Boron Mimetics: 1,2-Dihydro-1,2-azaborines Bind inside a Nonpolar Cavity of T4 Lysozyme

Rhodium-Mediated Stoichiometric Synthesis of Mono-, Bi-, and Bis-1,2-Azaborinines: 1-Rhoda-3,2-azaboroles as Reactive Precursors

A series of highly substituted 1,2‐azaborinines, including a phenylene‐bridged bis‐1,2‐azaborinine, was synthesized from the reaction of 1,2‐azaborete rhodium complexes with variously substituted alkynes. 1‐Rhoda‐3,2‐azaborole complexes, which are accessible by phosphine addition to the corresponding 1,2‐azaborete complexes, were also found to be suitable precursors for the synthesis of 1,2‐azaborinines and readily reacted with alkynyl‐substituted 1,2‐azaborinines to generate new regioisomers of bi‐1,2‐azaborinines, which feature directly connected aromatic rings. Their molecular structures, which can be viewed as boron‐nitrogen isosteres of biphenyls, show nearly perpendicular 1,2‐azaborinine rings. The new method using rhodacycles instead of 1,2‐azaborete complexes as precursors is shown to be more effective, allowing the synthesis of a wider range of 1,2‐azaborinines.

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Among aromatic compounds, borazarenes represent a significant class of isosteres in which carbon-carbon bonds have been replaced by B-N bonds.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.

Multi-Component Synthesis of Rare 1,3-Dihydro-1,3-azaborinine Derivatives: Application of a Bora-Nazarov Type Reaction

The twofold hydroboration products of (Fmes)BH₂ ⋅SMe₂ with a series of alkynes (2-butyne, arylethynes) react with two molar equiv of 2,6-dimethylphenyl isocyanide (CN-Xyl) at 80 °C to give rare examples of 1,3-azaborinine derivatives. A mechanistic study revealed a reaction course involving insertion of one isonitrile followed by a bora-Nazarov type ring-closure reaction and subsequent isonitrile insertion to give the respective 1,3-dihydro-1,3-azaborinines 5.

Inorganic Triphenylphosphine

A completely inorganic version of one of the most famous organophosphorus compounds, triphenylphosphine, has been prepared. A comparison of the crystal structures of inorganic triphenylphosphine, PBaz₃ (where Baz=B₃ H₂ N₃ H₃ ) and PPh₃ shows that they have superficial similarities and furthermore, the Lewis basicities of the two compounds are remarkably similar. However, their oxygenation and hydrolysis reactions are starkly different. PBaz₃ reacts quantitatively with water to give PH₃ and with the oxidizing agent ONMe₃ to give the triply-O-inserted product P(OBaz)₃ , an inorganic version of triphenyl phosphite; a corresponding transformation with PPh₃ is inconceivable. Thermodynamically, what drives these striking differences in the chemistry of PBaz₃ and PPh₃ is the great strength of the B-O bond.

The Dewar Isomer of 1,2-Dihydro-1,2-azaborinines: Isolation, Fragmentation, and Energy Storage

The photochemistry of 1,2-dihydro-1,2-azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring-opening reaction of the Dewar valence isomer back to the 1,2-dihydro-1-tert-butyldimethylsilyl-2-mesityl-1,2-azaborinine has an activation barrier of (27.0±1.2) kcal mol-1 . In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(-48±1) kcal mol-1 ) at room temperature.

Scope of the Thermal Ring-Expansion Reaction of Boroles with Organoazides

Electronic and steric factors have been investigated in the thermal ring expansion of boroles with organic azides, a reaction that provides access to highly arylated 1,2-azaborinines, BN analogues of benzene. Reactions of a variety of boroles and organic azides demonstrate that the synthetic method is quite general in furnishing 1,2-azaborinines, but the respective reaction rates reveal a strong dependence on the substituents on the two reactants. The products have been characterized by UV/Vis, electrochemical, NMR, and X-ray diffraction methods, clarifying their constitutions and highlighting substituent effects on the electronic structure of the 1,2-azaborinines. Furthermore, analysis of several possible mechanistic pathways for 1,2-azaborinine formation, as studied by DFT, revealed that a two-step mechanism involving azide-borole adduct formation and nitrene insertion is favored.

Medicinal Chemistry Profiling of Monocyclic 1,2-Azaborines

The first examples of biologically active monocyclic 1,2-azaborines have been synthesized and demonstrated to exhibit not only improved in vitro aqueous solubility in comparison with their corresponding carbonaceous analogues, but in the context of a CDK2 inhibitor, also improved biological activity and better in vivo oral bioavailability. This proof-of-concept study establishes the viability of monocyclic 1,2-azaborines as a novel pharmacophore with distinct pharmacological profiles that can help address challenges associated with solubility in drug development research.

Regioselective Catalytic and Stepwise Routes to Bulky, Functional-Group-Appended, and Luminescent 1,2-Azaborinines

The regioselective syntheses of 1,2-azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2-azaborinine ring can be selectively functionalized in the 4- and/or 6-position through control of the stepwise reaction sequence, allowing access to vinyl-functionalized and redox-active, luminescent, donor-functionalized 1,2-azaborinines. The electrochemistry and photochemistry of a tetraarylamine-substituted 1,2-azaborinine are studied. Cyclic voltammetry of this compound, relative to a non-B,N-substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.

Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles

The ability of anti-aromatic boroles to undergo ring expansion reactions with a broad range of substrates as an effective synthetic protocol towards a diverse array of 6-, 7-, and 8-membered boracycles is detailed in this review.

Oxygen insertion into boroles as a route to 1,2-oxaborines

The synthesis of 1,2-oxaborines is accomplishedviathe reaction of pentaarylboroles withN-methylmorpholine-N-oxideviaa 1,1-insertion reaction.

Diels-Alder Reactions of 1,2-Azaborines

Diels-Alder reactions employing 1,2-azaborine heterocycles as 1,3-dienes are reported. Carbocyclic compounds with high stereochemical and functional complexity are produced, as exemplified by the straightforward two-step synthesis of an amino allyl boronic ester bearing four contiguous stereocenters as a single diastereomer. Whereas electron-deficient dienophiles undergo irreversible Diels-Alder reactions, a reversible Diels-Alder reaction with the less electron-deficient methyl acrylate is observed. Both the N and the B substituent of the 1,2-azaborine exert significant influence on the [4+2] cycloaddition reactivity as well as the aromatic character of the heterocycle. The experimentally determined thermodynamic parameters of the reversible Diels-Alder reaction between 1,2-azaborines and methyl acrylate correlate with aromaticity trends and place 1,2-azaborines approximately between furan and thiophene on the aromaticity scale.

Boron-substituted 1,3-dihydro-1,3-azaborines: synthesis, structure, and evaluation of aromaticity

Getting the family together: A general synthetic strategy based on nucleophilic substitution provided B-substituted 1,3-dihydro-1,3-azaborines (see scheme), BN isosteres of arenes with potential for application in biomedicine and materials science. Experimental structural analysis and calculations suggest that the aromaticity of the 1,3-dihydro-1,3-azaborine heterocycle is intermediate between that of benzene and that of 1,2-dihydro-1,2-azaborine.

Formation of azaborines by photoelimination of B,N-heterocyclic compounds

Highly fluorescent π-conjugated polycyclic azaborines can be prepared from B,N-heterocyclic compounds with a BR2 -CH2 unit through the elimination of an R-H molecule (see scheme). These clean photoelimination reactions occur both in solution and in polymers doped with the precursors.

BN/CC isosteric compounds as enzyme inhibitors: N- and B-ethyl-1,2-azaborine inhibit ethylbenzene hydroxylation as nonconvertible substrate analogues

Good substrate gone bad! BN/CC isosterism of ethylbenzene leads to N-ethyl-1,2-azaborine and B-ethyl-1,2-azaborine. In contrast to ethylbenzene, which is the substrate for ethylbenzene dehydrogenase (EbDH), N-ethyl-1,2-azaborine (see scheme; Fc=Ferricenium tetrafluoroborate) and B-ethyl-1,2-azaborine are strong inhibitors of EbDH. Thus, the changes provided by BN/CC isosterism can lead to new biochemical reactivity.

Recent advances in azaborine chemistry

The chemistry of organoboron compounds has been primarily dominated by their use as powerful reagents in synthetic organic chemistry. Recently, the incorporation of boron as part of a functional target structure has emerged as a useful way to generate diversity in organic compounds. A commonly applied strategy is the replacement of a CC unit with its isoelectronic BN unit. In particular, the BN/CC isosterism of the ubiquitous arene motif has undergone a renaissance in the past decade. The parent molecule of the 1,2-dihydro-1,2-azaborine family has now been isolated. New mono- and polycyclic B,N heterocycles have been synthesized for potential use in biomedical and materials science applications. This review is a tribute to Dewar's first synthesis of a monocyclic 1,2-dihydro-1,2-azaborine 50 years ago and discusses recent advances in the synthesis and characterization of heterocycles that contain carbon, boron, and nitrogen.

Nucleophilic aromatic substitution reactions of 1,2-dihydro-1,2-azaborine

Could go either way: the addition of nucleophiles to the parent 1,2-dihydro-1,2-azaborine and subsequent quenching with an electrophile generates novel substituted 1,2-azaborine derivatives. Mechanistic studies are consistent with two distinct nucleophilic aromatic substitution pathways depending on the nature of the nucleophile.