Dehydrative condensation catalyses

Organoboron catalysis for direct amide/peptide bond formation

Amides and peptides are ubiquitous functional groups found in several natural and artificial materials, and they are essential for the advancement of life and material sciences. In particular, their relevance in clinical medicine and drug discovery has increased in recent years. Dehydrative condensation of readily available carboxylic acids with amines is the most "direct" method for amide synthesis; however, this methodology generally requires a stoichiometric amount of condensation agent (coupling reagent). Catalytic direct dehydrative amidation has become an "ideal" methodology for synthesizing amides from the perspective of green chemistry, with water as the only byproduct in principle, high atom efficiency, environmentally friendly, energy saving, and safety. Conversely, organoboron compounds, such as boronic acids, which are widely used in various industries as coupling reagents for Suzuki-Miyaura cross-coupling reactions or pharmaceutical structures, are environmentally friendly molecules that have low toxicity and are easy to handle. Based on the chemical properties of organoboron compounds, they have potential Lewis acidity and the ability to form reversible covalent bonds with dehydration, making them attractive as catalysts. This review explores studies on the development of direct dehydrative amide/peptide bond formation reactions from carboxylic acids using organoboron catalysis, classifying them based on chemical bonding and catalysis over approximately 25 years, from the early developmental days to 2023.

Visible Light-Promoted Oxidative Cross-Coupling of Alcohols to Esters

Ester is one of the most significant functional groups in organic chemistry and is enclosed in several valued molecules. Usually, esters are prepared through the acid-catalyzed esterification reaction of carboxylic acids with alcohols, transesterification of esters with alcohols, or via activation of carboxylic acids followed by the addition of alcohols. However, these procedures typically imply the excess use of reactants and harsh reaction conditions. Visible light-mediated photoreactions have been disclosed to display a safe, sustainable, and accessible alternative to traditional methods, and to lead new reactivity modes in organic procedures. In this context, we propose a transition metal-based and organic-based photocatalyst-free synthesis of esters from alcohols induced by visible light. The methodology can be carried out using sunlight or artificial visible light as a solar simulator or a blue LED source.

Cyclometalated Half-Sandwich Ruthenium Complexes: Preparation, Structure, and Catalytic Synthesis of Phenolic Esters from Aryl Halides without Using External CO under Air

A series of cyclometalated C,N-chelate half-sandwich ruthenium complexes based on N-heterocyclic ligands were prepared through a simple route with good yields. These air- and moisture-stable cyclometalated ruthenium complexes showed excellent catalytic efficiency in phenoxy carbonylation of aryl halides with phenyl formate derivatives as a CO source and phenol as a coupling partner under air. Ester products were obtained with high yields at room temperature and without the need for an inert atmosphere. The excellent catalytic activity, broad substrate range, and mild reaction conditions made this catalytic system potential for industrial production. In addition, DFT study has been carried out to elaborate the possible mechanism of this Ru-catalyzed reaction.

Photo-Induced Carbonylation of Aryl Bromides for the Synthesis of Aryl Esters and Amides Under Transition Metal-Free Conditions

In this work, we developed a photo-induced carbonylation of aryl bromides under transition metal-free conditions. The reaction shows good activity with alcohol and amine nucleophiles. Various esters and amides were formed from aryl halides and alcohols and amines under mild conditions in moderate to good yields.

Microwave Assisted Esterification of Aryl/Alkyl Acids Catalyzed by N-Fluorobenzenesulfonimide

The susceptibility of the carbonyl group towards nucleophilic attack affords the construction of various organic compounds. Thus, investigations of carbonyl activation applying greener methodologies are highly important. In the present work, among the investigated N-halo compounds, N-fluorobenzenesulfonimide (NFSi) has been found as an efficient and selective catalyst in the reaction of direct esterification of aryl and alkyl carboxylic acids supported by microwave (MW) irradiation. The comprehensive esterification of different benzoic acids and mono-, di- and tri-carboxy alkyl derivatives was performed, whereby significant reaction time reductions were achieved. The presented method used NFSi as an easily manipulatable, non-metal, water- and air-tolerant catalyst, allowing simple synthetic and isolation procedures and energy saving, compared to conventional methodologies. Importantly, in contrast to esterification under thermal conditions, where N-halo compounds behave as pre-catalysts, in the MW-supported protocol, a distinct reaction mechanism has been proposed that assumes NFSi as a sustainable catalyst. Moreover, a scale-up of the industrially important derivative was performed.

Ni(II)-Mediated Photochemical Oxidative Esterification of Aldehydes with Phenols

The photopromoted, Ni-catalyzed acceptorless dehydrogenation esterification of phenols and aromatic aldehydes has been achieved in an oxidant- and external photosensitizer-free manner. This reliable and atom-economical transformation was tolerant to a wide range of functional groups and proceeded efficiently to give various aryl benzoates in moderate to high yields. Additionally, this photocatalytic system displayed high activity for the hydrogen-evolution cross coupling of aliphatic aldehydes and phenols employing dual nickel and aromatic aldehyde catalysis.

Factors influencing pressure-driven membrane-assisted volatile fatty acids recovery and purification-A review

Volatile fatty acids (VFAs) are building block chemicals that can be produced through bioconversion of organic waste streams via anaerobic digestion as intermediate products. Purified VFAs are applicable in a wide range of industrial applications such as food, textiles, cosmetics, pharmaceuticals etc. production. The present review focuses on VFAs recovery methods and technologies such as adsorption, distillation, extraction, gas stripping, esterification and membrane based techniques etc., while presenting a discussion of their pros and cons. Moreover, a great attention has been given to the recovery of VFAs through membrane filtration as a promising sustainable clarification, fractionation and concentration approach. In this regard, a thorough overview of factors affecting membrane filtration performance for VFAs recovery has been presented. Filtration techniques such as nanofiltration and reverse osmosis have shown to be capable of recovering over 90% of VFAs content from organic effluent steams, proving the direct effect of membrane materials/surface chemistry, pore size and solution pH in recovery success level. Overall, this review presents a new insight into challenges and potentials of membrane filtration for VFAs recovery based on the effects of factors such as operational parameters, membrane properties and effluent characteristics.

Kinetic studies on Lewis acidic metal polyesterification catalysts - hydrolytic degradation is a key factor for catalytic performance

Kinetic analysis of polyesterification reactions using Lewis-acidic metal catalysts have been performed. While Sn-based catalysts are superior to Ti-based catalysts under neat polycondensation conditions (high [H₂O]), the result is inverted under azeotropic conditions (low [H₂O]). These findings show that the catalytic activity is crucially determined by the robustness of the catalyst against hydrolytic degradation.

Kinetic studies of Lewis acidic metal-catalyzed polyesterification reactions unveiled that titanium-based catalyst are prone to hydrolytic degradation, while n-BuSnOOH proved to be a robust catalyst.

Photochemical Activation of Aromatic Aldehydes: Synthesis of Amides, Hydroxamic Acids and Esters

A cheap, facile and metal-free photochemical protocol for the activation of aromatic aldehydes has been developed. Utilizing thioxanthen-9-one as the photocatalyst and cheap household lamps as the light source, a variety of aromatic aldehydes have been activated and subsequently converted in a one-pot reaction into amides, hydroxamic acids and esters in good to high yields. The applicability of this method was highlighted in the synthesis of Moclobemide, a drug against depression and social anxiety. Extended and detailed mechanistic studies have been conducted, in order to determine a plausible mechanism for the reaction.

Direct synthesis of amides from nonactivated carboxylic acids using urea as nitrogen source and Mg(NO3)2 or imidazole as catalysts

A new method for the direct synthesis of primary and secondary amides from carboxylic acids is described using Mg(NO3)2·6H2O or imidazole as a low-cost and readily available catalyst, and urea as a stable, and easy to manipulate nitrogen source. This methodology is particularly useful for the direct synthesis of primary and methyl amides avoiding the use of ammonia and methylamine gas which can be tedious to manipulate. Furthermore, the transformation does not require the employment of coupling or activating agents which are commonly required.

Kinetic Analysis as an Optimization Tool for Catalytic Esterification with a Moisture-Tolerant Zirconium Complex

This work describes the use of kinetics as a tool for rational optimization of an esterification process with down to equimolar ratios of reagents using a recyclable commercially available zirconocene complex in catalytic amounts. In contrast to previously reported group IV metal-catalyzed esterification protocols, the work presented herein circumvents the use of water scavengers and perfluorooctane sulfonate (PFOS) ligands. Insights into the operating mechanism are presented.

An Efficient Greener Approach for N-Acylation of Amines in Water Using Benzotriazole Chemistry

A straightforward, mild and cost-efficient synthesis of various arylamides in water was accomplished using versatile benzotriazole chemistry. Acylation of various amines was achieved in water at room temperature as well as under microwave irradiation. The developed protocol unfolds the synthesis of amino acid aryl amides, drug conjugates and benzimidazoles. The environmentally friendly synthesis, short reaction time, simple workup, high yields, mild conditions and free of racemization are the key advantages of this protocol.

Phosphorylation Organocatalysts Highly Active by Design

The activity of nucleophilic organocatalysts for alcohol/phenol phosphorylation was enhanced through attaching oligoether appendages to a benzyl substituent on imidazole- or aminopyridine-based active units, presumably because of stabilizing n-cation interactions of the ethereal oxygens with the positively charged aza-heterocycle in the catalytic intermediates, and was substantially higher than that of known benchmark catalysts for a range of substrates. Density functional theory calculations and the study of analogues having a lower potential for such stabilizing interactions support our hypothesis.

Direct Dehydrative Glycosylation Catalyzed by Diphenylammonium Triflate

Methods for direct dehydrative glycosylations of carbohydrate hemiacetals catalyzed by diphenylammonium triflate under microwave irradiation are described. Both armed and disarmed glycosyl-C1-hemiacetal donors were efficiently glycosylated in moderate to excellent yields without the need for any drying agents and stoichiometric additives. This method has been successfully applied to a solid-phase glycosylation.

1,3-Dibromo-5,5-dimethylhydantoin as a Precatalyst for Activation of Carbonyl Functionality

Activation of carbonyl moiety is one of the most rudimentary approaches in organic synthesis and is crucial for a plethora of industrial-scale condensation reactions. In esterification and aldol condensation, which represent two of the most important reactions, the susceptibility of the carbonyl group to nucleophile attack allows the construction of a variety of useful organic compounds. In this context, there is a constant need for development of and improvement in the methods for addition-elimination reactions via activation of carbonyl functionality. In this paper, an advanced methodology for the direct esterification of carboxylic acids and alcohols, and for aldol condensation of aldehydes using widely available, inexpensive, and metal-free 1,3-dibromo-5,5-dimethylhydantoin under neat reaction conditions is reported. The method is air- and moisture-tolerant, allowing simple synthetic and isolation procedures for both reactions presented in this paper. The reaction pathway for esterification is proposed and a scale-up of certain industrially important derivatives is performed.

Catalytic direct amidations in tert-butyl acetate using B(OCH2CF3)3

Catalytic direct amidation reactions have been the focus of considerable recent research effort, due to the widespread use of amide formation processes in pharmaceutical synthesis. However, the vast majority of catalytic amidations are performed in non-polar solvents (aromatic hydrocarbons, ethers) which are typically undesirable from a sustainability perspective, and are often poor at solubilising polar carboxylic acid and amine substrates. As a consequence, most catalytic amidation protocols are unsuccessful when applied to polar and/or functionalised substrates of the kind commonly used in medicinal chemistry. In this paper we report a practical and useful catalytic direct amidation reaction using tert-butyl acetate as the reaction solvent. The use of an ester solvent offers improvements in terms of safety and sustainability, but also leads to an improved reaction scope with regard to polar substrates and less nucleophilic anilines, both of which are important components of amides used in medicinal chemistry. An amidation reaction was scaled up to 100 mmol and proceeded with excellent yield and efficiency, with a measured process mass intensity of 8.

Formamide catalyzed activation of carboxylic acids - versatile and cost-efficient amidation and esterification

Formamide catalysis enables highly cost-efficient amide C–N and ester C–O bond formation through carboxylic acid chlorides as essential intermediates.

A novel, broadly applicable method for amide C–N and ester C–O bond formation is presented based on formylpyrrolidine (FPyr) as a Lewis base catalyst. Herein, trichlorotriazine (TCT), which is the most cost-efficient reagent for OH-group activation, was employed in amounts of ≤40 mol% with respect to the starting material (100 mol%). The new approach is distinguished by excellent cost-efficiency, waste-balance (E-factor down to 3) and scalability (up to >80 g). Moreover, high levels of functional group compatibility, which includes acid-labile acetals and silyl ethers, are demonstrated and even peptide C–N bonds can be formed. In comparison to reported amidation procedures using TCT, yields are considerably improved (for instance from 26 to 91%) and esterification is facilitated for the first time in synthetically useful yields. These significant enhancements are rationalized by activation by means of acid chlorides instead of less electrophilic acid anhydride intermediates.

Diboronic Acid Anhydrides as Effective Catalysts for the Hydroxy-Directed Dehydrative Amidation of Carboxylic Acids

The direct catalytic dehydrative amidation of β-hydroxycarboxylic acids with amines is described. A biphenyl-based diboronic acid anhydride with a B-O-B skeleton is shown to be an exceptionally effective catalyst for the reaction, providing β-hydroxycarboxylic amides in high to excellent yields with a low catalyst loading (minimum of 0.01 mol %, TON up to 7,500). This hydroxy-directed amidation shows excellent chemoselectivity and is applicable to gram-scale drug synthesis.

Base-catalyzed selective esterification of alcohols with unactivated esters

A practical and efficient base-catalyzed esterification has been developed for the facile synthesis of a broad range of esters from simple alcohols with unactivated tert-butyl esters. This protocol could be conducted at mild conditions, providing esters in high to excellent yields with good functional tolerance. Mechanistic studies provided evidence of an exchange of the tert-butyl alkoxide metal with the alcohol, producing a new alkoxide to participate in the transesterification reaction.

Visible light-induced transformation of aldehydes to esters, carboxylic anhydrides and amides

A transition metal- and organophotocatalyst free synthesis of esters, carboxylic anhydrides and amides from aldehydes induced by visible-light has been reported.

Visible light driven, nickel-catalyzed aryl esterification using a triplet photosensitiser thioxanthen-9-one

The nickel-catalyzed esterification of carboxylic acids with aryl bromides using thioxanthen-9-one as a photosensitizer provided aryl esters with excellent yields.

Mechanistic Insight into Asymmetric Hetero-Michael Addition of α,β-Unsaturated Carboxylic Acids Catalyzed by Multifunctional Thioureas

Carboxylic acids and their corresponding carboxylate anions are generally utilized as Brønsted acids/bases and oxygen nucleophiles in organic synthesis. However, a few asymmetric reactions have used carboxylic acids as electrophiles. Although chiral thioureas bearing both arylboronic acid and tertiary amine were found to promote the aza-Michael addition of BnONH₂ to α,β-unsaturated carboxylic acids with moderate to good enantioselectivities, the reaction mechanism remains to be clarified. Detailed investigation of the reaction using spectroscopic analysis and kinetic studies identified tetrahedral borate complexes, comprising two carboxylate anions, as reaction intermediates. We realized a dramatic improvement in product enantioselectivity with the addition of 1 equiv of benzoic acid. In this aza-Michael reaction, the boronic acid not only activates the carboxylate ligand as a Lewis acid, together with the thiourea NH-protons, but also functions as a Brønsted base through a benzoyloxy anion to activate the nucleophile. Moreover, molecular sieves were found to play an important role in generating the ternary borate complexes, which were crucial for obtaining high enantioselectivity as demonstrated by DFT calculations. We also designed a new thiourea catalyst for the intramolecular oxa-Michael addition to suppress another catalytic pathway via a binary borate complex using steric hindrance between the catalyst and substrate. Finally, to demonstrate the synthetic versatility of both hetero-Michael additions, we used them to accomplish the asymmetric synthesis of key intermediates in pharmaceutically important molecules, including sitagliptin and α-tocopherol.

Esterification of Aryl/Alkyl Acids Catalysed by N-bromosuccinimide under Mild Reaction Conditions

N-halosuccinimides (NXSs) are well-known to be convenient, easily manipulable and low-priced halogenation reagents in organic synthesis. In the present work, N-bromosuccinimide (NBS) has been promoted as the most efficient and selective catalyst among the NXSs in the reaction of direct esterification of aryl and alkyl carboxylic acids. Comprehensive esterification of substituted benzoic acids, mono-, di- and tri-carboxy alkyl derivatives has been performed under neat reaction conditions. The method is metal-free, air- and moisture-tolerant, allowing for a simple synthetic and isolation procedure as well as the large-scale synthesis of aromatic and alkyl esters with yields up to 100%. Protocol for the recycling of the catalyst has been proposed.

The Discovery of a Palladium(II)-Initiated Borono-Catellani Reaction

Reported is a novel palladium(II)-initiated Catellani-type reaction that utilizes widely accessible aryl boronic acids as the substrates instead of aryl halides, thereby greatly expanding the existing scope of this powerful transformation. This borono-Catellani reaction was promoted by cooperative catalysis between Pd(OAc)₂ and the inexpensive 5-norbornene-2-carbonitrile. Practicality is the striking feature of the reaction: it is run open to air at ambient temperature and no phosphine ligand is needed. This mild, chemoselective, and scalable protocol is compatible with a large range of readily available functionalized aryl boronic acids and bromides, as well as terminating olefins (50 examples, 39-97 % yields). Moreover, the orthogonal reactivity between the borono-Catellani and classical Catellani reaction was demonstrated. This work is expected to open new avenues for developing novel Catellani-type reactions.

Protecting-Group-Free Amidation of Amino Acids using Lewis Acid Catalysts

Amidation of unprotected amino acids has been investigated using a variety of 'classical" coupling reagents, stoichiometric or catalytic group(IV) metal salts, and boron Lewis acids. The scope of the reaction was explored through the attempted synthesis of amides derived from twenty natural, and several unnatural, amino acids, as well as a wide selection of primary and secondary amines. The study also examines the synthesis of medicinally relevant compounds, and the scalability of this direct amidation approach. Finally, we provide insight into the chemoselectivity observed in these reactions.

Palladium-Catalyzed Esterification of Carboxylic Acids with Aryl Iodides

The first palladium-catalyzed esterification of carboxylic acids with aryl iodides is described. A palladium-based catalytic system consisting of IBn^F (1,3-bis((pentafluorophenyl)methyl)imidazole-2-ylidene) ligand was found to significantly accelerate the aryl-O bond-forming esterification reaction. A series of aryl iodides and carboxylic acids undergoes a palladium-catalyzed coupling reaction to provide the corresponding aryl esters in moderate to good yields. In addition, sterically hindered aryl iodides and carboxylic acids were well-tolerated yielding the corresponding aryl esters.

Mechanistic insights into boron-catalysed direct amidation reactions

The generally accepted monoacyloxyboron mechanism of boron-catalysed direct amidation is brought into question in this study, and new alternatives are proposed.

The generally accepted monoacyloxyboron mechanism of boron-catalysed direct amidation is brought into question in this study, and new alternatives are proposed. We have carried out a detailed investigation of boron-catalysed amidation reactions, through study of the interaction between amines/carboxylic acids and borinic acids, boronic acids and boric acid, and have isolated and characterised by NMR/X-ray crystallography many of the likely intermediates present in catalytic amidation reactions. Rapid reaction between amines and boron compounds was observed in all cases, and it is proposed that such boron–nitrogen interactions are highly likely to take place in catalytic amidation reactions. These studies also clearly show that borinic acids are not competent catalysts for amidation, as they either form unreactive amino-carboxylate complexes, or undergo protodeboronation to give boronic acids. It therefore seems that at least three free coordination sites on the boron atom are necessary for amidation catalysis to occur. However, these observations are not consistent with the currently accepted ‘mechanism’ for boron-mediated amidation reactions involving nucleophilic attack of an amine onto a monomeric acyloxyboron intermediate, and as a result of our observations and theoretical modelling, alternative proposed mechanisms are presented for boron-mediated amidation reactions. These are likely to proceed via the formation of a dimeric B–X–B motif (X = O, NR), which is uniquely able to provide activation of the carboxylic acid, whilst orchestrating the delivery of the amine nucleophile to the carbonyl group. Quantum mechanical calculations of catalytic cycles at the B3LYP+D3/Def2-TZVPP level (solvent = CH₂Cl₂) support the proposal of several closely related potential pathways for amidation, all of which are likely to be lower in energy than the currently accepted mechanism.

Catalytic hydrogenation of carboxylic acids using low-valent and high-valent metal complexes

Carboxylic acids are ubiquitous in bio-renewable and petrochemical sources of carbon.