Palladium-catalyzed cross-coupling reactions of 4a,8a-azaboranaphthalene

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.

Impact of isoelectronic substitution on the excited state processes in polycyclic aromatic hydrocarbons: a joint experimental and theoretical study of 4a,8a-azaboranaphthalene

Substituting CC with the isoelectronic BN units is a promising approach to modify the optoelectronic properties of polycyclic aromatic hydrocarbons. While computational studies have already addressed trends in the electronic structure of the various isosteres, experimental data are still scarce. Here, the excited state spectroscopy and dynamics of 4a,8a-azaboranaphthalene were studied by picosecond time-resolved photoionization in a supersonic jet and analyzed with the aid of XMS-CASPT2 and time-dependent DFT calculations. A resonance-enhanced multiphoton ionization spectrum (REMPI) reveals the S1 origin at  = 33 830 ± 12 cm-1. Several vibrational bands were resolved and assigned by comparison with the computations. A [1+1] photoelectron spectrum via the S1 origin yielded an adiabatic ionization energy of 8.27 eV. Selected vibrational bands were subsequently investigated by pump-probe photoionization. While the origin as well as several low-lying vibronic states exhibit lifetimes in the ns-range, a monoexponential decay is observed at higher excitation energies, ranging from 400 ps at +1710 cm-1 to 13 ps at +3360 cm-1. The deactivation is attributed to an internal conversion of the optically excited S1 state via a barrier that gives access to a conical intersection (CI) to the S0 state. The doping significantly changes the energetic ordering of CIs and lowers the corresponding energy barrier for the associated deactivation pathway, as revealed by nudged elastic band (NEB) calculations.

Copper-catalyzed nitration of electron-deficient BN-naphthalene

Under Cu-catalysis, a regioselective nitration of 1,8-dihalogenated BN-naphthalene (ABN) compounds (4a-4c) has been established with the use of tert-butyl nitrite as the nitrating reagent. The syntheses of dihalo-ABN nitro products (6a-6c; halo = Cl, Br and I) were case-studied in conjunction with the first synthesis and characterization of diiodo-ABN compound 4c. The molecular structures of these compounds have been spectroscopically characterized and further confirmed by X-ray single crystal diffraction experiments. This method allows direct regioselective nitration of electron-deficient ABN systems, providing a step-economical entry to novel nitro-ABN structural motifs with potential applications in agrochemicals, materials sciences, and the medicinal and pharmaceutical industries.

Synthetic Doping of Acenaphthylene through BN/CC Isosterism and a Direct Comparison with BN-Acenaphthene

Boron/nitrogen-doped acenaphthylenes, a new class of BN-doped cyclopenta-fused polycyclic aromatic hydrocarbons, were synthesized via indole-directed C-H borylation. The reference molecule BN-acenaphthene was also synthesized in a similar manner. Both BN-acenaphthylene and BN-acenaphthene were unequivocally characterized by single-crystal X-ray analysis. The aromaticities of each ring in BN-acenaphthylenes were quantified by experimental and theoretical methods. Moreover, doping the BN unit into acenaphthylene can increase the LUMO level and decrease the HOMO level, resulting in wider HOMO-LUMO energy gaps. Furthermore, regioselective bromination of BN-acenaphthylene (B-Mes) afforded monobrominated BN-acenaphthylene in good yield. Subsequently, cross-coupling of brominated BN-acenaphthylene gave a series of BN-acenaphthylene derivatives. In addition, the photophysical properties of these BN-acenaphthylene derivatives can be fine-tuned by the substituents on the BN-acenaphthylene scaffold.

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.

"Spine Surgery" of Perylene Diimides with Covalent B-N Bonds toward Electron-Deficient BN-Embedded Polycyclic Aromatic Hydrocarbons

BN-embedded polycyclic aromatic hydrocarbons (PAHs) with unique optoelectronic properties are underdeveloped relative to their carbonaceous counterparts due to the lack of suitable and facile synthetic methods. Moreover, the dearth of electron-deficient BN-embedded PAHs further hinders their application in organic electronics. Here we present the first facile synthesis of novel perylene diimide derivatives (B₂N₂-PDIs) featuring n-type B-N covalent bonds. The structures of these compounds are fully confirmed through the detailed characterizations with NMR, MS, and X-ray crystallography. Further investigation shows that the introduction of BN units significantly modifies the photophysical and electronic properties of these B₂N₂-PDIs and is further understood with the aid of theoretical calculations. Compared with the parent perylene diimides (PDIs), B₂N₂-PDIs exhibit deeper highest occupied molecular orbital energy levels, new absorption peaks in the high-energy region, hypsochromic shift of absorption and emission maxima, and decrement of photoluminescent quantum yields. Single-crystal field-effect transistors based on B₂N₂-PDIs showcase an electron mobility up to 0.35 cm² V^-1 s^-1, demonstrating their potential application in optoelectronic materials.

BN-Embedded Perylenes

Transition metal catalyzed coupling reaction strategy has been utilized in the synthesis of two novel BN-perylenes starting from halogenated BN-naphthalene derivatives. The molecular structures and packing modes of BN-perylenes were confirmed by NMR spectroscopy and X-ray single-crystal diffraction experiments. Their photophysical properties were further investigated using UV-vis and fluorescence spectroscopy and DFT calculations. Interestingly, the isosteric BN-insertion in perylene system resulted in stronger π-π stacking interaction both in solid and solution phases. The synthesized BN-perylenes are proved to be highly stable and thus provide a new valuable platform for novel organic materials applications which is otherwise inaccessible to date.

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.

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.

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.

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.

Regioselective Functionalization of Stable BN-Modified Luminescent Tetraphenes for High-Resolution Fingerprint Imaging

A series of novel BN tetraphene derivatives have been prepared successfully for the first time via a post-functionalization strategy. The optical and electronic properties of these derivatives could be tuned systematically by the incorporation of different substituents on the main skeleton. The functionalized BN-containing luminogens have been explored for the detection of latent fingerprints (LFPs) on different substrates, including glass, aluminum foil, plastic, and ironware. This strategy provides great versatility in LFP imaging and good potential in elucidating the chemical information within LFPs, making the strategy valuable in forensic investigations.

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.

Expanding the functional group tolerance of cross-coupling in 1,2-dihydro-1,2-azaborines: installation of alkyl, alkenyl, aryl, and heteroaryl substituents while maintaining a B-H bond

Palladium-catalyzed Negishi cross-coupling of 3-bromo-1-(tert-butyldimethylsilyl)-1,2-dihydro-1,2-azaborine while maintaining the B-H functionality has been demonstrated. 17 examples, including dialkylzinc, alkyl-, alkenyl-, aryl-, as well as nitrogen-, sulfur-, and oxygen-containing heteroaryl-zinc halide reagents have been coupled to generate new C(3) substituted 1,2-azaborines in moderate to excellent yields.

Late-stage functionalization of BN-heterocycles

BN/CC isosterism has emerged as a viable strategy to expand the chemical space of organic molecules. In particular, the application of BN/CC isosterism to arenes has received significant attention due to the vast available chemical space provided by aromatic hydrocarbons. The synthetic efforts directed at assembling novel aromatic BN heterocycles have resulted in the discovery of new properties and functions in a variety of fields including biomedical research, medicinal chemistry, materials science, catalysis, and organic synthesis. This tutorial review specifically covers recent advances in synthetic technologies that functionalize assembled boron-nitrogen (BN) heterocycles and highlights their distinct reactivity and selectivity in comparison to their carbonaceous counterparts. It is intended to serve as a state-of-the-art compendium for readers who are interested in the reaction chemistry of BN heterocycles.

C-H Functionalization of BN-Aromatics Promoted by Addition of Organolithium Compounds to the Boron Atom

Addition of an organolithium compound to a BN-phenanthrene with embedded B and N atoms is proposed to result in coordination of RLi to the boron atom. This coordination, supported by NMR spectroscopy and DFT calculations, increases the nucleophilicity of the system in the β position to the N atom and is therefore a useful tool for promoting regioselective C-H functionalization of BN aromatics.

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.

Synthesis of functionalized helical BN-benzo[c]phenanthrenes

A novel parent BN-benzo[c]phenanthrene, with helical chirality and remarkable structural features, has been easily obtained in three steps with a global yield of 55%.

Friedel-Crafts Acylation Reactions of BN-Substituted Arenes

The acylation reaction of BN-arenes has been studied using BN-arene and acyl chloride in good to excellent yields, which led to the first synthesis of indanone BN-analogue. The BN-aromatic ketone products have been characterized by ¹H NMR spectroscopy with their molecular structures unambiguously confirmed by X-ray crystallography. The annulation reaction of BN-arenes promoted by AgBF₄ occurs in a completely regioselective manner and a mechanism for this transformation is proposed.

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.

The synthesis of BN-embedded tetraphenes and their photophysical properties

A series of intensely blue fluorescent BN-embedded tetraphene derivatives have been synthesized by the catalytic cyclization of BN-naphthalenes with alkynes.

Benzazaborinines as Novel Bioisosteric Replacements of Naphthalene: Propranolol as an Example

Two benzazaborinine analogues of propranolol were synthesized and extensively profiled in vitro and in vivo. These analogues showed potency and physicochemical and in vitro ADME-tox profiles comparable to propranolol. In addition, both benzazaborinine analogues showed excellent bioavailability and brain penetration following subcutaneous administration in a pharmacokinetic study in rats. These studies unveil the potential of aromatic azaborinines as bioisosteric replacements of naphthalene in drug discovery programs.

0-0 Energies Using Hybrid Schemes: Benchmarks of TD-DFT, CIS(D), ADC(2), CC2, and BSE/GW formalisms for 80 Real-Life Compounds

The 0-0 energies of 80 medium and large molecules have been computed with a large panel of theoretical formalisms. We have used an approach computationally tractable for large molecules, that is, the structural and vibrational parameters are obtained with TD-DFT, the solvent effects are accounted for with the PCM model, whereas the total and transition energies have been determined with TD-DFT and with five wave function approaches accounting for contributions from double excitations, namely, CIS(D), ADC(2), CC2, SCS-CC2, and SOS-CC2, as well as Green's function based BSE/GW approach. Atomic basis sets including diffuse functions have been systematically applied, and several variations of the PCM have been evaluated. Using solvent corrections obtained with corrected linear-response approach, we found that three schemes, namely, ADC(2), CC2, and BSE/GW allow one to reach a mean absolute deviation smaller than 0.15 eV compared to the measurements, the two former yielding slightly better correlation with experiments than the latter. CIS(D), SCS-CC2, and SOS-CC2 provide significantly larger deviations, though the latter approach delivers highly consistent transition energies. In addition, we show that (i) ADC(2) and CC2 values are extremely close to each other but for systems absorbing at low energies; (ii) the linear-response PCM scheme tends to overestimate solvation effects; and that (iii) the average impact of nonequilibrium correction on 0-0 energies is negligible.

Negishi Cross-Coupling Is Compatible with a Reactive B-Cl Bond: Development of a Versatile Late-Stage Functionalization of 1,2-Azaborines and Its Application to the Synthesis of New BN Isosteres of Naphthalene and Indenyl

The compatibility of the Negishi cross-coupling reaction with the versatile B-Cl functionality has been demonstrated in the context of late-stage functionalization of 1,2-azaborines. Alkyl-, aryl-, and alkenylzinc reagents have been utilized for the functionalization of the triply orthogonal precursor 3-bromo-1-(tert-butyldimethylsilyl)-2-chloro-1,2-dihydro-1,2-azaborine (2) to furnish new 2,3-substituted monocyclic 1,2-azaborines. This methodology has enabled the synthesis of previously elusive BN-naphthalene and BN-indenyl structures from a common intermediate.

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.

Late-Stage Functionalization of 1,2-Dihydro-1,2-azaborines via Regioselective Iridium-Catalyzed C-H Borylation: The Development of a New N,N-Bidentate Ligand Scaffold

The first general late-stage functionalization of monocyclic 1,2-azaborines at the C(6) position is described. Ir-catalyzed C-H borylation occurs regioselectively at the C(6) position of B-substituted 1,2-azaborines and is compatible with a range of substitution patterns at boron (e.g., hydride, alkoxide, alkyl, and aryl substituents). Subsequent Suzuki cross coupling with aryl- and heteroaryl bromides furnishes 1,2-azaborine-based biaryl compounds including 6-[pyrid-2-yl]-1,2-azaborines that represent novel κ(2)-N,N-bidentate ligands. The 6-[pyrid-2-yl]-B-Me-1,2-azaborine ligand has been demonstrated to form an emissive coordination complex with dimesitylboron that exhibits bathochromically shifted absorption and emission maxima and a higher photoluminescence quantum yield compared to its carbonaceous analogue.

4a,8a-Azaboranaphthalene-4-yl phosphine ligands: synthesis and electronic modulation in Suzuki–Miyaura coupling reactions

Design, synthesis and characterization of new BN-aromatic phosphine ligands are presented, demonstrating moderate catalytic activity in Suzuki reactions of C–Cl bond.

Unexpected and powerful effect of chlorobenzene in direct palladium-catalyzed cascade Sonogashira–hydroarylation reaction

A ubiquitous accelerating effect of chlorobenzene (PhCl) was observed unexpectedly in the Pd-catalyzed cascade Sonogashira–hydroarylation reaction.