Catalytic amide formation from non-activated carboxylic acids and amines

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Borinic acid catalysed peptide synthesis

The catalytic synthesis of peptides is a major challenge in the modern organic chemistry hindered by the well-established use of stoichiometric coupling reagents. Herein, we describe for the first time that borinic acid is able to catalyse this reaction under mild conditions with an improved activity compared to our recently developed thiophene-based boronic acid. This catalyst is particularly efficient for peptide bond synthesis affording dipeptides in good yields without detectable racemization.

Enzymatic Synthesis of Biobased Polyesters and Polyamides

Nowadays, "green" is a hot topic almost everywhere, from retailers to universities to industries; and achieving a green status has become a universal aim. However, polymers are commonly considered not to be "green", being associated with massive energy consumption and severe pollution problems (for example, the "Plastic Soup") as a public stereotype. To achieve green polymers, three elements should be entailed: (1) green raw materials, catalysts and solvents; (2) eco-friendly synthesis processes; and (3) sustainable polymers with a low carbon footprint, for example, (bio)degradable polymers or polymers which can be recycled or disposed with a gentle environmental impact. By utilizing biobased monomers in enzymatic polymerizations, many advantageous green aspects can be fulfilled. For example, biobased monomers and enzyme catalysts are renewable materials that are derived from biomass feedstocks; enzymatic polymerizations are clean and energy saving processes; and no toxic residuals contaminate the final products. Therefore, synthesis of renewable polymers via enzymatic polymerizations of biobased monomers provides an opportunity for achieving green polymers and a future sustainable polymer industry, which will eventually play an essential role for realizing and maintaining a biobased and sustainable society.

Atom-economic catalytic amide synthesis from amines and carboxylic acids activated in situ with acetylenes

Amide bond-forming reactions are of tremendous significance in synthetic chemistry. Methodological research has, in the past, focused on efficiency and selectivity, and these have reached impressive levels. However, the unacceptable amounts of waste produced have led the ACS GCI Roundtable to label 'amide bond formation avoiding poor atom economy' as the most pressing target for sustainable synthetic method development. In response to this acute demand, we herein disclose an efficient one-pot amide coupling protocol that is based on simple alkynes as coupling reagents: in the presence of a dichloro[(2,6,10-dodecatriene)-1,12-diyl]ruthenium catalyst, carboxylate salts of primary or secondary amines react with acetylene or ethoxyacetylene to vinyl ester intermediates, which undergo aminolysis to give the corresponding amides along only with volatile acetaldehyde or ethyl acetate, respectively. The new amide synthesis is broadly applicable to the synthesis of structurally diverse amides, including dipeptides.

Benzoisothiazolone Organo/Copper-Cocatalyzed Redox Dehydrative Construction of Amides and Peptides from Carboxylic Acids using (EtO)3P as the Reductant and O2 in Air as the Terminal Oxidant

Carboxylic acids and amine/amino acid reactants can be converted to amides and peptides at neutral pH within 5-36 h at 50 °C using catalytic quantities of a redox-active benzoisothiazolone and a copper complex. These catalytic "oxidation-reduction condensation" reactions are carried out open to dry air using O2 as the terminal oxidant and a slight excess of triethyl phosphite as the reductant. Triethyl phosphate is the easily removed byproduct. These simple-to-run catalytic reactions provide practical and economical procedures for the acylative construction of C-N bonds.

N-Heterocyclic Carbene-Mediated Microfluidic Oxidative Electrosynthesis of Amides from Aldehydes

A flow process for N-Heterocyclic Carbene (NHC)-mediated anodic oxidative amidation of aldehydes is described, employing an undivided microfluidic electrolysis cell to oxidize Breslow intermediates. After electrochemical oxidation, the reaction of the intermediate N-acylated thiazolium cation with primary amines is completed by passage through a heating cell to achieve high conversion in a single pass. The flow mixing regimen circumvented the issue of competing imine formation between the aldehyde and amine substrates, which otherwise prevented formation of the desired product. High yields (71-99%), productivities (up to 2.6 g h(-1)), and current efficiencies (65-91%) were realized for 19 amides.

Highly Efficient Preparation of Amides from Aminium Carboxylates Using N-(p-toluenesulfonyl) Imidazole

Treatment of aminium carboxylates with N-(p-toluenesulfonyl)imidazole in the presence of triethylamine in DMF at 100 °C afforded the corresponding amides in good to excellent yields. N-(p-Toluenesulfonyl)imidazole proved to be a highly efficient coupling reagent for the preparation of numerous structurally diverse primary, secondary and tertiary amides.

Palladium-catalyzed Suzuki–Miyaura coupling of amides by carbon–nitrogen cleavage: general strategy for amide N–C bond activation

A unified strategy for the palladium-catalyzed Suzuki–Miyaura cross-coupling of amides with boronic acids for the synthesis of ketones by N–C bond activation is reported.

Sterically-controlled intermolecular Friedel–Crafts acylation with twisted amides via selective N–C cleavage under mild conditions

Highly chemoselective Friedel–Crafts acylation of arenes with twisted amides by N–C bond cleavage is reported for the first time.

Direct oxidative amidation of aldehydes with amines catalyzed by heteropolyanion-based ionic liquids under solvent-free conditions via a dual-catalysis process

A heteropolyanion-based ionic liquid catalyzed oxidative amidation of aldehydes with amines via a dual-catalysis pathway has been reported.

Microwave-assisted heteropolyanion-based ionic liquid promoted sustainable protocol to N-heteroaryl amides via N-directing dual catalyzed oxidative amidation of aldehydes

A procedure for the synthesis of N-heteroaryl amides directly from oxidative amidation of aldehydes catalyzed by heteropolyanion-based ionic liquids has been reported. The proposed N-directing dual-catalysis mechanism was briefly investigated.

Isocyanide and Meldrum's acid-based multicomponent reactions in diversity-oriented synthesis: from a serendipitous discovery towards valuable synthetic approaches

Multicomponent reactions, which lead to synthesis of target compounds with inherent molecular diversity, greater efficiency and atom economy, are single step types of reactions made from three or more reactants attract the attention of all chemists.

One-pot biocatalytic amine transaminase/acyl transferase cascade for aqueous formation of amides from aldehydes or ketones

A novel biocatalytic amine transaminase/acyl transferase cascade for the formation of amides in aqueous solution has been developed.

BODIPY catalyzed amide synthesis promoted by BHT and air under visible light

A novel and efficient protocol for the synthesis of amides is reported via a BODIPY catalyzed oxidative amidation of aromatic aldehydes under visible light, with broad substrate scope and mild reaction conditions. Mechanistic studies reveal that dioxygen could be activated through both an ET and SET pathway to form the active peroxide intermediates.

Boronic acid-DMAPO cooperative catalysis for dehydrative condensation between carboxylic acids and amines

Arylboronic acid and DMAPO cooperatively catalyse the dehydrative condensation reaction between carboxylic acids and amines to give the corresponding amides under azeotropic reflux conditions. This cooperative use is much more effective than their individual use as catalysts.

Arylboronic acid and 4-(N,N-dimethylamino)pyridine N-oxide (DMAPO) cooperatively catalyse the dehydrative condensation reaction between carboxylic acids and amines to give the corresponding amides under azeotropic reflux conditions. This cooperative use is much more effective than their individual use as catalysts, and chemoselectively promotes the amide condensation of (poly)conjugated carboxylic acids. The present method is practical and scalable, and has been applied to the synthesis of sitagliptin and a drug candidate.

Carboxyl activation via silylthioesterification: one-pot, two-step amidation of carboxylic acids catalyzed by non-metal ammonium salts

The first organo-catalyzed silylthioesterification of a carboxylic acid and a commercially available mercaptoorganosilane results in the in situ production of an O-silylthionoester. Subsequent amine addition forms amides in an operationally simple one-pot procedure without removal of water. The scope and efficiency of these reactions with respect to the catalyst, carboxylic acid, amine, [Si─S] moiety, and solvent are investigated. A number of functionalities are tolerated in the two-step amidation including alkene, alkyne, alkyl and aryl halides, benzylic ethers, and heterocycles with free coordinating sites.

Base-catalyzed synthesis of aryl amides from aryl azides and aldehydes

Aryl amides are efficiently synthesized from the rearrangement of triazolines, which formed in the base-catalyzed azide–aldehyde cycloaddtion.

Aryl amides have been used as important compounds in pharmaceuticals, materials and in molecular catalysis. The methods reported to prepare aryl amides generally require very specific reagents, and the most popular carboxyl–amine coupling reactions demand stoichiometric activators. Herein, we report that aryl azides react with aldehydes under base-catalyzed conditions to yield aryl amides efficiently. Mechanistic investigations support the formation of triazoline intermediates via azide–enolate cycloaddition, which subsequently undergo rearrangement to give amides by either thermal decomposition (20–140 °C) or aqueous acid work-up at room temperature. The strategy does not require nucleophilic anilines and is especially efficient for highly electron-deficient aryl amides, including perfluoroaryl amides, which are otherwise challenging to synthesize.

B(OCH2CF3)3-mediated direct amidation of pharmaceutically relevant building blocks in cyclopentyl methyl ether

The use of B(OCH2CF3)3 for mediating direct amidation reactions of a wide range of pharmaceutically relevant carboxylic acids and amines is described, including numerous heterocycle-containing examples. An initial screen of solvents for the direct amidation reaction suggested that cyclopentyl methyl ether, a solvent with a very good safety profile suitable for use over a wide temperature range, was an excellent replacement for the previously used solvent acetonitrile. Under these conditions amides could be prepared from 18 of the 21 carboxylic acids and 18 of the 21 amines examined. Further optimisation of one of the low yielding amidation reactions (36% yield) via a design of experiments approach enabled an 84% yield of the amide to be obtained.

Boron-Catalyzed N-Alkylation of Amines using Carboxylic Acids

A boron-based catalyst was found to catalyze the straightforward alkylation of amines with readily available carboxylic acids in the presence of silane as the reducing agent. Various types of primary and secondary amines can be smoothly alkylated with good selectivity and good functional-group compatibility. This metal-free amine alkylation was successfully applied to the synthesis of three commercial medicinal compounds, Butenafine, Cinacalcet. and Piribedil, in a one-pot manner without using any metal catalysts.

Catalytic chemical amide synthesis at room temperature: one more step toward peptide synthesis

An efficient method has been developed for direct amide bond synthesis between carboxylic acids and amines via (2-(thiophen-2-ylmethyl)phenyl)boronic acid as a highly active bench-stable catalyst. This catalyst was found to be very effective at room temperature for a large range of substrates with slightly higher temperatures required for challenging ones. This methodology can be applied to aliphatic, α-hydroxyl, aromatic, and heteroaromatic acids as well as primary, secondary, heterocyclic, and even functionalized amines. Notably, N-Boc-protected amino acids were successfully coupled in good yields with very little racemization. An example of catalytic dipeptide synthesis is reported.

N,N-diethylurea-catalyzed amidation between electron-deficient aryl azides and phenylacetaldehydes

Urea structures, of which N,N-diethylurea (DEU) proved to be the most efficient, were discovered to catalyze amidation reactions between electron-deficient aryl azides and phenylacetaldehydes. Experimental data support 1,3-dipolar cycloaddition between DEU-activated enols and electrophilic phenyl azides, especially perfluoroaryl azides, followed by rearrangement of the triazoline intermediate. The activation of the aldehyde under near-neutral conditions was of special importance in inhibiting dehydration/aromatization of the triazoline intermediate, thus promoting the rearrangement to form aryl amides.

Ce(III) immobilised on aminated epichlorohydrin-activated agarose matrix – “green” and efficient catalyst for transamidation of carboxamides

The present study reports the preparation and characterisation of Ce(III) immobilised on an aminated epichlorohydrin-activated agarose matrix (CAEA) as a “green” catalyst. The catalyst was synthesised by the reaction of the epichlorohydrin-activated agarose matrix with ammonia solution, which was then treated with Ce(NO

Anionic phenoxy-amido rare-earth complexes as efficient catalysts for amidation of aldehydes with amines

A series of anionic organo-rare-earth amido complexes stabilized by dianionic phenoxy-amido ligands were synthesized, and their catalytic property for the amidation reactions of aldehydes with amines was explored.

Direct reductive coupling of secondary amides: chemoselective formation of vicinal diamines and vicinal amino alcohols

We report the first direct and chemoselective reductive homocoupling reaction of secondary amides and cross-coupling reaction of secondary amides with ketones. This method relies on the direct generation of α-amino carbon radicals from amides.

MnO2 catalyzed formylation of amines and transamidation of amides under solvent-free conditions

A facile and efficient MnO₂ catalyzed one-pot protocol for formylation of amine and transamidation of primary and secondary amides by amines has been developed.

An efficient mechanochemical synthesis of amides and dipeptides using 2,4,6-trichloro-1,3,5-triazine and PPh3

A mechanochemical method for amidation of carboxylic acids and urethane-protected (Fmoc, Cbz, Boc) α-amino acids has been developed as a facile, efficient, and eco-friendly route toward amides and dipeptides.