BN versus CC: How Similar Are They?

A convergent, modular approach to functionalized 2,1-borazaronaphthalenes from 2-aminostyrenes and potassium organotrifluoroborates

Azaborines are an important class of compounds with applications in both medicinal chemistry and materials science. The first borazaronaphthalene, 2-chloro-2,1-borazaronaphthalene, was reported in 1959; however, access to more highly functionalized substructures has been limited because of the harsh reaction conditions required to displace the chloride on boron. A convergent approach has been developed to synthesize disubstituted 2,1-borazaronaphthalenes from N-substituted 2-aminostyrenes and potassium organotrifluoroborates, where the potassium organotrifluoroborate is converted to the active R-BX2 species (X = Cl or F) in situ by addition of a fluorophile. Starting from aryl-, heteroaryl-, alkynyl-, alkenyl-, and alkyltrifluoroborates, a library of highly functionalized 2,1-borazaronaphthalenes were synthesized in one step under mild, transition-metal-free conditions.

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.

Thermodynamically Controlled, Dynamic Binding of Diols to a 1,2-BN Cyclohexane Derivative

The reversible covalent binding of diols to an N-Bn 1,2-BN cyclohexane has been studied by ¹¹B and ¹H NMR spectroscopy and single-crystal X-ray diffraction analysis. The activation barrier for the reversible B-N Lewis acid-base interaction has been measured by variable-temperature NMR with bound (2R,3R)-(-)-2,3-butanediol (T_c = -40 °C, ΔG^‡ = 11.2 ± 0.2 kcal mol^-1). Deuterium labeling experiments demonstrate that ligand exchange is reversible and rapid at room temperature, and competitive binding studies establish diol association as a thermodynamically controlled process.

Boron-substituted 1,3-dihydro-1,3-azaborines: synthesis, structure, and evaluation of aromaticity

Getting the family together: A general synthetic strategy based on nucleophilic substitution provided B-substituted 1,3-dihydro-1,3-azaborines (see scheme), BN isosteres of arenes with potential for application in biomedicine and materials science. Experimental structural analysis and calculations suggest that the aromaticity of the 1,3-dihydro-1,3-azaborine heterocycle is intermediate between that of benzene and that of 1,2-dihydro-1,2-azaborine.

Boron nitride nanotubes: biocompatibility and potential spill-over in nanomedicine

Boron nitride nanotubes (BNNTs) represent an innovative and extremely intriguing class of nanomaterials. Thanks to their special chemical and physical characteristics, they have already found a large number of applications in the field of nanotechnology, and recent studies have shown their possible exploitation in the biomedical domain, both as nanocarriers and, more interestingly, as nanotransducers. In this review, the latest findings on the interactions between BNNTs and living systems are summarized, starting with the major issues of their stabilization in physiological media and their functionalization with bioactive molecules. Thereafter the biocompatibility data which have so far been made available are discussed, and the need for further extensive and standardized tests is highlighted. Finally, the appealing diagnostic and therapeutic opportunities offered by BNNT-based systems are described, envisioning the potential spill-over effects of such 'smart' and 'active' nanoparticles in nanomedicine.

Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative

We have prepared a new borazine derivative that bears mesityl substituents at the boron centers and displays exceptional chemical stability. Detailed crystallographic and solid-state fluorescence characterizations revealed the existence of several polymorphs, each of which showed different emission profiles. In particular, a bathochromic shift is observed when going from the lower- to the higher-density crystal. Computational investigations of the conformational dynamics of borazine 1 in both the gas phase and in the solid state using molecular dynamics (MD) simulations showed that the conformation of the peripheral aryl groups significantly varies when going from an isolated molecule (in which the rings are able to flip over the 90° barrier at RT) to the crystals (in which the rotation is locked by packing effects), thus generating specific nonsymmetric intermolecular interactions in the different polymorphs. To investigate the optoelectronic properties of these materials by fabrication and characterization of light-emitting diodes (LEDs) and light-emitting electrochemical cells (LECs), borazine 1 was incorporated as the active material in the emissive layer. The current and radiance versus voltage characteristics, as well as the electroluminescence spectra reported here for the first time are encouraging prospects for the engineering of future borazine-based devices.

Formation of azaborines by photoelimination of B,N-heterocyclic compounds

Highly fluorescent π-conjugated polycyclic azaborines can be prepared from B,N-heterocyclic compounds with a BR2 -CH2 unit through the elimination of an R-H molecule (see scheme). These clean photoelimination reactions occur both in solution and in polymers doped with the precursors.

BN/CC isosteric compounds as enzyme inhibitors: N- and B-ethyl-1,2-azaborine inhibit ethylbenzene hydroxylation as nonconvertible substrate analogues

Good substrate gone bad! BN/CC isosterism of ethylbenzene leads to N-ethyl-1,2-azaborine and B-ethyl-1,2-azaborine. In contrast to ethylbenzene, which is the substrate for ethylbenzene dehydrogenase (EbDH), N-ethyl-1,2-azaborine (see scheme; Fc=Ferricenium tetrafluoroborate) and B-ethyl-1,2-azaborine are strong inhibitors of EbDH. Thus, the changes provided by BN/CC isosterism can lead to new biochemical reactivity.

The -BF-NH- link as a peptide-bond surrogate

A new peptidomimetic is proposed, resulting from substitution of the C═O carbonyl group by a B-F bond at the amide linkage. The effects of such chemical alteration are theoretically investigated through comparative calculations on dimethyl-fluoro-aminoborane H(3)C-BF-NH-CH(3) and N-methylacetamide H(3)C-CO-NH-CH(3), the simplest model of a peptide linkage. While little difference is found regarding size, electronic structure, and plaque rigidity, substantial distinctions are, however, observed between the polarities and association energies of the two compounds, with a B-F···H-N hydrogen bond estimated to be about one-third as strong as the natural C═O···H-N one. The conformational maps of the corresponding dipeptide models exhibit similarities and distinctions, which partly account for helical oligomer properties. Although capable of a high level of organization, the chains made of fluoro-aminoborane units show overall less structuration and more plasticity than their peptidic counterparts. Contrasts with fluorine-containing peptidomimetic 2-fluoro-2-butene are further underlined.

Four-component assembly of chiral N-B heterocycles with a natural product-like framework

The dative N-B bond was used to simply assemble heterocycles with a skeleton akin to the 5-oxofuro[2,3-b]furan motif. Twenty-five new N-B heterocycles were prepared via a highly efficient one-pot four-component reaction in yields and diastereoselectivities up to 95% and >97%, respectively. Several reaction intermediates were discovered using electrospray ionization mass spectroscopy which set the basis for the mechanism elucidation using DFT calculations.

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.

Synthesis of BN-fused polycyclic aromatics via tandem intramolecular electrophilic arene borylation

A tandem intramolecular electrophilic arene borylation reaction has been developed for the synthesis of BN-fused polycyclic aromatic compounds such as 4b-aza-12b-boradibenzo[g,p]chrysene (A) and 8b,11b-diaza-19b,22b-diborahexabenzo[a,c,fg,j,l,op]tetracene. These compounds adopt a twisted conformation, which results in a tight and offset face-to-face stacking array in the solid state. Time-resolved microwave conductivity measurements prove that the intrinsic hole mobility of A is comparable to that of rubrene, one of the most commonly used organic semiconductors, indicating that BN-substituted PAHs are potential candidates for organic electronic materials.