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

Site-Selective Distal C(sp3)-H Bromination of Aliphatic Amines as a Gateway for Forging Nitrogen-Containing sp3 Architectures

Herein, we disclose a new strategy that rapidly and reliably incorporates bromine atoms at distal, secondary C(sp3)-H sites in aliphatic amines with an excellent and predictable site-selectivity pattern. The resulting halogenated building blocks serve as versatile linchpins to enable a series of carbon-carbon and carbon-heteroatom bond-formations at remote C(sp3) sites, thus offering a new modular and unified platform that expediates the access to advanced sp3 architectures possessing valuable nitrogen-containing saturated heterocycles of interest in medicinal chemistry settings.

Regioselective α-Phosphonylation of Unprotected Alicyclic Amines

Unprotected alicyclic amines undergo α-C-H bond phosphonylation via a two-stage one-pot process involving the oxidation of amine-derived lithium amides with simple ketone oxidants, generating transient imines which are then captured with phosphites or phosphine oxides. Amines with an existing α-substituent undergo regioselective α'-phosphonylation. Amine α-arylation and α'-phosphonylation can be combined, generating a difunctionalized product in a single operation.

Palladium-Catalyzed Allylation of Endocyclic 1-Azaallyl Anions

Endocyclic 1-azaallyl anions engage allyl acetates in a palladium-catalyzed allylation followed by reduction to give unprotected 2-(hetero)aryl-3-allylpiperidines and 2-allyl-3-arylmorpholines, products not easily accessible by other means. The allyl group is then readily transformed into a variety of functional groups. Preliminary studies on the asymmetric variant of the reaction using an enantiomerically pure BI-DIME-type ligand provide the product with moderate enantioselectivity. Computational studies suggest that energy barriers of inner-sphere reductive elimination and outer-sphere nucleophilic substitution are almost the same, which makes both of them possible reaction pathways. In addition, the inner-sphere mechanism displays an enantiodiscriminating C-C bond forming step, while the outer-sphere mechanism is much less selective, which combined to give the asymmetric variant of the reaction moderate enantioselectivity.

Electrochemical synthesis of peptide aldehydes via C‒N bond cleavage of cyclic amines

Peptide aldehydes are crucial biomolecules essential to various biological systems, driving a continuous demand for efficient synthesis methods. Herein, we develop a metal-free, facile, and biocompatible strategy for direct electrochemical synthesis of unnatural peptide aldehydes. This electro-oxidative approach enabled a step- and atom-economical ring-opening via C‒N bond cleavage, allowing for homoproline-specific peptide diversification and expansion of substrate scope to include amides, esters, and cyclic amines of various sizes. The remarkable efficacy of the electro-synthetic protocol set the stage for the efficient modification and assembly of linear and macrocyclic peptides using a concise synthetic sequence with racemization-free conditions. Moreover, the combination of experiments and density functional theory (DFT) calculations indicates that different N-acyl groups play a decisive role in the reaction activity.

Palladium-Catalyzed Annulation of Tertiary Anilines with 3-Butenoic Acid via Dual C-H Bond Activation

Catalytic cyclization via dual C-H bond activation has evolved as a powerful strategy for building bi- and polycyclic molecules. Herein, a palladium-catalyzed annulation of tertiary anilines with 3-butenoic acid via N-α-C(sp3)-H and ortho-C(sp2)-H activation is described. The remarkable characteristics of this reaction include excellent diastereoselectivity, broad substrate scope, and good tolerance for some highly sensitive groups. In addition, the KIE experiment suggested that the C-H bond abscission is not the turnover-limiting step.

Copper-Nitroxyl-Catalyzed α-Oxygenation of Cyclic Secondary Amines Including Application to Late-Stage Functionalization

Cyclic secondary amines are prominent subunits in pharmaceutical compounds. Methods for direct functionalization of N-unprotected/unsubstituted piperidines and related heterocycles have limited precedent despite their potential to impact medicinal chemistry and organic synthesis. Herein, we report a Cu/nitroxyl co-catalyzed method for direct conversion of cyclic secondary amines to the corresponding lactams via aerobic dehydrogenation and oxidative coupling with water. The mild reaction conditions tolerate diverse functional groups, enabling application to molecules that cover broad chemical space. The method is showcased in selective functionalization of building blocks and complex molecules, including late-stage functionalization of bromodomain inhibitors.

C(sp3)-H cyclizations of 2-(2-vinyl)phenoxy-tert-anilines

1,5-hydride transfer-triggered cyclization reactions offering a robust method for C(sp3)-C(sp3) coupling and the synthesis of e.g. tetrahydroquinolines have been thoroughly investigated in the literature. Catalysts allowing milder reaction conditions or the development of enantioselective processes were important recent contributions to the field, as well as the studies on subtrates with oxygen or sulfur heteroatoms (besides the originally described nitrogen heterocycles). In a series of studies, we focused on expanded, higher order H-transfers/cyclizations by positioning the interacting substituents on distanced rings. Cyclizations of appropriately functionalized biaryl and fused bicyclic systems led to 7-9 membered rings. In the frame of this research, we set out to study the feasibility of the cyclization and the factors affecting it by in silico methods. The conclusions drawn from computational studies were complemented by cyclization screens on 2-(2-vinyl)phenoxy-tert-anilines and their CH2-expanded analogues, the results of which are presented here. Besides isolating the expected oxazonine products in several cases, we also observed a unique dimer formation, leading to an interesting 5-6-5 ring system.

Redox-neutral α-functionalization of pyrrolidines: facile access to α-aryl-substituted pyrrolidines

α-Aryl-substituted pyrrolidine moiety is found in many natural alkaloids. Starting from pyrrolidine, we were able to synthesize α-aryl-substituted pyrrolidines in one step using quinone monoacetal as the oxidizing agent and DABCO as the base. We also discovered the reaction condition needed to efficiently remove the N-aryl moiety from the α-arylated product. When the above reaction was carried out without the addition of an aryl nucleophile, the reaction of pyrrolidine and quinone monoacetal in 2,2,2-trifluoroethanol afforded octahydro-dipyrroloquinoline in high yield, which has the same skeleton as that of natural product incargranine B.

Formaldehyde-Mediated Hydride Liberation of Alkylamines for Intermolecular Reactions in Hexafluoroisopropanol

The ability of alkylamines to spontaneously liberate hydride ions is typically restrained, except under specific intramolecular reaction settings. Herein, we demonstrate that this reactivity can be unlocked through simple treatment with formaldehyde in hexafluoroisopropanol (HFIP) solvent, thereby enabling various intermolecular hydride transfer reactions of alkylamines under mild conditions. Besides transformations of small molecules, these reactions enable unique late-stage modification of complex peptides. Mechanistic investigations uncover that the key to these intermolecular hydride transfer processes lies in the accommodating conformation of solvent-mediated macrocyclic transition states, where the aggregates of HFIP molecules act as dexterous proton shuttles. Importantly, negative hyperconjugation between the lone electron pair of nitrogen and the antibonding orbital of amine's α C-H bond plays a critical role in the C-H activation, promoting its hydride liberation.

Mechanistic Investigation, Wavelength-Dependent Reactivity, and Expanded Reactivity of N-Aryl Azacycle Photomediated Ring Contractions

Under mild blue-light irradiation, α-acylated saturated heterocycles undergo a photomediated one-atom ring contraction that extrudes a heteroatom from the cyclic core. However, for nitrogenous heterocycles, this powerful skeletal edit has been limited to substrates bearing electron-withdrawing substituents on nitrogen. Moreover, the mechanism and wavelength-dependent efficiency of this transformation have remained unclear. In this work, we increased the electron richness of nitrogen in saturated azacycles to improve light absorption and strengthen critical intramolecular hydrogen bonding while enabling the direct installation of the photoreactive handle. As a result, a broadly expanded substrate scope, including underexplored electron-rich substrates and previously unsuccessful heterocycles, has now been achieved. The significantly improved yields and diastereoselectivities have facilitated reaction rate, kinetic isotope effect (KIE), and quenching studies, in addition to the determination of quantum yields. Guided by these studies, we propose a revised ET/PT mechanism for the ring contraction, which is additionally corroborated by computational characterization of the lowest-energy excited states of α-acylated substrates through time-dependent DFT. The efficiency of the ring contraction at wavelengths longer than those strongly absorbed by the substrates was investigated through wavelength-dependent rate measurements, which revealed a red shift of the photochemical action plot relative to substrate absorbance. The elucidated mechanistic and photophysical details effectively rationalize empirical observations, including additive effects, that were previously poorly understood. Our findings not only demonstrate enhanced synthetic utility of the photomediated ring contraction and shed light on mechanistic details but may also offer valuable guidance for understanding wavelength-dependent reactivity for related photochemical systems.

Alicyclic-Amine-Derived Imine-BF3 Complexes: Easy-to-Make Building Blocks for the Synthesis of Valuable α-Functionalized Azacycles

A new strategy to access α-functionalized alicyclic amines via their corresponding imine-BF3 complexes is reported. Isolable imine-BF3 complexes, readily prepared via dehydrohalogenation of N-bromoamines in a base-promoted/18-crown-6 catalyzed process followed by addition of boron trifluoride etherate, undergo reactions with a wide range of organometallic nucleophiles to afford α-functionalized azacycles. Organozinc and organomagnesium nucleophiles add at ambient temperatures, obviating the need for cryogenic conditions. In situ preparation of imine-BF3 complexes provides access to α-functionalized morpholines and piperazines directly from their parent amines in a single operation. α-Functionalized morpholines can be elaborated further, for instance by installing a second substituent in the α'-position.

β-C-H Allylation of Trialkylamines with Allenes Promoted by Synergistic Borane/Palladium Catalysis

Functionalization of the C(sp3 )-H bonds of trialkylamines is challenging, especially for reactions at positions other than the α position. Herein, we report a method for β-C(sp3 )-H allylation of trialkylamines. In these reactions, which involve synergistic borane/palladium catalysis, an enamine intermediate is first generated from the amine via α,β-dehydrogenation promoted by B(C6 F5 )3 and a base, and then the enamine undergoes palladium-catalyzed reaction with an allene to give the allylation product. Because the hydride and the proton resulting from the initial dehydrogenation are ultimately shuttled to the product by B(C6 F5 )3 and the palladium catalyst, respectively, these reactions show excellent atom economy. The establishment of this method paves the way for future studies of C-H functionalization of trialkylamines by means of synergistic borane/transition-metal catalysis.

Synthesis of dienes from pyrrolidines using skeletal modification

Saturated N-heterocyclic pyrrolidines are common in natural products, medicinal compounds and agrochemicals. However, reconstruction of their skeletal structures creating new chemical space is a challenging task, and limited methods exist for this purpose. In this study, we report a skeletal modification strategy for conversion of polar cyclic pyrrolidines into nonpolar linear dienes through a N-atom removal and deconstruction process. This involves N-sulfonylazidonation followed by rearrangement of the resulting sulfamoyl azide intermediates. This can be an energetically unfavorable process, which involves the formation of active C-C π bonds, the consumption of inert C-N and C-C σ bonds and the destruction of stable five-membered rings, but we have used it here to produce versatile conjugated and nonconjugated dienes with links of varying lengths. We also studied the application of this method in late-stage skeletal modification of bioactive compounds, formal traceless C(sp2)-H functionalization and formal N-atom deletion.

Palladium-Catalyzed Arylation of Endocyclic 1-Azaallyl Anions: Concise Synthesis of Unprotected Enantioenriched cis-2,3-Diarylpiperidines

Unprotected cis-2,3-diarylpiperidines are synthesized through an unprecedented palladium-catalyzed cross-coupling reaction between aryl halides and elusive endocyclic 1-azaallyl anions. These intermediates are generated in situ by the deprotonation of 2-aryl-1-piperideines, precursors that are readily prepared in two operations from simple piperidines. An asymmetric version of this reaction with (2R, 3R)-iPr-BI-DIME as the ligand provides products in moderate to good yields and enantioselectivities. This study significantly expands the synthetic utility of endocyclic 1-azaallyl anions.

Synthesis of Polycyclic Imidazoles via α-C-H/N-H Annulation of Alicyclic Amines

Secondary alicyclic amines are converted to their corresponding ring-fused imidazoles in a simple procedure consisting of oxidative imine formation followed by a van Leusen reaction. Amines with an existing α-substituent undergo regioselective ring-fusion at the α'-position. This method was utilized in a synthesis of fadrozole.

Oxoammonium salt-promoted diverse functionalization of saturated cyclic amines with dinucleophiles

Oxoammonium salt-promoted diverse functionalization of saturated cyclic amines with different dinucleophiles under mild conditions is presented. Specifically, when thiocyanate is used as a 1,3-dinucleophile, hexahydrothiazolo[4,5-b]pyridin-2(3H)-one derivatives are formed via the formation of the β-TEMPO-tethered cyclic iminium ion as a key intermediate. By contrast, when benzene-1,2-diamine is used as a 1,4-dinucleophile, 2-alkylquinoxaline derivatives are afforded via generation of the β-oxo cyclic iminium ion as a key intermediate. In addition, the usefulness of 2-alkylquinoxalines is showcased through their facile conversion into N-(2-oxo-2-(quinoxalin-2-yl)ethyl)nitrous amides featuring the synthetically useful N-NO moiety and the carbonyl group.

Nickel-Catalyzed Site-Selective Intermolecular C(sp3 )-H Amidation

A nickel-catalyzed site-selective intermolecular amidation of saturated C(sp³ )-H bonds is reported. This mild protocol exhibits a predictable reactivity pattern to incorporate amide functions at C(sp³ )-H sites adjacent to nitrogen and oxygen atoms in either cyclic or acyclic frameworks, thus offering a complementary reactivity profile to existing oxidative-type processes or metal-catalyzed C(sp³ )-N bond-forming reactions operating via two-electron manifolds.

B(C6F5)3-Catalyzed α,β-Difunctionalization and C-N Bond Cleavage of Saturated Amines with Benzo[c]isoxazoles: Access to Quinoline Derivatives

Herein, we disclose a strategy to realize α,β-difunctionalization and C-N bond cleavage of saturated amines with benzo[c]isoxazoles via a B(C₆F₅)₃-catalyzed consecutive hydrogen-borrowing and [4 + 2] cycloaddition followed by a C-N bond cleavage process. In general, the reactions proceed efficiently in the absence of any oxidant and metal catalyst to afford a broad range of quinoline derivatives starting from easily accessible substrates in an atom-economical manner.

Oxoammonium Salt-Promoted Multifunctionalization of Saturated Cyclic Amines Based On β-Oxo Cyclic Iminium Ion Intermediates

Herein we describe a convenient method for multiple C(sp³)-H bond functionalization of saturated cyclic amines through oxoammonium salt-promoted oxidation to afford a β-oxo cyclic iminium ion as a key intermediate, followed by cascade addition with thiocyanate and diverse N-, O-, and S-containing nucleophiles in the green solvent and EtOH. Notably, chiral spiro azapolyheterocycles were prepared enantioselectively (>20:1 dr, up to 99% ee) when cysteine or serine esters were used as substrates. Moreover, the concise late-stage modification of several natural product derivatives was accomplished using this method.

Regioselective α-Cyanation of Unprotected Alicyclic Amines

Secondary alicyclic amines are converted to α-aminonitriles via addition of TMSCN to their corresponding imines, intermediates that are produced in situ via the oxidation of amine-derived lithium amides with simple ketone oxidants. Amines with an existing α-substituent undergo regioselective α'-cyanation even if the C-H bonds at that site are less activated. Amine α-arylation can be combined with α'-cyanation to generate difunctionalized products in a single operation.

[2 + 3] Annulative Coupling of Tetrahydroisoquinolines with Aryliodonio diazo compounds To Access 1,2,4-Triazolo[3,4-a]isoquinolines

Base promoted one-pot annulative coupling of 1,2,3,4-tetrahydroisoquinolines (THIQs) with hypervalent iodine(III) species aryliodonio diazo compounds has been devised for the direct construction of 1,2,4-triazolo[3,4-a]isoquinoline derivatives at room temperature in open air for the first time. This approach involves [2 + 3] cascade annulation of nucleophilic THIQ with an electrophilic aryliodonio diazo compound via N-H and α-C1(sp³)-H difunctionalization of THIQ.

Photoinduced α-C-H Amination of Cyclic Amine Scaffolds Enabled by Polar-Radical Relay

Herein, we report a polar-radical relay strategy for α-C-H amination of cyclic amines with N-chloro-N-sodio-carbamates. The relay is initiated by in situ generation of cyclic iminium intermediate using N-iodosuccinimide (NIS) oxidant as an initiator, which then operates through a series of polar (addition and elimination) and radical (homolysis, hydrogen- and halogen atom transfer) reactions to enable the challenging C-N bond formation in a controlled manner. A broad range of α-amino cyclic amines were readily accessed with excellent regioselectivity, and the superb applicability was further demonstrated by functionalization of biologically relevant compounds.

N-Dealkylation of Amines

N-dealkylation, the removal of an N-alkyl group from an amine, is an important chemical transformation which provides routes for the synthesis of a wide range of pharmaceuticals, agrochemicals, bulk and fine chemicals. N-dealkylation of amines is also an important in vivo metabolic pathway in the metabolism of xenobiotics. Identification and synthesis of drug metabolites such as N-dealkylated metabolites are necessary throughout all phases of drug development studies. In this review, different approaches for the N-dealkylation of amines including chemical, catalytic, electrochemical, photochemical and enzymatic methods will be discussed.

I2-DMSO-Mediated N-H/α-C(sp3)-H Difunctionalization of Tetrahydroisoquinoline: Formal [2 + 2 + 1] Annulation for the Construction of Pyrrolo[2,1-a]isoquinoline Derivatives

An I₂-DMSO-mediated cascade reaction using methyl ketones and 1,2,3,4-tetrahydroisoquinolines (THIQs) as commercially available substrates has been developed for the construction of pyrrolo[2,1-a]isoquinoline derivatives. This metal-free process involves N-H/α-C(sp³)-H difunctionalization of THIQ. Two C-C bonds and one C-N bond are formed in one pot under mild conditions. Besides, a quaternary carbon center has been constructed in this transformation efficiently.

Site-Selective C(sp3)–H Functionalizations Mediated by Hydrogen Atom Transfer Reactions via α-Amino/α-Amido Radicals

Amines and amides, as N-containing compounds, are ubiquitous in pharmaceutically-active scaffolds, natural products, agrochemicals, and peptides. Amides in nature bear a key responsibility for imparting three-dimensional structure, such as in proteins. Structural modifications to amines and amides, especially at their positions α to N, bring about profound changes in biological activity oftentimes leading to more desirable pharmacological profiles of small drug molecules. A number of recent developments in synthetic methodology for the functionalizations of amines and amides omit the need of their directing groups or pre-functionalizations, achieving direct activation of the otherwise relatively benign C(sp3)–H bonds α to N. Among these, hydrogen atom transfer (HAT) has proven a very powerful platform for the selective activation of amines and amides to their α-amino and α-amido radicals, which can then be employed to furnish C–C and C–X (X = heteroatom) bonds. The abilities to both form these radicals and control their reactivity in a site-selective manner is of utmost importance for such chemistries to witness applications in late-stage functionalization. Therefore, this review captures contemporary HAT strategies to realize chemo- and regioselective amine and amide α-C(sp3)–H functionalization, based on bond strengths, bond polarities, reversible HAT equilibria, traceless electrostatic-directing auxiliaries, and steric effects of in situ-generated HAT agents.

1 Introduction

2 Functionalizations of Amines

3 Functionalizations of Carbamates

4 Functionalizations of Amides

5 Conclusion

Synthesis of Polycyclic Isoindolines via α-C-H/N-H Annulation of Alicyclic Amines

Relatively unstable cyclic imines, generated in situ from their corresponding alicyclic amines via oxidation of their lithium amides with simple ketone oxidants, engage aryllithium compounds containing a leaving group on an ortho-methylene functionality to provide polycyclic isoindolines in a single operation. The scope of this transformation includes pyrrolidine, piperidine, azepane, azocane, and piperazines.

Borane-catalyzed cascade Friedel–Crafts alkylation/[1,5]-hydride transfer/Mannich cyclization to afford tetrahydroquinolines

We report a redox-neutral annulation reaction of tertiary amines with electron-deficient alkynes under metal-free and oxidant-free conditions.

Electrochemical Desaturative β-Acylation of Cyclic N-Aryl Amines

Herein, we disclose a straightforward, robust, and simple route to access β-substituted desaturated cyclic amines via an electrochemically driven desaturative β-functionalization of cyclic amines. This transformation is based on multiple single-electron oxidation processes using catalytic amounts of ferrocene. The reaction proceeds in the absence of stoichiometric amounts of electrolyte under mild conditions, affording the desired products with high chemo- and regioselectivity. The reaction was tolerant of a broad range of substrates and also enables late-stage β-C(sp³ )-H acylation of potentially valuable products. Preliminary mechanistic studies using cyclic voltammetry reveal the key role of ferrocene as a redox mediator in the reaction.

Dehydrogenation/(3+2) Cycloaddition of Saturated Aza-Heterocycles via Merging Organic Photoredox and Lewis Acid Catalysis

Herein, we report a photoinduced dehydrogenation/(3+2) cycloaddition reaction by merging organic photoredox and Lewis acid catalysis, providing a straightforward and efficient approach for directly installing a benzofuran skeleton on the saturated aza-heterocycles. In this protocol, we also describe a novel organic photocatalyst (t-Bu-DCQ) with the advantages of a wider redox potential, easy synthesis, and a low price. Furthermore, the stepwise activation mechanism of dual C(sp³)-H bonds was demonstrated by a series of experimental and computational studies.

EATA Reaction Catalyzed by Copper Halides of Mixed Oxidation States and Its Application to Total Synthesis of Scorodonin

Naturally occurring conjugated allenynes are of general interest to the scientific community for their potent and various biological activities. The 1,5-H transfer of alka-1,4-diyn-3-yl amines would be one of the most straightforward yet challenging approach to this class of compounds since it may, in principle, form two regioisomeric products involving two different C-C triple bonds. Herein, a catalytic recipe of copper halides with mixed oxidation states, i.e., CuCl/CuBr 2 , has been identified to address the issues of the side reaction of conjugate addition and the selectivity of 1,5-H transfer of the key intermediate, alka-1,4-diyn-3-yl amines, in EATA (Enantioselective Allenation of Terminal Alkynes) reaction involving the conjugated 2-alkynals. This protocol could accommodate a wide range of functional groups providing a series of allenynes with a very high enantioselectivity (up to >99% ee). In addition, the enantioenriched allenynes can be readily transformed into various building blocks and applied to the highly enantioselective total synthesis of linear allenic natural product scorodonin for the first time. Mechanistic studies and DFT calculations elucidated the high regioselectivity for observed 1,5-H transfer within the intermediate of 1,4-diyn-3-yl amines. The calculated energy difference between two of the most stable transition states of 3.4 kcal/mol accounts for a selectivity of over 99:1, which is in perfect agreement with the experimental results.