CuH-Catalyzed Regioselective Intramolecular Hydroamination for the Synthesis of Alkyl-Substituted Chiral Aziridines

Catalytic asymmetric synthesis of 1,2-diamines

The asymmetric catalytic synthesis of 1,2-diamines has received considerable interest, especially in the last ten years, due to their presence in biologically active compounds and their applications for the development of synthetic building blocks, chiral ligands and organocatalysts. Synthetic strategies based on C-N bond-forming reactions involve mainly (a) ring opening of aziridines and azabenzonorbornadienes, (b) hydroamination of allylic amines, (c) hydroamination of enamines and (d) diamination of olefins. In the case of C-C bond-forming reactions are included (a) the aza-Mannich reaction of imino esters, imino nitriles, azlactones, isocyano acetates, and isothiocyanates with imines, (b) the aza-Henry reaction of nitroalkanes with imines, (c) imine-imine coupling reactions, and (d) reductive coupling of enamines with imines, and (e) [3+2] cycloaddition with imines. C-H bond forming reactions include hydrogenation of CN bonds and C-H amination reactions. Other catalytic methods include desymmetrization reactions of meso-diamines.

Nonactivated Aziridine Synthesis by Intermolecular Polarity-Mismatched Carboamination of Unactivated Alkenes with Unactivated Alkyl Halides

A general intermolecular polarity-mismatched carboamination reaction of unactivated alkenes with unactivated alkyl halides has been developed. A series of nonactivated alkyl-substituted aziridines were constructed in exclusive regioselectivity. The dual polarity-mismatched mechanism might be involved.

NQNHC Ligand-Enabled Cu(I)-Catalyzed Double Hydroamination: A Regio- and Chemoselective Bicyclization of o-Amino 1,6-Diyne

In this work, we have developed an efficient method for the intramolecular double hydroamination of aniline by employing o-amino 1,6-diyne as a potential starting material. This protocol enables easy access to bioactive motif 3,4-dihydro-1H-[1,4]oxazino[4,3-a]indole through an intramolecular cascade bicyclization and concomitant isomerization pathway in one pot. This transformation has been effectively achieved by utilizing a stereoelectronically tuned, π-accepting NHC-supported copper(I) system. During ligand optimization trials, naphthoquinone-annulated N-heterocyclic carbene, Nq(IDipp) [1,3-bis(2,6-diisopropylphenyl)-4,5-naphthoquino-imidazolidene]-supported copper(I) complexes of the type Nq(IDipp)CuX (X = Cl or I) were synthesized and fully characterized using various spectroscopic techniques. For this conversion, NHC plays a crucial role in providing the optimum electron density around the metal center. It is a highly regio- and chemoselective transformation with a high atom economy and uses cheap, environmentally benign copper-based catalysts. Furthermore, a plausible mechanism has been proposed on the basis of experimental observations and literature support.

Enantioselective Synthesis of β-Aminoboronic Acids via Borylalkylation of Enamides

Aminoboronic acids represent a class of significant compounds that have attracted significant attention in the fields of drug discovery and organic synthesis. Despite notable progress in their synthesis, the efficient construction of chiral β-aminoboronic acids with alkyl side chains remains a challenging endeavor. Here, we introduce an unprecedented nickel-catalyzed asymmetric borylalkylation of enamides, employing a simple chiral diamine ligand, readily available B2pin2, and alkyl halides as coupling partners. This reaction serves as an efficient platform for assembling a diverse range of β-aminoboronic acid derivatives with flexible alkyl side chains, displaying exceptional regio-, stereo-, and enantioselectivities. Moreover, this transformation exhibits a broad substrate scope and remarkable tolerance toward various functional groups. Theoretical calculations demonstrate that the benzyl group on the ligand is the key to the high enantiocontrol in this transformation. Additionally, we exemplify the practical application of this strategy through the concise synthesis of complex bioactive molecules.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Electrocatalytic Access to Azetidines via Intramolecular Allylic Hydroamination: Scrutinizing Key Oxidation Steps through Electrochemical Kinetic Analysis

Azetidines are prominent structural scaffolds in bioactive molecules, medicinal chemistry, and ligand design for transition metals. However, state-of-the-art methods cannot be applied to intramolecular hydroamination of allylic amine derivatives despite their underlying potential as one of the most prevalent synthetic precursors to azetidines. Herein, we report an electrocatalytic method for intramolecular hydroamination of allylic sulfonamides to access azetidines for the first time. The merger of cobalt catalysis and electricity enables the regioselective generation of key carbocationic intermediates, which could directly undergo intramolecular C-N bond formation. The mechanistic investigations including electrochemical kinetic analysis suggest that either the catalyst regeneration by nucleophilic cyclization or the second electrochemical oxidation to access the carbocationic intermediate is involved in the rate-determining step (RDS) of our electrochemical protocol and highlight the ability of electrochemistry in providing ideal means to mediate catalyst oxidation.

Direct N-Me Aziridination of Enones

The first direct general method for N-Me aziridination of electron-deficient olefins, enones, is described using N-methyl-O-tosylhydroxylamine as the aminating agent in the presence of a Cu(OTf)₂ catalyst. The aziridination of vinyl ketones, hitherto unknown for N-Me as well as N-H, has been achieved efficiently. The open-flask reaction is stereospecific, operationally simple, and additive-free. It also efficiently affords N-H aziridinated products under a similar reaction condition.

Hydroamination, Aminoboration, and Carboamination with Electrophilic Amination Reagents: Umpolung-Enabled Regio- and Stereoselective Synthesis of N-Containing Molecules from Alkenes and Alkynes

Nitrogen (N) is ubiquitously found in bioactive molecules, pharmaceutical agents, and organic functional materials. Accordingly, development of new C-N bond-forming catalysis has been one of the long-standing research subjects in synthetic organic chemistry. In this Perspective, recent advances in highly selective amination reactions with electrophilic amination reagents are described: by taking advantage of the concept of nitrogen umpolung, otherwise challenging aminofunctionalizations, such as hydroamination, aminoboration, and carboamination, of readily available feedstock-like alkenes and alkynes are possible, giving densely functionalized complex and often chiral alkylamines with high selectivity. The scope, limitations, and reaction mechanism are briefly summarized.

Asymmetric hydroamination with far fewer chiral species than copper centers achieved by tuning the structure of supramolecular helical catalysts

Mixing a BTA ligand (in black), a “sergeant” (in blue) and an achiral BTA additive (in orange) affords the amination product in 75% e.e. even though only one “sergeant” for ca. 10 copper centers are present in the supramolecular helical catalyst.

An umpolung-enabled copper-catalysed regioselective hydroamination approach to α-amino acids

A copper-catalysed regio- and stereoselective hydroamination of acrylates with hydrosilanes and hydroxylamines has been developed to afford the corresponding α-amino acids in good yields. The key to regioselectivity control is the use of hydroxylamine as an umpolung, electrophilic amination reagent. Additionally, a judicious choice of conditions involving the CsOPiv base and DTBM-dppbz ligand of remote steric hindrance enables the otherwise challenging C-N bond formation at the α position to the carbonyl. The point chirality at the β-position is successfully controlled by the Xyl-BINAP or DTBM-SEGPHOS chiral ligand with similarly remote steric bulkiness. The combination with the chiral auxiliary, (-)-8-phenylmenthol, also induces stereoselectivity at the α-position to form the optically active unnatural α-amino acids with two adjacent stereocentres.

Aziridine synthesis by coupling amines and alkenes via an electrogenerated dication

Aziridines-three-membered nitrogen-containing cyclic molecules-are important synthetic targets. Their substantial ring strain and resultant proclivity towards ring-opening reactions makes them versatile precursors of diverse amine products^1-3, and, in some cases, the aziridine functional group itself imbues important biological (for example, anti-tumour) activity^4-6. Transformation of ubiquitous alkenes into aziridines is an attractive synthetic strategy, but is typically accomplished using electrophilic nitrogen sources rather than widely available amine nucleophiles. Here we show that unactivated alkenes can be electrochemically transformed into a metastable, dicationic intermediate that undergoes aziridination with primary amines under basic conditions. This new approach expands the scope of readily accessible N-alkyl aziridine products relative to those obtained through existing state-of-the-art methods. A key strategic advantage of this approach is that oxidative alkene activation is decoupled from the aziridination step, enabling a wide range of commercially available but oxidatively sensitive⁷ amines to act as coupling partners for this strain-inducing transformation. More broadly, our work lays the foundations for a diverse array of difunctionalization reactions using this dication pool approach.

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

This review summarizes the most noteworthy achievements in the field of C–O and C–N bond formation by hydroalkoxylation and hydroamination reactions on unactivated alkenes (including 1,2- and 1,3-dienes) promoted by earth-abundant 3d transition metal catalysts based on manganese, iron, cobalt, nickel, copper and zinc. The relevant literature from 2012 until early 2021 has been covered.

Design, scope and mechanism of highly active and selective chiral NHC-iridium catalysts for the intramolecular hydroamination of a variety of unactivated aminoalkenes

Chiral, cationic NHC-iridium complexes are introduced as catalysts for the intramolecular hydroamination reaction of unactivated aminoalkenes. The catalysts show high activity in the construction of a range of 5- and 6-membered N-heterocycles, which are accessed in excellent optical purity, with various functional groups being tolerated with this system. A major deactivation pathway is presented and eliminated by using alternative reaction conditions. A detailed experimental and computational study on the reaction mechanism is performed providing valuable insights into the mode of action of the catalytic system and pointing to future modifications to be made for this catalytic platform.

Hydroxylamines as One-Atom Nitrogen Sources for Metal-Catalyzed Cycloadditions

Transition-metal-catalyzed C–N bond formation is one of the most important pathways to synthesize N-heterocycles. Hydroxylamines can be transformed into a nucleophilic reagent to react with a carbon cation or coordinate with a transition metal; it can also become an electrophilic nitrogen source to react with arenes, alkenes, and alkynes. In this short review, the progress made on transition-metal-catalyzed cycloadditions with hydroxylamines as a nitrogen source is summarized.

1 Introduction

2 Cycloaddition To Form Aziridine Derivatives

2.1 Intramolecular Cycloaddition To Form Aziridine Derivatives

2.2 Intermolecular Cycloaddition To Form Aziridine Derivatives

3 Cycloaddition To Form Indole Derivatives

4 Cycloaddition To Form Other N-Heterocycles

4.1 Aza-Heck-Type Amination Reactions

4.2 Nitrene Insertion Amination Reactions

4.3 Intramolecular Nucleophilic and Electrophilic Amination Reactions

5 Conclusion and Outlook

NiH-Catalyzed Proximal-Selective Hydroamination of Unactivated Alkenes

Reported herein is a modular, NiH-catalyzed system capable of proximal-selective hydroamination of unactivated alkenes with diverse amine sources. The key to the successful implementation of this approach is the promotion of NiH insertion into even highly substituted olefins via coordination of the bidentate directing group to the nickel complex. A wide range of primary and secondary amines can be installed in both internal and terminal unactivated alkenes with excellent regiocontrol under the optimized reaction conditions. This protocol is flexible and general for the preparation of a variety of valuable β- and γ-amino acid building blocks that would otherwise be difficult to synthesize. The utility of this transformation was further demonstrated by the site-selective late-stage modification of complex and medicinally relevant molecules. Combined experimental and computational studies illuminate the detailed reaction mechanism.

Cu-Catalyzed Regioselective C-H Alkylation of Benzimidazoles with Aromatic Alkenes

Herein we report a novel Cu-catalyzed regioselective C2-H alkylation of benzimidazoles with aromatic alkenes. The reaction features exclusive regioselectivity and broad substrate scope in the intermolecular alkylation of benzimidazoles with terminal and internal aromatic alkenes, constituting a modular access toward benzimidazole-containing 1,1-di(hetero)aryl alkanes. The intramolecular C2-H alkylation of benzimidazoles with aromatic alkenes has been achieved in an endo-selective manner. The enantioselective C2 alkylation of benzimidazoles has also been realized with moderate to good stereocontrol.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

CuH-Catalyzed Olefin Functionalization: From Hydroamination to Carbonyl Addition

In organic synthesis, ligand-modified copper(I) hydride (CuH) complexes have become well-known reagents and catalysts for selective reduction, particularly toward Michael acceptors and carbonyl compounds. Recently, our group and others have found that these hydride complexes undergo migratory insertion (hydrocupration) with relatively unactivated and electronically unpolarized olefins, producing alkylcopper intermediates that can be leveraged to forge a variety of useful bonds. The resulting formal hydrofunctionalization reactions have formed the basis for a resurgence of research in CuH catalysis. This Account chronicles the development of this concept in our research group, highlighting its origin in the context of asymmetric hydroamination, evolution to more general C-X bond-forming reactions, and applications in the addition of olefin-derived nucleophiles to carbonyl derivatives.Hydroamination, the formal insertion of an olefin into the N-H bond of an amine, is a process of significant academic and industrial interest, due to its potential to transform widely available alkenes and alkynes into valuable complex amines. We developed a polarity-reversed strategy for catalytic enantioselective hydroamination relying on the reaction of olefins with CuH to generate chiral organocopper intermediates, which are intercepted by electrophilic amine reagents. By engineering the auxiliary ligand, amine electrophile, and reaction conditions, the scope of this method has since been extended to include many types of olefins, including challenging internal olefins. Further, the scope of amine reagents has been expanded to enable the synthesis of primary, secondary, and tertiary amines as well as amides, N-alkylated heterocycles, and anilines. All of these reactions exhibit high regio- and stereoselectivity and, due to the mild conditions required, excellent tolerance for heterocycles and polar functional groups.Though the generation of alkylcopper species from olefins was originally devised as a means to solve the hydroamination problem, we soon found that these intermediates could react efficiently with an unexpectedly broad range of electrophiles, including alkyl halides, silicon reagents, arylpalladium species, heterocycles, and carbonyl derivatives. The general ability of olefins to function as precursors for nucleophilic intermediates has proved particularly advantageous in carbonyl addition reactions because it overcomes many of the disadvantages associated with traditional organometallic reagents. By removing the need for pregeneration of the nucleophile in a separate operation, CuH-catalyzed addition reactions of olefin-derived nucleophiles feature improved step economy, enhanced functional group tolerance, and the potential for catalyst control over regio- and stereoselectivity. Following this paradigm, feedstock olefins such as allene, butadiene, and styrene have been employed as reagents for asymmetric alkylation of ketones, imines, and aldehydes.

I2/TBHP mediated diastereoselective synthesis of spiroaziridines

A simple metal free protocol for the diastereoselective synthesis of spiroaziridines using easily accessible substrates is disclosed.

Cascade CuH-Catalysed Conversion of Alkynes to Enantioenriched 1,1-Disubstituted Products

Enantioenriched α-aminoboronic acids play a unique role in medicinal chemistry and have emerged as privileged pharmacophores in proteasome inhibitors. Additionally, they represent synthetically useful chiral building blocks in organic synthesis. Recently, CuH-catalyzed asymmetric alkene hydrofunctionalization has become a powerful tool to construct stereogenic carbon centers. In contrast, applying CuH cascade catalysis to achieve reductive 1,1-difunctionalization of alkynes remains an important, but largely unaddressed, synthetic challenge. Herein, we report an efficient strategy to synthesize α-aminoboronates via CuH-catalyzed hydroboration/hydroamination cascade of readily available alkynes. Notably, this transformation selectively delivers the desired 1,1-heterodifunctionalized product in favor of alternative homodifunctionalized, 1,2-heterodifunctionalized, or reductively monofunctionalized byproducts, thereby offering rapid access to these privileged scaffolds with high chemo-, regio- and enantioselectivity.

Asymmetric Synthesis of γ-Amino Alcohols by Copper-Catalyzed Hydroamination

Asymmetric synthesis of γ-amino alcohols from unprotected allylic alcohols by a copper-catalyzed hydroamination strategy has been developed. Using easily accessible starting materials, a range of chiral 1,3-amino alcohols were prepared with excellent regio- and enantioselectivity. Further, this protocol provided an efficient one-step method for the enantioselective synthesis of γ-amino alcohols in an intermolecular manner.

Enantioselective Synthesis of 4-Aminotetrahydroquinolines via 1,2-Reductive Dearomatization of Quinolines and Copper(I) Hydride-Catalyzed Asymmetric Hydroamination

A 1,2-reductive dearomatization of quinolines and copper(II) acetate monohydrate/( R, R)-Ph-BPE/P( p-tolyl)₃-catalyzed enantioselective hydroamination sequence was developed, affording diverse 4-amino-1,2,3,4-tetrahydroquinolines with high levels of enantioselectivity in either a stepwise or one-pot fashion. Pleasingly, internal cis-cyclic alkenes, which are challenging substrates in copper hydride-catalyzed enantioselective hydroamination reactions, were transformed efficiently under mild conditions.

Regio- and Enantioselective Synthesis of 1,2-Diamine Derivatives by Copper-Catalyzed Hydroamination

A highly regio- and enantioselective synthesis of 1,2-diamine derivatives from γ-substituted allylic pivalamides using copper-catalyzed hydroamination is reported. The N-pivaloyl group is essential, in both facilitating the hydrocupration step and suppressing an unproductive β-elimination from the alkylcopper intermediate. This approach enables an efficient construction of chiral differentially protected vicinal diamines under mild conditions with broad functional group tolerance.

Catalytic Asymmetric Synthesis of α-Arylpyrrolidines and Benzo-fused Nitrogen Heterocycles

Herein, we report a practical two-step synthetic route to α-arylpyrrolidines through Suzuki-Miyaura cross-coupling and enantioselective copper-catalyzed intramolecular hydroamination reactions. The excellent stereoselectivity and broad scope for the transformation of substrates with pharmaceutically relevant heteroarenes render this method a practical and versatile approach for pyrrolidine synthesis. Additionally, this intramolecular hydroamination strategy facilitates the asymmetric synthesis of tetrahydroisoquinolines and medium-ring dibenzo-fused nitrogen heterocycles.

Construction of N-Alkyl- and N-Arylaziridines from Unprotected Amines via C-H Oxidative Amination Strategy

A copper-promoted intramolecular C-H oxidative amination reaction between secondary amine (N-H) and C(sp³)-H at the benzylic position of azaarenes or α-position of ketones for the synthesis of aziridine derivatives has been developed. Moreover, a practical annulation of electron-deficient vinylarenes with an unprotected primary alkyl amine by a Yb(OTf)₃-CuI relay system has also been reported. The reactions were carried out with oxygen as the sole oxidant to give the N-alkyl- and N-arylaziridines in good yields.

Vinylphosphonium Salt-Mediated Reactions: A One-Pot Condensation Approach for the Highly cis-Selective Synthesis of N-Benzoylaziridines and the Green Synthesis of 1,4,2-Dioxazoles as Two Important Classes of Heterocyclic Compounds

An efficient, one-pot, and convenient approach for the reaction of the same precursors, trialkyl(aryl) phosphines, acetylene diesters, and benzhydroxamic acids has been developed to produce two important classes of heterocyclic compounds: N-benzoylaziridines and 1,4,2-dioxazoles. The strategy utilizes the intermediate solvation as a key step in product selectivity. The usefulness of the developed approach has been confirmed in the unprecedented highly cis-selective formation of the N-benzoylaziridines. In addition, the procedure provides a green alternative method for the synthesis of 1,4,2-dioxazoles employing a β-cyclodextrin nanoreactor in aqueous media.

A Practical Electrophilic Nitrogen Source for the Synthesis of Chiral Primary Amines by Copper-Catalyzed Hydroamination

A mild and practical method for the catalytic installation of the amino group across alkenes and alkynes has long been recognized as a significant challenge in synthetic chemistry. As the direct hydroamination of olefins using ammonia requires harsh conditions, the development of suitable electrophilic aminating reagents for formal hydroamination methods is of importance. Herein, we describe the use of 1,2-benzisoxazole as a practical electrophilic primary amine source. Using this heterocycle as a new amino group delivery agent, a mild and general protocol for the copper-hydride-catalyzed hydroamination of alkenes and alkynes to form primary amines was developed. This method provides access to a broad range of chiral α-branched primary amines and linear primary amines, as demonstrated by the efficient synthesis of the antiretroviral drug maraviroc and the formal synthesis of several other pharmaceutical agents.

Copper-catalysed synthesis of alkylidene 2-pyrrolinone derivatives from the combination of α-keto amides and alkynes

A Cu(i)-catalysed addition and cyclisation sequence has been developed for the synthesis of (E)-alkylidene pyrrolinone derivatives. The reactions incorporate simple α-keto amides and alkynes as substrates, and employ a commercially available Cu(i) catalyst. The process tolerates good variation of both starting materials, and delivers the desired pyrrolinones in good yields, with high levels of stereocontrol.