Concise Synthesis of Potassium Acyltrifluoroborates from Aldehydes through Copper(I)-Catalyzed Borylation/Oxidation

A Versatile Route to Acyl (MIDA)Boronates

A modular approach to highly functional acyl (MIDA)boronates is described. It involves the generation of the hitherto unknown radical derived from acetyl (MIDA)boronate and its capture by various alkenes, including electronically unbiased, unactivated alkenes. In contrast to the anion of acetyl (MIDA)boronate, which has not so far been employed in synthesis, the corresponding radical is well behaved and readily produced from the novel α-xanthyl acetyl (MIDA)boronate. This shelf-stable, easily prepared solid is a convenient acyl (MIDA)boronate transfer agent that provides a direct entry to numerous otherwise inaccessible structures, including latent 1,4-dicarbonyl derivatives that can be transformed into B(MIDA) substituted pyrroles and furans. A competition experiment indicated the acyl (MIDA)boronate substituted radical to be more stable than the all-carbon acetonyl radical but somewhat less reactive in additions to alkenes.

One-Step Synthesis of Acylborons from Acyl Chlorides through Copper-Catalyzed Borylation with Polystyrene-Supported PPh3 Ligand

We developed a one-step synthesis of acylborons from both readily available acyl chlorides and bis(pinacolato)diboron through copper(I)-catalyzed borylation. Under the reaction conditions using ^tBuOLi, polystyrene-supported triphenylphosphine as a copper ligand was found to promote the borylation of acyl chlorides while suppressing alcoholysis. This method enables the facile synthesis of potassium acyltrifluoroborates.

Copper-Catalyzed Borylation of Acyl Chlorides with an Alkoxy Diboron Reagent: A Facile Route to Acylboron Compounds

Herein, the copper‐catalyzed borylation of readily available acyl chlorides with bis(pinacolato)diboron, (B₂pin₂) or bis(neopentane glycolato)diboron (B₂neop₂) is reported, which provides stable potassium acyltrifluoroborates (KATs) in good yields from the acylboronate esters. A variety of functional groups are tolerated under the mild reaction conditions (room temperature) and substrates containing different carbon‐skeletons, such as aryl, heteroaryl and primary, secondary, tertiary alkyl are applicable. Acyl N‐methyliminodiacetic acid (MIDA) boronates can also been accessed by modification of the workup procedures. This process is scalable and also amenable to the late‐stage conversion of carboxylic acid‐containing drugs into their acylboron analogues, which have been challenging to prepare previously. A catalytic mechanism is proposed based on in situ monitoring of the reaction between p‐toluoyl chloride and an NHC‐copper(I) boryl complex as well as the isolation of an unusual lithium acylBpinOBpin compound as a key intermediate.

Preparation of Potassium Acyltrifluoroborates (KATs) from Carboxylic Acids by Copper-Catalyzed Borylation of Mixed Anhydrides

We report the preparation of potassium acyltrifluoroborates (KATs) from widely available carboxylic acids. Mixed anhydrides of carboxylic acids were prepared using isobutyl chloroformate and transformed to the corresponding KATs using a commercial copper catalyst, B₂ (pin)₂ , and aqueous KHF₂ . This method allows for the facile preparation of aliphatic, aromatic, and amino acid-derived KATs and is compatible with a variety of functional groups including alkenes, esters, halides, nitriles, and protected amines.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Mechanism-Based Design of Quinoline Potassium Acyltrifluoroborates for Rapid Amide-Forming Ligations at Physiological pH

Potassium acyltrifluoroborates (KATs) undergo chemoselective amide-forming ligations with hydroxylamines. Under aqueous, acidic conditions these ligations can proceed rapidly, with rate constants of ∼20 M^-1 s^-1. The requirement for lower pH to obtain the fastest rates, however, limits their use with certain biomolecules and precludes in vivo applications. By mechanistic investigations into the KAT ligation, including kinetic studies, X-ray crystallography, and DFT calculations, we have identified a key role for a proton in accelerating the ligation. We applied this knowledge to the design and synthesis of 8-quinolyl acyltrifluoroborates, a new class of KATs that ligates with hydroxylamines at pH 7.4 with rate constants >4 M^-1 s^-1. We trace the enhanced rate at physiological pH to unexpectedly high basicity of the 8-quinoline-KATs, which leads to their protonation even under neutral conditions. This proton assists the formation of the key tetrahedral intermediate and activates the leaving groups on the hydroxylamine toward a concerted 1,2-BF₃ shift that leads to the amide product. We demonstrate that the fast ligations at pH 7.4 can be carried out with a protein substrate at micromolar concentrations.

Catalytic Asymmetric Amino Acid and Its Derivatives by Chiral Aldehyde Catalysis

Amine acid transformation is an important chemical process in biological systems. As a well-developed and acknowledged tool, chiral aldehyde catalysis provides good catalytic activation and stereoselective control abilities in the asymmetric reaction of N-unprotected amino acid esters and amino acid esters analogs, in which the key to success is the design of the catalysts derived from chiral BINOL aldehyde, which is based on the face control of enolate intermediates. In this review, one of the co-catalytic systems that combined with a transition metal to form a multiplex catalytic system and the well-established multiplex stereocenters of chiral aldehyde catalysis have been reviewed. Finally, a novel organocatalysis is prospected.

α-Hydroxy boron-enabled regioselective access to bifunctional halo-boryl alicyclic ethers and α-halo borons

α-Hydroxy borons are an underutilized class of compounds and their only previous application involved oxidation into acylborons. Herein, we describe the synthesis of functionalized olefinic α-hydroxy borons and their utility to enable a novel and regioselective route to hitherto unknown bifunctional halo-boryl tetrahydrofurans/tetrahydropyrans and α-halo MIDA boronates. The orthogonally functionalized alicyclic ethers provided a building block-based approach for diversification of the tetrahydrofuran core.

Acrylic boronate: a multifunctional C3 building block for catalytic synthesis of rare organoborons and chemoselective heterobifunctional ligations

A novel C3 acylboron building block; acrylic boronate was successfully prepared and its versatility for catalytic synthesis of several previously inaccessible organoborons is described. Cross-metathesis and Pd-catalyzed coupling of acrylic boronate enabled two complementary routes to highly functionalized α,β-unsaturated acylborons and two new types of conjugated borylated products: α,β,γ,δ-unsaturated and bis-α,β unsaturated acylborons. The synthetic application of α,β-unsaturated acylborons was demonstrated for the first time, thereby providing a general and highly regioselective route to medicinally important 3-boryl pyrazoles. Acrylic boronate also provided a unique bis-electrophilic platform for rapid and chemoselective labeling of cysteines with acylboron tags which are potentially useful for site-selective functionalization and orthogonal ligation of proteins.

Rapid Access to Diverse Potassium Acyltrifluoroborates (KATs) through Late-Stage Chemoselective Cross-Coupling Reactions

Potassium acyltrifluoroborates (KATs) are opening up new avenues in chemical biology, materials science, and synthetic organic chemistry due to their intriguing reactivities. However, the synthesis of these compounds remains mostly complicated and time-consuming. Herein, we have developed chemoselective Pd-catalyzed approaches for the late-stage diversification of arenes bearing prefunctionalized KATs. These approaches feature chemoselective cross-coupling, rapid diversification, functional group tolerance, mild reaction conditions, simple operation, and high yields.

Catalytic Synthesis of Potassium Acyltrifluoroborates (KATs) from Boronic Acids and the Thioimidate KAT Transfer Reagent

We report the synthesis of potassium acyltrifluoroborates (KATs) by a palladium-catalyzed cross-coupling of boronic acids and the thioimidate KAT transfer reagent. The combination of widely available aryl- and vinylboronic acids with commercially available thioimidate 1 using catalytic Pd^II and a Cu^II additive enables the preparation of KATs in high yields and with good functional group tolerance. This formal insertion of CO into organoboronic acids can also be applied to boronic acid pinacol esters and potassium organotrifluoroborates using a slightly modified procedure. The cross-coupling can be telescoped into the one-pot synthesis of amides and α-aminotrifluoroborates by exploiting the unique chemistry of KATs and their trifluoroborate iminium (TIM) derivatives.

One-Step Synthesis of Acylboron Compounds via Copper-Catalyzed Carbonylative Borylation of Alkyl Halides*

A copper-catalyzed carbonylative borylation of unactivated alkyl halides has been developed, enabling efficient synthesis of aliphatic potassium acyltrifluoroborates (KATs) in high yields by treating the in situ formed tetracoordinated acylboron intermediates with aqueous KHF₂ . A variety of functional groups are tolerated under the mild reaction conditions, and primary, secondary, and tertiary alkyl halides are all applicable. In addition, this method also provides facile access to N-methyliminodiacetyl (MIDA) acylboronates as well as α-methylated potassium acyltrifluoroborates in a one-pot manner. Mechanistic studies indicate a radical atom transfer carbonylation (ATC) mechanism to form acyl halide intermediates that are subsequently borylated by (NHC)CuBpin.

Synthesis of Acylboron Compounds

Acylboron compounds are emerging as versatile functional groups with applications in multiple research fields. Their synthesis, however, is still challenging and requires innovative methods. This Minireview provides an overview on the obstacles of acylboron synthesis and highlights notable advances within the last three years on new strategies to overcome the challenges posed by the formation of acyl-boron bonds.

Recent Advances in the Synthesis of Acylboranes and Their Widening Applicability

The most common types of acylboranes are acyltrifluoroborates, acyl MIDA-boronates, and monofluoroacylboronates. Because of the increasing importance of these compounds in the past decade, we highlight the recently reported synthetic strategies to access acylboranes. In addition, an expanding array of their applications has been discovered, based on either the ability of acylboranes to enter rapid amide-forming ligations or the retained ketone-like character of the carbonyl group. Therefore, we also describe ground-breaking achievements where acylboranes were successfully put to use, such as their utility in biochemical, material, and medicinal sciences.

Potential Dental Biofilm Inhibitors: Dynamic Combinatorial Chemistry Affords Sugar-Based Molecules that Target Bacterial Glucosyltransferase

We applied dynamic combinatorial chemistry (DCC) to find novel ligands of the bacterial virulence factor glucosyltransferase (GTF) 180. GTFs are the major producers of extracellular polysaccharides, which are important factors in the initiation and development of cariogenic dental biofilms. Following a structure‐based strategy, we designed a series of 36 glucose‐ and maltose‐based acylhydrazones as substrate mimics. Synthesis of the required mono‐ and disaccharide‐based aldehydes set the stage for DCC experiments. Analysis of the dynamic combinatorial libraries (DCLs) by UPLC‐MS revealed major amplification of four compounds in the presence of GTF180. Moreover, we found that derivatives of the glucose‐acceptor maltose at the C1‐hydroxy group act as glucose‐donors and are cleaved by GTF180. The synthesized hits display medium to low binding affinity (K_D values of 0.4–10.0 mm) according to surface plasmon resonance. In addition, they were investigated for inhibitory activity in GTF‐activity assays. The early‐stage DCC study reveals that careful design of DCLs opens up easy access to a broad class of novel compounds that can be developed further as potential inhibitors.

Synthesis of secondary and tertiary amides without coupling agents from amines and potassium acyltrifluoroborates (KATs)

Although highly effective for most amide syntheses, the activation of carboxylic acids requires the use of problematic coupling reagents and is often poorly suited for challenging cases such as N-methyl amino acids. As an alternative to both secondary and tertiary amides, we report their convenient synthesis by the rapid oxidation of trifluoroborate iminiums (TIMs). TIMs are easily prepared by acid-promoted condensation of potassium acyltrifluoroborates (KATs) and amines and are cleanly and rapidly oxidized to amides with hydrogen peroxide. The overall transformation can be conducted either as a one-pot procedure or via isolation of the TIM. The unique nature of the neutral, zwitterionic TIMs makes possible the preparation of tertiary amides via an iminium species that would not be accessible from other carbonyl derivatives and can be conducted in the presence of unprotected functional groups including acids, alcohols and thioethers. In preliminary studies, this approach was applied to the late-stage modifications of long peptides and the iterative synthesis of short, N-methylated peptides without the need for coupling agents.

Oxidative amidation of potassium acyltrifluoroborates (KATs) and amines via trifluoroborate iminiums (TIMs) delivers amides without coupling agents. This unusual approach to amides can be applied for the late-stage modification of bioactive molecules and for solid-phase peptide synthesis.

Synthesis and Tunable Optical Properties of C,N-Chelated Borate Luminophores Derived from Potassium Acyltrifluoroborates

A new class of borate luminophores has been synthesized by a simple two-step reaction using potassium acyltrifluoroborates (KATs) as starting materials. The hydrazones obtained from reactions between KATs and 2-hydrazinopyridines followed by a cyclization resulted in the unprecedented formation of C,N-chelated six-membered bora-heterocycles. Under consideration of the results of DFT and TD-DFT calculations, four luminophores based on such bora-heterocycles are designed and synthesized, which exhibit a tunable fluorescence range from blue to red in the solid state. Moreover, one of the luminophores exhibits mechanofluorochromism from blue to yellow/green. As a result of the aforementioned mechanochromism of one of these luminophores, white-color emission was achieved by simply mixing the four luminophores.

Diversity-Oriented Synthesis of α-Functionalized Acylborons and Borylated Heteroarenes by Nucleophilic Ring Opening of α-Chloroepoxyboronates

The ring-opening reactions of N-methyliminodiacetyl (MIDA) α-chloroepoxyboronates with different nucleophiles allow the modular synthesis of a diverse array of organoboronates. These include seven types of α-functionalized acylboronates and seven types of borylated heteroarenes, some of which are difficult-to-access products using alternative methods. The common synthons, α-chloroepoxyboronates, could be viably synthesized by a two-step procedure from the corresponding alkenyl MIDA boronates. Mild reaction conditions, good functional-group tolerance, and generally good efficiency were observed. The utility of the products was also demonstrated.

Catalytic Synthesis of Potassium Acyltrifluoroborates (KATs) through Chemoselective Cross-Coupling with a Bifunctional Reagent

Potassium acyltrifluoroborates (KATs) are increasingly important functional groups, and general methods for their preparation are of great current interest. We report a bifunctional iminium reagent bearing both a tin nucleophile and a trifluoroborate, which was applied in chemoselective Pd⁰ -catalyzed Migita-Kosugi-Stille cross-coupling reactions owith aryl and vinyl halides. This method gives access to previously inaccessible aromatic and α,β-unsaturated acyltrifluoroborates, including precursors to amino-acid derived KATs.