The Redox-A3 Reaction

Photoredox-Catalyzed Alkynylation of C(sp3)-H Bonds Adjacent to a Nitrogen Atom of Tertiary Amines with Alkynyl Bromides

A novel and robust alkynylation of C(sp3)-H bonds adjacent to a nitrogen atom of tertiary amines with alkynyl bromides as radical alkynylating reagents has been realized under visible-light irradiation. A range variety of tertiary amines including N-arylamines and N-alkylamine have been coupled with both aromatic and aliphatic alkynyl bromides to furnish 51 examples of propargylamines in moderate to excellent yields (31-80% yields). The possible mechanism was a radical addition-elimination process based on preliminary mechanistic studies.

Metal-free synthesis of propargylamines via light-mediated persulfate activation and phase-transfer catalysis

We present a metal-free method to synthesize secondary and tertiary propargylamines from primary and secondary amines and alkynes using light-mediated persulfate activation and phase-transfer catalysis. Our method explores a tandem oxidative coupling/alkynylation reaction for the generation of diverse compounds, highlighting the sustainability of the process and its wide scope.

Atomically precise ultrasmall copper cluster for room-temperature highly regioselective dehydrogenative coupling

Three-component dehydrogenative coupling reactions represent important and practical methodologies for forging new C-N bonds and C-C bonds. Achieving highly all-in-one dehydrogenative coupling functionalization by a single catalytic system remains a great challenge. Herein, we develop a rigid-flexible-coupled copper cluster [Cu3(NHC)3(PF6)3] (Cu3NC(NHC)) using a tridentate N-heterocyclic carbene ligand. The shell ligand endows Cu3NC(NHC) with dual attributes, including rigidity and flexibility, to improve activity and stability. The Cu3NC(NHC) is applied to catalyze both highly all-in-one dehydrogenative coupling transformations. Mechanistic studies and density functional theory illustrate that the improved regioselectivity is derived from the low energy of ion pair with copper acetylide and endo-iminium ions and the low transition state, which originates from the unique physicochemical properties of the Cu3NC(NHC) catalyst. This work highlights the importance of N-heterocyclic carbene in the modification of copper clusters, providing a new design rule to protect cluster catalytic centers and enhance catalysis.

Regiospecific α-methylene functionalisation of tertiary amines with alkynes via Au-catalysed concerted one-proton/two-electron transfer to O2

Regioselective transformations of tertiary amines, which are ubiquitously present in natural products and drugs, are important for the development of novel medicines. In particular, the oxidative α-C-H functionalisation of tertiary amines with nucleophiles via iminium cations is a promising approach because, theoretically, there is almost no limit to the type of amine and functionalisation. However, most of the reports on oxidative α-C-H functionalisations are limited to α-methyl-selective or non-selective reactions, despite the frequent appearance of α-methylene-substituted amines in pharmaceutical fields. Herein, we develop an unusual oxidative regiospecific α-methylene functionalisation of structurally diverse tertiary amines with alkynes to synthesise various propargylic amines using a catalyst comprising Zn salts and hydroxyapatite-supported Au nanoparticles. Thorough experimental investigations suggest that the unusual α-methylene regiospecificity is probably due to a concerted one-proton/two-electron transfer from amines to O₂ on the Au nanoparticle catalyst, which paves the way to other α-methylene-specific functionalisations.

Synthesis of nitrogen-tethered 1,6-enynes through CuI/TFA catalysis

A new cascade process of the TFA-promoted Pictet–Spengler reaction and Cu-catalyzed intermolecular hydride transfer/alkynylation of an amine, formaldehyde solution, and an alkyne has been developed.

Room-Temperature Cu(II) Radical-Triggered Alkyne C-H Activation

A dimeric Cu(II) complex [Cu(II)2L2(μ2-Cl)Cl] (1) built from an asymmetric tridentate ligand (2-(((2-aminocyclohexyl)imino)methyl)-4,6-di-tert-butylphenol) and weakly coordinating anions has been synthesized and structurally characterized. In dichloromethane solution, 1 exists in a monomeric [Cu(II)LCl] (1') (85%)-dimeric (1) (15%) equilibrium, and cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) studies indicate structural stability and redox retention. Addition of phenylacetylene to the CH2Cl2 solution populates 1' and leads to the formation of a transient radical species. Theoretical studies support this notion and show that the radical initiates an alkyne C-H bond activation process via a four-membered ring (Cu(II)-O···H-Calkyne) intermediate. This unusual C-H activation method is applicable for the efficient synthesis of propargylamines, without additives, within 16 h, at low loadings and in noncoordinating solvents including late-stage functionalization of important bioactive molecules. Single-crystal X-ray diffraction studies, postcatalysis, confirmed the framework's stability and showed that the metal center preserves its oxidation state. The scope and limitations of this unconventional protocol are discussed.

Condensation-Based Methods for the C-H Bond Functionalization of Amines

This review aims to provide a comprehensive overview of condensation-based methods for the C-H bond functionalization of amines that feature azomethine ylides as key intermediates. These transformations are typically redox-neutral and share common attributes with classic name reactions such as the Strecker, Mannich, Friedel-Crafts, Pictet-Spengler, and Kabachnik-Fields reaction, while incorporating a redox-isomerization step. This approach provides an ideal platform to rapidly transform simple starting materials into complex amines.

Enantioselective decarboxylative Mannich reaction of β-keto acids with C-alkynyl N-Boc N,O-acetals: access to chiral β-keto propargylamines

The chiral keto-substituted propargylamines are an essential class of multifunctional compounds in the field of organic and pharmaceutical synthesis and have attracted considerable attention, but the related synthetic approaches remain limited. Therefore, a concise and efficient method for the enantioselective synthesis of β-keto propargylamines via chiral phosphoric acid-catalyzed asymmetric Mannich reaction between β-keto acids and C-alkynyl N-Boc N,O-acetals as easily available C-alkynyl imine precursors has been demonstrated here, affording a broad scope of β-keto N-Boc-propargylamines in high yields (up to 97%) with generally high enantioselectivities (up to 97 : 3 er).

α,α'-C-H Bond Difunctionalization of Unprotected Alicyclic Amines

A simple one-pot procedure enables the sequential, regioselective, and diastereoselective introduction of the same or two different substituents to the α- and α'-positions of unprotected azacycles. Aryl, alkyl, and alkenyl substituents are introduced via their corresponding organolithium compounds. The scope of this transformation includes pyrrolidines, piperidines, azepanes, and piperazines.

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

α-C-H Bond Functionalization of Unprotected Alicyclic Amines: Lewis-Acid-Promoted Addition of Enolates to Transient Imines

Enolizable cyclic imines, obtained in situ from their corresponding lithium amides by oxidation with simple ketone oxidants, are readily alkylated with a range of enolates to provide mono- and polycyclic β-aminoketones in a single operation, including the natural product (±)-myrtine. Nitrile anions also serve as competent nucleophiles in these transformations, which are promoted by BF3 etherate. β-Aminoesters derived from ester enolates can be converted to the corresponding β-lactams.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

Traceless Redox-Annulations of Alicyclic Amines

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-(nitromethyl)benzaldehyde. Benzoic acid acts as a promoter in these reactions, which involve concurrent amine α-C-H bond and N-H bond functionalization. Subsequent removal of the nitro group provides access to tetrahydroprotoberberines not accessible via typical redox-annulations. Also reported are decarboxylative annulations of ortho-(nitromethyl)benzaldehyde with proline and pipecolic acid.

L-Proline-catalyzed regioselective C1 arylation of tetrahydroisoquinolines through a multicomponent reaction under solvent-free conditions

Here we disclose the C1 arylation of tetrahydroisoquinolines (THIQ) through regioselective C(sp3)-H functionalization using a multicomponent reaction. The reaction was performed by reacting THIQ, aldehydes and aminopyrazoles or indoles under neat conditions with l-proline as a catalyst. The regioselectivity of the products was confirmed by X-ray analysis and spectroscopic data. The formation of an azomethine ylide intermediate is crucial for obtaining the regioselectivity.

Redox-Annulations of Cyclic Amines with ortho-Cyanomethylbenzaldehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-cyanomethylbenzaldehydes. These amine α-C-H bond functionalization reactions are promoted by acetic acid. The resulting β-aminonitriles can be converted to the corresponding β-aminoalcohols in diastereoselective fashion.

Gram-Scale Synthesis of 3-Sulfonyl Flavanones

In this paper, an easy-operational, high-yielding method for the gram-scale synthesis of 3-sulfonyl flavanones is described by a one-pot straightforward POCl₃ mediated intermolecular (5 + 1) annulation of the β-ketosulfones with an o-hydroxyaryl group (dual nucleophile) and arylaldehydes (dual electrophile) in refluxing toluene for 3 h. A plausible mechanism is proposed and discussed. This protocol provides a highly effective annulation via one carbon-oxygen (C-O) and one carbon-carbon (C-C) bond formations.

Divergent synthesis of N-heterocyclic 1,6-enynes through a zinc-catalyzed decarboxylative A3 reaction

A zinc-catalyzed decarboxylative A³ reaction of cyclic amino acids, α,β-unsaturated aldehydes and terminal alkynes has been developed. A series of functionalized N-heterocyclic 1,6-enynes have been successfully obtained with excellent regioselectivities through this novel approach. In addition, the utility of this straightforward process is demonstrated by the preparation of a polycyclic nitrogen-containing heterocyclic compound.

Synthetic Access to Secondary Propargylamines via a Copper-Catalyzed Oxidative Deamination/Alkynylation Cascade

A novel and selective cascade reaction of primary amines and alkynes for the synthesis of the corresponding secondary propargylamines is described. This protocol proceeds with a CuBr₂/TBHP system through a process of oxidative deamination of primary amines to imine and alkynylation, featuring a wide scope of substrates with good functional-group tolerance and operational simplicity. Additionally, the use of two different primary amines could also work smoothly using this protocol.

α-Functionalization of Cyclic Secondary Amines: Lewis Acid Promoted Addition of Organometallics to Transient Imines

Cyclic imines, generated in situ from their corresponding N-lithiated amines and a ketone hydride acceptor, undergo reactions with a range of organometallic nucleophiles to generate α-functionalized amines in a single operation. Activation of the transient imines by Lewis acids that are compatible with the presence of lithium alkoxides was found to be crucial to accommodate a broad range of nucleophiles including lithium acetylides, Grignard reagents, and aryllithiums with attenuated reactivities.

Redox-Annulations of Cyclic Amines with Electron-Deficient o-Tolualdehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-methyl-3,5-dinitrobenzaldehyde and closely related substrates. Acetic acid serves as the solvent and sole promoter of these transformations which involve dual C-H functionalization.

A metal-catalyzed new approach for α-alkynylation of cyclic amines

The first catalytic α-alkynylation of cyclic amines with the help of the N-propargylic group with an exclusive high E-stereoselectivity has been realized.

The first catalytic α-alkynylation of cyclic amines with the help of the N-propargylic group to afford 2-(1-alkynyl) N-allylic cyclic amines with an exclusive E-stereoselectivity for the in situ formed C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond has been realized. Based on mechanistic studies, it is proven that the reaction proceeds through metal-mediated anti-1,5-hydride transfer forming an iminonium intermediate, which accepts the addition of the in situ generated 1-alkynyl metal species. The synthetic application has also been demonstrated.

Csp3-H bond functionalization of amines via tunable iminium ions: divergent synthesis of trifluoromethylated arylamines

A series of tunable iminium ions, generated in situ by the condensation of 4-trifluoromethyl-p-quinols with cyclic amines, can lead to the divergent synthesis of trifluoromethylated arylamines in a single step via redox-neutral isomerization. The direct α- and β-functionalization of saturated amines can be achieved regioselectively under mild conditions.

Redox-Annulations of Cyclic Amines with 2-(2-Oxoethyl)malonates

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-(2-oxoethyl)malonates in the presence of catalytic amounts of benzoic acid. These reactions install a fully saturated five-membered ring and provide access to structures closely related to the natural products crispine A and harmicine.

Synthesis of Polycyclic Imidazolidinones via Amine Redox-Annulation

α-Ketoamides undergo redox-annulations with cyclic secondary amines, such as 1,2,3,4-tetrahydroisoquinoline, pyrrolidine, piperidine, and morpholine. Catalytic amounts of benzoic acid significantly accelerate these transformations. This approach provides polycyclic imidazolidinone derivatives in typically good yields.

Asymmetric Synthesis of Tetrahydroisoquinoline Alkaloids Using Ellman's Chiral Auxiliary

Chiral t-butylsulfinamide has been successfully employed for the stereoselective synthesis of 1-benzyl tetrahydroisoquinoline alkaloids. This is the first report on the synthesis of chiral 1-benzyltetrahydroisoquinoline natural products using tert-butylsulfinamide through a haloamide cyclization.

Acetic Acid Promoted Redox Annulations with Dual C-H Functionalization

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-alkylquinoline-3-carbaldehydes as well as the corresponding 4-alkyl isomers and pyridine analogues. These processes involve dual C-H bond functionalization. Acetic acid is used as a cosolvent and acts as the sole promoter of these transformations.

Enantioselective synthesis of naturally occurring isoquinoline alkaloids: (S)-(−)-trolline and (R)-(+)-oleracein E

The highly efficient asymmetric synthesis of a pair of enantiomers, (S)-(−)-trolline and (R)-(+)-oleracein E, has been achieved in five steps.

Controlling chemoselectivity in copper-catalyzed decarboxylative A3/A3 cross-couplings: direct formation of unsymmetrical 1,4-diamino-2-butynes

A direct cross-coupling of propiolic acid with two kinds of in situ formed iminiums has been achieved via a CuI/CuCl₂-catalyzed decarboxylative A³/A³ domino process.

Redox-Neutral Aromatization of Cyclic Amines: Mechanistic Insights and Harnessing of Reactive Intermediates for Amine α- and β-C-H Functionalization

Cyclic amines such as pyrrolidine and piperidine are known to undergo condensations with aldehydes to furnish pyrrole and pyridine derivatives, respectively. A combined experimental and computational study provides detailed insights into the mechanism of pyrrole formation. A number of reactive intermediates (e.g., azomethine ylides, conjugated azomethine ylides, enamines) were intercepted, outlining strategies for circumventing aromatization as a valuable pathway for amine C-H functionalization.