Enantioselective Synthesis of α-Aminoboronic Acid Derivatives via Copper-Catalyzed N-Alkylation

Due to burgeoning interest in the pharmaceutical industry in exploiting optically active α-aminoboronic derivatives as bioisosteres of α-amino acid derivatives, the discovery of methods for their catalytic asymmetric synthesis is an important challenge. Herein, we establish that a chiral copper catalyst (generated in situ from commercially available components) can achieve the enantioselective synthesis of α-aminoboronic derivatives via the coupling of two readily available partners, a carbamate and a racemic α-chloroboronate ester. Furthermore, we describe mechanistic studies that played a key role in the development of this new method and that provide insight into the optimized process.

Porous functional metal–organic frameworks (MOFs) constructed from different N-heterocyclic carboxylic ligands for gas adsorption/separation

This review mainly summarizes the recent progress of MOFs composed of N-heterocyclic carboxylate ligands in gas sorption/separation. This work may help to understand the relationship between the structures of MOFs and gas sorption/separation.

Enantioselective synthesis of α-aminoboronates by NiH-catalysed asymmetric hydroamidation of alkenyl boronates

Chiral α-aminoboronic acids and their derivatives are generally useful as bioactive compounds and some have been approved as therapeutic agents. Here we report a NiH-catalysed asymmetric hydroamidation process that with a simple amino alcohol ligand can easily produce a wide range of highly enantioenriched α-aminoboronates from alkenyl boronates and dioxazolones under mild conditions. The reaction is proposed to proceed by an enantioselective hydrometallation followed by an inner-sphere nitrenoid transfer and C–N bond forming sequence. The synthetic utility of this transformation was demonstrated by the efficient synthesis of a current pharmaceutical agent, Vaborbactam.

Enantioenriched α-aminoboronic acid, a structural unit in many bioactive molecules, is also a valuable synthon in organic synthesis. Here, the authors disclose a NiH-catalysed asymmetric hydroamidation process for their direct synthesis.

Enantioselective synthesis of cyclic α-aminoboronates via copper-catalyzed dearomative borylation of 4-quinolinols

A highly enantioselective and regioselective dearomative borylation of 4-quinolinols was developed using a Cu(I)/(R,R)-Ph-BPE catalyst for efficient synthesis of unprecedented heterocyclic α-amino boronates, a new class of compounds potentially relevant to drug discovery, in generally excellent yields and enantioselectivities. The products were also useful intermediates for highly functionalized tetrahydroquinolines and cyclic α-aminoboronate derivatives.

[Boronic Acid as a Promising Class of Chemical Entity for Development of Clinical Medicine for Targeted Therapy of Cancer]

The first medicine containing the boron element, bortezomib, was approved for clinical use just 18 years ago. The boronic acid substructure in bortezomib serves as an electrophilic functionality with high affinity for hydroxy groups, which are frequently found in catalytic sites of proteolytic enzymes, to create reversible covalent bonds with a slow dissociation rate. Today, boronic acid is considered an important molecule in the medicinal chemistry toolbox, which was promoted by the success of bortezomib and pioneering approaches to use boronic acid in the molecular design of serine protease inhibitors in the 1980s. In this review article, we first provide an overview of the development of bortezomib, and then summarize our achievements to construct boronic acid analogs of tyropeptin A, a naturally occurring proteasome inhibitor, with potent in vivo efficacy. Representative stereoselective synthetic methods of α-aminoboronic acid are also showcased.

Enantioselective Rhodium-Catalyzed Hydrogenation of (Z)-N-Sulfonyl-α-dehydroamido Boronic Esters

Highly enantioselective rhodium-catalyzed hydrogenation of (Z)-N-sulfonyl-α-dehydroamido boronic esters is realized for the first time using a JosiPhos-type ligand. This method has enabled convenient synthesis of a series of enantio-enriched N-sulfonyl-α-amido boronic esters in good yields and excellent enantioselectivities (up to 99% ee).

Alkoxide activation of tetra-alkoxy diboron reagents in C-B bond formation: a decade of unpredictable reactivity

Any attempt to facilitate a new generation of C-B bonds represents a useful tool in organic synthesis. In addition, if that approach highlights the nucleophilic character of boryl moieties in the absence of transition metal complexes, the challenge to create new reactive platforms becomes an opportunity. We have been deeply involved in the experimental and theoretical validation of C-B bond formation by means of alkoxide activation of tetra-alkoxy diboron reagents and here is presented a convenient guide to understand the concept and the applications.

α-Aminoboronates: recent advances in their preparation and synthetic applications

α-Aminoboronic acids and their derivatives are useful as bioactive agents. Thus far, three compounds containing an α-aminoboronate motif have been approved by the Food and Drug Administration (FDA) as protease inhibitors, and more are currently undergoing clinical trials. In addition, α-aminoboronic acids and their derivatives have found applications in organic synthesis, e.g. as α-aminomethylation reagents for the synthesis of chiral nitrogen-containing molecules, as nucleophiles for preparing valuable vicinal amino alcohols, and as bis-nucleophiles in the construction of valuable small molecule scaffolds. This review summarizes new methodology for the preparation of α-aminoboronates, including highlights of asymmetric synthetic methods and mechanistic explanations of reactivity. Applications of α-aminoboronates as versatile synthetic building blocks are also discussed.

Catalytic Chemo- and Enantioselective Transformations of gem-Diborylalkanes and (Diborylmethyl)metallic Species

Chemo- and stereoselective transformations of polyborylalkanes are powerful and efficient methods to access optically active molecules with greater complexity and diversity through programmed synthetic design. Among the various polyborylalkanes, gem-diborylalkanes have attracted much attention in organic chemistry as versatile synthetic handles. The notable advantage of gem-diborylalkanes lies in their ability to generate two key intermediates, α-borylalkyl anions and (gem-diborylalkyl) anions. These two different intermediates can be applied to various enantioselective reactions to rapidly access a diverse set of enantioenriched organoboron compounds, which can be further manipulated to generate various chiral molecule libraries via stereospecific C(sp³)-B bond transformations.In this Account, we summarize our recent contributions to the development of catalytic chemo- and stereoselective reactions using gem-diborylalkanes as versatile nucleophiles, which can be categorized as follows: (1) copper-catalyzed enantioselective coupling of gem-diborylalkanes with electrophiles and (2) the design and synthesis of (diborylmethyl)metallic species and their applications to enantioselective reactions. Since Shibata and Endo reported the Pd-catalyzed chemoselective Suzuki-Miyaura cross-coupling of gem-diborylalkanes with organohalides in 2014, Morken and Hall subsequently developed the first enantioselective analogous reactions using TADDOL-derived chiral phosphoramidite as the supporting ligand of a palladium catalyst. This discovery sparked interest in the catalytic enantioselective coupling of gem-diborylalkanes with electrophiles. Our initial studies focused on generating chiral (α-borylmethyl)copper species by enantiotopic-group-selective transmetalation of gem-diborylalkanes with chiral copper complexes and their reactions with various aldimines and ketimines to afford syn-β-aminoboronate esters with excellent enantio- and diastereoselectivity. Moreover, we developed the enantioselective allylation of gem-diborylalkanes that proceeded by reaction of in situ-generated chiral (α-borylalkyl)copper and allyl bromides. Mechanistic investigations revealed that the enantiotopic-group-selective transmetalation between gem-diborylalkanes and the chiral copper complex occurred through the open transition state rather than the closed transition state, thereby effectively generating chiral (α-borylmethyl)copper species. We also utilized (diborylmethyl)metallic species such as (diborylmethyl)silanes and (diborylmethyl)zinc halides in catalytic enantioselective reactions. We succeeded in developing the enantiotopic-group-selective cross-coupling of (diborylmethyl)silanes with aryl iodides to afford enantioenriched benzylic 1,1-silylboronate esters, which could be used for further consecutive stereospecific transformations to afford various enantioenriched molecules. In addition, we synthesized (diborylmethyl)zinc halides for the first time by the transmetalation of isolated (diborylmethyl)lithium and zinc(II) halides and their utilization to the synthesis of enantioenriched gem-diborylalkanes bearing a chiral center at the β-position via an iridium-catalyzed enantioselective allylic substitution process. In addition to our research efforts, we also include key contributions by other research groups. We hope that this Account will draw the attention of the synthetic community to gem-diboryl compounds and provide guiding principles for the future development of catalytic enantioselective reactions using gem-diboryl compounds.

Inverse Hydroboration of Imines with NHC-Boranes Is Promoted by Diphenyl Disulfide and Visible Light

We describe a simple and efficient procedure for nucleophilic borylation of imines in the absence of a photoredox catalyst. Visible light irradiation of an acetonitrile solution of an imine, an NHC-borane, and diphenyl disulfide (10 mol %) provides various stable α-amino NHC-boranes in good yields. The reaction proceeds via addition of a nucleophilic boryl radical to an imine, followed by hydrogen abstraction from thiophenol, which is generated from NHC-borane and diphenyl disulfide.

Iridium-Catalyzed Regio- and Enantioselective Borylation of Unbiased Methylene C(sp3 )-H Bonds at the Position β to a Nitrogen Center

Reported herein is the pyrazole-directed iridium-catalyzed enantioselective borylation of unbiased methylene C-H bonds at the position β to a nitrogen center. The combination of a chiral bidentate boryl ligand, iridium precursor, and pyrazole directing group was responsible for the high regio- and enantioselectivity observed. The method tolerated a vast array of functional groups to afford the corresponding C(sp³ )-H functionalization products with good to excellent enantioselectivity.

Tunable Synthesis of α-Amino Boronic Esters from Available Aldehydes and Amines through Sequential One-Pot Dehydration and Copper-Catalyzed Borylacylation

Copper-catalyzed multicomponent borylacylation of imines with acid chlorides and bis(pinacolato)diboron was developed for the preparation of synthetically useful and pharmacologically relevant α-amino boronic acid derivatives. Starting from a range of acid chlorides and imines with aryl, heteroaryl, and alkyl substituents, most of these ligand-free reactions proceeded smoothly at room temperature in moderate to good yields. Furthermore, a facile and convenient one-pot, multistep access to the direct synthesis of α-amino boronic acid derivatives from available aldehydes and amines was also developed.

Base-promoted domino-borylation-protodeboronation strategy

Since a nucleophilic sp2 boron species can be generated in situ under the combined action of an inorganic base, B2pin2 and methanol, research on base-promoted nucleophilic borylation of unsaturated compounds has attracted significant attention. A series of multi-borylated compounds, such as alkyl 1,2-bis(boronates), gem-diborylalkanes, and 1,1,2-tris(boronates), are constructed based on this strategy. These multi-borylated compounds can in turn undergo selective protodeboronation, creating a variety of useful boron-containing compounds. This Feature article documents the development of base-promoted domino-borylation-protodeboronation (DBP) strategies and their applications in organic synthesis.

Transition metal-free synthesis of alkyl pinacol boronates

This review systematically outlined the research in the area of transition metal free synthesis of alkyl pinacol boronates, which are versatile and important scaffolds to construct diverse organic compounds.

Synthesis of aminoboronic acid derivatives: an update on recent advances

Aminoboronic acids and their derivatives are particularly useful as drugs, probes and synthons. Recent developments in their synthesis are highlighted.

Highly Enantiospecific Borylation for Chiral α-Amino Tertiary Boronic Esters

Herein we report a highly efficient and enantiospecific borylation method to synthesize a wide range of enantiopure (>99 % ee) α-amino tertiary boronic esters. The configurationally stable α-N-Boc substituted tertiary organolithium species and pinacolborane (HBpin) underwent enantiospecific borylation at -78 °C with the formation of a new stereogenic C-B bond. This reaction has a broad scope, enabling the synthesis of various α-amino tertiary boronic esters in excellent yields and, importantly, with universally excellent enantiospecificity (>99 % es) and complete retention of configuration.

Enantioselective Synthesis of α-Amidoboronates Catalyzed by Planar-Chiral NHC-Cu(I) Complexes

The first highly selective catalytic hydroboration of alkyl-substituted aldimines to provide medicinally relevant α-amidoboronates is disclosed. The Cu(I)-catalyzed borylation proceeds with excellent facial selectivity when a set of planar-chiral N-heterocyclic carbenes (NHCs) were employed as ligands. Density functional theory computations suggest that interactions between BPin and the planar-chiral catalyst are responsible for the observed stereoselectivity. Important pharmacophores, such as the boronate analogue of isoleucine, can be prepared using a chromatography-free protocol starting from commercially available reagents. The application of these NHC ligands in these Cu(I)-catalyzed processes offers a significant contribution to existing strategies for laboratory-scale preparation of enantioenriched α-amidoboronates.

Synthesis of cyclic chiral α-amino boronates by copper-catalyzed asymmetric dearomative borylation of indoles

A copper(i)-catalyzed dearomative borylation of N-alkoxycarbonyl protected indole-3-carboxylates has been developed. The boron addition in this reaction occurred regioselectively at the 2-position of indoles followed by diastereoselective protonation, affording the corresponding stable cyclic chiral α-amino boronates (2-borylindolines) in moderate to good yields with excellent diastereo- and enantioselectivities. The product 2c could be used as a versatile precursor to undergo subsequent stereoselective transformations, delivering highly functionalized 2,3,3-trisubstituted chiral indolines.

Facile synthesis of α-aminoboronic acids from amines and potassium acyltrifluoroborates (KATs) via trifluoroborate-iminiums (TIMs)

We report the facile formation of trifluoroborate-iminiums (TIMs) from potassium acyltrifluoroborates (KATs) and the transformation of TIMs to α-aminotrifluoroborates by reduction or Grignard additions. Conditions for the hydrolysis of α-aminotrifluoroborates to α-aminoboronic acids, which are important biologically active compounds, were established. This new methodology allows access to sterically demanding α-aminoboronic acids that are not easily prepared with currently available methods. This work also introduces TIMs, that can be easily prepared and handled, as a new category of functional groups that serve as precursors to valuable organic compounds.

Super electron donors derived from diboron

Single-electron transfer is an important process in organic chemistry, in which a single-electron reductant (electron donor) acts as a key component. Compared with metal-based electron donors, organic electron donors have some unique advantages, such as tunable reduction ability and mild reaction conditions. The development of novel organic electron donors with good reduction ability together with ease of preparation is in high demand. Based on the pyridine-catalyzed radical borylation reaction developed in our laboratory, we have discovered that, the reaction system consisting of a diboron(4) compound, methoxide and a pyridine derivative could smoothly produce super electron donors in situ. Two boryl-pyridine based species, the major one being a trans-2H,2'H-[2,2'-bipyridine]-1,1'-diide borate complex and the minor one being a pyridine radical anion-borate complex, were observed and carefully characterized. These complexes were found to be organic super electron donors unprecedented in literature, and their formation mechanisms were studied by DFT calculations. The diboron/methoxide/pyridine system enables the preparation of organic super electron donors from easily accessible starting materials under mild conditions, which has the potential to be a general and practical single-electron reducing agent in organic synthesis.

Synergistic Photoredox Catalysis and Organocatalysis for Inverse Hydroboration of Imines

The first catalytic inverse hydroboration of imines with N-heterocyclic carbene (NHC) boranes has been realized by means of cooperative organocatalysis and photocatalysis. This catalytic combination provides a promising platform for promoting NHC-boryl radical chemistry under sustainable and radical-initiator-free conditions. The highly important functional-group compatibility and possible application in late-stage hydroborations represent an important step forward to an enhanced α-amino organoboron library.

Transition-metal-free B-B and B-interelement reactions with organic molecules

This review is a guided tour along the activation modes and reactivity of B-B, B-Si, B-N, B-S, B-Se and B-P reagents, in the absence of any transition metal complex. Here are disclosed the general concepts related to the homolytic and heterolytic cleavage of B-B and B-interelement bonds, as well as the generation of new C-B and C-interelement bonds, in a selective way. The greener consequences of these novel routes facilitate the gram scale preparations of target functionalised organic compounds. Intrinsic data about the suggested mechanisms and spectroscopic evidence that supports the innovative theories are provided along the review. Since this is a stimulating area of work that has emerged within the last decade, this overview serves as the basis to understand the new trends and hopefully to generate inspiration for future discoveries in the field.

B-B bond activation and NHC ring-expansion reactions of diboron(4) compounds, and accurate molecular structures of B2(NMe2)4, B2eg2, B2neop2 and B2pin2

In this detailed study we report on the structures of the widely employed diboron(4) compounds bis(pinacolato)diboron (B₂pin₂) and bis(neopentyl glycolato)diboron (B₂neop₂), as well as bis(ethylene glycolato)diboron (B₂eg₂) and tetrakis(dimethylamino)diboron (B₂(NMe₂)₄), and their reactivity, along with that of bis(catecholato)diboron (B₂cat₂) with backbone saturated and backbone unsaturared N-heterocyclic carbenes (NHCs) of different steric demand. Depending on the nature of the diboron(4) compound and the NHC used, Lewis-acid/Lewis-base adducts or NHC ring-expansion products stemming from B-B and C-N bond activation have been observed. The corresponding NHC adducts and NHC ring-expanded products were isolated and characterised via solid-state and solution NMR spectroscopy and X-ray diffraction. In general, we observed B-B bond and C-N bond activation at low temperature for B₂eg₂, at room temperature for B₂neop₂ and at higher temperature for B₂cat₂. The reactivity strongly depends on steric effects of the NHCs and the diboron(4) compounds, as well as on the corresponding Lewis-basicity and Lewis-acidity. Our results provide profound insight into the chemistry of these diboron(4) reagents with the nowadays ubiquitous NHCs, the stability of the corresponding NHC adducts and on B-B bond activation using Lewis-bases in general. We demonstrate that B-B bond activation may be triggered even at temperatures as low as -40 °C to -30 °C without any metal species involved. For example, the reactions of B₂eg₂ with sterically less demanding NHCs such as Me₂Im^Me and iPr₂Im in solution led to the corresponding ring-expanded products at low temperatures. Furthermore, boronium [L₂B(OR)₂]⁺ and borenium [LB(OR)₂]⁺ cations have been observed from the reaction of the bis-borate B₂eg₃ with the NHCs iPr₂Im and Me₂Im^Me, which led to the conclusion that the activation of bis-borates with NHCs (or Lewis-bases in general) might be a facile and simple route to access such species.

Reactivity of highly Lewis acidic diborane(4) towards pyridine and isocyanide: formation of boraalkene-pyridine complex and ortho-functionalized pyridine derivatives

The reaction of pinB-BMes2 (pin = pinacolato, Mes = 2,4,6-Me3C6H2) with Xyl-NC (Xyl = 2,6-Me2C6H3) and pyridine results in the formation of a pyridine-coordinated boraalkene that exhibits an intense color caused by an intramolecular charge-transfer interaction. In the presence of an excess of pyridine, the ortho C-H bond of pyridine was selectively functionalized to afford a quinoid compound or an isocyanide-coupled product. Based on the concentration effect, the reaction stoichiometry, and previously reported DFT calculations, a reaction mechanism that involves several rearrangement reactions was proposed. Using the present method, substituted pyridines and N-heterocycles afforded the corresponding functionalized derivatives. A subsequent hydrolysis of one of the resulting products furnished an aminomethylated pyridine derivative in two steps from parent pyridine.

A nano-catalytic approach for C–B bond formation reactions

Nanoparticle-catalysed borylation is one of the most convenient methods for the synthesis of organoboranes to overcome the confines of homogeneous catalysis such as recyclability and heavy metal contamination.