Heterocyclic Boron Acid Catalyzed Dehydrative Amidation of Aliphatic/Aromatic Carboxylic Acids with Amines

Borate-catalysed direct amidation reactions of coordinating substrates

The catalytic activity of different classes of boron catalysts was studied in amidation reactions with 4-phenylbutylamine/benzoic acid, and with 2-aminopyridine/phenylacetic acid. Whilst a simple boronic acid catalyst showed high catalytic activity with the former substrates, it was completely inactive in the latter reaction. In contrast, a borate ester catalyst was able to mediate the amidation of both substrate pairs with moderate activity. By screening a range of borate esters we were able to identify a novel borate catalyst that shows high reactivity with a range of challenging carboxylic acids/amine pairs, enabling catalystic amidation reactions to be achieved effectively with these industrially relevant compounds. The reactions can be performed on multigram scale with high levels of efficiency, and in situ catalyst generation from commercially available reagents renders the process readily accessible for everyday laboratory use. Further experiments showed that the deactivating effect of 2-aminopyridine on boronic acid catalysts was due to its ability to stabilise catalytically inactive boroxines.

Synthesis of N-methyl secondary amides via diboronic acid anhydride-catalyzed dehydrative condensation of carboxylic acids with aqueous methylamine

In this study, we present the first catalytic methodology for synthesizing N-methyl secondary amides via dehydrative condensation of hydroxycarboxylic acids with readily available and safe aqueous methylamine, employing diboronic acid anhydride (DBAA) as the catalyst. DBAA catalysis can also be applied to direct amidations using aqueous ethylamine or aqueous dimethylamine. Moreover, we demonstrate the applicability of this catalytic system for the concise synthesis of eight biologically active compounds containing β-amino alcohol motifs, including halostachine, synephrine, longimammine, phenylephrine, metanephrine, normacromerine, etilefrine, and macromerine.

An Electrochemical Design for a General Catalytic Carboxylic Acid Substitution Platform via Anhydrides at Room Temperature: Amidation, Esterification, and Thioesterification

An original concept for catalytic electrochemical dehydration has enabled a suite of acid substitutions, including amidation, esterification, and thioesterification, through a linchpin anhydride formed in situ. By avoiding stoichiometric dehydrating agents, this method addresses a leading challenge in organic synthesis and green chemistry. It also proceeds without acid additives at room temperature, accesses a diverse range of product structures, is easily scaled, and enabled the first example of catalytic peptide coupling at room temperature.

Aryl Borane as a Catalyst for Dehydrative Amide Synthesis

Tris(pentafluorophenyl)borane B(C6F5)3·H2O is reported as a catalyst for dehydrative amidation of carboxylic acids and amines. This protocol is applicable across a wide range of >35 substrates, including aromatic and aliphatic amines and acids, resulting in amides in ≤92% yields. The scalability of the reaction up to 10 mmol, along with the synthesis of drugs such as ibuprofen amide, moclobemide, and phenacetin, demonstrates the industrial potential of our protocol.

Improved Electrochemical Peptide Synthesis Enabled by Electron-Rich Triaryl Phosphines

While remarkable progress has been made in the development of peptide medicines, many problems related to peptide synthesis remain unresolved. Previously, we reported electrochemical peptide synthesis using a phosphine as a potentially recyclable coupling reagent. However, there was room for improvement from the point of view of reaction efficiency, especially in the carboxylic acid activation step and the peptide bond formation step. To overcome these challenges, we searched for the optimal phosphine. Among phosphines with various electronic properties, we found that electron-rich triaryl phosphines improved the reaction efficiency. Consequently, we successfully performed electrochemical peptide synthesis on sterically hindered and valuable amino acids. We also synthesized oligopeptides that were challenging with our previous method. Finally, we examined the effect of substituents on the phosphine cations, and gained some insights into reactivity, which will aid researchers designing reactions involving phosphine cations.

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.

Development of boronic acid catalysts for direct amidation of aromatic carboxylic acids using fluorescence-based screening

Direct amidation of carboxylic acids with amines holds significant importance; therefore, catalytic processes involving boronic acids have undergone extensive investigation. However, studies focused on the amidation of aromatic carboxylic acids remain limited. In this study, we introduce a fluorescence-based screening methodology employing an anthracene derivative probe, facilitating the rapid evaluation of various amidation catalysts. Using this approach, boronic acids were evaluated for their catalytic potential. Our findings reveal that 2-hydroxyphenylboronic acid (C7), previously deemed inefficient for aliphatic acids, effectively catalyzes the amidation of aromatic acids. The catalysts identified through this method consistently achieved high yields, reaching up to 98% across a broad spectrum of substrates.

One-Pot Oxidative Amidation of Aldehydes via the Generation of Nitrile Imine Intermediates

A one-pot procedure for the oxidative amidation of aldehydes via the in situ generation of reactive nitrile imine (NI) intermediates has been developed. Distinct from our progenitor processes, mechanistic and control experiments revealed that the NI undergoes rapid oxidation to an acyl diazene species, which then facilitates N-acylation of an amine. A range of substrates have been explored, including application in the synthesis of pharmaceutically relevant compounds.

Palladium-Catalyzed Chelation-Assisted Aldehyde C-H Bond Activation of Quinoline-8-carbaldehydes: Synthesis of Amides from Aldehydes with Anilines and Other Amines

A palladium-catalyzed chelation-assisted direct aldehyde C-H bond amidation of quinoline-8-carbaldehydes with an amine was developed under mild reaction conditions. A wide range of amides were obtained in good to excellent yields from aldehyde with a variety of aniline derivatives and aliphatic amines. Our methodology was successfully applied to synthesize known DNA intercalating agents and can be easily scaled up to a gram scale.

Borane-Pyridine: An Efficient Catalyst for Direct Amidation

Borane-pyridine acts as an efficient (5 mol%) liquid catalyst, providing improved solubility for the direct amidation of a wide range of aromatic and aliphatic carboxylic acids and amines to form secondary and tertiary carboxamides. Tolerance of potentially incompatible halo, nitro, and alkene functionalities has been demonstrated.

Hydroxy-directed peptide bond formation from α-amino acid-derived inert esters enabled by boronic acid catalysis

A boronic acid-catalyzed peptide bond formation from α-amino acid methyl esters is described. The catalysis showed high chemoselectivity for β-hydroxy-α-amino esters, affording the peptides in high to excellent yields with high functional group tolerance. This hydroxy-directed peptide bond formation could be applicable to oligopeptide syntheses. This is the first successful example of organoboron-catalyzed peptide bond formation from α-amino acid-derived inert esters.

Diarylborinic Acid-Catalyzed Ring Opening of cis-4-Hydroxymethyl-1,2-Cyclopentene Oxides: Synthesis of 1,2,4-Trisubstituted Cyclopentanes

A diarylborinic acid-catalyzed ring opening of cis-4-hydroxymethyl-1,2-cyclopentene oxides was developed with N-nucleophiles including anilines, benzotriazole, and alkylamines, as well as S-nucleophiles, affording 1,2,4-trisubstituted cyclopentane compounds containing a quaternary carbon center. The mechanism study indicated that the "half-cage" structure formed by the epoxide substrate and the catalyst prevents the nucleophiles from attacking the inner side of the "half-cage", resulting in the desired ring-opening product.

Development of fluorous boronic acid catalysts integrated with sulfur for enhanced amidation efficiency

A thermally stable, fluorous sulfur-containing boronic acid catalyst has been developed and was shown to efficiently promote dehydrative condensation between carboxylic acids and amines under environmentally friendly conditions. The methodology can be applied to aliphatic, aromatic and heteroaromatic acids as well as primary and secondary amines. N-Boc protected amino acids were also successfully coupled in good yields with very little racemization. The catalyst could be reused four times with no significant loss of activity.

Expeditious Access to the B3NO2 Heterocycle Enabling Modular Derivatization

DATB (1,3-dioxa-5-aza-2,4,6-triborinane) is a unique six-membered heterocycle exhibiting proficient catalytic activity in direct dehydrative amidation. Reported herein is an improved synthetic protocol for DATB derivatives featuring a concise two-step chromatography-free process. Suzuki-Miyaura coupling assembled 2,6-dibromoaniline derivatives and 1,2-phenylenediboronic acid to afford dimeric B-spiroborate salts. Acidic untying of the spiroborates gave rise to the DATB ring system with various substituents.