time as short as 5 min. Water can be used as the "green" nucleophile and solvent in the halohydroxylation and halo-oxidation reactions. Different types of products can be obtained by adjusting the reaction conditions. In addition, sunlight is proved to produce products with similar yields, representing a practical example of solar synthesis and providing an opportunity for solar energy utilization.

Catalytic asymmetric CO2 utilization reaction for the enantioselective synthesis of chiral 2-oxazolidinones

Catalytic asymmetric bromocyclizations of in situ generated carbamic acids from CO₂ and allylamines were achieved via the use of a BINOL-derived chiral bifunctional selenide catalyst bearing a hydroxy group. Chiral 2-oxazolidinone products as important pharmaceutical building blocks were obtained with good enantioselectivities by the present catalytic asymmetric CO₂ utilization reactions.

Enantioselective anti-Dihalogenation of Electron-Deficient Olefin: A Triplet Halo-Radical Pylon Intermediate

The textbook alkene halogenation reaction establishes straightforward access to vicinal dihaloalkanes. However, a robust catalytic method for dihalogenizing electron-deficient olefins in an enantioselective manner is still under development, and its mechanism remains controversial. Herein, we disclose efficient regio-, anti-diastereo-, and enantioselective dibromination, bromochlorination, and dichlorination reactions of enones catalyzed by a chiral N,N'-dioxide/Yb(OTf)₃ complex. With the combination of electrophilic halogen and halide salts as halogenating agents, an array of homo- and heterodihalogenated derivatives is achieved in moderate to good enantioselectivities. Moreover, DFT calculations reveal that a novel triplet halo-radical pylon intermediate is probable in accounting for the exclusive regio- and anti-diastereoselectivity.

Two Green Protocols for Halogenative Semipinacol Rearrangement

Semipinacol rearrangement is a special type of Wagner-Meerwein rearrangement that involves carbocation 1,2-rearrangement to provide carbonyl compounds with an α-quaternary carbon center. It has been strategically used for natural product synthesis and construction of highly congested quaternary carbons. Herein, we report a safe and green protocol that uses oxone/halide and Fenton bromide to achieve halogenative semipinacol rearrangement. The key feature of this method is the green in situ generation of reactive halogenating species from oxidation of halide with oxone or H₂O₂, which produces a nontoxic byproduct (potassium sulfate or water). Easy operation (insensitive to air and moisture) at room temperature without using special equipment adds additional advantage over previous methods.

Catalytic Efficiency of Primary α-Amino Amides as Multifunctional Organocatalysts in Recent Asymmetric Organic Transformations

Chiral primary α-amino amides, consisting of an adjacent enamine bonding site (Bronsted base site), a hydrogen bonding site (Bronsted acid site), and flexible bulky substituent groups to modify the steric factor, are proving to be extremely valuable bifunctional organocatalysts for a wide range of asymmetric organic transformations. Primary α-amino amides are less expensive alternatives to other primary amino organocatalysts, such as chiral diamines and cinchona-alkaloid-derived primary amines, as they are easy to synthesize, air-stable, and allow for the incorporation of a variety of functional groups. In recent years, we have demonstrated the catalytic use of simple primary α-amino amides and their derivatives as organocatalysts for the aldol reaction, Strecker reaction, Michael tandem reaction, allylation of aldehydes, reduction of N-Aryl mines, opening of epoxides, hydrosilylation, asymmetric hydrogen transfer, and N-specific nitrosobenzene reaction with aldehydes.

Indane-Based Chiral Aryl Chalcogenide Catalysts: Development and Applications in Asymmetric Electrophilic Reactions

Asymmetric electrophilic reactions provide an ideal method for the construction of chiral molecules by incorporating one or more functional groups into the parent substrates under mild conditions. However, due to the issues of the reactivities of electrophilic species and the possible racemization of chiral intermediates as well as the restriction of the chiral scaffolds of chiral catalysts, many limitations remain in this field, such as the narrow scopes of substrates and electrophiles as well as the limited types of nucleophiles and reactions. To overcome the limitations in the synthesis of diversified chiral molecules, we developed a series of indane-based chiral amino aryl chalcogenide catalysts. These catalysts are easily prepared based on the privileged chiral indane scaffold. They can provide an appropriate H-bonding effect by varying the amino protecting groups as well as offer a proper Lewis basicity and steric hindrance by adjusting different substituents on the aryl chalcogenide motifs. These features allow for them to meet the requirements of reactivity and the chiral environment of the reactions. Notably, they have been successfully applied to various asymmetric electrophilic reactions of alkenes, alkynes, and arenes, expanding the field of electrophilic reactions.Using these catalysts, we realized the enantioselective CF₃S-lactonization of olefinic carboxylic acids, enantioselective CF₃S-aminocyclization of olefinic sulfonamides, desymmetrizing enantioselective CF₃S-carbocyclization of gem-diaryl-tethered alkenes, enantioselective CF₃S-oxycyclization of N-allylamides, enantioselective intermolecular trifluoromethylthiolating difunctionalization and allylic C-H trifluoromethylthiolation of trisubstituted alkenes, formally the intermolecular CF₃S-oxyfunctionalization of aliphatic internal alkenes, intermolecular azidothiolation, oxythiolation, thioarylation of N-allyl sulfonamides, desymmetrizing enantioselective chlorocarbocyclization of aryl-tethered diolefins, enantioselective Friedel-Crafts-type electrophilic chlorination of N-allyl anilides, and enantioselective chlorocarbocyclization and dearomatization of N-allyl 1-naphthanilides. Additionally, the enantioselective electrophilic carbothiolation of alkynes to construct enantiopure carbon chirality center-containing molecules and axially chiral amino sulfide vinyl arenes and the electrophilic aromatic halogenation to produce P-chirogenic compounds can be accomplished. In these reactions, a bifunctional binding mode is proposed in the catalytic cycles, in which an acid-derived anion-binding interaction might exist and account for the high enantioselectivities of the reactions.In this Account, we demonstrate our achievements in asymmetric electrophilic reactions and share our thoughts on catalyst design, our understanding of asymmetric electrophilic reactions, and our perspectives in the field of chiral chalcogenide-catalyzed asymmetric electrophilic reactions. We hope that the experience we share will promote the design and development of other novel organocatalysts and new challenging reactions.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

Accessing Chiral 2,2-Disubstituted Morpholines via Organocatalytic Enantioselective Chlorocycloetherification

Chiral morpholine is an important scaffold in organic synthesis and a pharmacophore in medicinal chemistry. However, catalytic enantioselective procedure for the construction of morpholine remains sparse. We report herein a...

Anionic Chiral Co(III) Complexes Mediated Asymmetric Halocyclization─Synthesis of 5-Halomethyl Pyrazolines and Isoxazolines

An asymmetric synthesis of 5-halomethyl pyrazolines and isoxazolines which bear a tertiary stereocenter by catalytic halocyclization of β,γ-unsaturated hydrazones and ketoximes is described. By using Brønsted acids of anionic chiral Co(III) complexes as catalysts, a variety of chiral 5-halomethyl pyrazolines and isoxazolines were obtained in good yields with high enantioselectivities (up to 99% yield, 97:3 er). Preliminary bioassay results indicated that several isoxazoline derivatives exhibited significant antifungal activities.

Asymmetric Bromoaminocyclization and Desymmetrization of Cyclohexa-1,4-dienes through Anion Phase-Transfer Catalysis

The catalytic enantioselective desymmetrizing bromoaminocyclization of prochiral cyclohexa-1,4-dienes has been achieved by using chiral anion phase-transfer catalysis, providing a range of enantioenriched cis-3a-arylhydroindoles bearing an all-carbon quaternary stereocenter in good yields (up to 78%) and excellent enantioselectivities (up to 97% ee). Furthermore, the potential application of this methodology to natural product total synthesis was demonstrated by the asymmetric synthesis of (+)-Mesembrane.

Kinetic Resolution of Sulfinamides via Asymmetric N-Allylic Alkylation

An efficient kinetic resolution of sulfinamides via an asymmetric N-allylic alkylation reaction was realized using hydroquinine as a catalyst under mild conditions. The kinetic resolution of a range of Morita-Baylis-Hillman adducts and N-aryl tert-butylsulfinamides was highly effective. In addition, the synthetic utility of the protocol was demonstrated by a scaled-up reaction. Density functional theory calculations provide convincing evidence for the interpretation of stereoselection.

Efficient asymmetric syntheses of α-quaternary lactones and esters through chiral bifunctional sulfide-catalyzed desymmetrizing bromolactonization of α,α-diallyl carboxylic acids

Asymmetric halolactonizations are powerful methods for the syntheses of chiral lactones. Catalytic and highly enantioselective halolactonizations of α-allyl carboxylic acids, however, continue to present a formidable challenge. Herein, we report the chiral bifunctional sulfide-catalyzed desymmetrizing bromolactonizations of α,α-diallyl carboxylic acids. These reactions efficiently produced chiral α-quaternary lactones and esters.

Asymmetric Halocyclizations of 2-Vinylbenzyl Alcohols with Chiral FLPs

By the use of a chiral frustrated Lewis pair (FLP) consisting of a chiral-diene-derived borane and ^tBu₃P as the catalyst, an asymmetric halocyclization of 2-vinylbenzyl alcohols with NBS or NIS was successfully realized. A variety of optically active 1,3-dihydroisobenofuran derivatives were obtained in high yields with up to 87% ee and could be conveniently converted to other useful chiral compounds.

Catalytic Enantioselective Halocyclizations to Access Benzoxazepinones and Benzoxazecinones

We report a catalytic asymmetric halocyclization protocol to furnish benzoxazepinones and benzoxazecinones using (DHQ)₂PHAL as the catalyst. Various halogenated benzoxazepinones and benzoxazecinones were achieved in excellent yields and enantioselectivities under mild conditions. A cocrystal structure of the substrate and the catalyst was studied.

A Catalyst-Controlled Enantiodivergent Bromolactonization

A catalyst-controlled enantiodivergent bromolactonization of olefinic acids has been developed. Quinine-derived amino-amides bearing the same chiral core but different achiral aryl substituents were used as the catalysts. Switching the methoxy substituent in the aryl amide system from meta- to ortho-position results in a complete switch in asymmetric induction to afford the desired lactone in good enantioselectivity and yield. Mechanistic studies, including chemical experiments and density functional theory calculations, reveal that the differences in steric and electronic effects of the catalyst substituent alter the reaction mechanism.

A dynamic picture of the halolactonization reaction through a combination of ab initio metadynamics and experimental investigations

The halolactonization reaction is one of the most common electrophilic addition reactions to alkenes. The mechanism is generally viewed as a two-step pathway, which involves the formation of an ionic intermediate, in most cases a haliranium ion. Recently, an alternative concerted mechanism was proposed, in which the nucleophile of the reaction played a key role in the rate determining step by forming a pre-polarized complex with the alkene. This pathway was coined the nucleophile-assisted alkene activation (NAAA) mechanism. Metadynamics simulations on a series of model halolactonization reactions were used to obtain the full dynamic trajectory from reactant to product and investigate the explicit role of the halogen source and solvent molecules in the mechanism. The results in this work ratify the occasional preference of a concerted mechanism over the classic two-step transformation under specific reaction conditions. Nevertheless, as the stability of both the generated substrate cation and counter-anion increase, a transition towards the classic two-step mechanism was observed. NCI analyses on the transition states revealed that the activating role of the nucleophile is independent of the formation and stability of the intermediate. Additionally, the dynamic insights obtained from the metadynamics simulations and NCI analyses employed in this work, unveiled the presence of syn-directing noncovalent interactions, such as hydrogen bonding, between the alkenoic acid and the halogen source, which rationalized the experimentally observed diastereoselectivities. Explicit noncovalent interactions between the reactants and a protic solvent or basic additive are able to disrupt these syn-directing noncovalent interactions, affecting the diastereoselective outcome of the reaction.

The organocatalytic enantiodivergent fluorination of β-ketodiaryl-phosphine oxides for the construction of carbon-fluorine quaternary stereocenters

Commercially available cinchona alkaloids that can catalyze the enantiodivergent fluorination of β-ketodiarylphosphine oxides were developed to construct carbon-fluorine quaternary stereocenters. This protocol features a wide scope of substrates and excellent enantioselectivities, and it is scalable.

Catalytic enantioselective bromohydroxylation of cinnamyl alcohols

This work describes an effective enantioselective bromohydroxylation of cinnamyl alcohols with (DHQD)₂PHAL as the catalyst and H₂O as the nucleophile, providing a variety of corresponding optically active bromohydrins with up to 95% ee.

Optically active bromohydrins are obtained with up to 95% ee via asymmetric bromohydroxylation of cinnamyl alcohols with H₂O as nucleophile.

Halogen-Induced Controllable Cyclizations as Diverse Heterocycle Synthetic Strategy

In organic synthesis, due to their high electrophilicity and leaving group properties, halogens play pivotal roles in the activation and structural derivations of organic compounds. Recently, cyclizations induced by halogen groups that allow the production of diverse targets and the structural reorganization of organic molecules have attracted significant attention from synthetic chemists. Electrophilic halogen atoms activate unsaturated and saturated hydrocarbon moieties by generating halonium intermediates, followed by the attack of carbon-containing, nitrogen-containing, oxygen-containing, and sulfur-containing nucleophiles to give highly functionalized carbocycles and heterocycles. New transformations of halogenated organic molecules that can control the formation and stereoselectivity of the products, according to the difference in the size and number of halogen atoms, have recently been discovered. These unique cyclizations may possibly be used as efficient synthetic strategies with future advances. In this review, innovative reactions controlled by halogen groups are discussed as a new concept in the field of organic synthesis.

Chiral bifunctional sulfide-catalyzed asymmetric bromoaminocyclizations

A BINOL-derived chiral bifunctional sulfide catalyst bearing a phenylurea moiety was applied to enantioselective bromoaminocyclization reactions of 2-allylaniline derivatives, which provide optically active 2-substituted indoline products as important motifs for biologically active compounds. A protecting group on the nitrogen of the 2-allylaniline substrate was carefully optimized, and highly enantioselective reactions were achieved by employing the p-biphenylsulfonyl-protected substrates. The origin of the good level of enantioselectivity for the present bromoaminocyclization was also investigated on the basis of DFT calculations. The resultant optically active 2-(bromomethyl)indoline products could be transformed to various 2-substituted indolines with no loss of the optical purity.

Enantioselective Syntheses of 4H-3,1-Benzoxazines via Catalytic Asymmetric Chlorocyclization of o-Vinylanilides

The catalytic asymmetric halocyclization of alkene is a powerful and straightforward strategy for the synthesis of chiral heterocyclic compounds. Herein, an effective approach to chiral benzoxazine derivatives through organocatalyzed chlorocyclization of o-vinylanilides was reported. This method provides facile access to a series of chiral benzoxazines in good to excellent yields (up to 99% yield) and with high-level enantiocontrol (up to 92% ee).

Synthesis and cytotoxic evaluation of halogenated α-exo-methylene-lactones

α-exo-Methylene-γ-butyrolactones and α-exo-methylene-δ-valerolactones constitute an important group of natural and bioactive products. A simple and general protocol of halolactonization of dienoic acids to obtain various α-exo-methylene-lactones in excellent yields is described. The resulting halogenated α-exo-methylene-lactones were found to exhibit potent cytotoxic activities.

Regioselective Chlorolactonization of Styrene-Type Carboxylic Esters and Amides via PhICl2-Mediated Oxidative C-O/C-Cl Bond Formations

A facile method employing styrene-type carboxylic esters or amides in the presence of PhICl₂ in CH₃CN was developed to achieve the synthesis of 6-endo products 3,4-dihydroisocoumarins or 3,4-dihydroisocoumarin-1-imines in good to high yields. This metal-free regioselective intramolecular chlorolactonization process was proposed to involve a PhICl₂-mediated oxidative C-O bond formation followed by C-Cl bond formation.

Enantioselective Halolactonizations Using Amino-Acid-Derived Phthalazine Catalysts

Amino-acid-derived phthalazine catalysts have been designed and synthesized for enantioselective halolactonization of prochiral dienoic acids. The scope of the reaction is evidenced by 17 examples of spiro α-exo-methylene-halolactones with up to 99.8% enantiomeric excess. The resulting enantio-enriched spiro halolactone products are found to exhibit potent antitumor effects. In addition, both antipodes of products with equally excellent enantioselevity could be obtained since a pair of enantiomeric catalysts is guaranteed.

Organocatalytic, Enantioselective Dichlorination of Unfunctionalized Alkenes

The use of a new class of unsymmetrical cinchona-alkaloid-based, phthalazine-bridged organocatalysts enabled the highly enantioselective dichlorination of unfunctionalized alkenes. In combination with the electrophilic chlorinating agent 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) and triethylsilyl chloride (TES-Cl) as the source of nucleophilic chloride, 1-aryl-2-alkyl alkenes were dichlorinated with enantioselectivities of up to 94:6 er. Initial mechanistic investigations suggest that no free chlorine is formed, and by replacement of the chloride by fluoride, enantioselective chlorofluorinations of alkenes are possible.

Chiral Sulfide Catalysis for Desymmetrizing Enantioselective Chlorination

An unprecendented chiral sulfide catalyzed desymmetrizing enantioselective chlorination is disclosed. Various aryl-tethered diolefins and diaryl-tethered olefins afforded teralins and tricyclic hexahydrophenalene derivatives, respectively, bearing multiple stereogenic centers in high yields with excellent enantio- and diastereoselectivities. In contrast, the tertiary amine catalyst (DHQD)₂ PHAL led to a diastereomeric product. The products could be transformed into a variety of compounds, such as spiro-N-heterocycles.