Direct Synthesis of Enamides via Electrophilic Activation of Amides

A facile synthesis of α,β-unsaturated imines via palladium-catalyzed dehydrogenation

The dehydrogenation adjacent to an electron-withdrawing group provides an efficient access to α,β-unsaturated compounds that serving as versatile synthons in organic chemistry. However, the α,β-desaturation of aliphatic imines has hitherto proven to be challenging due to easy hydrolysis and preferential dimerization. Herein, by employing a pre-fluorination and palladium-catalyzed dehydrogenation reaction sequence, the abundant simple aliphatic amides are amendable to the rapid construction of complex molecular architectures to produce α,β-unsaturated imines. Mechanistic investigations reveal a Pd(0)/Pd(II) catalytic cycle involving oxidative H-F elimination of N-fluoroamide followed by a smooth α,β-desaturation of the in-situ generated aliphatic imine intermediate. This protocol exhibits excellent functional group tolerance, and even the carbonyl groups are compatible without any competing dehydrogenation, allowing for late-stage functionalization of complex bioactive molecules. The synthetic utility of this transformation has been further demonstrated by a diversity-oriented derivatization and a concise formal synthesis of (±)-alloyohimbane.

Nucleophilic Addition of Amides to Haloalkynes: Synthesis of (Z)-β-Halovinyl Amides as Dual Precursors of Alkylidene Carbenes and Allyl Halides

Toward a regioselective method for the synthesis of β-halovinyl amides, we developed a transition-metal-free nucleophilic addition reaction of amides to haloalkynes. The regioselective nucleophilic addition was achieved under solvent-free conditions using phosphonates to protonate the intermediate alkylidene carbenoids, thus suppressing their decomposition. Furthermore, we demonstrate that β-halovinyl amides can serve as dual precursors of allyl halides and alkylidene carbenes to obtain functionalized indoles and pyrrolidones.

A Neutral PCNHCP Co(I)-Me Pincer Complex as a Catalyst for N-Allylic Isomerization with a Broad Substrate Scope

Earth-abundant-metal catalyzed double bond transposition offers a sustainable and atom-economical route toward the synthesis of internal alkenes. With an emphasis specifically on internal olefins and ethers, the isomerization of allylic amines has been particularly under represented in the literature. Herein, we report an efficient methodology for the selective isomerization of N-allylic organic compounds, including amines, amides, and imines. The reaction is catalyzed by a neutral PCNHCP cobalt(I) pincer complex and proceeds via a π-allyl mechanism. The isomerization occurs readily at 80-90 °C, and it is compatible with a wide variety of functional groups. The in situ formed enamines could additionally be used for a one-pot inverse-electron-demand Diels-Alder reaction to furnish a series of diversely substituted heterobiaryls, which is further discussed in this report.

Cobalt-catalyzed double hydroboration of pyridines

Cobalt(ii) complexes were prepared from a modular phosphinopyridonate platform and applied to the hydroboration of pyridines. The synthetically useful, yet challenging, double hydroboration toward tetrahydropyridine derivatives was successfully performed with high activity and regiocontrol. This new method enabled the direct synthesis of N-heterocyclic allylic boronates from commercial pyridines and pinacolborane (HBpin). One-pot acetylation afforded the bench-stable borylated N-acetyl tetrahydropyridines in good yields. The synthetic utility of this procedure was demonstrated by a gram-scale double hydroboration-acetylation sequence followed by chemical diversification. Mechanistic experiments indicated metal-ligand cooperativity involving ligand-centered C-H activation and the intermediacy of a cobalt(iii) hydride species.

Cooperative Fe/Co-Catalyzed Remote Desaturation for the Synthesis of Unsaturated Amide Derivatives

Unsaturated amides represent common functional groups found in natural products and bioactive molecules and serve as versatile synthetic building blocks. Here, we report an iron(II)/cobalt(II) dual catalytic system for the syntheses of distally unsaturated amide derivatives. The transformation proceeds through an iron nitrenoid-mediated 1,5-hydrogen atom transfer (1,5-HAT) mechanism. Subsequently, the radical intermediate undergoes hydrogen atom abstraction from vicinal methylene by a cobaloxime catalyst, efficiently yielding β,γ- or γ,δ-unsaturated amide derivatives under mild conditions. The efficiency of Co-mediated HAT can be tuned by varying different auxiliaries, highlighting the generality of this protocol. Remarkably, this desaturation protocol is also amenable to practical scalability, enabling the synthesis of unsaturated carbamates and ureas, which can be readily converted into various valuable molecules.

Diversification of Acrylamide Polymers via Direct Transamidation of Unactivated Tertiary Amides

Postpolymerization modification offers a versatile strategy for synthesizing complex macromolecules, yet modifying acrylamide polymers like poly(N,N-dimethylacrylamide) (PDMA) is notoriously challenging due to the inherent stability and low reactivity of amide bonds. In this study, we unveil a novel approach for the direct transamidation of PDMA, leveraging recent advances in the transamidation of unactivated tertiary amide substrates. By exploiting photoiniferter polymerization, we extended this direct transamidation approach to ultrahigh-molecular-weight (UHMW) PDMA, showcasing the unprecedented postpolymerization modification of synthetic polymers exceeding 106 g/mol. We also designed acrylamide copolymers comprising both the moderately reactive N-methyl-N-phenyl tertiary amides, along with the less reactive, fully alkyl-substituted N,N-dimethyl amides inherent to PDMA. This disparate reactivity enabled a sequential, chemoselective transamidation by initially targeting the more reactive pendant aryl amides with less nucleophilic aromatic amines, and second, transamidating the untouched N,N-dimethyl amide moieties with more nucleophilic aliphatic amines, yielding a uniquely diversified acrylamide copolymer. This work not only broadens the scope of postpolymerization modification strategies by pioneering direct transamidation of unactivated amides but also provides a robust platform for the design of intricate macromolecules, particularly in the realm of UHMW polymers.

Oxidative Dehydrogenation of Alkanes through Homogeneous Base Metal Catalysis

Preparing valuable olefins from cheap and abundant alkane resources has long been a challenging task in organic synthesis, which mainly suffers from harsh reaction conditions and narrow scopes. Homogeneous transition metals catalyzed dehydrogenation of alkanes has attracted much attention for its excellent catalytic activities under relatively milder conditions. Among them, base metal catalyzed oxidative alkane dehydrogenation has emerged as a viable strategy for olefin synthesis for its usage of cheap catalysts, compatibility with various functional groups, and low reaction temperature. In this review, we discuss recent development of base metal catalyzed alkane dehydrogenation under oxidative conditions and their application in constructing complex molecules.

Coordinating activation strategy enables 1,2-alkylamidation of alkynes

The radical 1,2-difunctionalization reaction of alkynes has been evolved into a versatile approach for expeditiously increasing the complexity of the common feedstock alkyne. However, intermolecular 1,2-carboamidation with general alkyl groups is an unsolved problem. Herein, we show that a coordinating activation strategy could act as an efficient tool for enabling radical 1,2-alkylamidation of alkynes. With the employment of diacyl peroxides as both alkylating reagents and internal oxidants, a large library of β-alkylated enamides is constructed in a three-component manner from readily accessible amides and alkynes. This protocol exhibits broad substrate scope with good functional group compatibility and is amenable for late-stage functionalization of natural molecules and biologically compounds.

Tf2O-Promoted Regioselective Heteronucleophilic Ring-Opening Approaches of Tetrahydrofuran

The ring-opening functionalization strategy in tetrahydrofuran (THF) represents an ideal approach to access different valuable structures. Herein, we report different operationally simple, efficient, unique, and practical regioselective heteronucleophilic ring-opening strategies for the THF system. Tf2O, which is a strong electrophilic activator, was found to generate a THF triflate intermediate that triggers the nucleophilicity of nitriles (Nu1) and led to regioselective ring opening in the presence of different nucleophiles (Nu2). Furthermore, the synthesis of different heteronucleophilic ring-opening dimerization products was attributed to the nucleophilicity of Nu2. We also demonstrated that use of borane-tetrahydrofuran (BTHF) can achieve challenging hydride addition in a similar manner.

Structures and Reactivities of N-Alkenyl-Substituted Anilides: The "Magic" Methyl Effect on Alkene

Methyl substitution at the double bond of N-alkenyl anilides influences both the preferred conformation and the susceptibility to acidic hydrolysis. The R1-substituted amide favors the trans conformation, whereas amides substituted at R2 or R3 favor the cis conformation. Substitution at the R1 and R3 positions increases the ratio of the trans conformer. DFT study indicated that these conformational preferences can be explained in terms of substituent-induced torsion twisting of the N-alkenyl moiety relative to the amide plane. R1 substitution enhances the susceptibility to acidic hydrolysis, whereas R2 or R3 substitution increases the stability. The effect of the double bond on the conformational effect was showcased by contrasting the preferred conformation of R1-substituted anilide (trans) and hydrogenated N-isopropyl amide (cis).

Decarboxylative Enamide Synthesis from Carboxylic Acid and Alkenyl Isocyanate

Herein, we reported a protocol to access the enamide via employing carboxylic acid and alkenyl isocyanate as the precursors promoted by DMAP without involving any metal catalysts and dehydration reagents. This protocol is simple and practical and tolerates numerous functional groups. Considering the simplicity, the ready availability of both starting materials, and the significance of the enamides, we expect that this reaction will find broad application.

Merging Alkene Isomerization Enables Difunctionalization of Cyclic Enamines toward Ring-Fused Aminal Synthesis

Here we report a Pd-catalyzed isomerization of alicyclic allyl amine to achieve the unprecedented α,β-difunctionalization of synthetically inaccessible trisubstituted cyclic enamine. The dual role of in situ formed enamine intermediate allows for the intermolecular formal [4 + 2] reaction with acrylamide or isatoic anhydride to simultaneously construct the C-C bond and C-N bond, thus realizing the expedient construction of [4.3.0]-aminal with excellent diastereoselectivity and high atom economy.

Challenges and Breakthroughs in Selective Amide Activation

In contrast to ketones and carboxylic esters, amides are classically seen as comparatively unreactive members of the carbonyl family, owing to their unique structural and electronic features. However, recent decades have seen the emergence of research programmes focused on the selective activation of amides under mild conditions. In the past four years, this area has continued to rapidly develop, with new advances coming in at a fast pace. Several novel activation strategies have been demonstrated as effective tools for selective amide activation, enabling transformations that are at once synthetically useful and mechanistically intriguing. This Minireview comprises recent advances in the field, highlighting new trends and breakthroughs in what could be called a new age of amide activation.

Photoexcited Palladium-Initiated Remote Desaturation of N-Alkoxypyridinium Salts

1,5-Hydrogen atom transfer (HAT) is an effective strategy to achieve remote desaturation of nonfunctionalized alkanes. Herein, we report a photoinduced remote desaturation reaction of N-alkoxypyridinium salts, which serve as alkoxyl radical precursors. Mechanistic studies show that a single electron transfer between the excited palladium complex and a N-alkoxypyridinium salt initiates a radical chain process leading to desaturation of N-alkoxypyridinium salts. This chain mechanism is supported by the measurement of the quantum yield of this reaction (Φ = 82). This reaction is applicable to a range of N-alkoxypyridinium salts, including some complex molecule-derived ones.

A Metal-Free and Redox-Neutral Benzylic C-O Cyclization for Succinct Access of Helical Chromenes

A redox-neutral benzylic C-O cyclization under beneficial transition-metal-free conditions is reported. Key to the success of this process is the utilization of the Tf anion as the leaving group for achieving the redox-neutral transformation. This protocol delivers a series of captivating helical compounds having various functionality in good-to-excellent yields. It is particularly noteworthy that sterically hindered helical compounds are conformationally stable. In addition to simple helical chromenes, the bihelical multiple-ring systems which are potentially useful in material chemistry are also easily attained by employing this method. DFT calculation revealed that quinone intermediate is the key species, among four possible mechanisms, for accomplishing the desired cyclization via an oxa-6π-electrocyclization pathway.

A three-component difunctionalization of N-alkenyl amides via organophotoredox radical-polar crossover

Herein, we report a three-component organophotoredox coupling of N-alkenyl amides with α-bromocarbonyls and various nucleophiles. This transition metal-free difunctionalization protocol installs sequential C-C and C-Y (Y = S/O/N) bonds in alkenes. This reaction works with terminal and internal alkenes containing both cyclic and acyclic amides via radical-polar crossover.

Efficient catalyst-free direct amidation of non-activated carboxylic acids from carbodiimides

A novel and efficient catalyst- and activating agent-free amidation method via direct amidation of carboxylic acids where carbodiimides act as a reagent instead of an activating agent is reported. The reaction is conducted under non-traditional coupling conditions where a higher temperature is employed. Besides not using stoichiometric ratios of activating agent or catalyst, this approach is made even more attractive by occurring in the presence of the environmentally friendly and recyclable non-toxic solvent of DMSO. A wide variety of benzylic, aliphatic, α,β-unsaturated and aromatic carboxylic acids provide related amides in up to 95% yield. The excellent yield from a gram-scale reaction shows that this application is particularly convenient for larger-scale synthesis applications.

Electrochemical Aerobic Oxygenation and Nitrogenation of Cyclic Alkenes via C═C Bond Cleavage or Oxygenation and Azidation of Open-Chain Alkenes

An efficient strategy involving electrochemical C═C double-bond cleavage and functionalization of cyclic alkenes for the synthesis of ketonitriles is described. This transformation features environmentally friendly conditions and utilizes relatively safe TMSN₃ as the nitrogenation reagent and molecular oxygen as the oxidant. For the open-chain alkenes, the reaction gave 1,2-difunctionalized products. A wide range of cyclic alkenes and open-chain alkenes were found to be compatible, providing the corresponding ketonitriles and α-azido aromatic ketones in moderate to good yields.

Photoinduced carbamoylation reactions: unlocking new reactivities towards amide synthesis

The preparation of amide-containing compounds is among the most interesting and challenging topics for the synthetic community. Such relevance is given by their reactive aspects explored in the context of organic synthesis and by the direct application of these compounds as pharmaceuticals and useful materials, and their key roles in biological structures. A simple and straightforward strategy for the amide moiety installation is the use of carbamoyl radicals - this nucleophilic one-electron intermediate is prone to undergo a series of transformations, providing a range of structurally relevant derivatives. In this review, we summarize the latest advances in the field from the perspective of photoinduced protocols. To this end, their synthetic applications are organized accordingly to the nature of the radical precursor (formamides through HAT, 4-substituted-1,4-dihydropyridines, oxamic acids, and N-hydroxyphthalimido esters), the mechanistic aspects also being highlighted. The discussion also includes a recent approach proceeding via photolytic C-S cleavage of dithiocarbamate-carbamoyl intermediates. By exploring fundamental concepts, this material aims to offer an understanding of the topic, which will encourage and facilitate the design of new synthetic strategies applying the carbamoyl radical.

Ir-Catalyzed Chemoselective Reductive Condensation Reactions of Tertiary Amides with Active Methylene Compounds

The catalytic reductive condensation reactions of tertiary amides with active methylene compounds leading to multifunctionalized non-N-containing products is described. The reactions proceed through sequential iridium-catalyzed hydrosilylation of the amides followed by acid-mediated condensation with the active methylene compounds. This scalable method is broad in scope and shows remarkable chemoselectivity for the amide group in the presence of several sensitive or even more reactive functionalities such as ester, cyano, nitro, silyl dienol ether, and ketone.

γ C-H Functionalization of Amines via Triple H-Atom Transfer of a Vinyl Sulfonyl Radical Chaperone

A selective, remote desaturation has been developed to rapidly access homoallyl amines from their aliphatic precursors. The strategy employs a triple H-atom transfer (HAT) cascade, entailing (i) cobalt-catalyzed metal-HAT (MHAT), (ii) carbon-to-carbon 1,6-HAT, and (iii) Co-H regeneration via MHAT. A new class of sulfonyl radical chaperone (to rapidly access and direct remote, radical reactivity) enables remote desaturation of diverse amines, amino acids, and peptides with excellent site-, chemo-, and regioselectivity. The key, enabling C-to-C HAT step in this cascade was computationally designed to satisfy both thermodynamic (bond strength) and kinetic (polarity) requirements, and it has been probed via regioselectivity, isomerization, and competition experiments. We have also interrupted this radical transfer dehydrogenation to achieve γ-selective C-Cl, C-CN, and C-N bond formations.

Cobalt-catalyzed chemoselective dehydrogenation through radical translocation under visible light

The transformations that allow the direct removal of hydrogen from their corresponding saturated counterparts by the dehydrogenative strategy are a dream reaction that has remained largely underexplored. In this report, a straightforward and robust cobaloxime-catalyzed photochemical dehydrogenation strategy via intramolecular HAT is described for the first time. The reaction proceeds through an intramolecular radical translocation followed by the cobalt assisted dehydrogenation without needing any other external photosensitizers, noble-metals or oxidants. With this approach, a series of valuable unsaturated compounds such as α,β-unsaturated amides, enamides and allylic and homoallylic sulfonamides were obtained in moderate to excellent yields with good chemo- and regioselectivities, and the synthetic versatility was demonstrated by a range of transformations. And mechanistic studies of the method are discussed.

The dehydrogenative reactions proceeded through selective 1,n-hydrogen atom transfer (n = 5–7) for remote C–H activation by cobaloxime catalysis.

Highly Chemoselective Transamidation of Unactivated Tertiary Amides by Electrophilic N-C(O) Activation by Amide-to-Acyl Iodide Re-routing

The challenging transamidation of unactivated tertiary amides has been accomplished via cooperative acid/iodide catalysis. Most crucially, the method provides a novel manifold to re-route the reactivity of unactivated N,N-dialkyl amides through reactive acyl iodide intermediates, thus reverting the classical order of reactivity of carboxylic acid derivatives. This method provides a direct route to amide-to-amide bond interconversion with excellent chemoselectivity using equivalent amounts of amines. The combination of acid and iodide has been identified as the essential factor to activate the amide C-N bond through electrophilic catalytic activation, enabling the production of new desired transamidated products with wide substrate scope of both unactivated amides and amines, including late-stage functionalization of complex APIs (>80 examples). We anticipate that this powerful activation mode of unactivated amide bonds will find broad-ranging applications in chemical synthesis.

Ring-Expansion Reactions of Epoxy Amides and Enamides: Functionalized Azetidines, Dihydrofurans, Diazocanes, or Dioxa-3-azabicyclonon-4-enes?

Functionalized azetidines, 2,3-dihydrofurans, or the unorthodox dioxa-3-azabicyclonone-4-ene motifs are the products from transition metal-free reaction between N-oxiranylmethyl benzenesulfonamide and β-chloro-cinnamaldehyde, depending on whether one uses either NaI/K₂CO₃ or LiBr/K₂CO₃. These ring expansion reactions involve enamide (X-ray evidence) derived from N-oxiranylmethyl benzenesulfonamide and β-chloro-cinnamaldehyde as an intermediate. The N-oxiranylmethyl benzenesulfonamide itself upon heating gives readily separable and crystalline isomeric diazocanes that can be characterized by X-ray crystallography.

Boron Lewis Acid Catalyzed Intermolecular trans-Hydroarylation of Ynamides with Hydroxyarenes

An atom-economic protocol for the efficient and highly chemo- and stereoselective trans-hydroarylation of ynamides with hydroxyarenes catalyzed by B(C₆F₅)₃ has been developed. Use of readily available starting materials, low catalyst loading, mild reaction conditions, a broad substrate scope, ease of scale-up, and versatile functionalizations of the enamide products make this approach very practical and attractive.

Cu(II)-Catalyzed Synthesis of 4-(1,4,5,6-Tetrahydropyridin-3-yl)-1,4-dihydroisoquinolin-3-ones from 4-Diazoisoquinolin-3-ones

We report a simple, efficient, and highly selective C-H bond insertion of copper carbenes generated in situ from 4-diazo-1,4-dihydroisoquinolin-3-ones into β-C(sp²)-H bonds of N-sulfonyl enamides, which gave a series of 4-(1,4,5,6-tetrahydropyridin-3-yl)-1,4-dihydroisoquinolin-3(2H)-ones in good to excellent yields. Operationally simple and mild reaction conditions, a cheap catalyst, readily accessible starting materials, and a broad substrate scope are the merits of this reaction.

Synthesis of N-alkenylisoquinolinones via palladium-catalyzed cyclization/C4–O bond cleavage of oxazolidines

A palladium-catalyzed cascade ring-opening reaction to synthesize N-alkenylisoquinolinones via cyclization and C–O bond cleavage is reported.

Selective desaturation of amides: a direct approach to enamides

C(sp³)–H bond desaturation has been an attractive strategy in organic synthesis. Enamides are important structural fragments in pharmaceuticals and versatile synthons in organic synthesis. However, the dehydrogenation of amides usually occurs on the acyl side benefitting from enolate chemistry like the desaturation of ketones and esters. Herein, we demonstrate an Fe-assisted regioselective oxidative desaturation of amides, which provides an efficient approach to enamides and β-halogenated enamides.

A novel and regioselective N-α,β-desaturation and dehydrogenative N-β-halogenation of amides was developed. This chemistry with high selectivity and broad substrate scope provides an efficient approach to enamides from simple amides.

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.

Remote Radical Desaturation of Unactivated C-H Bonds in Amides

Desaturation of inert aliphatic C-H bonds in alkanes to form the corresponding alkenes is challenging. In this communication, a new and practical strategy for remote site-selective desaturation of amides via radical chemistry is reported. The readily installed N-allylsulfonylamide moiety serves as an N radical precursor. Intramolecular 1,5-hydrogen atom transfer from an inert C-H bond to the N-radical generates a translocated C-radical which is subsequently oxidized and deprotonated to give the corresponding alkene. The commercially available methanesulfonyl chloride is used as reagent and a Cu/Ag-couple as oxidant. The remote desaturation is realized on different types of unactivated sp³ -C-H bonds. The potential synthetic utility of this method is further demonstrated by the dehydrogenation of natural product derivatives and drugs.