Structures of Highly Twisted Amides Relevant to Amide N−C Cross‐Coupling: Evidence for Ground‐State Amide Destabilization

Herein, we show that acyclic amides that have recently enabled a series of elusive transition‐metal‐catalyzed N?C activation/cross‐coupling reactions are highly twisted around the N?C(O) axis by a new destabilization mechanism of the amide bond. A unique effect of the N‐glutarimide substituent, leading to uniformly high twist (ca. 90°) irrespective of the steric effect at the carbon side of the amide bond has been found. This represents the first example of a twisted amide that does not bear significant steric hindrance at the α‐carbon atom. The ¹⁵N NMR data show linear correlations between electron density at nitrogen and amide bond twist. This study strongly supports the concept of amide bond ground‐state twist as a blueprint for activation of amides toward N?C bond cleavage. The new mechanism offers considerable opportunities for organic synthesis and biological processes involving non‐planar amide bonds.     

Nickel‐Catalyzed Esterification of Aliphatic Amides

Recent studies have demonstrated that amides can be used in nickel‐catalyzed reactions that lead to cleavage of the amide C?N bond, with formation of a C?C or C?heteroatom bond. However, the general scope of these methodologies has been restricted to amides where the carbonyl is directly attached to an arene or heteroarene. We now report the nickel‐catalyzed esterification of amides derived from aliphatic carboxylic acids. The transformation requires only a slight excess of the alcohol nucleophile and is tolerant of heterocycles, substrates with epimerizable stereocenters, and sterically congested coupling partners. Moreover, a series of amide competition experiments establish selectivity principles that will aid future synthetic design. These studies overcome a critical limitation of current Ni‐catalyzed amide couplings and are expected to further stimulate the use of amides as synthetic building blocks in C?N bond cleavage processes.     

Nickel‐Catalyzed Decarbonylative Borylation of Amides: Evidence for Acyl C−N Bond Activation

A nickel/N‐heterocyclic carbene catalytic system has been established for decarbonylative borylation of amides with B₂nep₂ by C?N bond activation. This transformation shows good functional‐group compatibility and can serve as a powerful synthetic tool for late‐stage borylation of amide groups in complex compounds. More importantly, as a key intermediate, the structure of an acyl nickel complex was first confirmed by X‐ray analysis. Furthermore, the decarbonylative process was also observed. These findings confirm the key mechanistic features of the acyl C?N bond activation process.     

Structural Characterization of N‐Alkylated Twisted Amides: Consequences for Amide Bond Resonance and N−C Cleavage

Herein, we describe the first structural characterization of N‐alkylated twisted amides prepared directly by N‐alkylation of the corresponding non‐planar lactams. This study provides the first experimental evidence that N‐alkylation results in a dramatic increase of non‐planarity around the amide N?C(O) bond. Moreover, we report a rare example of a molecular wire supported by the same amide C=O‐Ag bonds. Reactivity studies demonstrate rapid nucleophilic addition to the N?C(O) moiety of N‐alkylated amides, indicating the lack of n_N to π^*_C=O conjugation. Most crucially, we demonstrate that N‐alkylation activates the otherwise unreactive amide bond towards σ N?C cleavage by switchable coordination.     

Efficient Synthesis of Diaryl Ketones by Nickel‐Catalyzed Negishi Cross‐Coupling of Amides by Carbon–Nitrogen Bond Cleavage at Room Temperature Accelerated by a Solvent Effect

The first Negishi cross‐coupling of amides for the synthesis of versatile diaryl ketones by selective C?N bond activation under exceedingly mild conditions is reported. The cross‐coupling was accomplished with bench‐stable, inexpensive precatalyst [Ni(PPh₃)₂Cl₂] that shows high functional‐group tolerance and enables the synthesis of highly functionalized diaryl ketone motifs. The coupling occurred with excellent chemoselectivity favoring the ketone (cf. biaryl) products. Notably, this process represents the mildest conditions for amide N?C bond activation accomplished to date (room temperature, <10 min). Considering the versatile role of polyfunctional biaryl ketone linchpins in modern organic synthesis, availability, and excellent functional‐group tolerance of organozinc reagents, this strategy provides a new platform for amide N?C bond/organozinc cross‐coupling under mild conditions.     

General One‐Pot Reductive gem‐Bis‐Alkylation of Tertiary Lactams/Amides: Rapid Construction of 1‐Azaspirocycles and Formal Total Synthesis of (±)‐Cephalotaxine

Amides are a class of highly stable and readily available compounds. The amide functional group constitutes a class of powerful directing/activating and protecting group for C? C bond formation. Tertiary tert‐alkylamine, including 1‐azaspirocycle is a key structural feature found in many bioactive natural products and pharmaceuticals. The transformation of amides into tert‐alkylamines generally requires several steps. In this paper, we report the full details of the first general method for the direct transformation of tertiary lactams/amides into tert‐alkylamines. The method is based on in situ activation of amide with triflic anhydride/2,6‐di‐tert‐butyl‐4‐methylpyridine (DTBMP), followed by successive addition of two organometallic reagents of the same or different kinds to form two C? C bonds. Both alkyl and functionalized organometallic reagents and enolates can be used as the nucleophiles. The method displayed excellent 1,2‐ and good 1,3‐asymmetric induction. Construction of 1‐azaspirocycles from lactams required only two steps or even one‐step by direct spiroannelation of lactams. The power of the method was demonstrated by a concise formal total synthesis of racemic cephalotaxine.     

Synthesis of Biaryls through Nickel‐Catalyzed Suzuki–Miyaura Coupling of Amides by Carbon–Nitrogen Bond Cleavage

The first Ni‐catalyzed Suzuki–Miyaura coupling of amides for the synthesis of widely occurring biaryl compounds through N?C amide bond activation is reported. The reaction tolerates a wide range of electron‐withdrawing, electron‐neutral, and electron‐donating substituents on both coupling partners. The reaction constitutes the first example of the Ni‐catalyzed generation of aryl electrophiles from bench‐stable amides with potential applications for a broad range of organometallic reactions.     

Bridging C−H Activation: Mild and Versatile Cleavage of the 8‐Aminoquinoline Directing Group

8‐Aminoquinoline has emerged as one of the most powerful bidentate directing groups in history of C?H activation within the last decade. However, cleavage of its robust amide bond has shown to be challenging in several cases, thus jeopardizing the general synthetic utility of the method. To overcome this limitation, we herein report a simple oxidative deprotection protocol. This transformation rapidly converts the robust amide to a labile imide, allowing subsequent cleavage in a simple one‐pot fashion to rapidly access carboxylic acids or amides as final products.     

General Olefin Synthesis by the Palladium‐Catalyzed Heck Reaction of Amides: Sterically Controlled Chemoselective NC Activation

Metal‐catalyzed reactions of amides proceeding via metal insertion into the N? CO bond are severely underdeveloped due to resonance stabilization of the amide bond. Herein we report the first Heck reaction of amides proceeding via highly chemoselective N? CO cleavage catalyzed by Pd⁰ utilizing amide bond ground‐state destabilization. Conceptually, this transformation provides access to a myriad of metal‐catalyzed transformations of amides via metal insertion/decarbonylation.     

Key Role of Intramolecular Metal Chelation and Hydrogen Bonding in the Cobalt‐Mediated Radical Polymerization of N‐Vinyl Amides

This work reveals the preponderance of an intramolecular metal chelation phenomenon in a controlled radical polymerization system involving the reversible trapping of the radical chains by a cobalt complex bis(acetylacetonato)cobalt(II). The cobalt‐mediated radical polymerization (CMRP) of a series of N‐vinyl amides was considered with the aim of studying the effect of the cobalt chelation by the amide moiety of the last monomer unit of the chain. The latter reinforces the cobalt? polymer bond in the order N‐vinylpyrrolidone<N‐vinyl caprolactam<N‐methyl‐N‐vinyl acetamide, and is responsible for the optimal control of the polymerizations observed for the last two monomers. Such a double linkage between the controlling agent and the polymer, through a covalent bond and a dative bond, is unique in the field of controlled radical polymerization and represents a powerful opportunity to fine tune the equilibrium between latent and free radicals. Possible hydrogen bond formation is also taken into account in the case of N‐vinyl acetamide and N‐vinyl formamide. These results are essential for understanding the factors influencing Co? C bond strength in general, and the CMRP mechanism in particular, but also for developing a powerful platform for the synthesis of new precision poly(N‐vinyl amide) materials, which are an important class of polymers that sustain numerous applications today.     

Carbon‐Centered Radical Addition to OC of Amides or Esters as a Route to CO Bond Formations

Among various types of radical reactions, the addition of carbon radicals to unsaturated bonds is a powerful tool for constructing new chemical bonds, in which the typical applied unsaturated substrates include alkenes, alkynes and imines. Carbonyl is perhaps the most common unsaturated group in nature. This work demonstrates a novel C?O bond formation through carbon‐centered radical addition to the carbonyl oxygen of amide or ester, in which amide and ester groups are easily activated through the radical process. EPR spectroscopy and radical clock experiments support the radical process for this transformation, and density functional theory (DFT) calculations support the possibility of carbon‐centered radical addition to the carbonyl oxygen of amides or esters.     

Recent Advances in the Metal‐Catalyzed Activation of Amide Bonds

The amide functional group is commonly found in peptides, proteins, pharmaceutical compounds, natural products, and polymers. The synthesis of amides is typically performed by using classical approaches that involve the reaction between a carboxylic acid and an amine in the presence of an activator. Amides are thought to be an inert functional group, because they are unsusceptible to nucleophile attack, owing to their low electrophilicity. The reason for this resistance is clear: the resonance stability of the amide bond. However, transition metal catalysis can circumvent this stability by selectively rupturing the N?C bond of the amide, thereby facilitating further cross‐coupling or other reactions. In this Focus Review, we discuss the recent advances in this area and present a summary of methods that have been developed for activating the amide N?C bond by using precious and non‐precious metals.     

Glycosidic linkage,N‐acetyl side‐chain,and other structural properties of methyl 2‐acetamido‐2‐deoxy‐β‐d‐glucopyranosyl‐(1→4)‐β‐d‐mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2‐acetamido‐2‐deoxy‐β‐d ‐glycopyranosyl‐(1→4)‐β‐d ‐mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono‐ and disaccharides bearing N‐acetyl side‐chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N‐acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen‐bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis–trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter‐residue hydrogen bonding and some bond angles in or proximal to β‐(1→4) O‐glycosidic linkages on linkage torsion angles ? and ψ. Hypersurfaces correlating ? and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.     

Diastereoselective [3+2] Annulation of Aromatic/Vinylic Amides with Bicyclic Alkenes through Cobalt‐Catalyzed C−H Activation and Intramolecular Nucleophilic Addition

A highly diastereoselective method for the synthesis of dihydroepoxybenzofluorenone derivatives from aromatic/vinylic amides and bicyclic alkenes is described. This new transformation proceeds through cobalt‐catalyzed C?H activation and intramolecular nucleophilic addition to the amide functional group. Transition‐metal‐catalyzed C?H activation reactions of secondary amides with alkenes usually lead to [4+2] or [4+1] annulation; to the best of our knowledge, this is the first time that a [3+2] cycloaddition is described in this context. The reaction proceeds under mild conditions and tolerates a wide range of functional groups. Mechanistic studies imply that the C?H bond cleavage may be the rate‐limiting step.     

Site‐Selective Remote Radical C−H Functionalization of Unactivated C−H Bonds in Amides Using Sulfone Reagents

A general and practical strategy for remote site‐selective functionalization of unactivated aliphatic C?H bonds in various amides by radical chemistry is introduced. C?H bond functionalization is achieved by using the readily installed N‐allylsulfonyl moiety as an N‐radical precursor. The in situ generated N‐radical engages in intramolecular 1,5‐hydrogen atom transfer to generate a translocated C radical which is subsequently trapped with various sulfone reagents to afford the corresponding C?H functionalized amides. The generality of the approach is documented by the successful remote C?N₃, C?Cl, C?Br, C?SCF₃, C?SPh, and C?C bond formation. Unactivated tertiary and secondary C?H bonds, as well as activated primary C?H bonds, can be readily functionalized by this method.     

A polarizable dipole–dipole interaction model for evaluation of the interaction energies for NH···OC and CH···OC hydrogen‐bonded complexes

In this article, a polarizable dipole–dipole interaction model is established to estimate the equilibrium hydrogen bond distances and the interaction energies for hydrogen‐bonded complexes containing peptide amides and nucleic acid bases. We regard the chemical bonds N? H, C?O, and C? H as bond dipoles. The magnitude of the bond dipole moment varies according to its environment. We apply this polarizable dipole–dipole interaction model to a series of hydrogen‐bonded complexes containing the N? H···O?C and C? H···O?C hydrogen bonds, such as simple amide‐amide dimers, base‐base dimers, peptide‐base dimers, and β‐sheet models. We find that a simple two‐term function, only containing the permanent dipole–dipole interactions and the van der Waals interactions, can produce the equilibrium hydrogen bond distances compared favorably with those produced by the MP2/6‐31G(d) method, whereas the high‐quality counterpoise‐corrected (CP‐corrected) MP2/aug‐cc‐pVTZ interaction energies for the hydrogen‐bonded complexes can be well‐reproduced by a four‐term function which involves the permanent dipole–dipole interactions, the van der Waals interactions, the polarization contributions, and a corrected term. Based on the calculation results obtained from this polarizable dipole–dipole interaction model, the natures of the hydrogen bonding interactions in these hydrogen‐bonded complexes are further discussed. © 2013 Wiley Periodicals, Inc.     

A General Catalytic Hydroamidation of 1,3‐Dienes: Atom‐Efficient Synthesis of N‐Allyl Heterocycles,Amides, and Sulfonamides

Transition‐metal‐catalyzed hydroamination reactions are sustainable and atom‐economical C? N bond‐forming processes. Although remarkable progress has been made in the inter‐ and intramolecular amination of olefins and 1,3‐dienes, related intermolecular reactions of amides are still much less known. Control of the regioselectivity without analogous telomerization is the particular challenge in the catalytic hydroamidation of alkenes and 1,3‐dienes. Herein, we report a general protocol for the hydroamidation of electron‐deficient N‐heterocyclic amides and sulfonamides with 1,3‐dienes and vinyl pyridines in the presence of a catalyst derived from [{Pd(π‐cinnamyl)Cl}₂] and ligand L7 or L10 . The reactions proceeded in good to excellent yield with high regioselectivity. The practical utility of our method is demonstrated by the hydroamidation of functionalized biologically active substrates. The high regioselectivity for linear amide products makes the procedure useful for the synthesis of a variety of allylic amides.     

Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)–NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)–N and C(acyl)–O bond cleavage reactions.     

Copper‐Catalyzed Aerobic Oxidative Inert CC and CN Bond Cleavage: A New Strategy for the Synthesis of Tertiary Amides

A copper‐catalyzed aerobic oxidative amidation reaction of inert C?C bonds with tertiary amines has been developed for the synthesis of tertiary amides, which are significant units in many natural products, pharmaceuticals, and fine chemicals. This method combines C?C bond activation, C?N bond cleavage, and C?H bond oxygenation in a one‐pot protocol, using molecular oxygen as the sole oxidant without any additional ligands.     

Copper‐Catalyzed Carbonylative Coupling of Cycloalkanes and Amides

Carbonylation reactions are a most powerful method for the synthesis of carbonyl‐containing compounds. However, most known carbonylation procedures still require noble‐metal catalysts and the use of activated compounds and good nucleophiles as substrates. Herein, we developed a copper‐catalyzed carbonylative transformation of cycloalkanes and amides. Imides were prepared in good yields by carbonylation of a C(sp³)?H bond of the cycloalkane with the amides acting as weak nucleophiles. Notably, this is the first report of copper‐catalyzed carbonylative C?H activation.