Recent advances in asymmetric intra- and intermolecular halofunctionalizations of alkenes

Catalytic Asymmetric Aza-Electrophilic Additions of 1,1-Disubstituted Styrenes

Electrophilic addition of alkenes is a textbook reaction that plays a pivotal role in organic chemistry. In the past decades, catalytic asymmetric variants of this important type of reaction have witnessed great achievements by the development of novel catalytic systems. However, enantioselective aza-electrophilic additions of unactivated alkenes, which could provide a transformative strategy for the preparation of synthetically significant nitrogen-containing compounds, still remain a formidable challenge. Herein, we have developed unprecedented Au(I)/NHC-catalyzed asymmetric aza-electrophilic additions of unactivated 1,1-disubstituted styrenes by the utilization of readily available dialkyl azodicarboxylates as electrophilic nitrogen sources. Based on this approach, a series of transformations, including [2 + 2] cycloaddition, intermolecular 1,2-oxyamination, and several types of intramolecular hydrazination-induced cyclizations, have been realized. These transformations provide a previously unattainable platform for the divergent synthesis of hydrazine derivatives, which could also be converted to other nitrogen-containing chiral synthons. Experimental and computational studies support the idea that carbocation intermediates are involved in reaction pathways.

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.

Kinetic Resolution of Electron-Deficient Bromohydrins via Copper(II)-Catalyzed C-C Bond Cleavage

Herein, we report a nonenzymatic kinetic resolution (KR) of α,β-unsaturated ketone-derived bromohydrins (up to s = 211) with N-bromosuccinimide (NBS) in the presence of a chiral Cu(II)-Box catalyst via the C-C bond cleavage of the fast reacting enantiomer. A one-pot synthesis-KR approach of the same has also been realized with excellent enantioselectivities (up to 99% ee). Both protocols are found to be effective for a variety of substrates, leading to enantioenriched bromohydrins. The synthetic utility of this process has been demonstrated by exploring a new strategy to convert the resolved enantiomer to an optically active epoxide.

Surmounting Byproduct Inhibition in an Intermolecular Catalytic Asymmetric Alkene Bromoesterification Reaction as Revealed by Kinetic Profiling

Kinetic profiling has shown that a (DHQD)₂PHAL-catalyzed intermolecular asymmetric alkene bromoesterification reaction is inhibited by primary amides, imides, hydantoins, and secondary cyclic amides, which are byproducts of common stoichiometric bromenium ion sources. Two approaches to resolving the inhibition are presented, enabling the (DHQD)₂PHAL loading to be dropped from 10 to 1 mol % while maintaining high bromoester conversions in 8 h or less. Iterative post-reaction recrystallizations enabled a homochiral bromonaphthoate ester to be synthesized using only 1 mol % (DHQD)₂PHAL.

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.

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.

Structure-Enantioselectivity Relationship (SER) Study of Cinchona Alkaloid Chlorocyclization Catalysts

Various structural elements of the Cinchona alkaloid dimers are interrogated to establish a structure-enantioselectivity relationship (SER) in three different halocyclization reactions. SER for chlorocyclizations of a 1,1-disubstituted alkenoic acid, a 1,1-disubstituted alkeneamide, and a trans-1,2-disubstituted alkeneamide showed variable sensitivities to linker rigidity and polarity, aspects of the alkaloid structure, and the presence of two or only one alkaloid side group defining the catalyst pocket. The conformational rigidity of the linker-ether connections was probed via DFT calculations on the methoxylated models, uncovering especially high barriers to ether rotation out of plane in the arene systems that include the pyridazine ring. These linkers are also found in the catalysts with the highest enantioinduction. The diversity of the SER results suggested that the three apparently analogous test reactions may proceed by significantly different mechanisms. Based on these findings, a stripped-down analogue of (DHQD)2PYDZ, termed "(trunc)2PYDZ", was designed, synthesized, and evaluated, showing modest but considerable asymmetric induction in the three test reactions, with the best performance on the 1,1-disubstituted alkeneamide cyclization. This first effort to map out the factors essential to effective stereocontrol and reaction promotion offers guidance for the simplified design and systematic refinement of new, selective organocatalysts.

Catalytic Site-, Diastereo-, and Enantioselective Cascade Iodocyclization of 2-Geranylarenols

A chiral amidophosphate-N-iodosuccinimide cooperative catalysis has been developed for the site-, diastereo-, and enantioselective iodocyclization of 2-geranylarenols with molecular iodine to give the corresponding iodo-containing polycyclic compounds with good levels of selectivity. This is the first example of a catalytic enantioselective iodocarbocyclization. A reactive chiral iodonium species is generated from molecular iodine via the dual halogen-bonding interactions with a chiral Lewis base and Lewis acid. The sterically demanding 3,3'-substituents of the chiral BINOL-derived amidophosphate are critical to induce the site-selective iodination at the less-hindered terminal alkenyl moiety of 2-geranylarenols.

Asymmetric Synthesis of α-Spiro-γ-lactones and α-Substituted γ-Lactones via Chiral Bifunctional Sulfide-Catalyzed Bromolactonizations

An efficient enantioselective synthesis of γ-chiral α-spiro-γ-lactones, which are important building blocks for pharmaceuticals, was achieved via BINOL-derived chiral bifunctional sulfide-catalyzed bromolactonizations of α-allyl carboxylic acids containing either hetero- or carbocyclic structures. Transformations of the resultant α-spiro-type bromolactonization product were examined to obtain optically active γ-functionalized α-spiro-γ-lactones. The utility of this catalytic system was also demonstrated in the asymmetric synthesis of α,α-diaryl- and dialkyl-substituted γ-lactones.

Strategies toward the Difunctionalizations of Enamide Derivatives for Synthesizing α,β-Substituted Amines

Enamide and enecarbamate derivatives containing a nucleophilic center at the β-position from their nitrogen atom as well as a latent electrophilic site at their α-position are interesting motifs in organic chemistry. This dual reactivity─analogous that of the enamines─enables difunctionalization and increased structural complexity. Furthermore, an electron-withdrawing group on nitrogen drastically increases their stability. In that respect, enamides and enecarbamates are excellent partners for multicomponent transformations, and our research primarily focuses on these compounds in particular.Difunctionalization generally occurs through the nucleophilic addition of the enecarbamate on an electrophile to form iminium, which can subsequently react with a nucleophilic species. Although potent, such an approach is highly challenging due to the low stability of the intermediate iminium, leading to undesired hydrolysis or oligomerization. Epimerization, competitivity, and compatibility issues between the reaction partners are additional hindrances to developing these methodologies. To overcome these limitations, we described many complementary strategies.To control the enantioselectivity of these transformations, chiral phosphoric acids were found to be particularly well-suited to activate multiple reactants due to the formation of a hydrogen bonds network, allowing for an organized transition state in a chiral pocket. Interestingly, when deprotonated as phosphates, they can also play the role of ligands for Lewis acidic metals.To avoid iminium oligomerization, we successfully used stabilized α-arylated enamides. However, this approach was restricted to a simple nucleophilic addition at the β-position. To achieve the difunctionalizations of α-unsubstituted derivatives, we explored reversibly linked nucleophile and electrophile to address their compatibility problem. Alternatively, we devised a sequential methodology for resolving the stability issue of the N-acyl iminium based on its intermediate trapping using a temporary nucleophile (alcohol or thiol). Interestingly, the trapping agent could further be displaced by the desired final α-substituent under Lewis acidic or photocatalytic activation. This led us to design new chiral and bifunctional phosphoric acid catalysts bearing chromophores to merge asymmetric organocatalysis and photochemistry.These photocatalysis studies incited us to focus on radical processes to manage original functionalizations that would not be feasible otherwise. β-Alkylation and β-trifluoromethylation of enecarbamates via visible-light-promoted atom transfer radical additions were successfully performed. As β-allylations remained unattainable with the precedent methods, we eventually turned our attention to cerium(IV)-mediated oxidative single electron transfers. It allowed for singly occupied molecular orbital activation of these substrates to elicit their umpolung reactivity.Thus, the functionalization of enecarbamate derivatives appears as a valid synthetic strategy for obtaining important building blocks for agrochemical, pharmaceutical, and cosmetic industries, including diamines, haloamines, aminotryptamines, and less accessible trifluoromethylated or allylic compounds.

Contra-thermodynamic halolactonization of lactam-tethered 5-aryl-4(E)-pentenoic acids for the flexible and stereocontrolled synthesis of fused lactam-halolactones

Halolactonization of alkenoic acids enables the construction of oxygen-heterocycles via intramolecular halonium-induced nucleophilic addition. Although the literature is currently inundated with halolactonizations of 5-aryl-4(E)-pentenoic acids that predictably afford the 6-endo cyclization adducts, methods that reliably alter the innate regioselectivity bias to instead deliver the thermodynamically less favored 5-exo cyclization products are relatively rare. Here, we attempt to bridge this gap and have found mild conditions for contra-thermodynamic halolactonization of lactam-tethered 5-aryl-4(E)-pentenoic acids that lead to the formation of trans-fused lactam-γ-lactones. The natural proclivity for these 5-aryl-4(E)-pentenoic acids to undergo 6-endo cyclization is overridden and 5-exo-trig cyclization predominates. The success of the approach hinges on the use of N,N-dimethylformamide (DMF) as the solvent and N-methylmorpholine oxide as the catalyst. The lactam-lactone products are synthesized in high diastereoselectivity, modularity, and chemoselectivity. Notably, most of the bicycles contain one benzylic quaternary stereocenter as well as an α-alkoxy quaternary stereocenter.

The contra-thermodynamic halolactonization of lactam-tethered 5-aryl-4(E)-pentenoic acids, under solvent- and catalyst-controlled conditions, has facilitated the efficient and stereocontrolled synthesis of halogenated fused γ-lactone-lactams.

Chiral Chalcogenide-Catalyzed Enantioselective Electrophilic Hydrothiolation of Alkenes

A new strategy for the construction of chiral sulfides by catalytic enantioselective hydrothiolation of alkenes via an electrophilic pathway has been developed. Using this strategy, cyclic and acyclic unactivated alkenes efficiently afforded various chiral products in the presence of electrophilic sulfur reagents and silanes through chiral chalcogenide catalysis. The obtained products were easily transformed into other types of valuable chiral sulfur-containing compounds. Mechanistic studies revealed that the superior construction of chiral thiiranium ion intermediate is the key to achieving such a transformation.

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.

Asymmetric Intermolecular Iodinative Difunctionalization of Allylic Sulfonamides Enabled by Organosulfide Catalysis: Modular Entry to Iodinated Chiral Molecules

Electrophilic halogenation of alkenes is a powerful transformation offering a convenient route for the construction of valuable functionalized molecules. However, as a highly important reaction in this field, catalytic asymmetric intermolecular iodinative difunctionalization remains a formidable challenge. Herein, we report that an efficient Lewis basic chiral sulfide-catalyzed approach enables this reaction. By this approach, challenging substrates such as γ,γ-disubstituted allylic sulfonamides and 1,1-disubstituted alkenes with an allylic sulfonamide unit undergo electrophilic iodinative difunctionalization to give a variety of iodine-functionalized chiral molecules in good yields with excellent enantio- and diastereoselectivities. A series of free phenols as nucleophiles are successfully incorporated into the substrates. Aside from phenols, primary and secondary alcohols, fluoride, and azide also serve as efficient nucleophiles. The obtained iodinated products are a good platform molecule, which can be easily transformed into various chiral compounds such as α-aryl ketones, chiral secondary amines, and aziridines via rearrangement or substitution. Mechanistic studies revealed that the chiral sulfide catalyst displays a superior effect on control of the reactivity of electrophilic iodine and the enantioselective construction of the chiral iodiranium ion intermediate and catalyst aggregates might be formed as a resting state in the reactions.

Catalytic Chemo-, Regio-, Diastereo-, and Enantioselective Bromochlorination of Unsaturated Systems Enabled by Lewis Base-Controlled Chloride Release

A new strategy is described for the Lewis base-catalyzed bromochlorination of unsaturated systems that is mechanistically distinct from prior methodologies. The novelty of this method hinges on the utilization of thionyl chloride as a latent chloride source in combination with as little as 1 mol % of triphenylphosphine or triphenylphosphine oxide as Lewis basic activators. This metal-free, catalytic chemo-, regio-, and diastereoselective bromochlorination of alkenes and alkynes exhibits excellent site selectivity in polyunsaturated systems and provides access to a wide variety of vicinal bromochlorides with up to >20:1 regio- and diastereoselectivity. The precision installation of Br, Cl, and I in various combinations is also demonstrated by simply varying the commercial halogenating reagents employed. Notably, when a chiral Lewis base promoter is employed, an enantioselective bromochlorination of chalcones is possible with up to a 92:8 enantiomeric ratio when utilizing only 1-3 mol % of (DHQD)₂PHAL.

Stereocontrolled access to δ-lactone-fused-γ-lactams bearing angular benzylic quaternary stereocenters

C-fused γ-lactam-lactones are resident in several bioactive molecules, including anticancer agents such as omuralide. In this embodiment, we report mild conditions for the catalytic halolactonization of lactam-tethered 5-aryl-4(E)-pentenoic acids. The use of dichloromethane as the solvent and Ph₃P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> S as the catalyst led to predominant 6-endo-trig cyclization and furnished the trans-fused-γ-lactam-δ-lactones. The transformation is modular, regioselective, chemoselective, and diastereoselective. The γ-lactam-δ-lactones bear angular quaternary benzylic stereocenters, which is noteworthy since the presence of a quaternary carbon in bioactive small molecules often promotes an element of conformational restriction that imparts potency, selectivity, and metabolic stability. The generated halogen and lactone motifs are important functional handles for late-stage diversification.

The catalytic halolactonization of readily affordable γ-lactam-tethered alkenoic acids has facilitated the site-selective, efficient, and stereocontrolled synthesis of halogenated fused γ-lactam-δ-lactones.

Mechanistic Details of Asymmetric Bromocyclization with BINAP Monoxide: Identification of Chiral Proton-Bridged Bisphosphine Oxide Complex and Its Application to Parallel Kinetic Resolution

The mechanism of our previously reported catalytic asymmetric bromocyclization reactions using 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) monoxide was examined in detail by the means of control experiments, NMR studies, X-ray structure analysis, and CryoSpray electrospray ionization mass spectrometry (ESI-MS) analysis. The chiral BINAP monoxide was transformed to a key catalyst precursor, proton-bridged bisphosphine oxide complex (POHOP·Br), in the presence of N-bromosuccinimide (NBS) and contaminating water. The thus-formed POHOP further reacts with NBS to afford BINAP dioxide and molecular bromine (Br₂) simultaneously in equimolar amounts. While the resulting Br₂ is activated by NBS to form a more reactive brominating reagent (Br₂─NBS), BINAP dioxide serves as a bifunctional catalyst, acting as both a Lewis base that reacts with Br₂─NBS to form a chiral brominating agent (P═O⁺─Br) and also as a Brønsted base for the activation of the substrate. By taking advantage of this novel concerted Lewis/Brønsted base catalysis by BINAP dioxide, we achieved the first regio- and chemodivergent parallel kinetic resolutions (PKRs) of racemic unsymmetrical bisallylic amides via bromocyclization.

HFIP-promoted halo-carbocyclizations of N- and O-tethered arene–alkene substrates to access all halo (X = Br, I, Cl)-functionalized tetrahydroquinoline and chroman cores

Herein, a HFIP-promoted mild and efficient method for the synthesis of all halo (X = Br, I, Cl)-functionalized tetrahydroquinoline and chroman building blocks is disclosed.

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...

Catalytic enantioselective oxysulfenylation of o-vinylanilides

Tf₂NH-assisted BINAM-derived thiophosphoramide catalysis has been accomplished for the enantioselective oxysulfenylation of o-vinylanilides with N-(aryl/alkylthio)imides. The developed reaction offers access to diverse substituted aryl/alkylthio tethered 3,1-benzoxazines in excellent yields and enantiomeric ratios. Furthermore, synthetic applications of benzoxazines and aryl/alkylthio moieties and a transition state model for the observed enantioselectivity are also discussed.

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.

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.

Are bis(pyridine)iodine(I) complexes applicable for asymmetric halogenation?

Enantiopure halogenated molecules are of tremendous importance as synthetic intermediates in the construction of pharmaceuticals, fragrances, flavours, natural products, pesticides, and functional materials. Enantioselective halofunctionalizations remain poorly understood and generally applicable procedures are lacking. The applicability of chiral trans-chelating bis(pyridine)iodine(i) complexes in the development of substrate independent, catalytic enantioselective halofunctionalization has been explored herein. Six novel chiral bidentate pyridine donor ligands have been designed, routes for their synthesis developed and their [N–I–N]⁺-type halogen bond complexes studied by ¹⁵N NMR and DFT. The chiral complexes encompassing a halogen bond stabilized iodenium ion are shown to be capable of efficient iodenium transfer to alkenes; however, without enantioselectivity. The lack of stereoselectivity is shown to originate from the availability of multiple ligand conformations of comparable energies and an insufficient steric influence by the chiral ligand. Substrate preorganization by the chiral catalyst appears a necessity for enantioselective halofunctionalization.

The enantioselectivity of the iodine(i) transfer process from chiral bis(pyridine)iodine(i) complexes to alkenes is explored.

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.

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

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

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.

Oxoammonium salts are catalysing efficient and selective halogenation of olefins, alkynes and aromatics

Electrophilic halogenation reactions have been a reliable approach to accessing organohalides. During the past decades, various catalytic systems have been developed for the activation of haleniums. However, there is still a short of effective catalysts, which could cover various halogenation reactions and broad scope of unsaturated compounds. Herein, TEMPO (2,2,6,6-tetramethylpiperidine nitroxide) and its derivatives are disclosed as active catalysts for electrophilic halogenation of olefins, alkynes, and aromatics. These catalysts are stable, readily available, and reactive enough to activate haleniums including Br⁺, I⁺ and even Cl⁺ reagents. This catalytic system is applicable to various halogenations including haloarylation of olefins or dibromination of alkynes, which were rarely realized in previous Lewis base catalysis or Lewis acid catalysis. The high catalytic ability is attributed to a synergistic activation model of electrophilic halogenating reagents, where the carbonyl group and the halogen atom are both activated by present TEMPO catalysis.

Organohalides are widely used as synthetic precursors and target products, but for various halogenation reactions there is a need for effective catalysts to activate commercially available haleniums. Here, the authors report that TEMPO and its derivatives are active catalysts for electrophilic halogenation of olefins, alkynes and aromatics, under mild reaction conditions and with good functional group tolerance.

Chlorocyclization/cycloreversion of allylic alcohols to vinyl chlorides

Unprecedented chlorination of allylic alcohols: a simple mix-and-go procedure for the cyclization/cycloreversion of secondary and tertiary allylic alcohols with chloronium ions under mild conditions and practical access to remote carbonyl vinyl chlorides.

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.

Ritter-enabled catalytic asymmetric chloroamidation of olefins

Intermolecular asymmetric haloamination reactions are challenging due to the inherently high halenium affinity (HalA) of the nitrogen atom, which often leads to N-halogenated products as a kinetic trap. To circumvent this issue, acetonitrile, possessing a low HalA, was used as the nucleophile in the catalytic asymmetric Ritter-type chloroamidation of allyl-amides. This method is compatible with Z and E alkenes with both alkyl and aromatic substitution. Mild acidic workup reveals the 1,2-chloroamide products with enantiomeric excess greater than 95% for many examples. We also report the successful use of the sulfonamide chlorenium reagent dichloramine-T in this chlorenium-initiated catalytic asymmetric Ritter-type reaction. Facile modifications lead to chiral imidazoline, guanidine, and orthogonally protected 1,2,3 chiral tri-amines.

Intermolecular haloamination reactions are challenging due to the high halenium affinity of the nitrogen atom. This is circumvented by using acetonitrile as an attenuated nucleophile, resulting in an enantioselective halo-Ritter reaction.

Supramolecular Halogen Bonds in Asymmetric Catalysis

Halogen bonding has received a significant increase in attention in the past 20 years. An important part of this interest has centered on catalytic applications of halogen bonding. Halogen bond (XB) catalysis is still a developing field in organocatalysis, although XB catalysis has outgrown its proof of concept phase. The start of this year witnessed the publication of the first example of a purely XB-based enantioselective catalytic reaction. While the selectivity can be improved upon, there are already plenty of examples in which halogen bonds, among other interactions, play a crucial role in the outcome of highly enantioselective reactions. This paper will give an overview of the current state of the use of XBs in catalytic stereoselective processes.