Tetramerization of BEB-Doped Phenalenyls to Obtain (BE)8-[16]Annulenes (E = N, O)

Two (BE)8-[16]annulenes were prepared and fully characterized by experimental and quantum-chemical means (1, E = N; 2, E = O). The 1,8-naphthalenediyl-bridged diborane(6) 3 served as their common starting material, which was treated with [Al(NH3)6]Cl3 to form 1 (91% yield) or with 1,8-naphthalenediboronic acid anhydride to form 2 (93% yield). As a result, the heteroannulenes 1 and 2 are supported by four aromatic "clamps" and may also be viewed as NH- or O-bridged cyclic tetramers of BNB- or BOB-doped phenalenyls. X-ray crystallography on mono-, di-, and tetraadducts 2·thf, 2·py2, and 2·py4 showed that 2 is an oligotopic Lewis acid (thf/py: tetrahydrofuran/pyridine donor). The applicability of 2 also as a Lewis basic ligand in coordination chemistry was demonstrated by the synthesis of the mononuclear Ag+ complex [Ag(py)2(2·py4)]+ and the dinuclear Pb2+ complex 6. During the assembly of 6, the rearrangement of 2 led to the formation of two (BO)9-macrocycles linked by two BOB-phenalenyls to form a nanometer-sized cage with four negatively charged, tetracoordinated B atoms. Both 1 and 2 show several redox waves in the cathodic regions of the cyclic voltammograms. An in-depth assessment of the consequences of electron injection on the aromaticity of 1 and 2 was achieved by electronic structure calculations. 1 and 2 are proposed to exhibit aromatic switching capabilities in the [16]annulene motif.

Rapid and Modular Access to Multifunctionalized 1,2-Azaborines via Palladium/Norbornene Cooperative Catalysis

1,2-Azaborines, a unique class of BN-isosteres of benzene, have attracted great interest across several fields. While significant advancements have been made in the postfunctionalization of 1,2-azaborines, challenges still exist for the selective functionalization of the C4 position and access to 1,2-azaborines with five or six independently installed substituents. Here we report a rapid and modular method for C3 and C4 difunctionalization of 1,2-azaborines using the palladium/norbornene (Pd/NBE) cooperative catalysis. Enabled by the C2 amide-substituted NBE, diverse 3-iodo-1,2-azaborines can be used as substrates, showing broad functional group tolerance. Besides ortho arylation, preliminary success of ortho alkylation has also been realized. In addition, a range of alkenes and nucleophiles can be employed for ipso C3 functionalization. The reaction is scalable, and various postfunctionalizations, including forming hexa-substituted 1,2-azaborines, have been achieved.

Air- and photo-stable luminescent carbodicarbene-azaboraacenium ions

Substitution of a C=C bond by an isoelectronic B-N bond is a well-established strategy to alter the electronic structure and stability of acenes. BN-substituted acenes that possess narrow energy gaps have attractive optoelectronic properties. However, they are susceptible to air and/or light. Here we present the design, synthesis and molecular structures of fully π-conjugated cationic BN-doped acenes stabilized by carbodicarbene ligands. They are luminescent in the solution and solid states and show high air and moisture stability. Compared with their neutral BN-substituted counterparts as well as the parent all-carbon acenes, these species display improved quantum yields and small optical gaps. The electronic structures of the azabora-anthracene and azabora-tetracene cations resemble higher-order acenes while possessing high photo-oxidative resistance. Investigations using density functional theory suggest that the stability and photo-physics of these conjugated systems may be ascribed to their cationic nature and the electronic properties of the carbodicarbene.

Synthesis, reduction and C-H activation chemistry of azaborinines with redox-active organoboryl substituents

The 2,3,4,5,6-pentaphenyl-1,2-azaborinin-1-yl (PPAB) potassium complex 1 undergoes facile salt metathesis with 9,10-dibromo-9,10-dihydroboraanthracene (DBABr2), 5-bromodibenzo[b,d]borole (DBBBr), 1-chlorotetraphenylborole (TPBCl) and dibromo(phenyl)borane (BBr2Ph) to yield the corresponding N-borylated azaborinines N-DBABr-PPAB (2, which hydrolyses and dimerises to the oxo-bridged N,N'-O(DBA)2-(PPAB)2, 3), N,N'-DBA-(PPAB)2 (4), N-DBB-PPAB (5), N-PPB-PPAB (7) and N-BBrPh-PPBA (9). Stepwise reduction of 4 yields the corresponding stable radical anion 4˙- and dianion 42-. One-electron reduction of 5 with KC8 yields the purple radical anion 5˙-, which forms a highly insoluble coordination polymer. 5˙- undergoes very slow radical intramolecular ortho-C-H activation at the C4-phenyl substituent of the PPAB moiety, yielding a BN-analogue of the 5,5'-spiro-bi[dibenzoborole] anion, [6]K. Compound 7 cannot be isolated and undergoes spontaneous and diastereoselective 2,5-anti-addition of the ortho-C-H bond of the PPAB C4-phenyl substituent to yield a novel BNB-analogue of the triply fused dihydrocyclopenta[l]phenanthrene cation, compound 8. Finally the one-electron reduction of 9 results in the ortho-C-H activation of the PPAB C4-phenyl substituent at an in situ-generated dicoordinate boryl anion (10), resulting in the formation of a BNB-analogue of 9H-fluorene, the borate 11-. DFT calculations provide a rationale for the diverse C-H activations observed in these reactions.

Theoretical investigation of tautomerism of 2- and 4-pyridones: origin, substituent and solvent effects

Computational investigation at the BHandHLYP/6-311+G(d,p) level of theory of the gas-phase tautomerism of 2- and 4-pyridones confirmed the slight prevalence of lactim in the case of the former, but its dominance in the case of the latter, as shown previously. Examination of aromaticity by using HOMA, EDDB, NBOdel, NICS and AICD led to the conclusion that tautomerization of 4-pyridone results in greater aromaticity gain. It is also driven by the Pauli repulsion relief, which was revealed by the tautomerization energy decomposition analysis. By contrast, in the case of 2-pyridone, lactim is favoured by orbital and electrostatic interactions and disfavoured by the Pauli repulsion. Aromaticity gain in this case is smaller. The position of the tautomeric equilibrium can be modulated by substituent inductive effects (Cl and F), inductive and resonance effects (NH2 and NO2), hydrogen bonding (NO2), and medium polarity, the increase of which increases lactam population.

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.

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.

Aromatic 1,2-Azaborinin-1-yls as Electron-Withdrawing Anionic Nitrogen Ligands for Main Group Elements

The 2-aryl-3,4,5,6-tetraphenyl-1,2-azaborinines 1-EMe₃ and 2-EMe₃ (E=Si, Sn; aryl=Ph (1), Mes (=2,4,6-trimethylphenyl, 2)) were synthesized by ring-expansion of borole precursors with N₃ EMe₃ -derived nitrenes. Desilylative hydrolysis of 1- and 2-SiMe₃ yielded the corresponding N-protonated azaborinines, which were deprotonated with nBuLi or MN(SiMe₃ )₂ (M=Na, K) to the corresponding group 1 salts, 1-M and 2-M. While the lithium salts crystallized as monomeric Lewis base adducts, the potassium salts formed coordination polymers or oligomers via intramolecular K⋅⋅⋅aryl π interactions. The reaction of 1-M or 2-M with CO₂ yielded N-carboxylate salts, which were derivatized by salt metathesis to methyl and silyl esters. Salt metathesis of 1-M or 2-M with methyl triflate, [Cp*BeCl] (Cp*=C₅ Me₅ ), BBr₂ Ar (Ar=Ph, Mes, 2-thienyl), ECl₃ (E=B, Al, Ga) and PX₃ (X=Cl, Br) afforded the respective group 2, 13 and 15 1,2-azaborinin-2-yl complexes. Salt metathesis of 1-K with BBr₃ resulted not only in N-borylation but also Ph-Br exchange between the endocyclic and exocyclic boron atoms. Solution ¹¹ B NMR data suggest that the 1,2-azaborinin-2-yl ligand is similarly electron-withdrawing to a bromide. In the solid state the endocyclic bond length alternation and the twisting of the C₄ BN ring increase with the sterics of the substituents at the boron and nitrogen atoms, respectively. Regression analyses revealed that the downfield shift of the endocyclic ¹¹ B NMR resonances is linearly correlated to both the degree of twisting of the C₄ BN ring and the tilt angle of the N-substituent. Calculations indicate that the 1,2-azaborinin-1-yl ligand has no sizeable π-donor ability and that the aromaticity of the ring can be subtly tuned by the electronics of the N-substituent.

The Rise of 1,4-BN-Heteroarenes: Synthesis, Properties, and Applications

BN-heteroarenes, which employ both boron and nitrogen in aromatic hydrocarbons, have gained great attention in the fields of organic chemistry and materials science. Nevertheless, the extensive studies on BN-heteroarenes are largely limited to 1,2-azaborine-based compounds with B-N covalent bonds, whereas 1,3- and 1,4-BN-heteroarenes are relatively rare due to their greater challenge in the synthesis. Recently, significant progresses have been achieved in the synthesis and applications of BN-heteroarenes featuring 1,4-azaborines, especially driven by their significant potential as multiresonant thermally activated delayed fluorescence (MR-TADF) materials. Therefore, it is timely to review these advances from the chemistry perspective. This review summarizes the synthetic methods and recent achievements of 1,4-azaborine-based BN-heteroarenes and discusses their unique properties and potential applications of this emerging class of materials, highlighting the value of 1,4-BN-heteroarenes beyond MR-TADF materials. It is hoped that this review would stimulate the conversation and cooperation between chemists who are interested in azaborine chemistry and materials scientists working in the fields of organic optoelectronics, metal catalysis, and carbon-based nanoscience etc.

Dithiophene-Fused Oxadiborepins and Azadiborepins: A New Class of Highly Fluorescent Heteroaromatics

Access to dithiophene-fused oxadiborepins and the first azadiborepins attained via a modular synthesis route are presented. The new compounds emit intense blue light, some of which demonstrate fluorescence quantum yields close to unity. Cyclic voltammetry (CV) revealed electrochemically reversible one-electron reduction processes. The weak aromatic character of the novel 1,2,7-azadiborepin ring is demonstrated with in-depth theoretical investigations using nucleus-independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations.

Single-Molecule Conductance of 1,4-Azaborine Derivatives as Models of BN-doped PAHs

The single-molecule conductance of a series of BN-acene-like derivatives has been measured by using scanning tunneling break-junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN-doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all-carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene-like compound, which is found to have a similar conductance, thus evidencing that B-N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state-of-the-art transport calculations.

From a 1,2-azaborinine to large BN-PAHs via electrophilic cyclization: synthesis, characterization and promising optical properties

A synthetic approach towards boron-nitrogen substituted polycyclic aromatic hydrocarbons (BN-PAHs) via electrophilic cyclization is described and it is shown that the variation of the rings' connectivity may tune the emission wavelengths effectively.

Aromatic nature of neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives

The aromatically relevant parameters of boron-rich inorganic benzenes-neutral and dianionic 1,4-diaza-2,3,5,6-tetraborinine derivatives (B4N2R6)-have been computationally estimated and evaluated from geometric, electronic, magnetic, and energetic points of view. The majority of the criteria (ASE, NICSzz, ELF, and PDI) indicate that the aromaticity of the neutral B4N2 benzene analogue stabilized by Lewis bases lies in between those of benzene and borazine. On the other hand, the aromaticity of the dianionic B4N2 benzene analogue 4' is controversial. The pronounced aromatic nature of 4' is supported by ELFπ, PDI, and NICSπzz, but ASE, the FiPC-NICS plot, and ACID oppose this. These data confirm that even with the same B4N2-skeletal framework of a 6π-system, the aromatic feature varies depending on the overall charge of the B4N2 systems.

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.

Triplet-State Structures, Energies, and Antiaromaticity of BN Analogues of Benzene and Their Benzo-Fused Derivatives

It is well known that benzene is aromatic in the ground state (the Hückel's rule) and antiaromatic in the first triplet (T₁) excited state (the Baird's rule). Whereas its BN analogues, the three isomeric dihydro-azaborines, have been shown to have various degrees of aromaticity in their ground state, almost no data are available for their T₁ states. Thus, the purpose of this work is to theoretically [B3LYP/6-311+G(d,p)] predict structures, energies, and antiaromaticity of T₁ dihydro-azaborines and some benzo-fused derivatives. Conclusions are based on spin density analysis, isogyric and hydrogenation reactions, HOMA, NICS, and ACID calculations. The results suggest that singlet-triplet energy gaps, antiaromaticity, and related excited-state properties of benzene, naphthalene, and anthracene could be tuned and controlled by the BN substitution pattern. While all studied compounds remain (nearly) planar upon excitation, the spin density distribution in T₁ 1,4-dihydro-azaborine induces a conformational change by which the two co-planar C-H bonds in the ground state become perpendicular to each other in the excited state. This predicted change in geometry could be of interest for the design of new photomechanical materials. Excitation of B-CN/N-NH₂ 1,4-azaborine would have a few effects: intramolecular charge transfer, aromaticity reversal, rotation, and stereoelectronic Umpolung of the amino group.

Aromaticity of unsaturated BEC4 heterocycles (E = N, P, As, Sb, O, S, Se, Te)

A compendium of pnictogen and chalcogen substituted boron heterocycles were assessed for their aromatic character by first principles density functional theory. Group-15 and Group-16 elements were placed at the ortho-, meta-, and para-positions of six-membered rings relative to boron to assess their impact on the aromaticity of the unsaturated heterocycles. Aromaticity was analyzed by a multidimensional approach using nuclear independent chemical shifts, gauge-including magnetically induced current, as well as natural bond orbital and natural resonance theory analyses. Based on these methods, we observe a general decline of aromaticity in heavier pnictaborines while the chalcogen analogues maintain relatively strong aromatic character. These general trends result from complementary π-π* natural bond order interactions that sustain resonance within the ring of each heterocycle establishing a pattern of cyclic delocalization. Consequently, natural resonance theory displays strong resonance, which is corroborated with the signed modulus of ring current, toroidal vortices of current maps, and elevated average induced current throughout the ring. The 1,3-configurations for pnictaborines and chalcogenaborines are generally more aromatic compared to the 1,2- and 1,4-isomers, which contain π-holes that limit diatropism within the heterocycles. However, an energetic trend favors the 1,2-heterocycles in both groups, with a few exceptions driven in large-part by π-donation of the lone pair on the heteroatom to the pz orbital on the adjacent boron resulting in stabilization. The importance of planarity for high aromaticity is demonstrated, especially in the pnictaborine isomers where pyramidalization at the pnictogen is favored, while bond regularity seems a less important criterion.

Bonded to Carbon or Nitrogen? This is a Question on the Regioselectivity in Hyperconjugative Aromaticity

In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. Although it has been extensively investigated in various reactions, the regioselectivity of hyperconjugative aromaticity on either main group systems or transition metal ones remains elusive due to the challenge of synthesizing the target products. Here we report a joint theoretical and experimental study on this issue. Theoretical calculations predicted that electron-withdrawing groups prefer an attachment to the sp³-hybridized carbon atom rather than the nitrogen atom in indoliums. For the electron-donating groups, the two isomers bonded to the sp³-hybridized carbon or nitrogen atom are almost isoenergetic. When both sp²- and sp³-hybridized carbon and nitrogen atoms in the five-membered ring of indoliums are considered, the isomer with the polyaurated substituents bonded to the sp³-hybridized carbon atom is thermodynamically more stable than that with the polyaurated substituents bonded to the sp³-hybridized nitrogen atom. This prediction is reasonably verified by experimental observation. Bond dissociation energy is found to be more important than aromaticity in rationalizing such a preference. Our findings could help experimentalists to design and realize more novel hyperconjugative aromatics.

All Non-Carbon B3 NO2 Exotic Heterocycles: Synthesis, Dynamics, and Catalysis

The B₃ NO₂ six-membered heterocycle (1,3-dioxa-5-aza-2,4,6-triborinane=DATB), comprising three different non-carbon period 2 elements, has been recently demonstrated to be a powerful catalyst for dehydrative condensation of carboxylic acids and amines. The tedious synthesis of DATB, however, has significantly diminished its utility as a catalyst, and thus the inherent chemical properties of the ring system have remained virtually unexplored. Here, a general and facile synthetic strategy that harnesses a pyrimidine-containing scaffold for the reliable installation of boron atoms is disclosed, giving rise to a series of Pym-DATBs from inexpensive materials in a modular fashion. The identification of a soluble Pym-DATB derivative allowed for the investigation of the dynamic nature of the B₃ NO₂ ring system, revealing differential ring-closing and -opening behaviors depending on the medium. Readily accessible Pym-DATBs proved their utility as efficient catalysts for dehydrative amidation with broad substrate scope and functional-group tolerance, offering a general and practical catalytic alternative to reagent-driven amidation.

The effect of two types of dibenzoannulation of pentalene on molecular energies and magnetically induced currents

The effect of two types of dibenzo-fusion of pentalene in the singlet and triplet states on its molecular energies and magnetically induced ring currents was examined via density functional calculations. The isomerization energy decomposition analysis (IEDA) together with the calculated aromaticity indices (NICS(1)zz, HOMA and FLUπ), estimation of resonance energies (RE) and extra cyclic resonance energies (ECRE) via the NBO method, and the NICS-XY-scans revealed that the π-electronic system is the most important factor controlling the molecular energies. The [a,f] topology features greater delocalization, which results in two opposing effects: larger ECRE, but weaker π-bonding. The latter is mainly responsible for the higher energy of [a,f]-dibenzopentalene (DBP) (ΔEiso = 21.7 kcal mol-1), with the other effects being σ-orbital and electrostatic interactions. The reversal of energetic stability in the triplet states (ΔEiso = -10.8 kcal mol-1) mainly comes from the reduced Pauli repulsion in [a,f]-DBP, which stabilizes the unpaired spin density over the central trimethylenemethane subunit vs. the central pentalene subunit in [a,e]-DBP. Although the [a,e] topology only reduces the diatropic and paratropic currents of the elementary subunits, benzene and pentalene, the [a,f] topology also creates strong global paratropicity involving the benzene rings. Both DBP isomers are characterized by global and smaller semi-global and local diatropic currents in the triplet state.

BN Polystyrenes: Emerging Optical Materials and Versatile Intermediates

BN polystyrenes are an emerging class of polyolefins functionalized with aromatic side chains in which at least one CC bond is replaced with a BN bond. This class of structures exhibits unusual photophysical properties relative to organic polymers. BN polystyrenes serve as intermediates in the preparation of functional polymers, including stereoregular polar polyolefins. The consequences of BN for CC bond substitution on reactivity and properties are highlighted.

Does aromaticity account for an enhanced thermodynamic stability? The case of monosubstituted azaborines and the stereoelectronic chameleonism of the NH2 group

This work was initiated by the increasing interest in BN/CC isosterism and by the long-lasting interest in the concepts of aromaticity and substituent effects. We have theoretically examined the aromaticity and stability of monosubstituted BN isosters of benzene, the three isomeric azaborines. The results provide insight into the effect of substitution on two basic molecular properties, which are influenced, here, by the substituent effects and by the B/N relationship in the ring. The results, along with other examples in the literature, also warn chemists that the general belief that aromaticity accounts for enhanced thermodynamic stability is not always true. The stability of cyclic, conjugated compounds depends on several effects, and only one of them is aromaticity. In addition, our calculations predict a switching of electronic properties of the NH2 group from the usual p-electron donor to a π-electron acceptor when it is moved from the B/C atoms to the nitrogen atom in all isomers, or C6 in 1,3-azaborine. This is the result of the conformational change that places the NLP in the plane of the ring and the NH bonds in a favourable spatial position to act as acceptors of π-electron density.

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.

Comparison of effects of charge delocalization and π-electron delocalization on the stability of monocyclic compounds

Aromaticities and stabilities of ortho, meta, and para isomers of some derivatives of benzene, C₅H₅^- and C₇H₇⁺ were studied and compared basis on the NICS index and their relative energies. For the benzene and C₇H₇⁺ derivatives, the ortho isomers with less stability were more aromatic. This discrepancy was also observed for the molecules with conjugated and non-conjugated π-electrons. However, for the charged conjugated systems, the structures with delocalized charge were more stable. Effect of electron withdrawing (EWGs) and electron donating groups (EDGs) on the electron delocalization and stability of the neutral and charged molecules was investigated. It was observed that the EWDs and EDGs change the stability trend of the neutral systems, while in the case of charged molecules, the isomers with delocalized charge were more stable regardless of the type of substituents. Although both π-electron delocalization and charge delocalization stabilize the aromatic and conjugated systems, effect of charge delocalization on the stability is more than that of π-electron delocalization.

Mono BN-substituted analogues of naphthalene: a theoretical analysis of the effect of BN position on stability, aromaticity and frontier orbital energies

An insight into the electronic structure changes driven by CC → BN substitution in a naphthalene system has been given by quantum chemical calculations.

Organometallic acids with azaborine, oxaborine, azaborole and oxaborole scaffolds

Organoboron compounds were used in the design of organometallic acids exhibiting a wide acidity range from weak acids to superacids.

Probing a General Rule towards Thermodynamic Stabilities of Mono BN-doped Lower Polyenes

The BN-doped organic analogues are interesting as aliphatic amineboranes for hydrogen storage, precursors for aromatic borazines and adsorbent cage azaboranes. However, BN-doped aliphatic polyenes remained undeveloped. Herein, we perform theoretical calculations on two mono BN-doped aliphatic lower polyenes, 1,3-butadiene and 1,3,5-hexatriene. A general rule is proposed, i.e., isomers with terminal nitrogen and directly BN-connected, N-B(R), in particular, are of significant thermodynamic stability as compared with their inverse analogues (where boron is at the terminal position). The N-B(R) type isomers are found to be the most stable ones in both polyenes. Isomers with terminal B and N are of intermediate stability. Highly destabilized isomers are those with one terminal methylene group and one terminal heteroatom in the butadiene series, and two terminal methylene groups in the hexatriene series. Rules established here may lead researchers to synthesize isomers with particular thermodynamic stability.

CC/B–N substitution in five membered heterocycles. A computational analysis

Novel five-membered azaboroles are aromatic, stable under neutral conditions, isomer stabilization energy is explained using σ-bond and aromatic stabilization energies.

Theoretical study of azido gauche effect and its origin

The strength and origin of the azido gauche effect were studied by ab initio calculations and compared with the well-known fluorine gauche effect.

A Modular Synthetic Approach to Monocyclic 1,4-Azaborines

A simple and general method for the synthesis of a wide range of monocyclic 1,4-azaborines, including the first examples containing B heteroatoms is described. Post-heterocycle-formation olefin isomerization was employed as a key strategy. This new synthetic method provides fundamental insight into the resonance stabilization and photophysical properties of 1,4-azaborines.