The synthesis and characterization of phenylacetylene tripodal compounds containing boroxine cores

AiZynth impact on medicinal chemistry practice at AstraZeneca

AstraZeneca chemists have been using the AI retrosynthesis tool AiZynth for three years. In this article, we present seven examples of how medicinal chemists using AiZynth positively impacted their drug discovery programmes. These programmes run the gamut from early-stage hit confirmation to late-stage route optimisation efforts. We also discuss the different use cases for which AI retrosynthesis tools are best suited.

Synthesis of Novel Multifunctional bora-Ibuprofen Derivatives

A unique class of β-boron-functionalized non-steroidal anti-inflammatory compound (pinB-NSAID) was previously synthesized via copper-catalyzed 1,2-difunctionalization of the respective vinyl arene with CO2 and B2pin2 reagents. Here, pinacolylboron-functionalized ibuprofen (pinB-ibuprofen) was used as a model substrate to develop the conditions for pinacol deprotection and subsequent boron functionalization. Initial pinacol-boronic ester deprotection was achieved by transesterification with diethanolamine (DEA) from the boralactonate organic salt. The resulting DEA boronate adopts a spirocyclic boralactonate structure rather than a diazaborocane–DABO boronate structure. The subsequent acid-mediated hydrolysis of DEA and transesterification/transamination provided a diverse scope of new boron-containing ibuprofen derivatives.

A Synthetic Approach to Dimetalated Arenes Using Flow Microreactors and the Switchable Application to Chemoselective Cross-Coupling Reactions

In spite of their potential utility, the chemistry of dimetalated arenes is still in its infancy because they are extremely difficult to synthesize. We report a novel method of synthesizing arenes bearing a boryl group and a metallic substituent, such as boryl, silyl, stannyl, or zincyl groups, in an integrated flow microreactor based on the generation and reactions of aryllithiums bearing a trialkyl borate moiety. The bimetallic arenes showed a remarkable chemoselectivity in palladium-catalyzed cross-coupling reactions. The selectivity was switched by the selection of the metal species that constitutes the dimetalated arenes as well as appropriate catalysts.

One-Pot Iridium Catalyzed C-H Borylation/Sonogashira Cross-Coupling: Access to Borylated Aryl Alkynes

Borylated aryl alkynes have been synthesized via one-pot iridium catalyzed C-H borylation (CHB)/Sonogashira cross-coupling of aryl bromides. Direct borylation of aryl alkynes encountered problems related to the reactivity of the alkyne under CHB conditions. However, tolerance of aryl bromides to CHB made possible a subsequent Sonogashira cross-coupling to access the desired borylated aryl alkynes.

Versatile Self-Adapting Boronic Acids for H-Bond Recognition: From Discrete to Polymeric Supramolecules

Because of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest in using their aryl derivatives in materials science and supramolecular chemistry has risen. Nevertheless, their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of the first double-H-bonded DD·AA-type complexes of a series of aromatic boronic acids that adopt a syn-syn conformation with suitable complementary H-bonding acceptor partners. The first determination of the association constant (K_a) of ortho-substituted boronic acids in solution showed that K_a for 1:1 association is in the range between 300 and 6900 M^-1. Crystallization of dimeric 1:1 and trimeric 1:2 and 2:1 complexes enabled an in-depth examination of these complexes in the solid state, proving the selection of the -B(OH)₂ syn-syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho-substituted boronic acids result in "flat" complexes. On the other hand, sterically demanding analogues bearing ortho substituents strive to retain their recognition properties by rotation of the ArB(OH)₂ moiety, forming "T-shaped" complexes. Solid-state studies of a diboronic acid and a tetraazanaphthacene provided for the first time the formation of a supramolecular H-bonded polymeric ribbon. On the basis of the conformational dynamicity of the -B(OH)₂ functional group, it is expected that these findings will also open new possibilities in metal-free catalysis or organic crystal engineering, where double-H-bonding donor boronic acids could act as suitable organocatalysts or templates for the development of functional materials with tailored organizational properties.

Spectroscopic and Computational Investigations of The Thermodynamics of Boronate Ester and Diazaborole Self-Assembly

The solution phase self-assembly of boronate esters, diazaboroles, oxathiaboroles, and dithiaboroles from the condensation of arylboronic acids with aromatic diol, diamine, hydroxythiol, and dithiol compounds in chloroform has been investigated by (1)H NMR spectroscopy and computational methods. Six arylboronic acids were included in the investigations with each boronic acid varying in the substituent at its 4-position. Both computational and experimental results show that the para-substituent of the arylboronic acid does not significantly influence the favorability of forming a condensation product with a given organic donor. The type of donor, however, greatly influences the favorability of self-assembly. (1)H NMR spectroscopy indicates that condensation reactions between arylboronic acids and catechol to give boronate esters are the most favored thermodynamically, followed by diazaborole formation. Computational investigations support this conclusion. Neither oxathiaboroles nor dithiaboroles form spontaneously at equilibrium in chloroform at room temperature. Computational results suggest that the effect of borylation on the frontier orbitals of each donor helps to explain differences in the favorability of their condensation reactions with arylboronic acids. The results can inform the use of boronic acids as they are increasingly utilized in the dynamic self-assembly of organic materials and as components in dynamic combinatorial libraries.

A method for the deprotection of alkylpinacolyl boronate esters

A two-step procedure for deprotection of alkylpinacolyl boronate esters via transesterification with diethanolamine followed by hydrolysis was successfully developed with the advantages of tolerance to various functional groups, short reaction time, and ease of product isolation.

Convenient methods for preparing pi-conjugated linkers as building blocks for modular chemistry

Simple, straightforward and optimized procedures for preparing extended pi-conjugated linkers are described. Either unsubstituted or 4-donor substituted pi-linkers bearing a styryl, biphenyl, phenylethenylphenyl, and phenylethynylphenyl pi-conjugated backbone are functionalized with boronic pinacol esters as well as with terminal acetylene moieties allowing their further use as building blocks in Suzuki-Miyaura or Sonogashira coupling reactions.

2,4,6-Tris-(4-fluoro-phen-yl)-2-(1-pyrid-yl)-boroxine

Crystals of the title compound, C₂₃H₁₇B₃F₃NO₃, were obtained unintentionally by slow evaporation of a chloro­form solution of the preformed boroxine–pyridine adduct. The mol­ecule contains three fluoro-substituted benzene rings, each bonded to one of the three B atoms of a six-membered boroxine ring. A pyridyl ring is also bound to one of the B atoms through a Lewis acid–base inter­action. The binding of the pyridyl substituent causes the otherwise planar boroxine ring to twist, resulting in a maximum torsion angle within the ring of 17.6 (2)°.

Effect of Para-Substituents and Solvent Polarity on the Formation of Triphenylboroxine·Amine Adducts

Density functional theory (B3LYP//6-311+G) calculations including Poisson-Boltzmann implicit solvent and NMR were used to study the formation of a series of para-substituted triphenylboroxine.amine adducts with respect to their phenylboronic acid monomers and free amine in solution. Our calculations suggest that the intermediate prior to forming trimer.amine is a dimer.amine adduct. Formation of dimer.amine can proceed via two pathways. Electron-donating substituents favor dimerization of two monomers before addition of the amine, and electron-withdrawing substituents favor formation of a monomer.amine adduct before addition of the second monomer. We also find that pi-electron acceptors destabilize formation of the dimer and trimer with respect to its monomers. Electron-withdrawing substituents favor adduct formation. Adduct formation is enthalpically stabilized by increasing the polarity of the solvent but differential solubility of the monomer compared to trimer.amine also has an effect on the equilibrium constant.