Synthesis, Functionalization, and Optical Properties of 1,2-Dihydro-1-aza-2-boraphenanthrene and Several Highly Fluorescent Derivatives

Synthesis, Structure, and Properties of Two Planar BN-Benzofluorenes

Two planar BN doped benzofluorenes (BN-BkF and BN-BjF) were synthesized separately through palladium-catalyzed intramolecular C-C or C-N coupling reactions. The structures of both BN-BkF and BN-BjF are unambiguously confirmed by X-ray crystallographic analysis. Moreover, the BN unit doping leads to a much lower HOMO level and higher LUMO level as compared to their carbon analogues BkF and BjF. In comparison to BkF and BjF, both BN-BkF and BN-BjF exhibit blue shifts in their ultraviolet-visible (UV-vis) absorption and fluorescence emission spectra. Furthermore, halogenation of BN-BkF and BN-BjF afforded monohalogenated BN-benzofluorenes in good yields. These monohalogenated BN-benzofluorenes can serve as convenient intermediates for palladium-catalyzed coupling reactions to yield a series of functionalized BN-benzofluorene derivatives. The UV-vis absorption and emission spectroscopies in dichloromethane of these BN-benzofluorene derivatives were studied, and the photophysics of these compounds exhibited a high degree of substituent dependency.

Ruthenium-Catalyzed Enyne Metathesis: An Entry to Functionalized Azaborine Heterocycles

An unprecedented and green synthetic route for the preparation of functionalized polycyclic azaborine skeletons via a ruthenium-mediated ring-closing enyne metathesis transformation of alkynyl B-anthranilamide (aam) is reported. A variety of B(aam)-bridge enyne derivatives were engaged with the Grubbs-II catalyst, affording the corresponding exo-cyclic compounds (28 novel derivatives). The methodology was efficiently optimized in dimethyl carbonate as green solvent and provided a direct access to functionalized boronated scaffolds displaying dienes which can be further functionalized. In addition, computational analysis of several of the cores synthesized in this study revealed an isoelectronic and isostructural relationship with potential isosteres.

BN-Embedded Aromatic Hydrocarbon Synthesis via Nucleophilic Diboration Reactions

Activation of bis(pinacolato)diboron with aromatic lithium amide promotes diboration of the proximal C-C triple bond, leading to BN-embedded aromatic compounds. In situ treatment of the initially generated spirocyclic borate intermediate with aqueous acid or organometallic reagents enables ligand installation on the endocyclic boron atom.

Synthesis of BN-Polyarenes by a Mild Borylative Cyclization Cascade

Reaction of BCl₃ with suitably substituted o-alkynylanilines promotes a cascade reaction in which BN-polycyclic compounds are obtained via the formation of two new cycles and three new bonds in a single operational step. The reaction is highly efficient and takes place at room temperature, providing a very mild and straightforward strategy for the preparation of BN-aromatic compounds, which can be further transformed into a variety of BN-PAHs with different polycyclic cores and substituents.

Concise Synthesis of BN-Dibenzo[f,k]tetraphenes with Different BN Substitution Positions and Direct Comparison with Their Carbonaceous Analogue

Two types of "parental" BN-dibenzo[f,k]tetraphenes (BNDBT-1 and BNDBT-2) have been synthesized via a transition-metal-catalyzed tandem cross-coupling reaction as key steps. Both BNDBT-1 and BNDBT-2 are fully characterized; one of them is unambiguously confirmed by a single X-ray crystal structure. Compared to its all-carbon analogue DBT, BNDBT-1 and BNDBT-2 exhibit a higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO) energy, while the BN doping position slightly influences the HOMO and LUMO energies of BNDBT-1 and BNDBT-2. Both BNDBT-1 and BNDBT-2 exhibit red-shifted absorption and emission spectra and higher emission efficiencies, as compared to their carbonaceous analogue DBT. Moreover, organic light emitting diodes were fabricated using BNDBT-1 and BNDBT-2 as emitters, demonstrating their potential applications.

The rapid construction of bis-BN dipyrrolyl[a,j]anthracenes and a direct comparison with a carbonaceous analogue

A series of bis-BN dipyrrolyl[a,j]anthracenes and a representative carbonaceous analogue have been synthesized. We studied the optical properties and OLED applications of these BN-PAHs and compared them with the carbonaceous counterpart.

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.

Enantioselective Synthesis of Oxazaborolidines by Palladium-Catalyzed N-H/B-H Double Activation of 1,2-Azaborines

A palladium-catalyzed N-H/B-H double activation of 1,2-dihydro-1,2-benzazaborines proceeded via cycloaddition with vinyl ethylene carbonate to produce polycyclic oxazaborolidines in 31-96 % yield. The key step in this process is the release of molecular hydrogen from a borate intermediate. Using a SPINOL-derived phosphoramidite as a chiral ligand, chiral oxazaborolidines were synthesized in good to high yields with excellent enantioselectivity (up to 95 % ee). The vinyl group of the resulting oxazaborolidine underwent metathesis, Heck reaction, and Wacker oxidation without affecting the oxazaborolidine framework.

1,10a-Dihydro-1-aza-10a-boraphenanthrene and 6a,7-Dihydro-7-aza-6a-boratetraphene: Two New Fluorescent BN-PAHs

Previously unknown 1,10a-dihydro-1-aza-10a-boraphenanthrene and 6a,7-dihydro-7-aza-6a-boratetraphene have been efficiently synthesized. Bromination of these BN-PAHs proceeds with complete regioselectivity, resulting in the formation of different substituted derivatives via cross-coupling reactions. These compounds exhibit rather high fluorescence quantum yields (up to ϕ_F = 0.80).

Synthesis, Properties, and Regioselective Functionalization of 9,9a-BN Anthracene

A novel 9,9a-BN anthracene 5 has been synthesized by the Ru-catalyzed electrocyclization of BN-aromatic enynes. The photophysical properties of 5 are different from those of all-carbon anthracene and other reported BN-anthracenes. The reactivity of 5 has been investigated by treating 5 with organolithium compounds, Br₂, or N-iodosuccinimide. The resulting halogenated compounds can be easily functionalized via cross-coupling reactions. UV-vis and fluorescence spectroscopy of 5 have been investigated to explore the photophysical properties of these BN-anthracenes.

A Crystalline B4N2 Dewar Benzene as a Building Block for Conjugated B,N-Chains

Dewar benzene, one of the isolable valence isomers of C₆H₆, has been extensively studied since its first synthesis in 1962. By contrast, the chemistry of inorganic congeners of Dewar benzene, which can be formally gained by replacing the skeletal carbon atoms with heteroatoms, has been less developed despite their peculiar structural and electronic features. Among them, the extant B,N-Dewar benzenes are limited to the B₃N₃ system. Herein, we report the development of the first example of an isolable B₄N₂ Dewar benzene, 3. As predicted by DFT calculations, a judicious selection of the substituents allows synthesizing 3. Single-crystal X-ray analysis, NMR, and computational studies confirmed that 3 possesses a high-lying B(sp³)-B(sp³) σ-bond at the bridgehead position. Reactions with ethylene and phenylacetylene proceeded smoothly under mild conditions, affording the fused B₄C₄N₂ ring systems (4 and 5). Structural characterization as well as DFT calculations revealed that compounds 4 and 5 involve a rigid and conjugated (BN)₄ tetraene scaffold. Formation of 4 and 5 demonstrates that 3 may serve as a building block for the construction of conjugated B,N-chains.

Synthesis, structure and properties of semi-internally BN-substituted annulated thiophenes

A series of semi-internally BN-substituted annulated thiophenes were synthesized from easily accessible 2,1-borazaronaphthalenes.

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].

Understanding the reactivity of polycyclic aromatic hydrocarbons and related compounds

This perspective article summarizes recent applications of the combination of the activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds such as cycloparaphenylenes, fullerenes and doped systems. To this end, we have selected representative examples to highlight the usefulness of this relatively novel computational approach to gain quantitative insight into the factors controlling the so far not fully understood reactivity of these species. Issues such as the influence of the size and curvature of the system on the reactivity are covered herein, which is crucial for the rational design of novel compounds with tuneable applications in different fields such as materials science or medicinal chemistry.

Expanding the BN-embedded PAH family: 4a-aza-12a-borachrysene

Previously unknown 4a-aza-12a-borachrysene has been synthesized in only four steps.

Borazatruxenes

We report the synthesis and characterization of a series of arene-borazine hybrids called borazatruxenes. These molecules are BN-isosteres of truxene whereby the central benzene core has been replaced by a borazine ring. The straightforward three step synthesis, stability and their chiroptical and electronic properties recommend them as new scaffolds for BN-carbon hybrid materials. Computational studies at DFT level, closely matching the experimental data, provided insights in the electronic structure of these molecules.

Remarkable effect of alkynyl substituents on the fluorescence properties of a BN-phenanthrene

A series of BN-phenanthrenes with substituents of a diverse nature have been synthesized by palladium-catalyzed cross-coupling reactions of a common chloro-substituted precursor, which was made from readily available materials in only four steps. Evaluation of the photophysical properties of the prepared compounds unveiled an impressive effect of the presence of alkynyl substituents on the fluorescence quantum yield, which improved from 0.01 in the parent compound to up to 0.65 in derivatives containing a triple bond.

A New Member of the BN-Phenanthrene Family: Understanding the Role of the B-N Bond Position

3,4-Dihydro-4-aza-3-boraphenanthrene, which shows the highest fluorescence quantum yield of all nonsubstituted BN-phenanthrenes reported to date (ϕ_F = 0.61), has been synthesized in only three steps (76% overall yield) from easily accessible 1-bromo-2-vinylnaphthalene, along with several substituted derivatives. The reactivity of these previously unknown BN-aromatic compounds toward organolithium compounds and bromine has been studied. This latter reaction affords bromo-substituted compounds that are suitable for further functionalization via Suzuki and Sonogashira couplings, with complete regioselectivity. The optical properties and excited state deactivation mechanisms of selected compounds were studied using computational methods.