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Main resonance structures and their weights of the three azaborine's isomer structures, as created using polyelectron population analysis (PEPA) 62
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The electron delocalization in 1,2-azaborine, 1,3-azaborine and 1,4-azaborine is studied using Canonical Molecular Orbital contributions to the Induced Magnetic Field (CMO-IMF) method and Polyelectron Population Analysis (PEPA). Contour maps of the out-of-plane component of the induced magnetic field (Bzind) of the π-system show that the three azab...
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... the The Journal of Physical Chemistry A Article probabilities (or, in general, the weights) of local valence bond- type (VB-type) resonance structures are calculated in the framework of PEPA using the hole-expansion methodology. 61 The results of the main resonance structures of the studied molecules are shown in Figure 7. The 1,3-azaborine (2) presents two main fully delocalized resonance structures having the same weights (23.6 and 23.3%). ...
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... 1,2-Azaborinine 1 has a NICS(1) zz value of −25.2 which is lower than that of benzene (−29.3) and benzyne (−33.5), but still suggests aromatic character that is slightly larger than that of 1,2- 85 The NICS(1) zz value of −1.1 and −3.1 computed for 2 is signicantly lower suggesting that the antiaromaticity expected on the basis of the formal electron count is not relevant (see Fig. S17 †). The two slightly differing values for 2 are due to the non-planar ring. ...
Iminoboranes have gathered immense attention due to their reactivity and potential applications as isoelectronic and isosteric alkynes. While cyclic alkynes are well investigated and useful reagents, cyclic iminoboranes are underexplored and their existence was inferred only via trapping experiments. We report the first direct spectroscopic evidence of a cyclic seven-membered iminoborane, 1-(tert-butyldimethylsilyl)-1H-1,3,2-diazaborepine 2, under cryogenic matrix isolation conditions. The amino-iminoborane 2 was photochemically generated in solid argon at 4 K from 2-azido-1-(tert-butyldimethylsilyl)-1,2-dihydro-1,2-azaborinine (3) and was characterized using FT-IR, UV-vis spectroscopy, and computational chemistry. The characteristic BN stretching vibration (1751 cm⁻¹) is shifted by about 240 cm⁻¹ compared to linear amino-iminoboranes indicating a significant weakening of the bond. The Lewis acidity value determined computationally (LAB = 9.1 ± 2.6) is similar to that of boron trichloride, and twelve orders of magnitude lower than that of 1,2-azaborinine (BN-aryne, LAB = 21.5 ± 2.6), a six-membered cyclic iminoborane. In contrast to the latter, the reduced ring strain of 2 precludes nitrogen fixation, but it unexpectedly allows facile (2 + 2) cycloaddition reaction with C2H4 under matrix isolation conditions at 30 K.
... but still suggests aromatic character that is slightly larger than that of 1,2-dihydro-1,2-azaborine (NICS(1)zz = -21.1). 85 The NICS(1)zz value of -1.1 and -3.1 computed for 2 is significantly lower suggesting that the antiaromaticity expected on the basis of the formal electron count is not relevant (see Figure 2c). The two slightly differing values for 2 are due to the non-planar ring. ...
Iminoboranes have gathered immense attention due to their reactivity and potential applications as isoelectronic and isosteric alkynes. While cyclic alkynes are well investigated and useful reagents, cyclic iminoboranes are underexplored and their existence was inferred only via trapping experiments. We report the first direct spectroscopic evidence of a cyclic seven-membered iminoborane, 1-(tert-butyldimethylsilyl)-1-H-1,3,2-diazaborepine 2, under cryogenic matrix isolation conditions. The amino-iminoborane 2 was photochemically generated in solid argon at 4 K from 1-(tert-butyldimethylsilyl)-2-azido-1,2-dihydro-1,2-azaborine (3) and was characterized using FT-IR, UV-Vis spectroscopy, and computational chemistry. The characteristic BN stretching vibration (1751 cm-1) is shifted by about 240 cm-1 compared to linear amino-iminoboranes indicating a significant weakening of the bond. The Lewis acidity value determined computationally (LAB = 9.1 ± 2.6) is similar to that of boron trichloride, and twelve orders of magnitude lower than that of 1,2-azaborinine (BN-aryne, LAB = 21.5 ± 2.6), a six-membered cyclic iminoborane. In contrast to the latter, the reduced ring strain of 2 precludes nitrogen fixation, but it unexpectedly allows facile (2 + 2) cycloaddition reaction with C2H4 under matrix isolation conditions at 30 K. The reaction of 2 with ethene provides the first example of a (2 + 2) cycloaddition reaction of an iminoborane.
... 28 The effect of heteroatoms on the T 1 state Baird aromaticity of [6]CPP has been reported in a computational study where the phenylene units were changed successively to 1,4-azaborines, 29 a monocycle which is less Huckel-aromatic in S 0 than benzene. 30 Yet, such replacements in the cases investigated led to attenuated Baird aromaticity according to nucleus independent chemical shifts (NICS's) calculated with the PBE0 functional. Can other heterocycles lead to increased Baird aromaticity? ...
The aromaticity of cyclic 4nπ-electron molecules in their first ππ* triplet state (T1), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T1 state Baird aromaticity in macrocyclic compounds, [n]CM's, which are cyclic oligomers of four different monocycles (M = p-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 π-electrons in their main conjugation paths we find that for their T1 states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ([n]CFU's) retain Baird aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a
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[n]CFU with a specific n is more strongly Baird-aromatic than the analogous
3
[n]CPP while the magnetic indices tell the opposite. To construct large T1 state Baird-aromatic [n]CM's, the design should be such that the T1 state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hückel aromaticity in S0 than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T1 state aromaticity, we reveal the analogy to the Hückel aromaticity of the corresponding closed-shell dications yet observe stronger Hückel aromaticity in the macrocyclic dications than Baird aromaticity in the T1 states of the neutral macrocycles.
... These BN heterocycles have a variable degree of aromaticity and stability: the most thermodynamically stable compounds are 1,2-azaborines, the least stable compounds are 1,3-derivatives. The delocalization of π-electrons has a reverse tendency: the most aromatic structure is typical for 1,3-azaborine, whereas 1,4-isomer is the least aromatic derivative [65]. ...
Among the known allylborating agents, amine adducts of allylic triorganoboranes can be considered as the most advanced smart reagents since they combine high activity with stability and selectivity. The new reactivity profile of these compounds offers additional opportunities for both the laboratory and industrial application of allylboranes. The current review presents a short analysis of the promising routes of application of the amine adducts, which include, for example, the catalytic synthesis of functionalized allylarenes, homoallylamines, and the synthesis of BN isosteres of carbon aromatic systems.
... 26 The effect of heteroatoms on the T1 state Baird-aromaticity of [6]CPP has been reported in a computational study where the phenylene units were changed successively to 1,4azaborines, 27 a monocycle which is less Hückel-aromatic in S0 than benzene. 28 Yet, such replacements in the cases investigated led to attenuated Baird-aromaticity according to nucleus independent chemical shifts (NICS's) calculated with the PBE0 functional. Can other heterocycles lead to increased Baird-aromaticity? by addition of two non-bonding same-spin -electrons to [9]CPP 2+ which has a Hückelaromatic MCP with 4 × 9 -2 (i.e., 4 × 8 + 2) spin-paired -electrons. ...
... in which the T1 state of a neutral 4n-electron cycle is viewed as that of the corresponding Hückel-aromatic dication with 4n -2 spin-paired -electrons plus two non-bonding same-spin -electrons. 29 This description was utilized to rationalize the aromaticity/antiaromaticity switch that occurs when going from S0 to T1 in a [26]-and [28]porphyrinoid pair. 30,31 Indeed, the potential Baird-aromaticity of a neutral [n]CPP in its T1 state could therefore closely resemble that of the corresponding [n]CPP dication ( Figure 1B), species for which in-plane Hückel-aromaticity with 4n -2 -electrons has been demonstrated. ...
p>The aromaticity of cyclic 4 n p-electron molecules in their first pp* triplet state (T<sub>1</sub>), labelled Baird-aromaticity, has gained growing attention in the last decade. Here we explore computationally the limitations of T<sub>1</sub> state Baird-aromaticity in macrocyclic compounds, [ n ]CM ’s, which are cyclic oligomers of four different monocycles (M = para -phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 p-electrons in their main conjugation paths we find that for their T<sub>1</sub> states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ( [ n ]CFU ’s) retain Baird-aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a <sup>3</sup>[ n ]CFU with a specific n is more strongly Baird-aromatic than the analogous <sup>3</sup>[ n ]CPP while the magnetic indices tell the opposite. To construct large T<sub>1</sub> state Baird-aromatic [ n ]CM ’s the design should be such that the T<sub>1</sub> state Baird-aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel-aromaticity of one or a few monocycles and semi-localized triplet diradical character. Monomers with lower Hückel-aromaticity in S<sub>0</sub> than benzene ( e.g. , furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g. , methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T<sub>1</sub> state aromaticity we reveal the analogy to the Hückel-aromaticity of the corresponding closed-shell dications, yet, observe stronger Hückel-aromaticity in the macrocyclic dications than Baird-aromaticity in the T<sub>1</sub> states of the neutral macrocycles. </p
A survey of the chemical literature published between 2008 and 2018 dealing with the synthesis, structural investigations and chemical reactivity of six-membered boracycles with two or more heteroatoms is presented. The chapter outlines the advances in the chemistry of monocyclic, fused and polycyclic species and should be read in conjunction with earlier chapters in CHEC I (1984, vol. 1, p. 629), CHEC II (1996, vol. 6, p. 1155) and CHEC III (2008, vol. 9, p. 961). Fully aromatic and both partially and fully reduced species are included. New routes to the synthesis of BN-isosteres of benzene, naphthalene and BN-polycyclic and ring-fused BN heteroaromatic compounds, including modern synthetic technique are described. The reactivity and substituent modification procedures are covered. Also included are sections on theoretical methods and thermodynamic aspects such as aromaticity and thermodynamic stability. Applications of polyheteroatom six-membered boracycles in material and medicinal chemistry are discussed.
Heterocycles containing both N and B heteroatoms in their structure were first reported by Dewar in the middle of the last century. However, they received little attention until the early years of this century, when several groups revisited these compounds due to their interest in BN/CC isosterism. As a result of these systematic studies, very significant advances have been made in our understanding of the chemistry of these BN-heterocycles. The purpose of this review is to summarize the most significant advances in the last two decades as regards the development of synthetic strategies and studies of their reactivity, as well as to provide an overview of their general properties and main applications.
In this chapter, we discuss selected energetics and thermochemical aspects of species with carbon–boron bonds. Although they lack a carbon–boron bond, atomic boron, its anion, and binary metal borides—the hypothetical alkali metal borides MB—and structurally complex metal borides are briefly introduced. Consideration of species with carbon–boron bonds begins with binary species such as the gas‐phase diatomic BC and the solid B4C and proceeds to the highly reactive methyl and phenyl derivatives of univalent borylene, BH. The question of regular behavior as homologous series is addressed for the relatively stable and long‐known classes of compounds, trialkylboranes and carboranes, with their normal (threefold) and higher boron‐coordination numbers. Trivalent boron and nitrogen species are then compared, first as organoboranes and amines, then as their organoborate and ammonium ions, and also as their “dimer” anions and cations. Pyridine borane complexes are likewise discussed and compared with benzene derivatives. Species with boron in the ring, borabenzenes and their amine complexes, are much more poorly understood. However, aromatic derivatives with both boron and nitrogen in the ring have evinced considerable interest, and the derivatives of the isomeric 1,2‐, 1,3‐, and 1,4‐azaborinines are described at some length. The chapter closes with species that contain oxygen along with boron, carbon, and hydrogen. Borane carbonyl (BH3CO), boronic acid derivatives, and boroxines are discussed.
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 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.
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.
