Electrophilic aromatic substitution of a BN indole

N-Directed Two-Fold Bromoboration of Diynes Enables Access to Brominated BN-Embedded PAHs

N-directed 2-fold bromoboration reactions of diynes with BBr3 have been developed, allowing the access to novel internally BN-doped polycyclic aromatic hydrocarbons from readily available precursors under mild conditions. Computational investigations identified three potential reaction mechanisms, each involving either BBr3 or [BBr4]-, with low activation barriers (ΔG‡ < 16 kcal/mol) for all pathways. The resulting brominated products can be further functionalized through various cross-coupling protocols, enabling the synthesis of highly luminescent emitters with quantum yield exceeding 90.

From propenolysis to enyne metathesis: tools for expedited assembly of 4a,8a-azaboranaphthalene and extended polycycles with embedded BN

The synthesis of BN-containing molecules, which have an interesting isosteric relationship to their parent all-C cores, has drawn a great deal of attention as an avenue to alter and tune molecular function. Nevertheless, many cores with embedded BN are still hard to synthesize, and thus, further effort is required in this direction. Herein, we present an integrated approach to BN-containing polycycles rooted in an exceptionally clean B-N condensation of amines with a tri-allylborane. Having released propene as the only byproduct, the resulting BN precursors are seamlessly telescoped into BN-containing polycyclic cores via a set of additional methodologies, either developed here ad-hoc or applied for the first time for the synthesis of BN-cycles. As the "sharpening stone" of the process, BN-embedded naphthalene, which has previously only been obtained in low yield, can now be synthesized efficiently through propenolysis, ring-closing metathesis and a new high-yielding aromatization. As a more advanced application, an analogously obtained BN-containing bis-enyne is readily converted to BN-containing non-aromatic tetra-, penta- and hexacyclic structures via ring-closing enyne metathesis, followed by the Diels-Alder cycloaddition. The resulting air-sensitive structures are easily handled by preventive hydration (quaternization) of their B-N bridge; reverting this hydration restores the original Bsp2-Nsp2 structure. In the future, these structures may pave the way to BN-anthracenes and other π-extended BN-arenes.

Gad S, M. El-Mehasseb I, E. El-Kelany K. Isosterism in pyrrole via azaboroles substitution, a theoretical investigation for electronic structural, stability and aromaticity

This work uses ab-initio CBS-QB3 and density functional theory (B3LYP) to analyze the structure, stability, and aromaticity of all isosteric nitrogen-boron pyrroles. The mono-NB unit substituted group of the isosteric NB pyrrole has four isosteres, whereas the multi-NB unit substituted group has two isosteres. These two groups make up all isosteric NB pyrrole. For structural, energetic, magnetic, and electron delocalization criteria, the results highlight the predominance of the PN3B2 isostere and its greater stability over other conformers. In addition, the global reactivity indices, ESP, HOMO-LUMO, and NBO charges have all been estimated to forecast the active side's electron donation and acceptance. These isosteres are categorized as weak electrophiles and marginal nucleophiles. NB-isosteres have poorer stability, HOMO-LUMO gap, and aromaticity than the parent (pyrrole). In general, NB compounds with more ring sharing are less aromatic than NB molecules with less ring sharing. The current study is anticipated to help in understanding of the chemistry of NB substituted molecules and their experimental identification and characterization.

Wittig/B─H insertion reaction: A unique access to trisubstituted Z-alkenes

The Wittig reaction, which is one of the most effective methods for synthesizing alkenes from carbonyl compounds, generally gives thermodynamically stable E-alkenes, and synthesis of trisubstituted Z-alkenes from ketones presents notable challenges. Here, we report what we refer to as Wittig/B─H insertion reactions, which innovatively combine a Wittig reaction with carbene insertion into a B─H bond and constitute a promising method for the synthesis of thermodynamically unstable trisubstituted Z-boryl alkenes. Combined with the easy transformations of boryl group, this methodology provides efficient access to a variety of previously unavailable trisubstituted Z-alkenes and thus provides a platform for discovery of pharmaceuticals. The unique Z-selectivity of the reaction is determined by the maximum overlap of the orbitals between the B─H bond of the borane adduct and the alkylidene carbene intermediate in the transition state.

Minor bioactive indoles from kimchi mirror the regioselectivity in indole-3-carbinol oligomerization

Kimchi is a globally consumed food with diverse health-benefits, but the low-abundance bioactive compounds in kimchi remain largely neglected. Here we show that kimchi contains a family of low-abundance (0.5-1.6 μg/g, dried weight) high-order indole oligomers derived from indole-3-carbinol (I3C), a breakdown product released from cruciferous vegetables used for producing the traditional subsidiary food. The structure determination of such complex molecules was accomplished by synthesizing linear indole oligomers as standard materials followed by the LC-HR-MS analysis. One indole tetramer (LTe2) is substantially toxic to tumor MV4-11 (IC₅₀ = 1.94 μM) and THP-1 cells (IC₅₀ = 7.12 μM). Collectively, the work adds valuable information to the knowledge package about kimchi, and may inspire the generation of indole-based molecules, to which many drugs belong.

Three-Component Reaction to 1,4,2-Diazaborole-Type Heteroarene Systems

The borane FmesBH₂ reacts in a three-component reaction with an isonitrile and a small series of organonitriles to give rare examples of the class of dihydro-1,4,2-diazaborole derivatives. In a related way, annulated BN-indolizine derivatives became conveniently available, as were dihydro-1,4,2-oxaza- or thiazaborole derivatives. The nucleophilic framework of a dihydro-1,4,2-diazaborole example allowed for an uncatalyzed acylation reaction. It also served as a 1,3-dipolar reagent and underwent a [3+2] cycloaddition/[4+2] cycloreversion sequence when treated with methyl propiolate to give the respective pyrrole product. The [3+2] cycloaddition product of a dihydro-1,4,2-diazaborole derivative with N-phenylmaleimide was isolated and its heterobicyclo[2.2.1]heptane derived structure characterized by X-ray diffraction.

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.

Chiral Phosphoric Acid-Catalyzed Remote Control of Axial Chirality at Boron-Carbon Bond

The previously elusive catalytic enantioselective construction of axially chiral B-aryl-1,2-azaborines with a C-B stereogenic axis has been realized through a chiral phosphoric acid-catalyzed desymmetrization strategy reported herein. The electrophilic aromatic substitution reaction of 3,5-disubsituted phenols with diazodicarboxamides could afford these axially chiral structures in good efficiency with excellent enantiocontrol. The efficient long-range stereochemical control is achieved by multiple well-defined H-bonding interactions between chiral phosphoric acid and both substrates. Meanwhile, the reaction duration could be markedly shortened with weakly acidic N-H in 1,2-azaborine acting as H-bond donor. The scalability of the reaction and facile cleavage of the N-N bond in the product further demonstrated the practicality of this method.

Multi-component synthesis of dihydro-1,3-azaborinine derived oxindole isosteres

We report on the synthesis of the first examples of 1,3-azaborinine derived oxindole systems, the BN-isosteres of the important compound class of the oxindoles. Hydroboration of terminal aryl acetylenes with FmesBH2, followed by treatment with 2 equiv. of a glycine ester derived isonitrile gave a small series of 1,3-azaborinine derived oxindoles. The new BN-oxindoles show interesting photophysical behavior.

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.

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

The isoelectronic replacement of C[double bond, length as m-dash]C bonds with -B[double bond, length as m-dash]N+ bonds in polycyclic aromatic hydrocarbons (PAHs) is a widely used tool to prepare novel optoelectronic materials. Far less well explored are corresponding B,O-doped PAHs, although they have a similarly high application potential. We herein report on the modular synthesis of B,N- and B,O-doped PAHs through the [Au(PPh3)NTf2]-catalyzed 6-endo-dig cyclization of BN-H and BO-H bonds across suitably positioned C[triple bond, length as m-dash]C bonds in the key step. Readily available, easy-to-handle o-alkynylaryl boronic and borinic acids serve as starting materials, which are either cyclized directly or first converted into the corresponding aminoboranes and then cyclized. The reaction even tolerates bulky mesityl substituents on boron, which later kinetically protect the formed B,N/O-PAHs from hydrolysis or oxidation. Our approach is also applicable for the synthesis of rare doubly B,N/O-doped PAHs. Specifically, we prepared 1,2-B,E-naphthalenes and -anthracenes, 1,5-B2-2,6-E2-anthracenes (E = N, O) as well as B,O2-containing and unprecedented B,N,O-containing phenalenyls. Selected examples of these compounds have been structurally characterized by X-ray crystallography; their optoelectronic properties have been studied by cyclic voltammetry, electron spectroscopy, and quantum-chemical calculations. Using a new unsubstituted (B,O)2-perylene as the substrate for late-stage functionalization, we finally show that the introduction of two pinacolatoboryl (Bpin) substituents is possible in high yield and with perfect regioselectivity via an Ir-catalyzed C-H borylation approach.

Cation-π binding ability of BN indole

A BN indole-containing aromatic scaffold has been synthesized and the cation-π binding ability characterized by nuclear magnetic resonance (NMR) monitored titrations. The resulting chemical shifts were analyzed using a non-linear curve fitting procedure and the extracted association constants (Ka's) compared with the natural indole scaffold. Computations were also performed to support our findings. This work shows that incorporation of a B-N bond in place of a C-C bond in an aromatic system slightly lowers the cation-π binding ability of the arene's π-system with simple cations.

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.

Cascade Dehydrogenative Hydroboration for the Synthesis of Azaborabenzofulvenes

Tandem dehydrogenative hydroboration has been established to be highly effective in the synthesis of BN isosteres of benzofulvene and derivatives. The scope of this synthetic method is applicable to a variety of substrates. Spectroscopic and computational studies indicate that the new azaborabenzofulvenes have similar electronic properties as their carbonaceous analogues.

Microwave Spectra, Structure, and the Aromatic Character of 1-Chloroborepin

High resolution microwave spectra for the somewhat unstable compound 1-chloroborepin were measured in the 5-10 GHz range using a pulsed beam Fourier transform microwave spectrometer. Transitions were assigned and measured for three isotopologues, which include the most abundant isotopologue, ¹¹B³⁵Cl, and the less abundant ¹⁰B³⁵Cl and ¹¹B³⁷Cl isotopologues. The molecular parameters (MHz) determined for the ¹¹B³⁵Cl isotopologue are A = 3490.905(35), B = 1159.38520(79), C = 870.59492(56), 1.5χ_aa (¹¹B) = -0.220(22), 0.25(χ_bb - χ_cc) (¹¹B) = -1.5300(99), 1.5χ_aa (³⁵Cl) = -54.572(33), and 0.25(χ_bb - χ_cc) (³⁵Cl) = 4.7740(79). The inertial defect is calculated to be Δ = -0.174 amu Ų from the experimental rotational constants, indicating a planar structure with some out of plane vibrational motion. An extended Townes-Dailey analysis was performed on the ¹¹B and ³⁵Cl nuclei to determine the electron occupations in the valence hybridized orbitals using the experimental quadrupole coupling strengths. From the analysis it was determined that Cl is sharing some electron density with the empty p-orbital on B. The B-Cl bond length determined from the data is 1.798(1) Å, and the B-C bond lengths are 1.533(10) Å. The structural parameters and electronic structure properties of 1-chloroborepin are consistent with an aromatic boron-containing molecule.

Synthesis, Optical Properties, and Regioselective Functionalization of 4a-Aza-10a-boraphenanthrene

4a-Aza-10a-boraphenanthrene has been synthesized in only four steps from commercially available materials with a remarkable overall yield of 62%. In contrast to other BN-isosteres of phenathrene, this isomer is weakly fluorescent, which has been explained by means of computational studies that found a low energy conical intersection for the nonradiative deactivation of the excited state. Moreover, a completely regioselective functionalization of 4a-aza-10a-boraphenanthrene at C-1 by reaction with activated electrophiles has been achieved.

Microwave spectra, molecular structure, and aromatic character of 4a,8a-azaboranaphthalene

The microwave spectra for seven unique isotopologues of 4a,8a-azaboranaphthalene [hereafter referred to as BN-naphthalene] were measured using a pulsed-beam Fourier transform microwave spectrometer. Spectra were obtained for the normal isotopologues with (10)B, (11)B, and all unique single (13)C and the (15)N isotopologue (with (11)B), in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the (11)B(14)N isotopologue are A = 3042.712 75(43) MHz, B = 1202.706 57(35) MHz, C = 862.220 13(35) MHz, DJ = 0.06(1) kHz, 1.5χaa ((14)N) = 2.5781(61) MHz, 0.25(χbb - χcc) ((14)N) = - 0.1185(17) MHz, 1.5χaa (11B) = - 3.9221(75) MHz, and 0.25(χbb - χcc) ((11)B) = - 0.9069(24) MHz. The experimental inertial defect is Δ = - 0.159 amu Å(2), which is consistent with a planar structure for the molecule. The B-N bond length from the experimentally determined structure is 1.47 Å, which indicates π-bonding character between the B and N. The measured quadrupole coupling strengths provide important and useful information about the bonding, orbital occupancy, and aromatic character for this aromatic molecule. Extended Townes-Dailey analyses were used to determine the B and N electron sp(2)-hybridized and p-orbital occupations. These results are compared with electron orbital occupations from the natural bond orbital option in theoretical calculations. From the analyses, it was determined that BN-naphthalene has aromatic character similar to that of other N-containing aromatics. The results are compared with similar results for B-N bonding in 1,2-dihydro-1,2-azaborine and BN-cyclohexene. Accurate and precise structural parameters were obtained from the microwave measurements on seven isotopologues and from high-level G09 calculations.

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes

Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η(4)-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B-C bond coupling en route to the rhodium η(4)-1,2-azaborete complex and the selective oxidative cleavage of the B-N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene.

Design of Small Intramolecular Singlet Fission Chromophores: An Azaborine Candidate and General Small Size Effects

We report the first attempt to design small intramolecular singlet fission chromophores, with the aid of quantum chemistry and explicitly simulating the time evolution of state populations using quantum dynamics method. We start with three previously proposed azaborine-substituted intermolecular singlet fission chromophores. Through analyzing their frontier orbital amplitudes, we select a BN-substituted azulene as the building block. Covalently connecting two such monomers and tuning their relative configuration, we examine three dimers. One dimer is found to be an eminent candidate: the triplet-pair state is quickly formed within 1 ps, and the two triplets are ready to be disentangled. We elucidate the general small size effects in intramolecular singlet fission and focus on specific aspects which should be taken care of when manipulating the fission rate through steric hindrance.

Seeking small molecules for singlet fission: a heteroatom substitution strategy

We design theoretically small molecule candidates for singlet fission chromophores, aiming to achieve a balance between sufficient diradical character and kinetic persistence. We develop a perturbation strategy based on the captodative effect to introduce diradical character into small π-systems. Specifically, this can be accomplished by replacing pairs of not necessarily adjacent C atoms with isoelectronic and isosteric pairs of B and N atoms. Three rules of thumb emerge from our studies to aid further design: (i) Lewis structures provide insight into likely diradical character; (ii) formal radical centers of the diradical must be well-separated; (iii) stabilization of radical centers by a donor (N) and an acceptor (B) is essential. Following the rules, we propose candidate molecules. Employing reliable multireference calculations for excited states, we identify three likely candidate molecules for SF chromophores. These include a benzene, a napthalene, and an azulene, where four C atoms are replaced by a pair of B and a pair of N atoms.

UV-photoelectron spectroscopy of BN indoles: experimental and computational electronic structure analysis

We present a comprehensive electronic structure analysis of two BN isosteres of indole using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of external BN indole I and fused BN indole II have been recorded, assessed by density functional theory calculations, and compared with natural indole. The first ionization energies of these indoles are natural indole (7.9 eV), external BN indole I (7.9 eV), and fused BN indole II (8.05 eV). The computationally determined molecular dipole moments are in the order: natural indole (2.177 D) > fused BN indole II (1.512 D) > external BN indole I (0.543 D). The λ_max in the UV–vis absorption spectra are in the order: fused BN indole II (292 nm) > external BN indole I (282 nm) > natural indole (270 nm). The observed relative electrophilic aromatic substitution reactivity of the investigated indoles with dimethyliminium chloride as the electrophile is as follows: fused BN indole II > natural indole > external BN indole I, and this trend correlates with the π-orbital coefficient at the 3-position. Nucleus-independent chemical shifts calculations show that the introduction of boron into an aromatic 6π-electron system leads to a reduction in aromaticity, presumably due to a stronger bond localization. Trends and conclusions from BN isosteres of simple monocyclic aromatic systems such as benzene and toluene are not necessarily translated to the bicyclic indole core. Thus, electronic structure consequences resulting from BN/CC isosterism will need to be evaluated individually from system to system.

Rhodium-catalyzed B-H activation of 1,2-azaborines: synthesis and characterization of BN isosteres of stilbenes

The first example of catalytic B-H activation of azaborines leading to a new family of stilbene derivatives through dehydrogenative borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes have been synthesized using this method, including a BN isostere of a biologically active stilbene. It is demonstrated that BN/CC isosterism in the context of stilbenes can lead to significant changes in the observed photophysical properties such as higher quantum yield and a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart 6g is consistent with a stronger charge-transfer character of the excited state exhibited by 3g in which the 1,2-azaborine heterocycle serves as a better electron donor than the corresponding arene.

Protecting group-free synthesis of 1,2-azaborines: a simple approach to the construction of BN-benzenoids

The protecting group-free synthesis of a versatile 1,2-azaborine synthon 5 is described. Previously inaccessible 1,2-azaborine derivatives, including the BN isostere of phenyl phenylacetate and BN1 triphenylmethane were prepared from 5 and characterized. The structural investigation of BN phenyl phenylacetate revealed the presence of a unique NH-carbonyl hydrogen bond that is not present in the corresponding carbonaceous analogue. The methyne CH in BN triphenylmethane was found to be less acidic than the corresponding proton in triphenylmethane. The gram-quantity synthesis of the parent 1,2-azaborine 4 was demonstrated, which enabled the characterization of its boiling point, density, refractive index, and its polarity on the ET(30) scale.

BN isosteres of indole

Indole is a heterocycle of great importance to biological systems and materials applications. Synthesis of indole and its derivatives has been a major focus of research for over a century. BN/CC isosterism is an emerging strategy for expanding the structural diversity of indole-based compounds. Two classes of BN indoles have been reported to date: the well-studied "external" BN indoles (or 1,3,2-benzodiazaborolines), and the recently reported "fused" BN indoles. This perspective presents the history of both classes of indole isosteres, with a general overview of their synthesis, functionalization, and properties.

A polycyclic borazine radical cation: [1,2-B2{1,2-(MeN)2C6H4}2]˙+

The radical cation, [1,2-B₂{1,2-(MeN)₂C₆H₄}₂]˙⁺, has been prepared and characterised by X-ray crystallography, ESR and DFT calculations.

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.

A 1,3-dihydro-1,3-azaborine debuts

We present the first synthesis and characterization of a 1,3-dihydro-1,3-azaborine, a long-sought BN isostere of benzene. 1,3-Dihydro-1,3-azaborine is a stable structural motif with considerable aromatic character as evidenced by structural analysis and its reaction chemistry. Single crystal X-ray analysis indicates bonding consistent with significant electron delocalization. 1,3-Dihydro-1,3-azaborines also undergo nucleophilic substitutions at boron and electrophilic aromatic substitution reactions. In view of the versatility and impact of aromatic compounds in the biomedical field and in materials science, the present study further expands the available chemical space of arenes via BN/CC isosterism.

Density functional theoretical investigation of the aromatic nature of BN substituted benzene and four ring polyaromatic hydrocarbons

We have studied the topological and local aromaticity of BN-substituted benzene, pyrene, chrysene, triphenylene and tetracene molecules. The nucleus-independent chemical shielding (NICS), harmonic oscillator model of aromaticity (HOMA), para-delocalization index (PDI) and aromatic fluctuation index (FLU) have been calculated to quantify aromaticity in terms of magnetic and structural criteria. We find that charge separations due to the introduction of heteroatoms largely affect both the local and topological aromaticity of these molecules. Our studies show that the presence of any kind of heteroatom in the ring not only reduces the local delocalization in the six membered ring, but also affects strongly the topological aromaticity. In fact, the relative orders of the topological and local aromaticity depend strongly on the position of the heteroatoms in the structure. In general, more ring shared BN containing molecules are less aromatic than the less ring shared BN molecules. In addition our results provide evidence that the structural stability of the molecule is dominated by the σ bond rather than the π bond.