A robust, efficient, and highly enantioselective method for synthesis of homopropargyl amines

Copper-Catalyzed Asymmetric Allylation of N-Aryl Aldimines

The copper-catalyzed enantioselective allylation reaction of N-aryl aldimines has been developed using a combination of Cu(OAc)2 and SPINOL-based phosphonamidite. This protocol significantly broadens the substrate scope, such that imines bearing various ortho-substituents on the N-aryl were converted smoothly into homoallylic amines in up to 99% yield and 98% ee. Taking advantage of the diversity of the N-aryl motif, three kinds of N-heterocyclic compounds were constructed, respectively, from the corresponding homoallylic amines in merely one step.

Evaluation of helicene-derived 2,2'-bipyridine N-monoxide catalyst for the enantioselective propargylation of N-acylhydrazones with allenyltrichlorosilane

Helicene-derived 2,2'-bipyridine N-monoxide was evaluated as a Lewis base catalyst for the enantioselective propargylation of N-acylhydrazones with allenyltrichlorosilane. The helicene-derived catalyst provided moderate-to-good reactivity and enantioselectivity for a range of acylhydrazones. This study represents the first example of the catalytic asymmetric propargylation of non-activated acylhydrazones.

Asymmetric Reactions of N-Phosphonyl/Phosphoryl Imines

The asymmetric reactions of imines continued to attract the attention of the scientific community for decades. However, the stereoselective reactions of N-phosphonyl/phosphoryl imines remained less explored as compared to other N-substituted imines. The chiral auxiliary-based asymmetric-induction strategy with N-phosphonyl imines could effectively generate enantio- and diastereomeric amine, α,β-diamine, and other products through various reactions. On the other hand, the asymmetric approach for the generation of chirality through the utilization of optically active ligands, along with metal catalysts, could be successfully implemented on N-phosphonyl/phosphoryl imines to access numerous synthetically challenging chiral amine scaffolds. The current review critically summarizes and reveals the literature precedence of more than a decade to highlight the major achievements existing to date that can display a clear picture of advancement as well drawbacks in this area.

Iron-Catalyzed Borylation of Propargylic Acetates for the Synthesis of Multisubstituted Allenylboronates

A novel iron-catalyzed borylation of propargylic acetates leading to allenylboronates has been developed. The method allows the preparation of a variety of di-, tri- and tetrasubstituted allenylboronates at room temperature with good functional group compatibility. Stereochemical studies show that an anti-S_N 2' displacement of acetate by boron occurs; this also allows transfer of chirality to yield enantiomerically enriched allenylboronates. The synthetic utility of this protocol was further substantiated by transformations of the obtained allenylboronates including oxidation and propargylation.

Designing successful monodentate N-heterocyclic carbene ligands for asymmetric metal catalysis

Chiral ligands are of particular importance in asymmetric transition-metal catalysis. Although the development of effective chiral monodentate N-heterocyclic carbenes (NHCs) has been slow, a growing amount of papers have been published in recent years showing their unique efficiency as chiral ancillary ligands. Herein we provide an overview of NHC structures that accomplish high levels of enantioselectivity (≥90% ee) and give guidelines to their use and thoughts on the future of this field.

Ir-Catalyzed Regio- and Enantioselective Hydroalkynylation of Trisubstituted Alkene to Access All-Carbon Quaternary Stereocenters

The stereoselective construction of all-carbon quaternary stereocenters, especially acyclic ones, represents an important challenge in organic synthesis. In particular, homopropargyl amides with a quaternary stereocenter β to a nitrogen atom are valuable synthetic intermediates, which could be transformed to diverse chiral structures through alkyne transformations. However, highly enantioselective synthetic methods for homopropargyl amides with a β quaternary stereocenter are extremely rare. We report here unprecedented substrate-directed, iridium-catalyzed enantioselective hydroalkynylations of trisubstituted alkenes to form an acyclic all-carbon quaternary stereocenter β to a nitrogen atom. The hydroalkynylation of enamide occurred with unconventional selectivity, favoring the more hindered reaction site. Homopropargyl amides with β-stereocenters were prepared in high regio- and enantioselectivities. Combined experimental and computational studies revealed the origin of the regio- and enantioselectivities.

A copper-catalyzed asymmetric oxime propargylation enables the synthesis of the gliovirin tetrahydro-1,2-oxazine core

The bicyclic tetrahydro-1,2-oxazine subunit of gliovirin is synthesized through a diastereoselective copper-catalyzed cyclization of an N-hydroxyamino ester. Oxidative elaboration to the fully functionalized bicycle was achieved through a series of mild transformations. Central to this approach was the development of the first catalytic, enantioselective propargylation of an oxime to furnish a key N-hydroyxamino ester intermediate.

Stereocontrolled Addition of Scrambling ortho-Sulfinyl Carbanions: Easy Access to Homopropargylamines and α-Allenylamines

An unprecedented behavior of ortho-sulfinylpropargyl carbanions in the presence of optically active sulfinylimines affords two different families of compounds: this peculiar chemodivergency is importantly affected by the nature of the employed base, and assisted by the configuration of the electrophile, displaying no alteration in the stereocontrol of both reactions. α-Allenylamines are formed exclusively, using R-sulfinyl aldimines as electrophiles, while homopropargylamines result when S-sulfinyl aldimines are employed.

Enantio- and Diastereoselective Synthesis of Homopropargyl Amines by Copper-Catalyzed Coupling of Imines, 1,3-Enynes, and Diborons

An efficient, enantio- and diastereoselective, copper-catalyzed coupling of imines, 1,3-enynes, and diborons is reported. The process shows broad substrate scope and delivers complex, chiral homopropargyl amines; useful building blocks on the way to biologically-relevant compounds. In particular, functionalized homopropargyl amines bearing up to three contiguous stereocenters can be prepared in a single step.

Copper-catalyzed synthesis of allenylboronic acids. Access to sterically encumbered homopropargylic alcohols and amines by propargylboration

Tri- and tetrasubstituted allenylboronic acids were prepared via a new versatile copper-catalyzed methodology. The densely functionalized allenylboronic acids readily undergo propargylboration reactions with ketones and imines without any additives. Catalytic asymmetric propargylborylation of ketones is demonstrated with high stereoselectivity allowing for the synthesis of highly enantioenriched tertiary homopropargyl alcohols. The reaction is suitable for kinetic resolution of racemic allenylboronic acids affording alkynes with adjacent quaternary stereocenters.

Electronically Activated Organoboron Catalysts for Enantioselective Propargyl Addition to Trifluoromethyl Ketones

A broadly applicable, practical, scalable, efficient and highly α- and enantioselective method for addition of a silyl-protected propargyl moiety to trifluoromethyl ketones has been developed. Reactions, promoted by 2.0 mol % of a catalyst that is derived in situ from a readily accessible aminophenol compound at ambient temperature, were complete after only 15 minutes at room temperature. The desired tertiary alcohols were isolated in up to 97 % yield and 98.5:1.5 enantiomeric ratio. Alkyl-, alkenyl-, alkynyl-, aryl- or heteroaryl-substituted trifluoromethyl ketones can be used. Utility is highlighted by application to a transformation that is relevant to enantioselective synthesis of BI 653048, a compound active against rheumatoid arthritis.

Enantioselective copper catalysed, direct functionalisation of allenes via allyl copper intermediates

We explore the breadth of the copper catalysed functionalisation of allenes, which enables efficient access to enantioenriched, densely functionalised molecules.

The direct functionalisation of allenes under copper catalysis enables efficient access to enantioenriched, densely functionalised molecules. In this review we explore the breadth and depth of a versatile reaction manifold, which involves the element-cupration of allenes to generate allyl copper intermediates that are subsequently coupled with diverse arrays of electrophiles.

A 7-Step Formal Asymmetric Total Synthesis of Strictamine via an Asymmetric Propargylation and Metal-Mediated Cyclization

Herein is shown how a novel catalytic asymmetric propargylation of 3,4-dihydro-β-carboline, followed by a designed Au(I)/Ag(I)-mediated 6-endo-dig cyclization, can directly deliver the indolenine-fused methanoquinolizidine core of the akuammiline alkaloid strictamine in its native oxidation state, ultimately achieving a 7-step formal asymmetric total synthesis. Also demonstrated are how the cyclization products can rearrange into vincorine-type skeletons and a further use for the developed propargylation with the first catalytic asymmetric total synthesis of decarbomethoxydihydrogambirtannine.

One-pot synthesis of imidazolinium salts via the ring opening of tetrahydrofuran

One-pot synthesis of C2-hydroxypropyl-substituted imidazolinium salts, active catalysts for the aza-Diels–Alder reaction, via the ring opening of THF is reported.

Copper-Catalyzed Asymmetric Propargylation of Cyclic Aldimines

The copper-catalyzed asymmetric propargylation of cyclic aldimines is reported. The influence of the imine trimer to inhibit the reaction was identified, and equilibrium constants between the monomer and trimer were determined for general classes of imines. Asymmetric propargylation of a diverse series of N-alkyl and N-aryl aldimines was achieved with good to high asymmetric induction. The utility was demonstrated by a titanium catalyzed hydroamination and reduction to generate the chiral indolizidines (-)-crispine A and (-)-harmicine.

Enantioselective Generation of Adjacent Stereocenters in a Copper-Catalyzed Three-Component Coupling of Imines, Allenes, and Diboranes

A highly enantio- and diastereoselective copper-catalyzed three-component coupling affords the first general synthesis of homoallylic amines bearing adjacent stereocenters from achiral starting materials. The method utilizes a commercially available NHC ligand and copper source, operates at ambient temperature, couples readily available simple imines, allenes, and diboranes, and yields high-value homoallylic amines that exhibit versatile amino, alkenyl, and boryl units.

Copper-Catalyzed Enantioselective Addition of Styrene-Derived Nucleophiles to Imines Enabled by Ligand-Controlled Chemoselective Hydrocupration

The copper-catalyzed intermolecular enantioselective addition of styrenes to imines has been achieved under mild conditions at ambient temperature. This process features the use of styrenes as latent carbanion equivalents via the intermediacy of catalytically generated benzylcopper derivatives, providing an effective means for accessing highly enantiomerically enriched amines bearing contiguous stereocenters. Mechanistic studies shed light on the origin of the preferential styrene hydrocupration in the presence of an imine with the Ph-BPE-derived copper catalyst.

Iridium-Catalyzed Enantioselective Hydroalkynylation of Enamides for the Synthesis of Homopropargyl Amides

Reported is an iridium-catalyzed asymmetric hydroalkynylation of enamides with terminal alkynes. The reaction occurs regioselectively at the β-position of an enamide to produce homopropargyl amides. Good to high enantioselectivity was observed with an iridium complex ligated by a chiral bis(phosphine) ligand. This method provides a straightforward route to synthesize chiral homopropargyl amides with a stereocenter β to the amide.

Synthesis of homopropargylamines from 2-cyanoazetidines

Mild generation of vinylidene carbene from α-amino nitrile is the basis of a new synthesis of homopropargylamines.

Silver-Catalyzed Enantioselective Propargylation Reactions of N-Sulfonylketimines

The enantioselective silver-catalyzed propargylation of N-sulfonylketimines is described. This reaction proceeds in high yield and excellent enantiomeric ratio and is compatible with a wide variety of diaryl- and alkylketimines. Synthetic transformations of homopropargylic products via enyne ring-closing metathesis, Sonogashira cross-coupling, and reduction reactions proceed with high stereochemical fidelity. Both allenyl and propargyl borolane reagents can be used to obtain homopropargylic products, a distribution most consistent with a mechanism involving transmetalation of the silver catalyst with the borolane reagent.

N-Substituted tertiary and O-substituted quaternary carbon stereogenic centers by site-, diastereo- and enantioselective vinylogous Mannich reactions

A readily accessible small-molecule phosphine, derived from commercially available starting materials such as an enantiomerically pure amino acid, serves as the precursor to a Ag-based chiral complex that can be prepared and used in situ to promote a variety of enantioselective vinylogous Mannich (EVM) reactions that involve siloxypyrroles as reaction partners. Transformations with unsubstituted nucleophilic components proceed efficiently and with exceptional site- (γ vs α-addition), diastereo- and enantioselectivity [up to 98% yield, generally >98:2 γ/α and diastereomeric ratio (dr) and up to 99:1 enantiomeric ratio (er)]. The first examples of efficient, diastereo- and enantioselective vinylogous Mannich additions with 5-methyl-substituted siloxyfuran, resulting in the formation of O-substituted quaternary carbon stereogenic centers are presented as well. Appreciable efficiency and diastereo- and enantioselectivity (up to >98:2 dr and >99:1 er) is accompanied by formation of α-addition products that can be oxidatively removed.

Component-selective and stereocontrolled one-step three-component reaction among aldehydes, amines, and allenyl boronic acids or allenyl pinacolboronates

A one-step, three-component condensation of allenyl boronic acids or allenyl pinacolboronates with amines and aldehydes affords α-allenyl or α-propargyl α-amino acids and anti-β-amino alcohols. This process gives the allenyl or propargyl product depending on the amine and boron components. Secondary amines generate exclusively α-allenyl α-amino acids, while primary aliphatic amines lead to α-propargyl α-amino acids. Secondary aliphatic amines react with chiral α-hydroxy aldehydes and allenyl boron derivatives to form stereoselectively allenyl anti-β-amino alcohol products.

N-Phosphonyl/phosphinyl imines and group-assisted purification (GAP) chemistry/technology

Group-assisted purification (GAP) chemistry, which can provide various chiral amines and other functionalities without the use of column chromatography or recrystallization; products are consistently obtained with excellent stereocontrol.

Copper-catalyzed enantioselective allylic alkylation of terminal alkyne pronucleophiles

The copper-catalyzed enantioselective allylic alkylation of terminal alkynes with primary allylic phosphates was developed by the use of a new chiral N-heterocyclic carbene ligand bearing a phenolic hydroxy group at the ortho position of one of the two N-aryl groups. This reaction occurred with excellent γ-branch regioselectivity and high enantioselectivity, forming a controlled stereogenic center at the allylic/propargylic position. Various terminal alkynes, including silyl, aliphatic, and aromatic alkynes, could be used directly without premetalation of the C(sp)-H bond. On the basis of the results of experiments using an isomeric secondary allylic phosphate, which gave a branched product through an α-selective substitution reaction with retention of configuration, a reaction pathway involving 1,3-allylic migration of Cu in a ([σ + π]-allyl)copper(III) species is proposed.

NHC-Cu-catalyzed addition of propargylboron reagents to phosphinoylimines. Enantioselective synthesis of trimethylsilyl-substituted homoallenylamides and application to the synthesis of S-(-)-cyclooroidin

A catalytic method for the efficient and enantioselective addition of a 1-trimethylsilyl-substituted allene moiety to phosphinoylimines is presented. Transformations are promoted by 5.0 mol % of a copper complex of an N-heterocyclic carbene in the presence of a propargylboron reagent that can be readily prepared in multigram quantities. Within 10 min of reaction, products are obtained in up to 91% yield, 98% allene (vs propargyl) selectivity, and 98:2 enantiomeric ratio. An assortment of aldimines serve as suitable substrates. The phosphinoyl and silyl groups can be removed efficiently and orthogonally. The silylallene moiety may be transformed to versatile derivatives that are difficult to access via nonsilylated allenes. The special features and utility of the approach are highlighted through a succinct enantioselective synthesis of marine alkaloid S-(-)-cyclooroidin.

An efficient, practical, and enantioselective method for synthesis of homoallenylamides catalyzed by an aminoalcohol-derived, boron-based catalyst

A practical catalytic method for enantioselective addition of an allene unit to aldimines is disclosed. Transformations are promoted by an in-situ-generated B-based catalyst that is derived from a simple, robust, and readily accessible (in multigram quantities) chiral aminoalcohol. A range of aryl-, heteroaryl-, and alkyl-substituted homoallenylamides can be obtained in 66–91% yield and 84:16 to >99:1 enantiomeric ratio through reactions performed at ambient temperature and in the presence of 0.1–3.0 mol% of the chiral catalyst and a commercially available allenylboron reagent. The catalytic protocol does not require strict anhydrous conditions, can be performed on gram scale, and promotes highly selective addition of an allenyl unit (vs a propargyl group). The utility of the approach is demonstrated through development of succinct approaches to syntheses of anisomycin and epi-cytoxazone.

Anionic and zwitterionic copper(I) complexes incorporating an anionic N-heterocyclic carbene decorated with a malonate backbone: synthesis, structure and catalytic applications

The anionic malonate-derived N-heterocyclic carbenes (maloNHCs) react cleanly and rapidly with copper chloride to generate the anionic complexes of type [(maloNHC)CuCl]·Li, which crystallize in the solid state either in an oligomeric trimer arrangement or in polymeric helixes depending on the substitution pattern and the solvent. Ten zwitterionic heteroleptic Cu(I) complexes combining the anionic maloNHC and a neutral imidazol-2-ylidene are also obtained in a very selective manner and fully characterized. Whereas the anionic complexes are relatively active catalysts for the hydrosilylation of carbonyl compounds, the zwitterionic complexes reveal to be efficient and extremely robust pre-catalysts for the intramolecular cyclopropanation reaction of a diazo ester and outperform the corresponding cationic Cu(i) complexes with classical imidazol-2-ylidenes.

A broadly applicable NHC-Cu-catalyzed approach for efficient, site-, and enantioselective coupling of readily accessible (pinacolato)alkenylboron compounds to allylic phosphates and applications to natural product synthesis

A set of protocols for catalytic enantioselective allylic substitution (EAS) reactions that allow for additions of alkenyl units to readily accessible allylic electrophiles is disclosed. Transformations afford 1,4-dienes that contain a tertiary carbon stereogenic site and are promoted by 1.0–5.0 mol % of a copper complex of an N-heterocyclic carbene (NHC). Aryl- as well as alkyl-substituted electrophiles bearing a di- or trisubstituted alkene may be employed. Reactions can involve a variety of robust alkenyl–(pinacolatoboron) [alkenyl–B(pin)] compounds that can be either purchased or prepared by various efficient, site-, and/or stereoselective catalytic reactions, such as cross-metathesis or proto-boryl additions to terminal alkynes. Vinyl-, E-, or Z-disubstituted alkenyl-, 1,1-disubstituted alkenyl-, acyclic, or heterocyclic trisubstituted alkenyl groups may be added in up to >98% yield, >98:2 S_N2′:S_N2, and 99:1 enantiomeric ratio (er). NHC–Cu-catalyzed EAS with alkenyl–B(pin) reagents containing a conjugated carboxylic ester or aldehyde group proceed to provide the desired 1,4-diene products in good yield and with high enantioselectivity despite the presence of a sensitive stereogenic tertiary carbon center that could be considered prone to epimerization. In most instances, the alternative approach of utilizing an alkenylmetal reagent (e.g., an Al-based species) represents an incompatible option. The utility of the approach is illustrated through applications to enantioselective synthesis of natural products such as santolina alcohol, semburin, nyasol, heliespirone A, and heliannuol E.

Facile access to imidazole and imidazolium substituted dibenzo-diazocines

The C₂-symmetrical 2,8- and 4,10-diimidazo methano dibenzo diazocines were synthesized and converted into alkyl imidazolium salts.

A Ru-catalyzed one-pot synthesis of homopropargylic amines from alkyl azides under photolytic conditions

A new synthetic method for homopropargylic amines from alkyl azides is presented.

Design, synthesis, and applications of chiral N-2-phenyl-2-propyl sulfinyl imines for group-assisted purification (GAP) asymmetric synthesis

A new chiral (Rs)-2-phenyl-2-propyl sulfinamide has been designed and synthesized; its derived aldimines and ketimines have been applied for asymmetric addition reaction with allylmagnesium bromide. The reaction was conveniently performed at room temperature to give a series of homoallylic amines in high yields (up to quant) and diastereoselectivity (up to >99% de). The pure products were obtained by relying on group-assisted purification (GAP) chemistry to avoid traditional purification methods of column chromatography or recrystallization. The conversion of disulfide to (R(s))-thiosulfinate which contains a newly generated polar group was also confirmed to be of the GAP chemistry in which washing crude product can generate pure enantiomer. The absolute stereochemistry has been determined by X-ray analysis.