Direct Amidation of Esters by Ball Milling*

Synthesis of Core-Functionalised Naphthalenediimides from Naphthalenetetracarboxylic Dianhydride using a Vibratory Ball Mill: Bromination, Imidization and Heck-Type Reactions

The synthesis of 9 core-functionalised naphthalene diimide (c-NDI) residues is reported via a 3-step synthesis from naphthalenetetracarboxylic dianhydride using only mechanochemical activation. Selective dibromination and subsequent diimidization were achieved for the first time in a vibratory ball mill, resulting in the key structural intermediate, 2,6-dibromonaphthalenediimide (DBND), which is the basis for elaboration into a multitude of organic electronic materials. Our new synthesis of DBND is achieved in just 5 hours reaction time over two steps compared to typical solution state times of 24 hours. Subsequent Heck-type cross coupling reactions, with a range of styrene residues, produced a series of c-NDIs in good yields. The Heck-type reactions are rapid (1.5 hours), require no additional heating or solvent and are tolerant of atmospheric moisture and air.

Rapid Synthesis of Primary Amides Using Ammonia Borane

We report the rapid synthesis of primary amides by directly using commercially available ammonia borane (NH3·BH3), sodium hexamethyldisilazide (NaHMDS), and esters. The success of this protocol relies on NH3·BH3 as the nitrogen source being considerably more convenient and NaHMDS being an excellent proton abstractor but not participating in the nucleophilic addition reaction. The reaction had a wide substrate scope containing bioactive molecules, and most of the substrates were efficiently amidated over 90% yields.

Navigating Ball Mill Specifications for Theory-to-Practice Reproducibility in Mechanochemistry

The rising prospects of mechanochemically assisted syntheses hold promise for both academia and industry, yet they face challenges in understanding and, therefore, anticipating respective reaction kinetics. Particularly, dependencies based on variations in milling equipment remain little understood and globally overlooked. This study aims to address this issue by identifying critical parameters through kinematic models, facilitating the reproducibility of mechanochemical reactions across the most prominent mills in laboratory settings, namely planetary and mixer mills. Through a series of selected experiments replicating major classes of organic, organometallic, transition metal-catalyzed, and inorganic reactions from literature, we rationalize the independence of kinematic parameters on reaction kinetics when the accumulated energy criterion is met. As a step forward and to facilitate the practicability of our findings, we provide a freely accessible online tool[†] that allows the calculation of respective energy parameters for different planetary and mixer mills. Our work advances the current understanding of mechanochemistry and lays the foundation for future rational exploration in this rapidly evolving field.

Solvent Minimized Synthesis of Amides by Reactive Extrusion

Herein, we report on the translation of a small scale ball-milled amidation protocol into a large scale continuous reactive extrusion process. Critical components to the successful translation were: a) understanding how the different operating parameters of a twin-screw extruder should be harnessed to control prolonged continuous operation, and b) consideration of the physical form of the input materials. The amidation reaction is applied to 36 amides spanning a variety of physical form combinations (liquid-liquid, solid-liquid and solid-solid). Following this learning process, we have developed an understanding for the translation of each physical form combination and demonstrated a 7-hour reactive extrusion process for the synthesis of an amide on 500 gram scale (1.3 mols of product).

Chemoselective Approach to Versatile Acyl Fluorides by Photoinduced Activation of p-Methoxybenzyl Esters

A new strategy for the metal-free photoinduced activation of p-methoxybenzyl esters is developed using Selectfluor and benzil for the generation of acyl fluoride intermediates that enable various transformations. The highlight of this activation method is its high chemoselectivity in the presence of other functionalities, such as esters, amides, and ketones. A synthetic application for the preparation of peptide mimetics that possess two different amide units is also described.

Meeting the UN Sustainable Development Goals with Mechanochemistry

Chemistry traditionally relies on reactions in solution, but this method is increasingly problematic due to the scale of chemical processes and their economic and environmental impact. Handling residual chemical waste, including solvents, incurs significant costs and environmental pressure. Conversely, novel chemical approaches are needed to address pressing societal issues such as climate change, energy scarcity, food insecurity, and waste pollution. Mechanochemistry, a sustainable chemistry discipline that uses mechanical action to induce chemical reactivity without bulk solvents, is a hot topic in academic research on sustainable and green chemistry. Given its fundamentally different working principles from solution chemistry, mechanochemistry offers more efficient chemical processes and the opportunity to design new chemical reactions. Mechanochemistry has a profound impact on many urgent issues facing our society and it is now necessary to use mechanochemistry to address them. This Minireview aims to provide a guide for using mechanochemistry to meet the United Nations (UN) Sustainable Development Goals (SDGs), thereby contributing to a prosperous society. Detailed analysis shows that mechanochemistry connects with most UN SDGs and offers more cost-efficiency than other approaches together with a superior environmental performance.

BF3·OEt2-mediated transamidation of unprotected primary amides under solvent-free conditions

A BF3·OEt2-mediated transamidation between unactivated amides and amines is reported, enabling access to diverse secondary and tertiary amides under transition-metal-free and solvent-free conditions. The operationally simple procedure provides a novel manifold for converting amide-amide bonds with excellent chemoselectivity. In particular, a series of amides including challenging thioamides enable direct transamidation to products with modest to excellent yields. Meanwhile, additional experiments were conducted to elucidate the mechanism of this transformation, and a plausible mechanism was proposed based on the results and related literature.

A Catalytic Approach for the Synthesis of Peptide-Oligonucleotides Conjugates in Aqueous Solution or On-Column

Peptide-oligonucleotide conjugates (POCs) are covalent architectures composed of a DNA or RNA molecules linked to a peptide. These constructs have found widespread applications ranging from hybrid nanomaterials to gene-targeted therapies. Considering the important role of POCs, a new catalytic approach for their preparation is reported here, that could be applied either on solid support in anhydrous media, or post-synthetically in aqueous buffer. Single amino acids, peptides and cell penetrating peptides (CPPs) were conjugated to various oligo(ribo)nucleotides with high conversions and good isolated yields. The applicability of the method was demonstrated on more than 35 examples including an analogue of a commercial therapeutic oligonucleotide. Other conjugation partners, such as deoxycholic acid and biotin were also successfully conjugated to oligonucleotides. To highlight the potential of this catalytic approach, these conditions have been applied to iterative processes, which is of high interest for the development of DNA-Encoded Libraries.

Current progress in functionalization of cellulose nanofibers (CNFs) for active food packaging

There is a growing interest in utilizing renewable biomass resources to manufacture environmentally friendly active food packaging, against the petroleum-based polymers. Cellulose nanofibers (CNFs) have received significant attention recently due to their sustainability, biodegradability, and widely available sources. CNFs are generally obtained through chemical or physical treatment, wherein the original surface chemistry and interfacial interactions can be changed if the functionalization process is applied. This review focuses on promising and sustainable methods of functionalization to broaden the potential uses of CNFs in active food packaging. Novel aspects, including functionalization before, during and after cellulose isolation, and functionalization during and after material processing are addressed. The CNF-involved structural construction including films, membranes, hydrogels, aerogels, foams, and microcapsules, is illustrated, which enables to explore the correlations between structure and performance in active food packaging. Additionally, the enhancement of CNFs on multiple properties of active food packaging are discussed, in which the interaction between active packaging systems and encapsulated food or the internal environment are highlighted. This review emphasizes novel approaches and emerging trends that have the potential to revolutionize the field, paving the way for advancements in the properties and applications of CNF-involved active food packaging.

Process Mass Intensity (PMI): A Holistic Analysis of Current Peptide Manufacturing Processes Informs Sustainability in Peptide Synthesis

Small molecule therapeutics represent the majority of the FDA-approved drugs. Yet, many attractive targets are poorly tractable by small molecules, generating a need for new therapeutic modalities. Due to their biocompatibility profile and structural versatility, peptide-based therapeutics are a possible solution. Additionally, in the past two decades, advances in peptide design, delivery, formulation, and devices have occurred, making therapeutic peptides an attractive modality. However, peptide manufacturing is often limited to solid-phase peptide synthesis (SPPS), liquid phase peptide synthesis (LPPS), and to a lesser extent hybrid SPPS/LPPS, with SPPS emerging as a predominant platform technology for peptide synthesis. SPPS involves the use of excess solvents and reagents which negatively impact the environment, thus highlighting the need for newer technologies to reduce the environmental footprint. Herein, fourteen American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable (ACS GCIPR) member companies with peptide-based therapeutics in their portfolio have compiled Process Mass Intensity (PMI) metrics to help inform the sustainability efforts in peptide synthesis. This includes PMI assessment on 40 synthetic peptide processes at various development stages in pharma, classified according to the development phase. This is the most comprehensive assessment of synthetic peptide environmental metrics to date. The synthetic peptide manufacturing process was divided into stages (synthesis, purification, isolation) to determine their respective PMI. On average, solid-phase peptide synthesis (SPPS) (PMI ≈ 13,000) does not compare favorably with other modalities such as small molecules (PMI median 168-308) and biopharmaceuticals (PMI ≈ 8300). Thus, the high PMI for peptide synthesis warrants more environmentally friendly processes in peptide manufacturing.

Cesium Carbonate Promoted Direct Amidation of Unactivated Esters with Amino Alcohol Derivatives

Cesium carbonate promoted direct amidation of unactivated esters with amino alcohols was developed without the use of transition-metal catalysts and coupling reagents. This method enabled the synthesis of several serine-containing oligopeptides and benzamide derivatives with yields up to 90%. The methodology proceeds under mild reaction conditions and exhibits no racemization for most naturally occurring amino acid substrates. The reaction demonstrates good compatibility with primary alkyl and benzyl esters and broad tolerance for a range of amino acid substrates with nonpolar and protected side chains. The hydroxy group on the amine nucleophile was found to be critical for the reaction to be successful. A likely mechanism involving cesium coordination to the substrates enabling the subsequent proximity-driven acyl transfer was proposed. The practicality of this approach was demonstrated in the preparation of a biologically active nicotinamide derivative in a reasonable yield.

The low loading of metal in metal-organic framework-derived NiNx@NC promotes amide formation through C-N coupling

The pyrolysis of Ni-substituted zeolitic imidazolate framework-8 produces NiNx@NC with an ultra-low loading of Ni (7.4 × 10-6 mol%). The Ni-N coordination, subnanometer particle size, and uniform distribution of NiNx on the NC support lead to excellent catalytic activity (TON = 2702) and selectivity for the amination of aldehydes to produce amides.

Multifunctional soybean protein isolate-graft-carboxymethyl cellulose composite as all-biodegradable and mechanically robust mulch film for "green" agriculture

Multifunctional mulch films with robust mechanical behaviors of biopolymer-based biodegradable mulch materials were highly demanded in promoting the development of "green" agriculture. Herein, a sort of mechanically robust and all-biodegradable soybean protein isolate-graft‑sodium carboxymethyl cellulose composite mulch film was innovatively proposed through the amidation reactions between -COOH on protonated sodium carboxymethyl cellulose and -NH2 on soybean protein isolate. Arising from the reinforced intermolecular interactions upon chemical covalent bonds and physical hydrogen bonds, the maximum tensile strength and the elongation at break were increased from 10.61 MPa and 20.67 % for sodium carboxymethyl cellulose film to 42.15 MPa and 24.8 % for the optimized soybean protein isolate-graft‑sodium carboxymethyl cellulose composite mulch film, respectively. In addition, experimental results showed that the optimized soybean protein isolate-graft‑sodium carboxymethyl cellulose composite mulch film possesses soil moisture retention and controlled urea release properties. When employed as mulch film in practice, the cabbage seed presents higher germination when soil was covered with this versatile mulch film compared to commercial low-density polyethylene mulch film. Our discoveries build a prototype for the manufacture of eco-friendly mulch films with high mechanical strength, soil moisture retention, controlled urea release features.

Atom-Economical Synthesis of N-Arylamides Utilizing Isopropenyl Esters with Heterogeneous Acid Catalysts

Existing methods for the catalytic synthesis of N-arylamides are limited by a narrow substrate scope, high catalyst costs, and complicated purification processes of products. To overcome these limitations, this study developed an ecofriendly method for the synthesis of N-arylamides using isopropenyl esters. Isopropenyl esters activated using heterogeneous acid catalysts reacted smoothly even with less reactive arylamines to afford N-arylamides in high yields. This method exhibits a wide substrate scope and is applicable for the synthesis of various N-arylamides (33 examples, 46-99% yield). The developed method enabled the obtainment of high-purity products with a facile workup procedure and showed excellent process mass intensity values due to the reduction of chemical waste.

Ball milling synthesis of S-quinolyl xanthates via coupling of haloquinolines with potassium O-alkyl xanthates

An environmentally benign protocol that provides various S-quinolyl xanthates via a ball milling enabled cross coupling reaction of haloquinolines and readily available potassium O-alkyl xanthates is first reported. The reaction proceeded well under mild, transition metal- and solvent-free conditions, making it an attractive method for the introduction of xanthates into the quinoline scaffold.

Supramolecular intermediates in thermo-mechanochemical direct amidations

We present a solvent-free thermo-mechanochemical approach for the direct coupling of carboxylic acids and amines, which avoids activators and additives. Detailed analysis of the reactions by ex situ and in situ monitoring methods led to the observation, isolation, and characterisation of multicomponent crystalline intermediates that precede the formation of amides. We applied our methodology for the quantitative synthesis of the active pharmaceutical ingredient moclobemide.

Tinkering with Mechanochemical Tools for Scale Up

Mechanochemistry provides an environmentally benign platform to develop more sustainable chemical processes by limiting raw materials, energy use, and waste generation while using physically smaller equipment. A continuously growing research community has steadily showcased examples of beneficial mechanochemistry applications at both the laboratory and the preparative scale. In contrast to solution-based chemistry, mechanochemical processes have not yet been standardized, and thus scaling up is still a nascent discipline. The purpose of this Minireview is to highlight similarities, differences and challenges of the various approaches that have been successfully applied for a range of chemical applications at various scales. We hope to provide a discussion starting point for those interested in further developing mechanochemical processes for commercial use and/or industrialisation.

Non-activated Esters as Reactive Handles in Direct Post-Polymerization Modification

Non-activated esters are prominently featured functional groups in polymer science, as ester functional monomers display great structural diversity and excellent compatibility with a wide range of polymerization mechanisms. Yet, their direct use as a reactive handle in post-polymerization modification has been typically avoided due to their low reactivity, which impairs the quantitative conversion typically desired in post-polymerization modification reactions. While activated ester approaches are a well-established alternative, the modification of non-activated esters remains a synthetic and economically valuable opportunity. In this review, we discuss past and recent efforts in the utilization of non-activated ester groups as a reactive handle to facilitate transesterification and aminolysis/amidation reactions, and the potential of the developed methodologies in the context of macromolecular engineering.

Green metrics in mechanochemistry

The development of new green methodologies and their broader adoption for promoting sustainable development in chemistry laboratories and industry play a significant role in society, due to the economic importance of chemistry and its widespread presence in everyday life. Therefore, a sustainable approach to chemistry contributes to the well-being of the worldwide population and complies with the United Nations Sustainable Development Goals (UN SDGs) and the European Green Deal. The review highlights how batch and continuous mechanochemical methods are an eco-friendly approach for organic synthesis, with a lower environmental footprint in most cases, compared to solution-based procedures. The assessment is objectively based on the use of green metrics (e.g., atom and real atom economy, E-factor, process mass intensity, material parameter recovery, Eco-scale, stoichiometric factor, etc.) and indicators (e.g. DOZN tool and life cycle assessment, LCA, studies) applied to organic transformations such as synthesis of the amide bond, carbamates, heterocycles, active pharmaceutical ingredients (APIs), porphyrins, porous organic polymers (POPs), metal- or acid-catalysed processes, multicomponent and condensation reactions, rearrangements, etc. The generalized absence of bulk solvents, the precise control over the stoichiometry (i.e., using agents in a stoichiometrically rather than in excess), and the more selective reactions enabling simplified work-up procedures are the distinctive factors, marking the superiority of mechanochemical processes over solution-based chemistry.

A sustainable metal and base-free direct amidation of esters using water as a green solvent

Herein, we report a simple and efficient synthetic approach for direct amidation of esters via C(acyl)-O bond cleavage without any additional reagents or catalysts, using only water as a green solvent. Subsequently, the reaction byproduct is recovered and utilized for the next phase of ester synthesis. This method emphasized metal-free, additive-free, and base-free characteristics making it a new, sustainable, and eco-friendly way to realize direct amide bond formation. In addition, the synthesis of the drug molecule diethyltoluamide and the Gram-scale synthesis of a representative amide are demonstrated.

Amidation by reactive extrusion for the synthesis of active pharmaceutical ingredients teriflunomide and moclobemide

The solventless synthesis of an amide was performed in a twin-screw extruder in the presence of a coupling agent, providing a high yielding and productive method. The reaction conditions were optimized to prepare APIs, teriflunomide and moclobemide.

Electrochemical Amidation: Benzoyl Hydrazine/Carbazate and Amine as Coupling Partners

An electrochemical amidation of benzoyl hydrazine/carbazate and primary/secondary amine as coupling partners via concomitant cleavage and formation of C(sp²)-N bonds has been achieved. This methodology proceeds under metal-free and exogenous oxidant-free conditions producing N₂ and H₂ as byproducts. Mechanistic studies reveal the in situ generations of both acyl and N-centered radicals from benzoyl hydrazines and amines. The utility of this protocol is demonstrated through a large-scale, and synthesis of bezafibrate, a hyperlipidemic drug.

Mechanistic Understanding of KOtBu-Mediated Direct Amidation of Esters with Anilines: An Experimental Study and Computational Approach

A sustainable and cost-effective protocol has been reported for the synthesis of amide bonds from unactivated esters and non-nucleophilic amines promoted by potassium tert -butoxide under aerobic conditions. The reaction proceeds under relatively mild conditions, encompassing wide substrate scope. A combined experimental and quantum chemical study has been performed to shed light on the mechanism, which implied that a radical pathway is operating for the present protocol.

Design Strategy for Vulcanization Accelerator of Diphenylguanidine/Cyclodextrin Inclusion Complex for Natural Rubber Latex Foam with Enhancing Performance

Vulcanization is an essential process to obtain high-performance rubber products. Diphenylguanidine (DPG) is often used as the secondary accelerator in the vulcanization process of natural rubber (NR) latex. However, DPG would make NR latex emulsion exhibit gelation, resulting in the negative vulcanization efficiency. In addition, exposure to DPG might lead to some physiological diseases during the production process of DPG doped NR latex. Hydroxypropyl-β-cyclodextrin (HP-β-CD) with the hydrophobic interior and hydrophilic exterior has the advantages of good water solubility, high bioavailability, reliable stability, and low toxicity. In this study, the inclusion complex of diphenylguanidine-hydroxypropyl-β-cyclodextrin (DPG-HP-β-CD) is prepared by ball milling with a host-guest molar ratio of 1 : 1, which has also been applied to the foaming process of NR latex. The mechanical properties of DPG-HP-β-CD inclusion complex/natural rubber latex foam (DPG-HP-β-CD/NRLF) have been significantly improved, including the tensile strength, elongation at break, hardness, compression set, resilience, and antiaging performance. Further, the usage of DPG has been reduced, leading to the reduction of toxicity and environmental hazards.

Solvent‐Free Synthesis of Core‐Functionalised Naphthalene Diimides by Using a Vibratory Ball Mill: Suzuki, Sonogashira and Buchwald–Hartwig Reactions

Solvent‐free synthesis by using a vibratory ball mill (VBM) offers the chance to access new chemical reactivity, whilst reducing solvent waste and minimising reaction times. Herein, we report the core functionalisation of N,N’‐bis(2‐ethylhexyl)‐2,6‐dibromo‐1,4,5,8‐naphthalenetetracarboxylic acid (Br₂‐NDI) by using Suzuki, Sonogashira and Buchwald–Hartwig coupling reactions. The products of these reactions are important building blocks in many areas of organic electronics including organic light‐emitting diodes (OLEDs), organic field‐effect transistors (OFETs) and organic photovoltaic cells (OPVCs). The reactions proceed in as little as 1 h, use commercially available palladium sources (frequently Pd(OAc)₂) and are tolerant to air and atmospheric moisture. Furthermore, the real‐world potential of this green VBM protocol is demonstrated by the double Suzuki coupling of a monobromo(NDI) residue to a bis(thiophene) pinacol ester. The resulting dimeric NDI species has been demonstrated to behave as an electron acceptor in functioning OPVCs.

Base-promoted direct amidation of esters: beyond the current scope and practical applications

The base-promoted direct amidation of unactivated esters is among the most useful reactions for amide bond formation in contemporary organic chemistry. The intensive research in this area has led to the development of a number of new methods to achive this transformation. However, to date, the existing literature is more methodological and in many instances lacks practical directions. Therefore, the full potential of this transformation is yet to be revealed by broadening the substrate scope. In a search for new practical applications of the amidation reaction, herein we present a comprehensive study of a number of base-promoted direct amidations that encompass a wide range of amines and esters. Furthermore, we applied our findings in the synthesis of phosphoramidates and several industrially relevant products.

Plasma flow chemistry for direct N-acylation of amines by esters

The direct N-acylation of amines by esters in a microreactor is described using argon plasma, thereby producing amides in good to excellent yields.

Mechanochemical Nucleophilic Substitution of Alcohols via Isouronium Intermediates

An expansion of the solvent-free synthetic toolbox is essential for advances in the sustainable chemical industry. Mechanochemical reactions offer a superior safety profile and reduced amount of waste compared to conventional solvent-based synthesis. Herein a new mechanochemical method was developed for nucleophilic substitution of alcohols using fluoro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate (TFFH) and K₂ HPO₄ as an alcohol-activating reagent and a base, respectively. Alcohol activation and reaction with a nucleophile were performed in one milling jar via reactive isouronium intermediates. Nucleophilic substitution with amines afforded alkylated amines in 31-91 % yields. The complete stereoinversion occurred for the S_N 2 reaction of (R)- and (S)-ethyl lactates. Substitution with halide anions (F^- , Br^- , I^- ) and oxygen-centered (CH₃ OH, PhO^- ) nucleophiles was also tested. Application of the method to the synthesis of active pharmaceutical ingredients has been demonstrated.

Mechanochemical Organocatalysis: Do High Enantioselectivities Contradict What We Might Expect?

Ball mills input energy to samples by pulverising the contents of the jar. Each impact on the sample or wall of the jar results in an instantaneous transmission of energy in the form of a temperature and pressure increase (volume reduction). Conversely, enantioselective organocatalytic reactions proceed through perceived delicate and well-organised transition states. Does there exist a dichotomy in the idea of enantioselective mechanochemical organocatalysis? This Review provides a survey of the literature reporting the combination of organocatalytic reactions with mechanochemical ball milling conditions. Where possible, direct comparisons of stirred in solution, stirred neat and ball milled processes are drawn with a particular focus on control of stereoselectivity.

Mechanochemically-Assisted Synthesis of Polyimides

Polyimides were obtained in 99 % yield in under 1 h through the "beat and heat" approach, involving solvent-free vibrational ball milling and a thermal treatment step. The influence of a plethora of additives was explored, such as Lewis acids, Lewis bases, and dehydrating agents, and the mechanochemical reaction was identified to run via a polyamic acid intermediate. The protocol was adopted to a range of substrates inaccessible through solution-based processes, including perylene tetracarboxylic acid dianhydride and melamine. Furthermore, quantum chemical calculations were conducted to identify the water removal as the crucial step in the reaction mechanism. The presented method is substantially faster and more versatile than the solution-based process.