Highly Enantioselective Iridium-Catalyzed Coupling Reaction of Vinyl Azides and Racemic Allylic Carbonates

Iridium-Catalyzed Asymmetric Allylic Alkylation of Boron Enolates to Construct Acyclic All-Carbon Quaternary Stereocenters

Enolates are ubiquitous intermediates in organic synthesis. Among them, boron enolates exhibit distinctive reactivity patterns and selectivities due to the presence of a boron atom, making their synthesis highly attractive. Although methods for accessing ketone- or ester-derived boron enolates are well-developed, much less progress has been made in the development of aldehyde-derived boron enolates due to aldehydes' high tendency toward self-condensation. Therefore, the practical applications of aldehyde-derived boron enolates are significantly hindered. We present herein an efficient method for the preparation of aldehyde-derived boron enolates via the 1,2-hydroboration of ketenes with boranes, avoiding the use of acidic R2BCl/R2BOTf and bases and leading to improved functional group tolerance. Utilizing this convenient protocol, we developed an Ir-catalyzed asymmetric allylic alkylation of boron enolates, yielding a wide array of chiral aldehydes bearing acyclic all-carbon quaternary centers with high chemo-, regio-, and enantioselectivity, which are prevalent in various natural products and bioactive molecules. The synthetic utility and practicality of this method are demonstrated through gram-scale reactions and asymmetric syntheses of the ent-5HT1 A antagonist as well as biological activity studies in inhibiting the growth of plant pathogens.

Ligand-Accelerated, Copper-Catalyzed Aerobic Oxidative Dehydrogenation of Primary Amines to Nitriles

A highly efficient copper-catalyzed oxidative dehydrogenation of primary amines to access nitriles has been developed. We found that DMAP was an efficient ligand for copper-catalyzed oxidation and molecular oxygen was a green oxidizing agent. This reaction exhibited excellent functional group compatibility and a broad substrate scope. Various benzylic, allylic, and aliphatic amines were selectively and effectively oxidized to the corresponding nitriles in high yields (up to 100%). The ligand DMAP not only accelerated the reaction rate but also enhanced the stability of the catalyst. The practicality of the reaction was illustrated on a gram scale, even at a lower catalyst loading and/or under bubble air conditions.

Iridium-Catalyzed Asymmetric Allylation of Silyl Enol Ethers Derived from α-Ketoesters and α-Diketones with Allylic Alcohols

We report an asymmetric allylation of silyl enol ethers derived from α-ketoesters with allylic alcohols by an iridium/chiral (P,olefin)-ligand catalyst. Allylation with a broad range of silyl enol ethers and allylic alcohols formed the allylated products with excellent enantioselectivities. Silyl enol ethers derived from α-diketones as well as α-ketoesters were also compatible with the asymmetric allylation to achieve nearly perfect enantioselectivities. The utility of allylated products bearing an α-ketoester group and an α-diketone group is demonstrated by transformation to valuable functional groups, exemplified by α-aminoester, aldehyde, 2(1H)-quinoxalinone, and quinoxaline.

Cobalt-catalyzed reductive cross-coupling: a review

Transition-metal-catalyzed reductive cross-coupling is highly efficient for forming C-C bonds. It earns its limelight from its application by coupling unreactive electrophilic substrates to synthesize a variety of carbon-carbon bonds with various hybridizations (sp, sp2, and sp3), late-stage functionalization, and bioactive molecules' synthesis. Reductive cross-coupling is challenging to bring selectivity but promising approach. Cobalt is comparatively more affordable than other highly efficient metals e.g., palladium and nickel but cobalt catalysis is still facing efficacy challenges. Researchers are trying to harness the maximum out of cobalt's catalytic properties. Shortly, with efficiency achieved combined with the affordability of cobalt, it will revolutionize industrial applications. This review gives insight into the core of cobalt-catalyzed reductive cross-coupling reactions with a variety of substrates forming a range of differently hybridized coupled products.

Divergent Reactivity of Iminyl Radicals in Four Interrupted Pathways for the Synthesis of Cyclic/Acyclic Ketones and N-Heterocycles from Vinyl Azides and Phenylacetic Acids

Herein, we report the Ag-catalyzed substrate-controlled interrupted radical pathways of the iminyl radical. The benzylic groups played a crucial role in pathway selection involving a series of dimerization and hydrolysis, a 1,5-H shift followed by cascade radical cyclization, and direct N-(sp2)/aromatization reactions that provide access to diverse cyclic/acyclic ketones, quinolines, and phenanthridine derivatives. Contrary to previous reports, mechanistic investigations with high-resolution mass spectrometry analysis uncovered the involvement of rare azine, oxime, and β-functionalized vinyl azide intermediates.

Cobalt-Catalyzed Acceptorless Dehydrogenation of Primary Amines to Nitriles

The direct double dehydrogenation from primary amines to nitriles without an oxidant or hydrogen acceptor is both intriguing and challenging. In this paper, we describe a non-noble metal catalyst capable of realizing such a transformation with high efficiency. A cobalt-centered N,N-bidentate complex was designed and employed as a metal-ligand cooperative dehydrogenation catalyst. Detailed kinetic studies, control experiments, and DFT calculations revealed the crucial hydride transfer, proton transfer, and hydrogen evolution processes. Finally, a tandem outer-sphere/inner-sphere mechanism was proposed for the dehydrogenation of amines to nitriles through an imine intermediate.

Enantioselective Ir-Catalyzed Allyl Alkylation/Semipinacol Rearrangement

The semipinacol rearrangement is a powerful and versatile method for constructing all-carbon quaternary stereocenters. The development of catalytic asymmetric semipinacol rearrangements using multifunctionalizable electrophiles remains highly sought-after in organic synthesis. In this study, a catalytic enantioselective allylic cation-induced semipinacol rearrangement reaction was presented that enables the simultaneous construction of two skipped chiral carbon centers. Chiral Ir(I)-(P,olefin) and Sc(OTf)3 catalysts cooperatively initiate the asymmetric allylic alkylation of alkenyl cyclobutanols with allylic alcohols, triggering ring expansion of the cyclobutanol moiety through a stereoselective 1,2-alkyl migration. The reaction afforded a range of cyclopentanones bearing an α-quaternary carbon that is adjacent to a chiral allyl scaffold. The products were applied to synthesize enantioenriched fused tricyclopentanoids bearing four stereogenic carbon centers.

Iridium/Acid Dual-Catalyzed Enantioselective Aza-ene-type Allylic Alkylation of Nitro Ketene Aminals with Racemic Allylic Alcohols

The enantioselective allylic alkylation of nitro ketene aminals with racemic allylic alcohols was realized by iridium/acid dual catalysis. An allyl group was installed on the α-position of nitro ketene aminals in a branched-selective manner in high efficiency with excellent enantioselectivities (93-99% ee). The protocol was applied to the late-stage modification of neonicotinoid insecticides, which directly furnished a novel neonicotinoid analogue with good insecticidal activity against Aphis craccivora (LC50 = 6.40 mg/L). On the basis of the control experiment, an aza-ene-type allylic alkylation reaction mechanism was proposed.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Stereodivergent Construction of 1,3-Chiral Centers via Tandem Asymmetric Conjugate Addition and Allylic Substitution Reaction

Herein, we report a synthesis of cyclohexanones bearing multi-continuous stereocenters by combining copper-catalyzed asymmetric conjugate addition of dialkylzinc reagents to cyclic enones with iridium-catalyzed asymmetric allylic substitution reaction. Good to excellent yields, diastereoselectivity and enantioselectivity can be obtained. Unlike the stereodivergent construction of adjacent stereocenters (1,2-position) reported in the literature, the current reaction can achieve the stereodivergent construction of nonadjacent stereocenters (1,3-position) by a proper combination of two chiral catalysts with different enantiomers.

Regio- and Enantioselective Allylic Cyanomethylation by Synergistic Rhodium and Silane Catalysis

Rh/silane-cocatalyzed regio- and enantioselctive allylic cyanomethylation with inert acetonitrile directly has been developed. Addition of a catalytic amount neutral silane reagent as an acetonitrile anion carrier is essential for the success of this reaction. The synthesis of mono- and bis-allylation products can be switched by adjusting the size of substituents on the silane, ligands, and temperature. Chiral homoallylic nitriles could be synthesized in above 20:1 branch/linear ratio, up to 98% yield and >99% ee.

Ir-Catalyzed Asymmetric Cascade Allylation/Spiroketalization Reaction for Stereoselective Synthesis of Oxazoline-Spiroketals

An asymmetric cascade allylation/spiroketalization reaction between 2-(1-hydroxyallyl)phenols and 5-methyleneoxazolines is accomplished by using a chiral Ir(I) catalyst derived from commercially available iridium precursor and the Carreira ligand. This protocol furnishes a class of structurally novel and unique oxazoline-spiroketals in up to 86% yield, >99% ee and >20:1 dr. Moreover, control experiments reveal that 4,4-disubstitution on 5-methyleneoxazolines is necessary to avoid the aromatization and for the spiroketalization to occur. On the basis of this, a plausible reaction mechanism is illustrated.

Hydroxy-directed iridium-catalyzed enantioselective formal β-C(sp2)-H allylic alkylation of α,β-unsaturated carbonyls

Hydroxy-directed iridium-catalyzed enantioselective formal β-C(sp2)-H allylic alkylation of kojic acid and structurally related α,β-unsaturated carbonyl compounds is developed. This reaction, catalyzed by an Ir(i)/(P,olefin) complex, utilizes the nucleophilic character of α-hydroxy α,β-unsaturated carbonyls, to introduce an allyl group at its β-position in a branched-selective manner in good to excellent yield with uniformly high enantioselectivity (up to >99.9 : 0.1 er). To the best of our knowledge, this report represents the first example of the use of kojic acid in a transition metal catalyzed highly enantioselective transformation.

Enantioselective Synthesis of Functionalized Tetrahydropyridines through Iridium-Catalyzed Formal [5+1] Annulation

An efficient iridium-catalyzed asymmetric formal [5+1] annulation by in situ generation of enamines as N-nucleophiles for the synthesis of tetrahydropyridine derivatives is disclosed. The methodology offers direct access to a wide variety of chiral tetrahydropyridine derivatives in moderate to good yields and excellent enantioselectivity.

Synthesis of gem-Disulfonyl Enamines via an Iminyl-Radical-Mediated Formal 1,3-HAT/Radical Coupling Cascade

We herein report the first example of an iminyl-radical-mediated formal 1,3-HAT/radical coupling cascade of vinyl azides leading to the synthesis of tetrasubstituted gem-disulfonyl enamines. It is possible to employ a variety of vinyl azides and sulfinate salt coupling elements without sacrificing effectiveness and scalability. The combination of experimental studies and DFT calculations showed that this reaction proceeds via a radical addition/formal 1,3-HAT/radical coupling mechanism.

Palladium-Catalyzed Oxidative Nonclassical Heck Reaction of Arylhydrazines with Allylic Alcohols via C-N Bond Cleavage: Access to β-Arylated Carbonyl Compounds

An efficient palladium-catalyzed oxidative nonclassical Heck reaction of arylhydrazines with allylic alcohols via C-N bond cleavage has been successfully developed. This method provides a series of β-arylated carbonyl compounds with broad functional group tolerance under base-free, simple, and mild open air reaction conditions. In the reaction, arylhydrazines with the smaller molecular weight of the leaving group were employed as the "green" arylation reagent, which released N₂ and water as the byproducts under air. Mechanistic studies suggested that an aryl radical process and Pd-H complex migration reinsertion were involved. Moreover, the synthesis of the antiarrhythmic drug propafenone was completed with this transformation as the key step.

Iridium-Catalyzed Asymmetric Allylic Substitution of Methyl Azaarenes

Herein, an Ir-catalyzed asymmetric allylic substitution reaction of methyl azaarenes is described. Azaarenes such as (benzo)thiazole, oxazole, benzoimidazole, pyridine, and (iso)quinoline are all tolerated. The corresponding chiral azaarene derivatives are obtained in good yields with high enantioselectivity (up to 96 % yield and 99 % ee). The utilization of the Knochel reagent TMPZnBr⋅LiBr warrants the in situ formation of benzylic nucleophiles without additional activating reagents. ¹ H NMR studies suggested a two-fold function of the Knochel reagent in this reaction. The synthetic utility of this method has been showcased by a concise enantioselective synthesis of an allosteric protein kinase modulator.

Regio- and Enantioselective γ-Allylic Alkylation of In Situ-Generated Free Dienolates via Scandium/Iridium Dual Catalysis

An unprecedented asymmetric γ-allylic alkylation of free dienolates via Sc/Ir dual catalysis is reported, which affords a range of synthetically versatile γ-allylic crotonaldehydes in high efficiency with excellent chemo-, regio-, and enantioselectivities. The dienolates bearing no essential auxiliary groups were generated in situ by scandium triflate-mediated Meinwald rearrangement of vinyloxiranes atom-economically. With the assistance of computational density functional theory calculations, a Sc/Ir bimetallic catalytic cycle was proposed to illustrate the reaction mechanism.

Iridium-Catalyzed Enantioselective and Chemodivergent Allenylic Alkylation of Vinyl Azides for the Synthesis of α-Allenylic Amides and Ketones

The first enantioselective synthesis of α-allenylic amides and ketones through allenylic alkylation of vinyl azides is reported. In these chemodivergent reactions, cooperatively catalyzed by a Ir^I /(phosphoramidite,olefin) complex and Sc(OTf)₃ , vinyl azides act as the surrogate for both amide enolates and ketone enolates. The desiccant (molecular sieves) plays a crucial role in controlling the chemodivergency of this enantioconvergent and regioselective reaction: Under otherwise identical reaction conditions, the presence of the desiccant led to α-allenylic amides, while its absence resulted in α-allenylic ketones. Utilizing racemic allenylic alcohols as the alkylating agent, the overall process represents a dynamic kinetic asymmetric transformation (DyKAT), where both the products are formed with the same absolute configuration. To the best of our knowledge, this is the first example of the use of vinyl azide as the ketone enolate surrogate in an enantioselective transformation.

Recent advances in iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands

This minireview describes the recent progress of iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands realizing highly enantioselective carbon–carbon and carbon–heteroatom bond formation.

S-triggered Schmidt-type rearrangement of vinyl azides to access N-aryl-(trifluoromethylsulfinyl)acetamides

A novel S-induced Schmidt-type rearrangement of vinyl azides with CF3SO2Na is developed for synthesis of N-arylated 2-(trifluoromethylsulfinyl)acetamieds, which is mediated by triphosgene (BTC) under mild reaction conditions.

Synthesis of 2-monosubstituted indolin-3-ones by cine-substitution of 3-azido-2-methoxyindolines

We report herein the formal cine-substitution/hydrolysis of 3-azidoindole intermediates generated from 3-azido-2-methoxyindolines (AZINs). This protocol enables the introduction of both various carboxylic acid and alcohol into indolin-3-ones at the C2-position,...

cis-3-Azido-2-methoxyindolines as safe and stable precursors to overcome the instability of fleeting 3-azidoindoles

Use of 3-azidoindoles in organic synthesis remains a difficult task owing to their instabilities. Herein, we report a general and concise approach for tackling this problem by using 3-azidoindole surrogates. The surrogates are bench-stable, presumably due to the observed intramolecular O-N_β bonding. The resultant fleeting intermediates undergo capturing in situ to afford 3-substitued indoles through formal ipso-substitution of the azide group by nucleophiles. In these investigations, we found that the fleeting 3-azidoindoles show a C3-electrophilic character for the first time.

Iodine-Mediated Cyclization of Enamines to Imidazole-4-Carboxylic Derivatives with Sequential Removal of Nitrogen Atoms from TMSN3

An iodine-mediated oxidative [4+1] cyclization of enamines with TMSN₃ for the synthesis of 2,5-disubstituted imidazole-4-carboxylic derivatives has been developed. The mechanistic studies revealed that the reaction proceeds through a sequential removal of two nitrogen atoms from TMSN₃. The synthetic utility was further demonstrated with a gram-scale reaction and various derivatization transformations of the products.

Synthesis of α-substituted indolylacetamide using acetonitriles as acetamide enolate equivalents through O-transfer reactions

We introduce readily available ammonium hemiaminals as O-transfer reagents and commercially available acetonitriles as a primary amide enolate precursor. The combination serves as an amide enolate equivalent, thereby providing one-pot access to α-substituted indolylacetamides. A broad substrate scope and good functional group tolerance as well as gram-scale synthesis make this protocol highly attractive. Mechanistic experiments suggest that the cyano group is trapped by a hydroxy group of hemiaminals en route to the desired primary amides under metal-free conditions.

Iridium-Catalyzed Enantioconvergent Allylation of a Boron-Stabilized Organozinc Reagent

An iridium-catalyzed enantioconvergent coupling of the versatile boron-stabilized organozinc reagent BpinCH₂ZnI with a racemic branched allylic carbonate has been developed here, which differs from our previous work by using 1,1-bisborylmethane through the kinetic resolution process. The reaction has a broad substrate scope, and various chiral homoallylic organoboronic esters could be obtained in good yields with excellent enantioselectivities. The synthetic practicability of the products was demonstrated by their conversion to other useful families of compounds.

An aryl thiol-vinyl azide coupling reaction and a thiol-vinyl azide coupling/cyclization cascade: efficient synthesis of β-ketosulfides and arene-fused 5-methylene-2-pyrrolidinone derivatives

The addition reaction of thiol to vinyl azide has been extensively studied. Variously substituted aryl thiols are all viable for this coupling process. The scope of the other partner is successfully expanded from α-aryl vinyl azide to α-alkyl vinyl azide. A thiol-vinyl azide coupling/cyclization cascade is realized with substituted aryl vinyl azides carrying a 2-methoxycarbonyl group. The value of β-ketosulfide products was demonstrated by its application in S-heterocycle synthesis.

Ni-Catalyzed radical cyclization of vinyl azides with cyclobutanone oxime esters to access cyanoalkyl containing quinoxalin-2(1H)-ones

A nickel-catalyzed cascade addition/cyclization of 2-azido-N-arylacrylamides and cyclobutanone oxime esters for the construction of 3-cyanoalkylated quinoxalin-2(1H)-ones is developed. This reaction proceeds under mild conditions with good functional group tolerance and broad substrate scope. A preliminary mechanistic experiment indicated that the cyanoalkyl radical might be involved in this transformation.

Regioselective Synthesis of 2,4-Diaryl-6-trifluoromethylated Pyridines through Copper-Catalyzed Cyclization of CF3-Ynones and Vinyl Azides

A novel copper-catalyzed cyclization of readily available vinyl azides with CF₃-ynones is steadily achieved under mild conditions to furnish the versatile 2,4-diaryl-6-trifluoromethylated pyridine products, which are of great interest in medicinal chemistry. The generation of the vinyl iminophosphorane intermediates from vinyl azides through the Staudinger-Meyer reaction ensures the subsequent 1,4-addition process with CF₃-ynones in this transformation.

Enantioselective Allylation of Alkenyl Boronates Promotes a 1,2-Metalate Rearrangement with 1,3-Diastereocontrol

Alkenyl boronates add to Ir(π-allyl) intermediates with high enantioselectivity. A 1,2-metalate shift forms a second C-C bond and sets a 1,3-stereochemical relationship. The three-component coupling provides tertiary boronic esters that can undergo multiple additional functionalizations. An extension to trisubstituted olefins sets three contiguous stereocenters.

2-Nitroallyl carbonate-based green bifunctional reagents for catalytic asymmetric annulation reactions

2-Nitroallylic carbonates, a new class of "green" 1,3-bielectrophilic reagents for organic synthesis and catalysis, have been prepared. The bifunctional tertiary amine-catalyzed asymmetric [3 + 3] annulations of cyclic enols with these reagents occur much faster than corresponding reactions with 2-nitroallylic esters and produce no acidic by-products poisoning the catalyst. Furthermore, 2-nitroallylic carbonates enable highly enantioselective one-pot synthesis of a variety of fused dihydropyrane derivatives from available precursors bearing pharmacophoric fragments.

Ni-Catalyzed asymmetric hetero-Diels–Alder reactions of conjugated vinyl azides: synthesis of chiral azido polycycles

We report herein an efficient Ni(ii)/Feng ligand-catalyzed asymmetric hetero-Diels–Alder reaction between conjugated vinyl azides with carbonyl groups, providing a synthesis of complicated chiral azido polycycles in high yields and excellent enantioselectivities.

Electrochemical Tandem Fluoroalkylation-Cyclization of Vinyl Azides: Access to Trifluoroethylated and Difluoroethylated N-Heterocycles

A transition-metal- and oxidant-free electrochemical strategy for radical fluoroalkylation of vinyl azides was developed. The reaction was carried out under mild conditions by using inexpensive and bench-stable R_fSO₂Na (R_f = CF₃, CF₂H) as fluorination reagents. Depending on the starting material, both the electrochemical radical cyclization and dearomatization products could be obtained. This method provides a green and safe approach to synthesize fluorinated nitrogen heterocycles.

Iridium-Catalyzed Enantioselective α-Allylic Alkylation of Amides Using Vinyl Azide as Amide Enolate Surrogate

Among the unstabilized enolates used as nucleophiles in iridium-catalyzed asymmetric allylic alkylation reactions, amide enolates are the least explored. Vinyl azides are now employed as amide enolate surrogates in Ir-catalyzed asymmetric allylic alkylation with branched allylic alcohols as the allylic electrophile. Competing reaction pathways are suppressed through the systematic tuning of the steric and electronic properties of vinyl azide to effect the α-allylic alkylation of secondary acetamides with high atom economy, exclusive branched selectivity, and mostly excellent enantioselectivity.

Catalytic asymmetric synthesis of 3,2'-pyrrolinyl spirooxindoles via conjugate addition/Schmidt-type rearrangement of vinyl azides and (E)-alkenyloxindoles

A catalytic asymmetric conjugate addition/Schmidt-type rearrangement of vinyl azides and (E)-alkenyloxindoles was realized. It afforded a variety of optically active 3,2'-pyrrolinyl spirooxindoles with high yields (up to 98%), and excellent diastereo- and enantioselectivities (up to 98% ee, >19 : 1 dr), even at the gram-scale in the presence of a chiral N,N'-dioxide-nickel(ii) complex. In addition, a possible catalytic cycle and transition state model were proposed to rationalize the stereoselectivity.