Method for Accessing Nitrogen-Containing, B-Heteroaryl-Substituted 2,1-Borazaronaphthalenes

Removing Neighboring Ring Influence in Monocyclic B-OH Diazaborines: Properties and Reactivity as Phenolic Bioisosteres with Dynamic Hydroxy Exchange

The design of small molecules with unique geometric profiles or molecular connectivity represents an intriguing yet neglected challenge in modern organic synthesis. This challenge is compounded when emphasis is placed on the preparation of new chemotypes that have distinct and practical functions. To expand the structural diversity of boron-containing heterocycles, we report herein the preparation of novel monocyclic hemiboronic acids, diazaborines. These compounds have enabled the study of a pseudoaromatic boranol-containing (B-OH) ring free of influence from an appended aromatic system. Synthetic and spectroscopic studies have provided insight into the aromatic character, Lewis acidic nature, chemical reactivity, and unique ability of the exocyclic B-OH unit to participate in hydroxy exchange, suggesting their use in organocatalysis and as reversible covalent inhibitors. Moreover, density functional theory and nucleus-independent chemical shift calculations reveal that the aromatic character of the boroheterocyclic ring is increased significantly in comparison to known bicyclic benzodiazaborines (naphthoid congeners), consequently leading to attenuated Lewis acidity. Direct structural comparison to a well-established biaryl isostere, 2-phenylphenol, through X-ray crystallographic analysis reveals that N-aryl derivatives are strikingly similar in size and conformation, with attenuated logP values underscoring the value of the polar BNN unit. Their potential application as low-molecular-weight scaffolds in drug discovery is demonstrated through orthogonal diversification and preliminary antifungal evaluation (Candida albicans), which unveiled analogs with low micromolar inhibitory concentration.

Assessing the Role of BN-Embedding Position in B2N2-Perylenes

Incorporating heteroatoms can effectively modulate the molecular optoelectronic properties. However, the fundamental understanding of BN doping effects in BN-embedded polycyclic aromatic hydrocarbons (PAHs) is underexplored, lacking rational guidelines to modulate the electronic structures through BN units for advanced materials. Herein, a concise synthesis of novel B2N2-perylenes with BN doped at the bay area is achieved to systematically explore the doping effect of BN position on the photophysical properties of PAHs. The shift of BN position in B2N2-perylenes alters the π electron conjugation, aromaticity and molecular rigidness significantly, achieving substantially higher electron transition abilities than those with BN doped in the nodal plane. It is further clarified that BN position dominates the photophysical properties over BN orientation. The revealed guideline here may apply generally to novel BN-PAHs, and aid the advancement of BN-PAHs with highly-emissive performance.

Modular synthesis of 1,2-azaborines via ring-opening BN-isostere benzannulation

1,2-Azaborines represent a unique class of benzene isosteres that have attracted interest for developing pharmaceuticals with better potency and bioavailability. However, it remains a long-standing challenge to prepare monocyclic 1,2-azaborines, particularly multi-substituted ones, in an efficient and modular manner. Here we report a straightforward method to directly access diverse multi-substituted 1,2-azaborines from readily available cyclopropyl imines/ketones and dibromoboranes under relatively mild conditions. The reaction is scalable, shows a broad substrate scope, and tolerates a range of functional groups. The utility of this method is demonstrated in the concise syntheses of BN isosteres of a PD-1/PD-L1 inhibitor and pyrethroid insecticide, bifenthrin. Combined experimental and computational mechanistic studies suggest that the reaction pathway involves boron-mediated cyclopropane ring-opening and base-mediated elimination, followed by an unusual low-barrier 6π-electrocyclization accelerated by the BN/CC isomerism. This method is anticipated to find applications for the synthesis of BN-isostere analogues in medicinal chemistry, and the mechanistic insights gained here may guide developing other boron-mediated electrocyclizations.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.

A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes

A novel 1,2-azaborine (i.e., 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, 1a) has been synthesized and used for the first time as a B-N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (i.e., 2a, 3, and 4) of general formula [Ir(C^∧N)₂(N^∧NB)], where C^∧N indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex 2a was compared to the isoelectronic C═C iridium(III) complex 2b, obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand 1b. Due to the dual cyclometalation mode of such C═C ligand, the isomeric complex 2c was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of 2a was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (e.g., λ_max = 523 vs 577 nm for 2a vs 2b, in acetonitrile solution at 298 K). On the other hand, the ³LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its C═C pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical C═C counterparts (e.g., photoluminescence quantum yield (PLQY) = 16 vs 22% for 2a vs 2b, in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.

Synthesis of isoxazoloazaborines via gold(i)-catalyzed propargyl aza-Claisen rearrangement/borylative cyclization cascade

Isoxazoloazaborines have been synthesized from 4-N-propargylaminoisoxazole via gold(i)-catalyzed propargyl aza-Claisen rearrangement followed by electrophilic borylative cyclization in 27–86% yields.

Parent B2 N2 -Perylenes with Different BN Orientations

Introducing BN units into polycyclic aromatic hydrocarbons expands the chemical space of conjugated materials with novel properties. However, it is challenging to achieve accurate synthesis of BN-PAHs with specific BN positions and orientations. Here, three new parent B₂ N₂ -perylenes with different BN orientations are synthesized with BN-naphthalene as the building block, providing systematic insight into the effects of BN incorporation with different orientations on the structure, (anti)aromaticity, crystal packing and photophysical properties. The intermolecular dipole-dipole interaction shortens the π-π stacking distance. The crystal structure, (anti)aromaticity, and photophysical properties vary with the change of BN orientation. The revealed BN doping effects may provide a guideline for the synthesis of BN-PAHs with specific stacking structures, and the synthetic strategy employed here can be extended toward the synthesis of larger BN-embedded PAHs with adjustable BN patterns.

Selectivity in the Elaboration of Bicyclic Borazarenes

Among aromatic compounds, borazarenes represent a significant class of isosteres in which carbon-carbon bonds have been replaced by B-N bonds. Described herein is a summary of the selective reactions that have been developed for known systems, as well as a summary of computationally-based predictions of selectivities that might be anticipated in reactions of yet unrealized substructures.

Synthesis and biological evaluation of an antibacterial azaborine retinoid isostere. Tetrahedron Lett 2020;62. [PMID: 33390617 DOI: 10.1016/j.tetlet.2020.152667]

Our continued synthetic interest in this class of retinoids, CD437 and its analogs, against methicillin-resistant Staphylococcus aureus (MRSA) has brought us to explore further isosteric substitutions within the scaffold. Although our previous findings have shown promising activity against gram-positive pathogens, their therapeutic viability remained an issue. Specifically, through preliminary analysis, our best performing compound, analog 2, displayed low solubility within serum as well as high affinity for retinoid binding proteins with a concentration dependent relationship. To circumvent this issue, we proposed a class of analogs containing an azaborine substitution in place of the naphthalene moiety. Azaborines have a nitrogen-boron bond substituting a carbon-carbon double bond that alters the electronics of the parent scaffold. This motif has been explored successfully in cancer research but to the best of our knowledge has yet to be applied to antibiotics. Herein, we describe the synthesis of the desired analogs, antimicrobial activity, and surprising physiochemical properties.

BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH₄/Me₃SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me₃SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me₃Si)₂NR' or 5 with H₂NR' or H₂O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7^•] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7^•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

Intramolecular (directed) electrophilic C-H borylation

The intramolecular C-H borylation of (hetero)arenes and alkenes using electrophilic boranes is a powerful transition metal free methodology for forming C-B bonds. These C-H borylation reactions are preceded by intermolecular bond (both dative and covalent) formation, with examples proceeding via initial C-B and N-B bond formation dominating this field thus both are discussed in depth herein. Less prevalent intramolecular electrophilic C-H borylation reactions that proceed by intermolecular O-B, S-B and P-B bond formation are also summarised. Mechanistic studies are presented that reveal two mechanisms for C-H borylation, (i) electrophilic aromatic substitution (prevalent with B-X electrophiles); (ii) σ-bond metathesis mediated (prevalent with B-H and B-R electrophiles). To date, intramolecular electrophilic C-H borylation is utilised mainly for accessing boron containing conjugated organic materials, however recent developments, summarized herein alongside early studies, have highlighted the applicability of this methodology for forming synthetically versatile organo-boronate esters and boron containing bioactives. The multitude of synthetic procedures reported for intramolecular electrophilic C-H borylation contain many common features and this enables key requirements for successful C-H borylation and the factors effecting regioselectivity and substrate scope to be identified, discussed and summarized.

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized and incorporated into proteins.

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized through the functionalization of BN-indole. The spectroscopic properties of BN-tryptophan are reported with respect to the natural tryptophan, and the incorporation of BN-tryptophan into proteins expressed in E. coli using selective pressure incorporation is described. This work shows that a cellular system can recognize the unnatural, BN-containing tryptophan. More importantly, it presents the first example of an azaborine containing amino acid being incorporated into proteins.

Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2-N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the "aromatic metamorphosis" of indoles into 1,2-benzazaborins via the insertion of boron into the C2-N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

Four-coordinate triarylborane synthesis via cascade B-Cl/C-B cross-metathesis and C-H bond borylation

To develop a simple and efficient synthetic method for four-coordinate triarylboranes, we herein describe a tandem highly selective B-Cl/C-B cross-metathesis of two of the same or different arylboranes and C-H bond borylation to synthesize four-coordinate triarylboranes with a broad substrate scope. By switching substituent groups of the target molecules, different emission wavelengths can be achieved from 467 nm to 583 nm with aggregation-induced emission (AIE) properties.

The State of the Art in Azaborine Chemistry: New Synthetic Methods and Applications

Boron-nitrogen heteroarenes hold great promise for practical application in many areas of chemistry. Enduring interest in realizing this potential has in turn driven perennial innovation with respect to these compounds' synthesis. This Perspective discusses in detail the most recent advances in methods pertaining to the preparation of BN-isosteres of benzene, naphthalene, and their derivatives. Additional focus is placed on the progress enabled by these syntheses toward functional utility of such BN-heterocycles in biochemistry and pharmacology, materials science, and transition-metal-based catalysis. The prospects for future research efforts in these and related fields are also assessed.

A modular route to boron doped PAHs by combining borylative cyclisation and electrophilic C-H borylation

Heteroatom doping into polyaromatic hydrocarbons (PAHs) is a powerful approach for modifying key physical properties, however, there are extremely few modular routes that enable facile formation of B-, B2- and B,N-(specifically not containing direct B-N bonds) doped PAHs despite the growing importance of these materials. Sequential, one pot borylative cyclisation/intramolecular electrophilic C-H borylation of naphthyl-alkynes provides a simple new route to access novel B-, B,N- and B2-doped (PAHs). The initial products, dihydronaphthalene/dihydroquinoline B-mesityl PAHs, were reacted with [Ph3C][BF4]/pyridyl base to form the oxidised B-, and B,N-doped PAHs. However, for B-triisopropylphenyl (Trip) PAH congeners oxidation has to be performed prior to Trip installation due to preferential oxidation of an isopropylaryl moiety to the styrene. This alternative sequence enables access to Trip-B-PAHs and to structurally constrained B and B2-PAHs. Analysis of the solid state structures and optoelectronic properties of these PAHs confirm that frontier orbital energies, extended packing structures, Stokes shift and quantum yields all can be rationally modified using this methodology. The simplicity of this synthetic approach makes it a powerful tool for rapidly generating novel bench stable boron doped PAHs, which is important for facilitating further structure-property relationship studies and the wider utilisation of these materials in optoelectronic applications.

The Least Stable Isomer of BN Naphthalene: Toward Predictive Trends for the Optoelectronic Properties of BN Acenes

The least stable isomer of the parental BN naphthalene series has been synthesized in a simple four-step sequence. Its experimental electronic structure characterization via UV-PES, cyclic voltammetry, and UV-vis spectroscopy in direct comparison with three other known BN naphthalene isomers has established two guiding principles for predicting the electronic structures of BN acene compounds: (1) Orientational BN isomers have similar HOMO-LUMO gaps. (2) For each pair of orientational BN isomers, the more thermodynamically stable compound has the lower HOMO energy. Furthermore, we demonstrate that BN/CC isosterism in the context of BN-9,1-Naph can impact crystal packing to favor a cofacial π-stack motif.

Synthesis, computational, and spectroscopic analysis of tunable highly fluorescent BN-1,2-azaborine derivatives containing the N-BOH moiety

Methods to tune the luminescence wavelength and the quantum yields by controlling the power and location of electron-donor and acceptor substituents on the ring system.