Kinetic Modeling of the Nickel-Catalyzed Esterification of Amides

Highly efficient esterification of carboxylic acids with O-H nucleophiles through acid/iodide cooperative catalysis

The esterification of carboxylic acids is an important reaction for preparing esters which find wide applications in various research fields. In this manuscript, we report an acid/iodide cooperative catalytic method which enables highly efficient esterification of carboxylic acids with a wide range of equivalent O-H nucleophiles including both alcohols and weak nucleophilic phenols. Under the reaction conditions, both aromatic and aliphatic carboxylic acids including those bearing functional groups work well, furnishing the corresponding esters in good to high yields. Moreover, this reaction is scalable and applicable to the modification of bioactive molecules. These results demonstrate the synthetic value of this new reaction in organic synthesis.

DABCO-catalyzed esterification of N-pivaloyl-activated amides

A previously elusive organocatalytic protocol for the esterification of amides is disclosed. DABCO (10 mol%) is identified as an effective catalyst for the esterification of N-pivaloyl amides. Although N-pivaloyl amides are nearly planar (amide bond twist (τ) = 4.54° and pyramidalization (χN = 6.39°)) and resonance stabilized, esterification is achieved with high efficiency. The developed protocol is generic, phenols, thiophenols, aliphatic alcohols, and thiols were identified as effective substrates. Furthermore, the reaction features a broad substrate scope and excellent functional group tolerance. To exemplify the practical applicability of the developed protocol, the esterification of bioactive natural products, pterostilbene and menthol, is demonstrated. In addition, a series of competitive experiments were conducted to establish the reactivity pattern of alcohols, thiols, and phenols, which could serve as selectivity principles for future synthetic design. Our findings signify a notable advancement in utilizing amides as versatile synthetic building blocks in organic synthesis under metal-free conditions.

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

Enantioselective nickel-catalyzed Mizoroki-Heck cyclizations of amide electrophiles

Amide cross-couplings that rely on C-N bond activation by transition metal catalysts have emerged as valuable synthetic tools. Despite numerous discoveries in this field, no catalytic asymmetric variants have been disclosed to date. Herein, we demonstrate the first such transformation, which is the Mizoroki-Heck cyclization of amide substrates using asymmetric nickel catalysis. This proof-of-concept study provides an entryway to complex enantioenriched polycyclic scaffolds and advances the field of amide C-N bond activation chemistry.

Distinguishing Competing Mechanistic Manifolds for C(acyl)-N Functionalization by a Ni/N-Heterocyclic Carbene Catalyst System

Carboxylic acid derivatives are appealing alternatives to organohalides as cross-coupling electrophiles for fine chemical synthesis due to their prevalence in biomass and bioactive small molecules as well as their ease of preparation and handling. Within this family, carboxamides comprise a versatile electrophile class for nickel-catalyzed coupling with carbon and heteroatom nucleophiles. However, even state-of-the-art C(acyl)-N functionalization and cross-coupling reactions typically require high catalyst loadings and specific substitution patterns. These challenges have proven difficult to overcome, in large part due to limited experimental mechanistic insight. In this work, we describe a detailed mechanistic case study of acylative coupling reactions catalyzed by the commonly employed Ni/SIPr catalyst system (SIPr = 1,3-bis(2,6-di-isopropylphenyl)-4,5-dihydroimidazol-2-ylidine). Stoichiometric organometallic studies, in situ spectroscopic measurements, and crossover experiments demonstrate the accessibility of Ni(0), Ni(I), and Ni(II) resting states. Although in situ precatalyst activation limits reaction efficiency, the low concentrations of active, SIPr-supported Ni(0) select for electrophile-first (closed-shell) over competing nucleophile-first (open-shell) mechanistic manifolds. We anticipate that the experimental insights into the nature and controlling features of these distinct pathways will accelerate rational improvements to cross-coupling methodologies involving pervasive carboxamide substrate motifs.

Amide N-C Bond Activation: A Graphical Overview of Acyl and Decarbonylative Coupling

This Graphical Review provides an overview of amide bond activation achieved by selective oxidative addition of the N-C(O) acyl bond to transition metals and nucleophilic acyl addition, resulting in acyl and decarbonylative coupling together with key mechanistic details pertaining to amide bond distortion underlying this reactivity manifold.

Chiral Auxiliary-Directed Site-Selective Deprotonation of the Cubane Skeleton

The first diastereoselective synthesis of trisubstituted cubanes was achieved using a chiral auxiliary. To establish chirality within the cubane skeleton, at least three substituents must be introduced at the appropriate positions. Ready conversion of cubane carboxylic acid to a chiral amide followed by sequential ortho-selective deprotonations and electrophilic trapping afforded the corresponding 1,2,3-trisubstituted cubanes with high diastereoselectivity. This route opens new possibilities for the preparation of enantio-enriched cubanes.

Nickel-Catalyzed Reductive Borylation of Enaminones via C(sp2)-N Bond Cleavage

The cleavage and transformation of alkenyl C(sp²)-N bonds is a significant synthetic challenge. Herein we described an unprecedented nickel-catalyzed reductive borylation of enaminones to synthesize β-ketone boronic esters. Notably, B₂pin₂ played the dual role in this process, and water served as a hydrogen source, which was transferred to target products. The air-stable nickel catalyst was applied to the cleavage of alkenyl C(sp²)-N bonds, concomitant with the reductive process of the alkenyl boronic ester intermediates, on the basis of the mechanism study.

An Efficient and Straightforward Approach for Accessing Thioesters via Palladium-Catalyzed C-N Cleavage of Thioamides

We first report the coupling of activated thioamides with alcohols to efficiently form thioesters via palladium-catalyzed C-N cleavage strategy. The new approach employs the thioamides as thioacylating reagent to give...

PCl3-mediated transesterification and aminolysis of tert-butyl esters via acid chloride formation

A PCl3-mediated conversion of tert-butyl esters into esters and amides in one-pot under air is developed. This novel protocol is highlighted by the synthesis of skeletons of bioactive molecules and gram-scale reactions. Mechanistic studies revealed that this transformation involves the formation of an acid chloride in situ, which is followed by reactions with alcohols or amines to afford the desired products.

Highly economical and direct amination of sp3 carbon using low-cost nickel pincer catalyst

Developing more efficient routes to achieve C-N bond coupling is of great importance to industries ranging from products in pharmaceuticals and fertilizers to biomedical technologies and next-generation electroactive materials. Over the past decade, improvements in catalyst design have moved synthesis away from expensive metals to newer inexpensive C-N cross-coupling approaches via direct amine alkylation. For the first time, we report the use of an amide-based nickel pincer catalyst (1) for direct alkylation of amines via activation of sp3 C-H bonds. The reaction was accomplished using a 0.2 mol% catalyst and no additional activating agents other than the base. Upon optimization, it was determined that the ideal reaction conditions involved solvent dimethyl sulfoxide at 110 °C for 3 h. The catalyst demonstrated excellent reactivity in the formation of various imines, intramolecularly cyclized amines, and substituted amines with a turnover number (TON) as high as 183. Depending on the base used for the reaction and the starting amines, the catalyst demonstrated high selectivity towards the product formation. The exploration into the mechanism and kinetics of the reaction pathway suggested the C-H activation as the rate-limiting step, with the reaction second-order overall, holding first-order behavior towards the catalyst and toluene substrate.

Nickel-Catalyzed Conversion of Amides to Carboxylic Acids

We report the conversion of amides to carboxylic acids using nonprecious metal catalysis. The methodology strategically employs a nickel-catalyzed esterification using 2-(trimethylsilyl)ethanol, followed by a fluoride-mediated deprotection in a single-pot operation. This approach circumvents catalyst poisoning observed in attempts to directly hydrolyze amides using nickel catalysis. The selectivity and mildness of this transformation are shown through competition experiments and the net-hydrolysis of a complex valine-derived substrate. This strategy addresses a limitation in the field with regard to functional groups accessible from amides using transition metal-catalyzed C-N bond activation and should prove useful in synthetic applications.

Ni-Catalyzed Suzuki-Miyaura Cross-Coupling of Aliphatic Amides on the Benchtop

Suzuki-Miyaura cross-couplings of amides offer an approach to the synthesis of ketones that avoids the use of basic or pyrophoric nucleophiles. However, these reactions require glovebox manipulations, thus limiting their practicality. We report a benchtop protocol for Suzuki-Miyaura cross-couplings of aliphatic amides that utilizes a paraffin capsule containing a Ni(0) precatalyst and NHC ligand. This methodology is broad in scope, is scalable, and provides a user-friendly approach to convert aliphatic amides to alkyl-aryl ketones.

Cesium Carbonate Catalyzed Esterification of N-Benzyl-N-Boc-amides under Ambient Conditions

We report a general activated amide to ester transformation catalyzed by Cs₂CO₃. Using this approach, esterification proceeds under relatively mild conditions and without the need for a transition metal catalyst. This method exhibits broad substrate scope and represents a practical alternative to existing esterification strategies. The synthetic utility of this protocol is demonstrated via the facile synthesis of crown ether derivatives and the late-stage modification of a representative natural product and several sugars in reasonable yields.

Breaking Amide C-N Bonds in an Undergraduate Organic Chemistry Laboratory

An undergraduate organic chemistry laboratory experiment involving the breakage of amide C-N bonds is reported. Whereas amides are typically considered stable species due to well-established resonance effects, this experiment allows students to cleave the amide C-N bond in a nickel-catalyzed esterification process. Moreover, students perform the experiment on the benchtop using a commercially available paraffin wax capsule containing the necessary nickel precatalyst and N-heterocyclic carbene ligand. The laboratory procedure introduces students to several modern topics in organic chemistry that are not otherwise well-represented in typical undergraduate organic chemistry curricula, such as amide bond cleavage, transition metal-catalyzed cross-coupling reactions, and nonprecious-metal catalysis.

An Efficient, One-Pot Transamidation of 8-Aminoquinoline Amides Activated by Tertiary-Butyloxycarbonyl

The efficient, one-pot access to the transamidation of 8-aminoquinoline (8-AQ), notorious for its harsh removal conditions, has been widely employed as an auxiliary in C⁻H functionalization reactions due to its strong directing ability. In this study, the facile and mild Boc protection of the corresponding 8-AQ amide was critical to activate the amide C(acyl)⁻N bond by twisting its geometry to lower the amidic resonance energy. Both aryl and alkyl amines proceeded transamidation in one-pot, user-friendly conditions with excellent yields.

Palladium/copper-catalyzed decarbonylative heteroarylation of amides via C–N bond activation

A novel strategy for the synthesis of 2-arylated oxazole derivatives via palladium/copper-catalyzed decarbonylative heteroarylation of amides via C–N bond activation by ground-state destabilization is reported.

Computational studies on Ni-catalyzed amide C–N bond activation

This review summarizes the mechanistic models of Ni-catalyzed amide C–N bond cleavage and discusses their applications in related transformations.

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.

Modifications to the Aryl Group of dppf-Ligated Ni σ‑Aryl Precatalysts: Impact on Speciation and Catalytic Activity in Suzuki-Miyaura Coupling Reactions

There is currently significant interest in the development of efficient nickel precatalysts for cross-coupling. In this work, 14 nickel(II) precatalysts of the form (dppf)Ni(aryl)(X) (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) were synthesized. In particular, both the electronic and steric properties of the aryl group were modified to understand how this affects precatalyst activation. Using EPR spectroscopy, it was demonstrated that the amount of off-cycle nickel(I) species which are formed via comproportionation during precatalyst activation varies depending on the nature of the aryl group. For example, sterically bulky aryl groups reduce comproportionation. Additionally, the catalytic activity of the family of precatalysts was evaluated in five different Suzuki-Miyaura coupling reactions. The results from these catalytic studies provide information about how precatalyst structure affects catalytic efficiency, which may be useful for the rational design of improved nickel precatalysts for cross-coupling.

Nickel-Catalyzed Amide Bond Formation from Methyl Esters

Despite being one of the most important and frequently run chemical reactions, the synthesis of amide bonds is accomplished primarily by wasteful methods that proceed by stoichiometric activation of one of the starting materials. We report a nickel-catalyzed procedure that can enable diverse amides to be synthesized from abundant methyl ester starting materials, producing only volatile alcohol as a stoichiometric waste product. In contrast to acid- and base-mediated amidations, the reaction is proposed to proceed by a neutral cross coupling-type mechanism, opening up new opportunities for direct, efficient, chemoselective synthesis.