Breaking Amides using Nickel Catalysis

Computational Study of Mechanism and Thermodynamics of Ni/IPr-Catalyzed Amidation of Esters

Nickel catalysis has shown remarkable potential in amide C⁻N bond activation and functionalization. Particularly for the transformation between ester and amide, nickel catalysis has realized both the forward (ester to amide) and reverse (amide to ester) reactions, allowing a powerful approach for the ester and amide synthesis. Based on density functional theory (DFT) calculations, we explored the mechanism and thermodynamics of Ni/IPr-catalyzed amidation with both aromatic and aliphatic esters. The reaction follows the general cross-coupling mechanism, involving sequential oxidative addition, proton transfer, and reductive elimination. The calculations indicated the reversible nature of amidation, which highlights the importance of reaction thermodynamics in related reaction designs. To shed light on the control of thermodynamics, we also investigated the thermodynamic free energy changes of amidation with a series of esters and amides.

Well-Defined Palladium(II)-NHC Precatalysts for Cross-Coupling Reactions of Amides and Esters by Selective N-C/O-C Cleavage

Transition-metal-catalyzed cross-coupling reactions represent a most powerful tool for the rapid construction of C-C and C-X bonds available to synthetic chemists. Recently, tremendous progress has been made in the burgeoning area of cross-coupling reactions of amides and esters enabled by regio- and chemoselective acyl C-X (X = N, O) cleavage using well-defined Pd(II)-NHC complexes. The use of N-heterocyclic carbenes as ligands in palladium-catalyzed cross-couplings permits reactions of amides and esters that were previously impossible using palladium or could be achieved only under harsh conditions. These reactions provide an attractive method to synthetic chemists to manipulate the traditionally inert amide and ester bonds with the broad cross-coupling generality inherent to palladium catalysis. Research in the area of cross-coupling of stable acyl electrophiles can be broadly categorized by the type of electrophile undergoing the cross-coupling. Recent studies have shown that cross-coupling of amides by transition-metal catalysis represents one of the most straightforward and wide-ranging ways of manipulating the classically inert amide bonds into generic acyl-metal intermediates that can be systematically exploited in cross-coupling reactions as a new paradigm in organic synthesis. The key to achieving high chemoselectivity of the process is control of amidic resonance (n_N to π_C═O^* conjugation, rotation of ca. 15-20 kcal/mol in planar amides), enabling oxidative addition of the N-C amide bond to a metal in a rational and predictable manner. This mode of catalysis has been extended to C(acyl)-O cross-coupling reactions of aryl esters, where selective C-O bond cleavage is accomplished through a rational match of aryl ester electrophiles and nucleophilic metal catalysts. These two types of transition-metal-catalyzed cross-coupling reactions represent an attractive concept in synthetic chemistry because of the ubiquity of esters and amides as precursors in organic synthesis. Furthermore, the high stability of amides and esters provides unprecedented opportunities for orthogonal cross-coupling strategies in the presence of other electrophiles. In this Account, we highlight advances that have taken place in the past few years in the field of cross-coupling of amides and esters, focusing on both (1) the stereoelectronic properties of well-defined Pd(II)-NHC complexes that have been critical to realize this challenging cross-coupling manifold and (2) the role of the isomerization barrier of the acyl electrophiles undergoing the cross-coupling. In a broader sense, the chemistry described here provides a practical approach to functionalize common amide and ester functional groups in organic synthesis and establishes straightforward access to acyl-metal intermediates that enable nonconventional cross-coupling strategies.

Unique Stereoselective Homolytic C-O Bond Activation in Diketopiperazine-Derived Alkoxyamines by Adjacent Amide Pyramidalization

Simple monocyclic diketopiperazine (DKP)-derived alkoxyamines exhibit unprecedented activation of a remote C-O bond for homolysis by amide distortion. The combination of strain-release-driven amide planarization and the persistent radical effect (PRE) enables a unique, irreversible, and quantitative trans→cis isomerization under much milder conditions than typically observed for such homolysis-limited reactions. This isomerization is shown to be general and independent of the steric and electronic nature of both the amino acid side chains and the substituents at the DKP nitrogen atoms. Homolysis rate constants are determined, and they significantly differ for both the labile trans diastereomers and the stable cis diastereomers. To reveal the factors influencing this unusual process, structural features of the kinetic trans diastereomers and thermodynamic cis diastereomers are investigated in the solid state and in solution. X-ray crystallographic analysis and computational studies indicate substantial distortion of the amide bond from planarity in the trans-alkoxyamines, and this is believed to be the cause for the facile and quantitative isomerization. Thus, these amino-acid-derived alkoxyamines are the first examples that exhibit a large thermodynamic preference for one diastereomer over the other upon thermal homolysis, and this allows controlled switching of configurations and configurational cycling.

Pd-Catalyzed Decarbonylative C-H Coupling of Azoles and Aromatic Esters

A decarbonylative C-H coupling of azoles and aromatic esters by palladium catalysis is described. Our previously reported Ni-catalyzed C-H coupling of azoles and aromatic esters has a significant drawback regarding the substrate scope. Herein, we employ palladium catalysis instead of nickel, resulting in a broader substrate scope in terms of azoles and aromatic esters.

Transition-Metal-Free Esterification of Amides via Selective N-C Cleavage under Mild Conditions

A general, transition-metal-free, and operationally simple method for esterification of amides by a highly selective cleavage of N-C(O) bonds under exceedingly mild conditions is reported. The reaction is characterized by broad substrate scope and excellent functional group tolerance. The potential of this mild esterification is highlighted by late-stage diversification of natural products and pharmaceuticals. Conceptually, the metal-free acyl functionalization of amides represents a significant step forward as a practical alternative to ligand exchange in acylmetal intermediates.

Activation of diverse carbon-heteroatom and carbon-carbon bonds via palladium(II)-catalysed β-X elimination

Chemists' ability to synthesize structurally complex, high-value organic molecules from simple starting materials is limited by methods to selectively activate and functionalize strong alkyl C(sp³) covalent bonds. Recent activity has focused on the activation of abundant C-O, C-N and C-C bonds via a mechanistic paradigm of oxidative addition of a low-valent, electron-rich transition metal. This approach typically employs nickel(0), rhodium(I), ruthenium(0) and iron catalysts under conditions finely tuned for specific, electronically activated substrates, sometimes assisted by chelating functional groups or ring strain. By adopting a redox-neutral strategy involving palladium(II)-catalysed C-H activation followed by β-heteroatom/carbon elimination, we describe here a catalytic method to activate alkyl C(sp³)-oxygen, nitrogen, carbon, fluorine and sulfur bonds with high regioselectivity. Directed hydrofunctionalization of the resultant palladium(II)-bound alkene leads to formal functional group metathesis. The method is applied to amino acid upgrading with complete regioselectivity and moderate to high retention of enantiomeric excess. Low-strain heterocycles undergo strong-bond activation and substitution, giving ring-opened products.

Integrating Metal-Catalyzed C-H and C-O Functionalization To Achieve Sterically Controlled Regioselectivity in Arene Acylation

One major goal of organometallic chemists is the direct functionalization of the bonds most recurrent in organic molecules: C-H, C-C, C-O, and C-N. An even grander challenge is C-C bond formation when both precursors are of this category. Parallel to this is the synthetic goal of achieving reaction selectivity that contrasts with conventional methods. Electrophilic aromatic substitution (EAS) via Friedel-Crafts acylation is the most renowned method for the synthesis of aryl ketones, a common structural motif of many pharmaceuticals, agrochemicals, fragrances, dyes, and other commodity chemicals. However, an EAS synthetic strategy is only effective if the desired site for acylation is in accordance with the electronic-controlled regioselectivity of the reaction. Herein we report steric-controlled regioselective arene acylation with salicylate esters via iridium catalysis to access distinctly substituted benzophenones. Experimental and computational data indicate a unique reaction mechanism that integrates C-O activation and C-H activation with a single iridium catalyst without an exogenous oxidant or base. We disclose an extensive exploration of the synthetic scope of both the arene and the ester components, culminating in the concise synthesis of the potent anticancer agent hydroxyphenstatin.

Transition-Metal-Catalyzed Decarbonylative Coupling Reactions: Concepts, Classifications, and Applications

Transition metal-catalyzed decarbonylative coupling reactions have emerged as a powerful alternative to conventional cross-coupling protocols due to the advantages associated with the use of carbonyl-containing functionalities as coupling electrophiles instead of commonly used organohalides or sulfates. A wide variety of novel transformations based on this concept have been successfully achieved, including decarbonylative carbon-carbon and carbon-heteroatom bond forming reactions. In this Review, we summarize the recent progress in this field and present a comprehensive overview of metal-catalyzed decarbonylative coupling reactions with carbonyl derivatives.

Decarbonylative Methylation of Aromatic Esters by a Nickel Catalyst

A Ni-catalyzed decarbonylative methylation of aromatic esters was achieved using methylaluminums as methylating agents. Dimethylaluminum chlorides uniquely worked as the methyl source. Because of the Lewis acidity of aluminum reagents, less reactive alkyl esters could also undergo the present methylation. By controlling the Lewis acidity of aluminum reagents, a chemoselective decarbonylative cross-coupling between alkyl esters and phenyl esters was successful.

Suzuki-Miyaura Coupling of Simple Ketones via Activation of Unstrained Carbon-Carbon Bonds

Here, we describe that simple ketones can be efficiently employed as electrophiles in Suzuki-Miyaura coupling reactions via catalytic activation of unstrained C-C bonds. A range of common ketones, such as cyclopentanones, acetophenones, acetone and 1-indanones, could be directly coupled with various arylboronates in high site-selectivity, which offers a distinct entry to more functionalized aromatic ketones. Preliminary mechanistic study suggests that the ketone α-C-C bond was cleaved via oxidative addition.

Catalytic Method for the Synthesis of C-N-Linked Bi(heteroaryl)s Using Heteroaryl Ethers and N-Benzoyl Heteroarenes

C-N-linked bi(heteroaryl)s are synthesized by a rhodium-catalyzed N-heteroarylation reaction of N-benzoyl heteroarenes including azoles/azolones, pyridones, cyclic ureas, and cyclic imides using heteroaryl aryl ethers. The reaction involves the covalent bond-exchange reaction of N-CO and HetAr-O bonds without using metal bases and exhibits a broad applicability, giving diverse C-N-linked bi(heteroaryl)s containing five- and six-membered heteroarenes. The N-heteroarylation of N-H azoles/azolones and pyridone proceeds at higher reaction temperatures.

A Two-Step Procedure for the Overall Transamidation of 8-Aminoquinoline Amides Proceeding via the Intermediate N-Acyl-Boc-Carbamates

Herein a two-step strategy for achieving overall transamidation of 8-aminoquinoline amides has been explored. In this protocol, the 8-aminoquinoline amides were first treated with Boc₂O and DMAP to form the corresponding N-acyl-Boc-carbamates, which were found to be sufficiently reactive to undergo subsequent aminolysis with different amines in the absence of any additional reagents or catalysts. To demonstrate the utility of this approach, it was applied on a number of 8-aminoquinoline amides from the recent C-H functionalization literature, enabling access to a range of elaborate amide derivatives in good to high yields.

Cross-coupling of aromatic esters and amides

Catalytic cross-coupling reactions of aromatic esters and amides have recently gained considerable attention from synthetic chemists as de novo and efficient synthetic methods to form C-C and C-heteroatom bonds. Esters and amides can be used as diversifiable groups in metal-catalyzed cross-coupling: in a decarbonylative manner, they can be utilized as leaving groups, whereas in a non-decarbonylative manner, they can form ketone derivatives. In this review, recent advances of this research topic are discussed.

Pd-PEPPSI: a general Pd-NHC precatalyst for Buchwald-Hartwig cross-coupling of esters and amides (transamidation) under the same reaction conditions

Amides are of fundamental interest in many fields of chemistry involving organic synthesis, chemical biology and biochemistry. Here, we report the first catalytic Buchwald-Hartwig coupling of both common esters and amides by highly selective C(acyl)-X (X = O, N) cleavage to rapidly access aryl amide functionality via a cross-coupling strategy. Reactions are promoted by versatile, easily prepared, well-defined Pd-PEPPSI type precatalysts, and proceed in good to excellent yields and with excellent chemoselectivity for the acyl bond cleavage. The method is user friendly because it employs commercially-available, moisture- and air-stable precatalysts. Notably, for the first time we demonstrate selective C(acyl)-N and C(acyl)-O cleavage/Buchwald-Hartwig amination under the same reaction conditions, which allows for streamlining amide synthesis by avoiding restriction to a particular acyl metal precursor. Of broad interest, this study opens the door to using a family of well-defined Pd(ii)-NHC precatalysts bearing pyridine "throw-away" ligands for the selective C(acyl)-amination of bench-stable carboxylic acid derivatives.

Ligand-Controlled Chemoselective C(acyl)-O Bond vs C(aryl)-C Bond Activation of Aromatic Esters in Nickel Catalyzed C(sp2)-C(sp3) Cross-Couplings

A ligand-controlled and site-selective nickel catalyzed Suzuki-Miyaura cross-coupling reaction with aromatic esters and alkyl organoboron reagents as coupling partners was developed. This methodology provides a facile route for C(sp²)-C(sp³) bond formation in a straightforward fashion by successful suppression of the undesired β-hydride elimination process. By simply switching the phosphorus ligand, the ester substrates are converted into the alkylated arenes and ketone products, respectively. The utility of this newly developed protocol was demonstrated by its wide substrate scope, broad functional group tolerance and application in the synthesis of key intermediates for the synthesis of bioactive compounds. DFT studies on the oxidative addition step helped rationalizing this intriguing reaction chemoselectivity: whereas nickel complexes with bidentate ligands favor the C(aryl)-C bond cleavage in the oxidative addition step leading to the alkylated product via a decarbonylative process, nickel complexes with monodentate phosphorus ligands favor activation of the C(acyl)-O bond, which later generates the ketone product.

Nickel-Catalyzed Phosphine Free Direct N-Alkylation of Amides with Alcohols

Herein, we developed an operational simple, practical, and selective Ni-catalyzed synthesis of secondary amides. Application of renewable alcohols, earth-abundant and nonprecious nickel catalyst facilitates the transformations, releasing water as byproduct. The catalytic system is tolerant to a variety of functional groups including nitrile, allylic ether, and alkene and could be extended to the synthesis of bis-amide, antiemetic drug Tigan, and dopamine D2 receptor antagonist Itopride. Preliminary mechanistic studies revealed the participation of a benzylic C-H bond in the rate-determining step.

Nickel-Catalyzed Suzuki-Miyaura Coupling of Aliphatic Amides

We report the Ni-catalyzed Suzuki-Miyaura coupling of aliphatic amide derivatives. Prior studies have shown that aliphatic amide derivatives can undergo Ni-catalyzed carbon-heteroatom bond formation but that Ni-mediated C-C bond formation using aliphatic amide derivatives has remained difficult. The coupling disclosed herein is tolerant of considerable variation with respect to both the amide-based substrate and the boronate coupling partner and proceeds in the presence of heterocycles and epimerizable stereocenters. Moreover, a gram-scale Suzuki-Miyaura coupling/Fischer indolization sequence demonstrates the ease with which unique polyheterocyclic scaffolds can be constructed, particularly by taking advantage of the enolizable ketone functionality present in the cross-coupled product. The methodology provides an efficient means to form C-C bonds from aliphatic amide derivatives using nonprecious-metal catalysis and offers a general platform for the heteroarylation of aliphatic acyl electrophiles.

Direct/Reversible Amidation of Troponyl Alkylglycinates via Cationic Troponyl Lactones and Mechanistic Insights

The conversion of troponyl alkylglycinate acid/ester/amide derivatives (Trag acid/ester/amide) into cationic troponyl lactones (CTLs) in the presence of trifluoroacetic acid and their amidation with amines is described. The reversible amidation of Trag amides, that is, the cleavage and reformation of the Trag amide bond via CTLs is demonstrated. The direct amidation of Trag esters with the amino group of amino acid esters/peptide esters via CTLs is achieved. The direct amidation of the amine group of hydroxyl amino acid esters is selective over esterification. The Trag amide bond is stable under basic ester hydrolysis and Fmoc removal conditions. Hence, the troponyl alkylglycinates could be applicable as protecting groups for amine functionality of amino acids and peptides. The reaction mechanism was investigated by using a deuterium probe and studied by NMR and electrospray ionisation mass spectrometry techniques. Deuterium incorporation at α-CH₂ strongly supported the formation of CTLs via ketene intermediates.

Reversible Twisting of Primary Amides via Ground State N-C(O) Destabilization: Highly Twisted Rotationally Inverted Acyclic Amides

Since the seminal studies by Pauling in 1930s, planarity has become the defining characteristic of the amide bond. Planarity of amides has central implications for the reactivity and chemical properties of amides of relevance to a range of chemical disciplines. While the vast majority of amides are planar, nonplanarity has a profound effect on the properties of the amide bond, with the most common method to restrict the amide bond relying on the incorporation of the amide function into a rigid cyclic ring system. In a major departure from this concept, here, we report the first class of acyclic twisted amides that can be prepared, reversibly, from common primary amides in a single, operationally trivial step. Di-tert-butoxycarbonylation of the amide nitrogen atom yields twisted amides in which the amide bond exhibits nearly perpendicular twist. Full structural characterization of a range of electronically diverse compounds from this new class of twisted amides is reported. Through reactivity studies we demonstrate unusual properties of the amide bond, wherein selective cleavage of the amide bond can be achieved by a judicious choice of the reaction conditions. Through computational studies we evaluate structural and energetic details pertaining to the amide bond deformation. The ability to selectively twist common primary amides, in a reversible manner, has important implications for the design and application of the amide bond nonplanarity in structural chemistry, biochemistry and organic synthesis.

Photoredox-Catalyzed Decarboxylative Alkylation of Silyl Enol Ethers To Synthesize Functionalized Aryl Alkyl Ketones

Photoredox-catalyzed decarboxylative alkylation of silyl enol ethers has been developed. Diverse functionalized aryl alkyl ketones were afforded in modest to good yields using N-(acyloxy)phthalimide as an easy access alkyl radical source under mild and operationally simple conditions. The excellent performance of drug molecules such as fenbufen and indomethacin and naturally occurring carboxylic acids such as stearic acid and dehydrocholic acid further demonstrated the practicability of the reaction.

Ni-Catalyzed dehydrogenative coupling of primary and secondary alcohols with methyl-N-heteroaromatics

Here we report the first base-metal catalyzed dehydrogenative coupling of primary (aromatic, heteroaromatic, and aliphatic) and secondary alcohols with methyl-N-heteroaromatics to form various C(sp³)-alkylated N-heteroaromatics.

One-pot transition-metal free transamidation to sterically hindered amides

A highly efficient one-pot transamidation of primary amides has been developed under transition-metal free conditions, generating a variety of amides including hindered amides in good yield (up to 86%) catalyzed by CsF.

Decarbonylative cross-coupling of amides

We present recent advances and key developments in the field of decarbonylative cross-coupling reactions of amides by a formal double N–C/C–C bond activation as well as discuss future challenges and potential applications for this exciting field.