Electrophilic Borylation Research Articles

Preparation, Structure, Reactivity, Lewis Acidic and Fluorescence Properties of Arylpyridine Based Boron C,N-Chelates Featuring Weakly Coordinating Anions.

Herein, we present the preparation of a series of electronically and/or sterically distinct borenium-type species based on a simple 2-arylpyridine scaffold. Corresponding arylpyridine was firstly subjected to electrophilic borylation (BBr3/i-Pr2NEt) and formed BBr2 chelate was reduced with LiAlH4 to yield arylpyridine boron dihydride. Elimination of one hydride led to Lewis acidic borenium-like products. Four methods of hydride elimination were evaluated and influence of counterions on reactivity, Lewis acidic and luminescent properties was assessed both experimentally and computationally. Arylpyridine chelates featuring weakly coordinating counterions exhibit fluorescent properties upon UV irradiation. Several general trends were inferred to modulate emission wavelength and fluorescence quantum yield. Based on our observations, we have devised and prepared borenium-type fluorophores with yellow-green fluorescence and quantum yields up to 93 %.

Borylation-Reduction-Borylation for the Formation of 1,4-Azaborines.

Given the current interest in materials containing 1,4-azaborine units, the development of new routes to these structures is important. Carbonyl directed electrophilic borylation using BBr3 is a facile method for the ortho-borylation of N,N-diaryl-amide derivatives. Subsequent addition of Et3SiH results in carbonyl reduction and then formation of 1,4-azaborines that can be protected in situ using a Grignard reagent. Overall, borylation-reduction-borylation is a one-pot methodology to access 1,4-azaborines from simple precursors.

Synthesis, Structure, and Properties of a Nitrogen-Boron-Nitrogen-Embedded Polycyclic π-System Containing a Pleiaheptalene Framework.

A novel polycyclic π-system (1) featuring both a pleiaheptalene framework (a three-fused heptagon system) and nitrogen-boron-nitrogen (NBN) unit was constructed by electrophilic borylation. A combined experimental and computational study demonstrated that 1 has a highly twisted π-backbone with approximate C2 symmetry, which can undergo conformational isomerization at room temperature in contrast to pleiaheptalene. It was also found that 1 can bind the fluoride ion in the solution, which induces changes in the absorption and emission spectra.

Pushing Up the Size Limit of Boron‐doped peri‐Acenes: Modular Synthesis and Characterizations

AbstractHeteroatom‐doped peri‐acenes (PAs) have recently attracted considerable attention considering their fascinating physical properties and chemical stability. However, the precise sole addition of boron atoms along the zigzag edges of PAs remains challenging, primarily due to the limited synthetic approach. Herein, we present a novel one‐pot modular synthetic strategy toward unprecedented boron‐doped PAs (B‐PAs), including B‐[4,2]PA (1 a‐2), B‐[4,3]PA (1 b‐2) and B‐[7,2]PA (1 c‐3) derivatives, through efficient intramolecular electrophilic borylation. Their chemical structures are unequivocally confirmed with a combination of mass spectrometry, NMR, and single‐crystal X‐ray diffraction analysis. Notably, 1 b‐2 exhibits an almost planar geometry, whereas 1 a‐2 displays a distinctive bowl‐like distortion. Furthermore, the optoelectronic properties of this series of B‐PAs are thoroughly investigated by UV/Vis absorption and fluorescence spectroscopy combined with DFT calculation. Compared with their parent all‐carbon analogs, the obtained B‐PAs exhibit high stability, wide energy gaps, and high photoluminescence quantum yields of up to 84 %. This study reveals the exceptional ability of boron doping to finely tune the physicochemical properties of PAs, showcasing their potential applications in optoelectronics.

Pushing Up the Size Limit of Boron-doped peri-Acenes: Modular Synthesis and Characterizations.

Heteroatom-doped peri-acenes (PAs) have recently attracted considerable attention considering their fascinating physical properties and chemical stability. However, the precise sole addition of boron atoms along the zigzag edges of PAs remains challenging, primarily due to the limited synthetic approach. Herein, we present a novel one-pot modular synthetic strategy toward unprecedented boron-doped PAs (B-PAs), including B-[4,2]PA (1 a-2), B-[4,3]PA (1 b-2) and B-[7,2]PA (1 c-3) derivatives, through efficient intramolecular electrophilic borylation. Their chemical structures are unequivocally confirmed with a combination of mass spectrometry, NMR, and single-crystal X-ray diffraction analysis. Notably, 1 b-2 exhibits an almost planar geometry, whereas 1 a-2 displays a distinctive bowl-like distortion. Furthermore, the optoelectronic properties of this series of B-PAs are thoroughly investigated by UV/Vis absorption and fluorescence spectroscopy combined with DFT calculation. Compared with their parent all-carbon analogs, the obtained B-PAs exhibit high stability, wide energy gaps, and high photoluminescence quantum yields of up to 84 %. This study reveals the exceptional ability of boron doping to finely tune the physicochemical properties of PAs, showcasing their potential applications in optoelectronics.

BN-Embedded Cycloarenes: One-Pot Borylation Synthesis, Photoelectric Properties, and Application in Perovskite Solar Cells.

Incorporating heteroatoms, such as nitrogen, oxygen, and/or sulfur atoms, into cycloarenes can effectively regulate their molecular geometries and (opto)electronic properties. However, the rarity of cycloarenes and heterocycloarenes limits the further exploitation of their applications. Herein, we designed and synthesized the first examples of boron and nitrogen (BN)-doped cycloarenes (BN-C1 and BN-C2) via one-pot intramolecular electrophilic borylation of imine-based macrocycles. BN-C2 adopts a bowl-shaped conformation, while BN-C1 possesses a planar geometry. Accordingly, the solubility of BN-C2 was significantly improved by replacing two hexagons in BN-C1 with two N-pentagons, due to the creation of distortions away from planarity. Various experiments and theoretical calculations were carried out for heterocycloarenes BN-C1 and BN-C2, demonstrating that the incorporated BN bonds diminish the aromaticity of 1,2-azaborine units and their adjacent benzenoid rings but preserve the dominant aromatic properties of pristine kekulene. Importantly, when two additional electron-rich nitrogen atoms were introduced, the highest occupied molecular orbital energy level of BN-C2 was elaborately lifted compared with that of BN-C1. As a result, the energy-level alignment of BN-C2 with the work function of the anode and the perovskite layer was suitable. Therefore, for the first time, heterocycloarene (BN-C2) was explored as a hole-transporting layer in inverted perovskite solar cell devices, in which the power conversion efficiency reached 14.4%.

Highly Efficient and Stable Blue OLEDs Based on B—N Bonds Embedded 6,12‐Diphenyl‐5,11‐dihydroindolo[3,2‐b]carbazole with Narrowband Emission and Extended Lifetime†

Comprehensive SummaryEmitters with narrowband spectra are of great importance nowadays because of the growing demand for ultra‐high‐definition displays in the fields of panel displays and solid‐state lighting. Though the reported multiple resonance (MR) emitters have been widely studied with extremely sharp spectra and color‐tunable emissions, the electrophilic borylation synthetic strategies and stable narrowband blue devices still face great challenges. Herein, by virtue of the advantages of both the MR effect of conventional indolocarbazole skeleton and the easy‐to‐access of B—N covalent bonds, the linear tert‐butyl benzene and carbazole groups modified 6,12‐diphenyl‐5,11‐dihydroindolo[3,2‐b]carbazole derivatives (tPh[BN] and Cz[BN]) containing two B—N covalent bonds are easily constructed through amine‐directed double borylation strategy. Both emitters behaved bright blue emission with full‐width‐at‐half‐maximum (FWHM) of 20—27 nm in solution, high fluorescence quantum efficiency over 90%, and excellent stability toward moisture. By employing triplet‐triplet annihilation electroluminescent mechanism, blue OLED devices based on them showed a small FWHM of 48 nm and maximum external quantum efficiency of ~7.1% with negligible efficiency roll‐off, and long operational lifetime (LT80) of 87 h at an initial brightness of 1000 cd·m−2, demonstrating the enormous application potential of B—N bonds embedded emitters in achieving ideal narrowband blue emitters.

Amides as modifiable directing groups in electrophilic borylation.

Amide directed C-H borylation using ≥two equiv. of BBr3 forms borenium cations containing a R2N(R')C[double bond, length as m-dash]O→B(Ar)Br unit which has significant Lewis acidity at the carbonyl carbon. This enables reduction of the amide unit to an amine using hydrosilanes. This approach can be applied sequentially in a one-pot electrophilic borylation-reduction process, which for phenyl-acetylamides generates ortho borylated compounds that can be directly oxidised to the 2-(2-aminoethyl)-phenol. Other substrates amenable to the C-H borylation-reduction sequence include mono and diamino-arenes and carbazoles. This represents a simple method to make borylated molecules that would be convoluted to access otherwise (e.g. N-octyl-1-BPin-carbazole). Substituent variation is tolerated at boron as well as in the amide unit, with diarylborenium cations also amenable to reduction. This enables a double C-H borylation-reduction-hydrolysis sequence to access B,N-polycyclic aromatic hydrocarbons (PAHs), including an example where both the boron and nitrogen centres contain functionalisable handles (N-H and B-OH). This method is therefore a useful addition to the metal-free borylation toolbox for accessing useful intermediates (ArylBPin) and novel B,N-PAHs.

Two Directing Groups Used for Metal Catalysed Meta-C-H Functionalisation Only Effect Ortho Electrophilic C-H Borylation.

Two templates used in meta-directed C-H functionalisation under metal catalysis do not direct meta-C-H borylation under electrophilic borylation conditions. Using BCl3 only Lewis adduct formation with Lewis basic sites in the template is observed. While combining BBr3 and the template containing an amide linker only led to amide directed ortho C-H borylation, with no pyridyl directed meta borylation. The amide directed borylation is selective for the ortho borylation of the aniline derived unit in the template, with no ortho borylation of the phenylacetyl ring - which would also form a six membered boracycle - observed. In the absence of other aromatics amide directed ortho borylation on to phenylacetyl rings can be achieved. The absence of meta-borylation using two templates indicates a higher barrier to pyridyl directed meta borylation relative to amide directed ortho borylation and suggests that bespoke templates for enabling meta-directed electrophilic borylation may be required.

Electrophilic C-H Borylation of Aza[5]helicenes Leading to Bowl-Shaped Quasi-[7]Circulenes with Switchable Dynamics.

The intrinsic relationship between helicenes and circulenes is of fundamental interest and importance in molecular engineering. Herein, electrophilic borylation of phenanthroline-derived aza[5]helicenes is presented, resulting in the incorporation of a boryl unit into two termini of helicenes to afford quasi-[7]circulenes. Their bowl-shaped structures were determined by X-ray diffraction. UV-vis absorption and fluorescence spectroscopy, as well as electrochemical measurements and DFT calculations, gave insight into their electronic properties. Variable-temperature NMR studies and DFT calculations revealed bowl-to-bowl inversion at room temperature and bowl-to-helix equilibria at elevated temperature, highlighting the important role of B ← N bond strength in tuning their dynamic properties.

Divergent Synthesis of Double Heterohelicenes as Strong Chiral Double Hydrogen-Bonding Donors.

We developed a very efficient and expandable divergent approach initiated by a direct electrophilic borylation at phenyl rings to synthesize a series of double heterohelicenes. Their π-extended structures with pristine zigzag nitrogen (N)-boron (B)-nitrogen (N) edges offer them substantial physical properties and strong double hydrogen-bond donating capability. The isolated (P,P) and (M,M) enantiomers exhibit circularly polarized luminescence in response to hydrogen-bonding interactions.

Stable, π-conjugated radical anions of boron–nitrogen dihydroindeno[1,2-b]fluorenes

We have recently reported the synthesis and application of boron–nitrogen dihydroindeno[1,2-b]fluorene derivatives as acceptors in organic photovoltaic devices. Their modest observed efficiencies may be related to the properties of their reduced congeners. In this work, we report two new members of this family of compounds prepared via the electrophilic borylation of 2,5-di- p-tolylpyrazine followed by an arylation of the boron center with ZnAr2 reagents. Two derivatives, 1 (Ar = 2,4,6-F3C6H2) and 2 (Ar = C6F5) were synthesized, and their radical anions, 1•− and 2•−, were formed via chemical reductions with CoCp*2 and CoCp2, respectively. Through comparison of structural parameters, as well as spectroscopic and computational data, the unpaired electron in the radical anions is localized in the planar core of the molecule, and dimerization is disfavored as a result. However, unlike the neutral starting materials, 1•− and 2•− are reactive toward ambient atmosphere. These observations suggest that the reduced compounds are stable toward intrinsic degradation pathways but subject to extrinsic degradation in device operation.

The selective regulation of borylation site based on one-shot electrophilic C–H borylation reaction, achieving highly efficient narrowband organic light-emitting diodes

The electrophilic borylation site of borylation reaction can be selectively controlled by varying the steric hindrance and electron-donating ability of the substrate substituent. The device based on the synthesized compound exhibits EQE of 21.6% with FWHM of 28 nm and CIE coordinates of (0.135, 0.094). The work provides an effective way for the development of high-performance MR-TADF emitter. • The electrophilic borylation site of one-shot electrophilic C–H borylation reaction can be selectively controlled. • New compounds tDPAC-BN and tDMAC-BN exhibited high PLQYs, narrow FWHMs, and satisfactory TADF properties. • The device based on tDPAC-BN achieved EQE of 21.6% with FWHM of 28 nm and CIE coordinates of (0.135, 0.094). Developing novel thermally activated delayed fluorescence (TADF) materials with a small full-width at half-maximum (FWHM) is very important for the fabrication of wide gamut and high-resolution displays of organic light-emitting diodes (OLEDs). In this work, we report two blue TADF emitters (tDPAC-BN and tDMAC-BN) with narrow FWHM based on a one-shot electrophilic C–H borylation reaction by selecting acridan-containing arylamine derivatives as the starting materials of borylation. The rigid skeleton structure significantly reduces vibrational motion, endowing tDPAC-BN and tDMAC-BN with high PLQYs (94.4% and 89.7%) and narrow FWHMs (19 nm and 26 nm in toluene). The electroluminescent devices employing tDPAC-BN and tDMAC-BN as emitters exhibit external quantum efficiency (EQE) of 21.6% and 22.3%, and CIE coordinates of (0.135, 0.094) and (0.116, 0.186), respectively. More importantly, we found the electrophilic borylation site of borylation reaction can be controlled by varying the steric hindrance effect and electron-donating ability of the substrate substituent. The different HOMO distribution originated from the differences in the substrate substituent accounts for different cyclization mode for tDPAC-BN and tDMAC-BN. Therefore, the work provides a useful strategy for broadening the range of high-performance TADF materials with narrow FWHM.

Linear Extension of Anthracene via B←N Lewis Pair Formation: Effects on Optoelectronic Properties and Singlet O2Sensitization

AbstractThe functionalization of polycyclic aromatic hydrocarbons (PAHs) via B←N Lewis pair formation offers an opportunity to judiciously fine‐tune the structural features and optoelectronic properties, to suit the demands of applications in organic electronic devices, bioimaging, and as sensitizers for singlet oxygen generation. We demonstrate that the N‐directed electrophilic borylation of 2,6‐di(pyrid‐2‐yl)anthracene offers access to linearly extended acene derivativesPy‐BR(R=Et, Ph, C6F5). In comparison to indeno‐fused 9,10‐diphenylanthracene, the formal “BN for CC” replacement inPy‐BRselectively lowers the LUMO, resulting in a much reduced HOMO–LUMO gap. An even more extended conjugated system with seven six‐membered rings in a row (Qu‐BEt) is obtained by borylation of 2,6‐di(quinolin‐8‐yl)anthracene. FluorinatedPy‐BPfshows particularly advantageous properties, including relatively lower‐lying HOMO and LUMO levels, strong yellow‐green fluorescence, and effective singlet oxygen sensitization, while resisting self‐sensitized conversion to its endoperoxide.

Linear Extension of Anthracene via B←N Lewis Pair Formation: Effects on Optoelectronic Properties and Singlet O2 Sensitization.

The functionalization of polycyclic aromatic hydrocarbons (PAHs) via B←N Lewis pair formation offers an opportunity to judiciously fine-tune the structural features and optoelectronic properties, to suit the demands of applications in organic electronic devices, bioimaging, and as sensitizers for singlet oxygen generation. We demonstrate that the N-directed electrophilic borylation of 2,6-di(pyrid-2-yl)anthracene offers access to linearly extended acene derivatives Py-BR (R=Et, Ph, C6 F5 ). In comparison to indeno-fused 9,10-diphenylanthracene, the formal "BN for CC" replacement in Py-BR selectively lowers the LUMO, resulting in a much reduced HOMO-LUMO gap. An even more extended conjugated system with seven six-membered rings in a row (Qu-BEt) is obtained by borylation of 2,6-di(quinolin-8-yl)anthracene. Fluorinated Py-BPf shows particularly advantageous properties, including relatively lower-lying HOMO and LUMO levels, strong yellow-green fluorescence, and effective singlet oxygen sensitization, while resisting self-sensitized conversion to its endoperoxide.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds.

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)2 P→BBr2 + species.

Metal‐Free Phosphorus‐Directed Borylation of C(sp2)−H Bonds

AbstractSpectacular progress has recently been achieved in transition metal‐catalyzed C−H borylation of phosphines as well as directed electrophilic C−H borylation. As shown here, P‐directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine–boranes. It operates under metal‐free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N‐phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post‐derivatizing the borane moiety). NMR monitoring supports the involvement of P‐stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π‐arene/boron interactions in the (biphenyl)(i‐Pr)2P→BBr2+ species.

A Modular Cascade Synthetic Strategy Toward Structurally Constrained Boron-Doped Polycyclic Aromatic Hydrocarbons.

A novel synthetic strategy was developed for the construction of difficult‐to‐access structurally constrained boron‐doped polycyclic aromatic hydrocarbons (sc‐B‐PAHs) via a cascade reaction from the readily available ortho‐aryl‐substituted diarylalkynes. This domino process involves borylative cyclization, 1,4‐boron migration and successive two‐fold electrophilic borylation. Two types of sc‐B‐PAHs bearing B‐doped [4]helicene (1 a‐1 i) or BN‐doped [4]helicene (1 n‐1 t) and double [4]helicene (1 u‐1 v) are constructed by this cascade reaction. Remarkably, this synthetic strategy is characterized by modest yields (20–50 %) and broad substrate scope (18 examples) with versatile functional group tolerance. The resultant sc‐B‐PAHs show good stability under ambient conditions and are thoroughly investigated by X‐ray crystallography, UV/Vis absorption and fluorescence spectroscopy, and cyclic voltammetry. Interestingly enough, BN‐doped [4]helicene 1 o forms a unique alternating π‐stacked dimer of enantiomers within a helical columnar superstructure, while BN‐doped double [4]helicene 1 u establishes an unprecedented π‐stacked trimeric sandwich structure with a rare 2D lamellar π‐stacking. The synthetic approach reported herein represents a powerful tool for the rapid generation of novel sc‐B‐PAHs, which are highly attractive for the elucidation of the structure‐property relationship and for potential optoelectronic applications.

A Modular Cascade Synthetic Strategy Toward Structurally Constrained Boron‐Doped Polycyclic Aromatic Hydrocarbons

AbstractA novel synthetic strategy was developed for the construction of difficult‐to‐access structurally constrained boron‐doped polycyclic aromatic hydrocarbons (sc‐B‐PAHs) via a cascade reaction from the readily available ortho‐aryl‐substituted diarylalkynes. This domino process involves borylative cyclization, 1,4‐boron migration and successive two‐fold electrophilic borylation. Two types of sc‐B‐PAHs bearing B‐doped [4]helicene (1 a‐1 i) or BN‐doped [4]helicene (1 n‐1 t) and double [4]helicene (1 u‐1 v) are constructed by this cascade reaction. Remarkably, this synthetic strategy is characterized by modest yields (20–50 %) and broad substrate scope (18 examples) with versatile functional group tolerance. The resultant sc‐B‐PAHs show good stability under ambient conditions and are thoroughly investigated by X‐ray crystallography, UV/Vis absorption and fluorescence spectroscopy, and cyclic voltammetry. Interestingly enough, BN‐doped [4]helicene 1 o forms a unique alternating π‐stacked dimer of enantiomers within a helical columnar superstructure, while BN‐doped double [4]helicene 1 u establishes an unprecedented π‐stacked trimeric sandwich structure with a rare 2D lamellar π‐stacking. The synthetic approach reported herein represents a powerful tool for the rapid generation of novel sc‐B‐PAHs, which are highly attractive for the elucidation of the structure‐property relationship and for potential optoelectronic applications.

Construction of 1,2,3-Benzodiazaborole by Electrophilic Borylation of Azobenzene and Nucleophilic Dialkylative Cyclization

1,2,3-Benzodiazaboroles can be conveniently prepared from azobenzenes by a two-step protocol involving electrophilic ortho-borylation with BBr3 and dialkylative cyclization with the Grignard reagent. The methodology provides a diverse range of products equipped with functionalities from azobenzenes containing substituents (Me, t-Bu, F, Cl, Br, I, and OCF3) and a series of Grignard reagents (alkyl- and arylmagnesium reagents). Moreover, this study displays the moderate aromaticity of the B-N-N-containing five-membered ring and mechanistic investigations of the cyclization reaction.