Less Is More: N(BOH)2 Configuration Exhibits Higher Reactivity than the B3NO2 Heterocycle in Catalytic Dehydrative Amide Formation

Gem-Diboronic Acids: A Motif for Anion Recognition in Competitive Media

Gem-diboronic acid derivatives have previously been demonstrated to facilitate substrate binding within a metal's secondary coordination sphere, but their use as an anion recognition motif has not been explored. Here, we introduce the gem-diboronic acid motif as a highly effective group for recognition of oxoanions in aqueous solution. Anion receptors based on this motif demonstrate higher binding affinities than other common neutral motifs such as amides and (thio)ureas, and display a unique selectivity profile. Studies with a receptor bearing two gem-diboronic acid groups, receptor 2, indicate that the interaction with anions is highly directional. Despite its simplicity, receptor 2 represents one of the most selective receptors for malonate reported to date.

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.

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.

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.

Butterfly-Shaped Dibenz[a,j]anthracenes: Synthesis and Photophysical Properties

A strategy for the synthesis of dibenz[a,j]anthracenes (DBAs) from cyclohexa-2,5-diene-1-carboxylic acids is presented. Our approach involves sequential C-H olefination, cycloaddition, and decarboxylative aromatization. In the key step for DBA skeleton construction, the bis-C-H olefination products, 1,3-dienes, are utilized as substrates for [4 + 2] cycloaddition with benzyne. This concise synthetic route allows for regioselective ring formation and functional group introduction. The structural features and photophysical properties of the resulting DBA molecules are discussed.

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.