From synthesis to function via iterative assembly of N-methyliminodiacetic acid boronate building blocks

Anthranilamide (aam)-substituted diboron: palladium-catalyzed selective B(aam) transfer

An unsymmetrical diboron bearing an anthranilamide (aam) substituent was synthesized and was converted into arylboranes via Pd-catalyzed selective B(aam) transfer.

Synthesis of bench-stable solid triorganoindium reagents and reactivity in palladium-catalyzed cross-coupling reactions

Triorganoindium reagents can be isolated as bench-stable solid R₃In(DMAP) complexes and show excellent reactivity in palladium-catalyzed cross-coupling reactions.

Automating drug discovery

Small-molecule drug discovery can be viewed as a challenging multidimensional problem in which various characteristics of compounds - including efficacy, pharmacokinetics and safety - need to be optimized in parallel to provide drug candidates. Recent advances in areas such as microfluidics-assisted chemical synthesis and biological testing, as well as artificial intelligence systems that improve a design hypothesis through feedback analysis, are now providing a basis for the introduction of greater automation into aspects of this process. This could potentially accelerate time frames for compound discovery and optimization and enable more effective searches of chemical space. However, such approaches also raise considerable conceptual, technical and organizational challenges, as well as scepticism about the current hype around them. This article aims to identify the approaches and technologies that could be implemented robustly by medicinal chemists in the near future and to critically analyse the opportunities and challenges for their more widespread application.

B(MIDA)-Containing Diborons

Unsymmetrical B(MIDA)-containing diborons of potential synthetic utility were found to be synthesized from the readily available (neop)B-B(neop) and (HO)₂B-B(OH)₂, and the procedure was extended to the generation of symmetrical (MIDA)B-B(MIDA). NMR and X-ray crystal structure studies revealed that B(MIDA) units of all of the diborons obtained in this study were in rigid tetrahedral environment.

Catalytic B-H Bond Insertion Reactions Using Alkynes as Carbene Precursors

Herein, we report transition-metal-catalyzed B-H bond insertion reactions between borane adducts and alkynes to afford organoboron compounds in excellent yields under mild reaction conditions. This successful use of alkynes as carbene precursors in these reactions constitutes a new route to organoboron compounds. The starting materials are safe and readily available, and the reaction exhibits 100% atom-economy. Moreover, an asymmetric version catalyzed by chiral dirhodium complexes produced chiral boranes with excellent enantioselectivity (up to 96% ee). This is the first report of highly enantioselective heteroatom-hydrogen bond insertion reactions of metal carbenes generated in situ from alkynes. The chiral products of the reaction could be easily transformed to widely used borates and diaryl methanol compounds without loss of optical purity, which demonstrates its potential utility in organic synthesis. A kinetics study indicated that the Cu-catalyzed B-H bond insertion reaction is first order with respect to the catalyst and the alkyne and zero order with respect to the borane adduct, and no kinetic isotopic effect was observed in the reaction of the adduct. These results, along with density functional theory calculations, suggest that the formation of the Cu carbene is the rate-limiting step and that the B-H bond insertion is a fast, concerted process.

Chemoselective oxidation of aryl organoboron systems enabled by boronic acid-selective phase transfer

We report the direct chemoselective Brown-type oxidation of aryl organoboron systems containing two oxidizable boron groups. Basic biphasic reaction conditions enable selective formation and phase transfer of a boronic acid trihydroxyboronate in the presence of boronic acid pinacol (BPin) esters, while avoiding speciation equilibria. Spectroscopic investigations validate a base-promoted phase-selective discrimination of organoboron species. This phenomenon is general across a broad range of organoboron compounds and can also be used to invert conventional protecting group strategies, enabling chemoselective oxidation of BMIDA species over normally more reactive BPin substrates. We also demonstrate the selective oxidation of diboronic acid systems with chemoselectivity predictable a priori. The utility of this method is exemplified through the development of a chemoselective oxidative nucleophile coupling.

An efficient methodology to introduce o-(aminomethyl)phenyl-boronic acids into peptides: alkylation of secondary amines

Current approaches for incorporating boronic acids into peptides require one of the following: the synthesis of commercially unavailable pinacol-protected boronate ester amino acid building blocks, amidation of small-molecule amine-containing boronic acids, or reductive amination of amine residues with 2-formylphenyl boronic acid. These methods have drawbacks, such as the use of excess starting materials, the lack of reactive-site specificity, or the inability to add multiple boronic acids in solution. In addition, several of these approaches do not allow for incorporation of the critical o-aminomethyl functionality that allows for binding of sacharrides under physiological conditions. In this work, we report three methods to functionalize synthetic peptides with boronic acids using solid-phase and solution-phase chemistries by alkylating a secondary amine with o-(bromomethyl)phenylboronic acid. Solution-phase chemistries afforded the highest yields, and were used to synthesize seven complex biotinylated multi-boronic acid peptides.

A one-pot tandem chemoselective allylation/cross-coupling via temperature control of a multi-nucleophile/electrophile system

Chemoselective control of a multi-reactive system allows two sequential C–C bond formations via two distinct reactivity modes, accessing pharmaceutical and natural product scaffolds.

Sequential and iterative Pd-catalyzed cross-coupling reactions in organic synthesis

ABSTRACT

Sequential and iterative Pd-catalyzed cross-coupling reactions can be performed in which the order of C-C bond formations can be controlled either by the attenuated leaving groups of the multireactive substrate or by specific catalyst/ligand combinations. This tutorial review gives an overview about recent developments in this field and the various strategies used for the assembly of oligoarenes and -alkenes.

GRAPHICAL ABSTRACT

Soluble polymer supports for homogeneous catalysis in flow reactions

The use of polyisobutylene and poly(4-dodecylstyrene) bound catalysts that contain transition metal or organocatalysts for cyclopropanation, ring-closing metathesis, and nucleophilic catalysis in flow chemistry schemes is described and compared with similar catalysts used in batch reactions. These Rh(II) carboxylate catalysts, N-heterocyclic carbene-ligated Ru(II) benzylidene catalysts, and analogs of 4-dimethylaminopyridine catalysts were used in reactions in heptane in flow and then separated in a gravity based liquid/liquid separation using a biphasic heptane/acetonitrile mixture. The less dense catalyst-containing phase in that separation was continuously used in flow with fresh substrate solution. Leaching of catalysts, yields, and turnover frequencies in these flow reactions were comparable with prior results obtained with the same phase isolable catalysts in batch reactions.

Piperidine-based glycodendrons as protein N-glycan prosthetics

The generation of homogeneously glycosylated proteins is essential for defining glycoform-specific activity and improving protein-based therapeutics. We present a novel glycodendron prosthetic which can be site-selectively appended to recombinant proteins to create 'N-glycosylated' glycoprotein mimics. Using computational modeling, we designed the dendrimer scaffold and protein attachment point to resemble the native N-glycan architecture. Three piperidine-melamine glycodendrimers were synthesized via a chemoenzymatic route and attached to human growth hormone and the F_c region of human IgG. These products represent a new class of engineered biosimilars bearing novel glycodendrimer structures.

A General Protocol for the Polycondensation of Thienyl N-Methyliminodiacetic Acid Boronate Esters To Form High Molecular Weight Copolymers

Thienyl di-N-methyliminodiacetic acid (MIDA) boronate esters are readily synthesized by electrophilic C–H borylation producing bench stable crystalline solids in good yield and excellent purity. Optimal conditions for the slow release of the boronic acid using KOH as the base in biphasic THF/water mixtures enables the thienyl MIDA boronate esters to be extremely effective homo-bifunctionalized (AA-type) monomers in Suzuki–Miyaura copolymerizations with dibromo-heteroarenes (BB-type monomers). A single polymerization protocol is applicable for the formation of five alternating thienyl copolymers that are (or are close analogues of) state of the art materials used in organic electronics. The five polymers were produced in excellent yields and with high molecular weights comparable to those produced using Stille copolymerization protocols. Therefore, thienyl di-MIDA boronate esters represent bench stable and low toxicity alternatives to highly toxic di-trimethylstannyl AA-type monomers that are currently ubiquitous in the synthesis of these important alternating copolymers.

Selective and Serial Suzuki-Miyaura Reactions of Polychlorinated Aromatics with Alkyl Pinacol Boronic Esters

Among cross-coupling reactions, the Suzuki–Miyaura transformation stands out because of its practical advantages, including the commercial availability and low toxicity of the required reagents, mild reaction conditions, and functional group compatibility. Nevertheless, few conditions can be used to cross-couple alkyl boronic acids or esters with aryl halides, especially 2-pyridyl halides. Herein, we describe two novel Suzuki–Miyaura protocols that enable selective conversion of polychlorinated aromatics, with a focus on reactions to convert 2,6-dichloropyridines to 2-chloro-6-alkylpyridines or 2-aryl-6-alkylpyridines.

Copper mediated carbometalation reactions

Since the first discovery of carbocupration of alkynes in the 1970s a tremendous amount of research has been carried out in this field. The exceptionally high selectivities obtained attribute to the great synthetic value of carbocupration reactions. This tutorial review will present the most important features of carbocupration of alkynes and highlight the most relevant reviews. Then a comprehensive review of copper mediated carbometalation of cyclopropenes will follow. The latter method has received much attention over the last decade as it allows the highly selective construction of poly-substituted cyclopropanes which can be transformed into acyclic derivatives bearing one or multiple tertiary or quaternary carbon stereocenters.

MIDA boronates are hydrolysed fast and slow by two different mechanisms

MIDA boronates (N-methylimidodiacetic boronic acid esters) serve as an increasingly general platform for building-block-based small molecule construction, largely due to the dramatic and general rate differences with which they are hydrolysed under various basic conditions. Yet the mechanistic underpinnings of these rate differences have remained unclear, hindering efforts to address current limitations of this chemistry. Here we show that there are two distinct mechanisms for this hydrolysis: one is base-mediated and the other neutral. The former can proceed more than three orders of magnitude faster, and involves rate-limiting attack at a MIDA carbonyl carbon by hydroxide. The alternative ‘neutral’ hydrolysis does not require an exogenous acid/base and involves rate-limiting B-N bond cleavage by a small water cluster, (H₂O)_n. The two mechanisms can operate in parallel, and their relative rates are readily quantified by ¹⁸O incorporation. Whether hydrolysis is ‘fast’ or ‘slow’ is dictated by the pH, the water activity (a_w), and mass-transfer rates between phases. These findings stand to rationally enable even more effective and widespread utilisation of MIDA boronates in synthesis.

Palladium-Catalyzed Oxidative Carbocyclization-Borylation of Enallenes to Cyclobutenes

A highly efficient palladium-catalyzed oxidative borylation of enallenes was developed for the selective formation of cyclobutene derivatives and fully-substituted alkenylboron compounds. Cyclobutenes are formed as the exclusive products in MeOH in the presence of H2 O and Et3 N, whereas the use of AcOH leads to alkenylboron compounds. Both reactions showed a broad substrate scope and good tolerance for various functional groups, including carboxylic acid ester, free hydroxy, imide, and alkyl groups. Furthermore, transformations of the borylated products were conducted to show their potential applications.

Stereocontrolled Synthesis of Vinyl Boronates and Vinyl Silanes via Atom-Economical Ruthenium-Catalyzed Alkene-Alkyne Coupling

The synthesis of vinyl boronates and vinyl silanes was achieved by employing a Ru-catalyzed alkene-alkyne coupling reaction of allyl boronates or allyl silanes with various alkynes. The double bond geometry in the generated vinyl boronates can be remotely controlled by the juxtaposing boron- and silicon groups on the alkyne substrate. The synthetic utility of the coupling products has been demonstrated in a variety of synthetic transformations, including iterative cross-coupling reactions, and a Chan-Lam-type allyloxylation followed by a Claisen rearrangement. A sequential one-pot alkene-alkyne-coupling/allylation-sequence with an aldehyde to deliver a highly complex α-silyl-β-hydroxy olefin with a handle for further functionalization was also realized.

A Cu/Pd Cooperative Catalysis for Enantioselective Allylboration of Alkenes

A cooperative Cu/Pd-catalyzed asymmetric three-component reaction of styrenes, B2(pin)2, and allyl carbonates was reported. This reaction, in the presence of chiral CuOAc/SOP and achiral Pd(dppf)Cl2 catalysts, occurs smoothly with high enantioselectivities (up to 97% ee) . The allylboration products, which contain alkene (or diene) unite and alkylboron group, are easily functionalized. The utility of this protocol was demonstrated through the synthesis of an antipsychotic drug, (-)-preclamol.

David and Goliath: chemical perturbation of eukaryotes by bacteria

Environmental microbes produce biologically active small molecules that have been mined extensively as antibiotics and a smaller number of drugs that act on eukaryotic cells. It is known that there are additional bioactives to be discovered from this source. While the discovery of new antibiotics is challenged by the frequent discovery of known compounds, we contend that the eukaryote-active compounds may be less saturated. Indeed, despite there being far fewer eukaryotic-active natural products these molecules interact with a far richer diversity of molecular and cellular targets.