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

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.

CuCl2 -Catalyzed α-Chloroketonation of Aromatic Alkenes via Visible-Light-Induced LMCT

Here we report a copper-catalyzed protocol for the synthesis of α-chloroketones from aromatic alkenes including electron-deficient olefins under visible-light irradiation. Preliminary mechanistic studies show that the peroxo Cu(II) species is the key intermediate and hydroperoxyl (HOO⋅) and chlorine (Cl⋅) radicals can be generated by ligand-to-metal charge transfer (LMCT).

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.

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.

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.

Formyl MIDA Boronate: C1 Building Block Enables Straightforward Access to α-Functionalized Organoboron Derivatives

Formyl MIDA boronate has been known to be an elusive type of acylboronate that has not been obtained to date. In this work, an approach to the one-pot preparation and chemical transformations of formyl MIDA boronate were developed to provide new types of α-functionalized organoboron compounds. Among them are acylboronate reagents which present boron-substituted analogues of ynones and β-dicarbonyl compounds. The developed synthetic procedures, utilizing formyl MIDA boronate, are tolerant to diverse functional groups, making this reagent an advantageous C₁ building block for extending the scope of organoboron chemistry.

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.