Synthesis of B-trisubstituted borazines via the rhodium-catalyzed hydroboration of alkenes with N,N′,N″-trimethyl or N,N′,N″-triethylborazine

Hydroboration of terminal and internal alkenes with N,N′,N″-trimethyl- and N,N′,N″-triethylborazine was carried out at 50 °C in the presence of a rhodium(I) catalyst. Addition of dppb or DPEphos (1 equiv.) to RhH(CO)(PPh₃)₃ gave the best catalyst for hydroboration of ethylene at 50 °C, resulting in a quantitative yield of B,B′,B″-triethyl-N,N′,N″-trimethylborazine. On the other hand, a complex prepared from (t-Bu)₃P (4 equiv.) and [Rh(coe)₂Cl]₂ gave the best yield for hydroboration of terminal or internal alkenes.     

Cycled-fused-borazines—MNDO and AM1 treatments

The Hückel type cycled-fused-borazines which are analogous to cyclacenes are considered for the semiempirical molecular orbital treatment at the level of MNDO and AM1 type calculations. The structures having 3–10 arenoid rings (R) are considered. It has been found that they should be thermally stable and exogenic. Moreover, these properties get more and more as R increases. The systems exhibit no cryptoannulenic effect on any of the properties presently studied, showing that the peripheral circuits, unlike the ones in cyclacenes, are not influential.     

Synthesis of new borazines containing crosslinkable groups

Three new functional group containing borazine derivatives, 2,4,6-tri（allylamino）borazine, 2,4,6-tri（3-ethynylanilino）borazine, and 2,4,6-tri（4-propargyl oxyanilino）borazine were synthesized by aminolysis reaction of 2,4,6-trichloroborazine ClaB3N3H3（TCB）. The new compounds were characterized by infrared （IR） spectra, nuclear magnetic resonance （NMR）, and mass spectrometry （MS）.     

Borazine: to be or not to be aromatic

Aromaticity of borazine, which has been subject of controversial discussions, is addressed. Beside a short review on aromaticity of borazine we report a detailed analysis of two molecular fields, the induced magnetic field (B ^ind) and the electron localization function (ELF). The induced magnetic field of borazine shows a long-range shielding cone perpendicular to the molecular plane, as in benzene, but lower in magnitude. Contrary to benzene, borazine shows two weakly paratropic regions, one of them inside the ring, and the second one enveloping the boron atoms. It is necessary to separate σ and π contributions to identify whether borazine exhibits π-aromatic character comparable to benzene. Nucleus-independent chemical shift (NICS) isolines show that the σ electrons are much stronger localized than π electrons, their local paramagnetic contributions generate a short-range response and a paratropic (deshielding) region in the ring center (similar to an anti-aromatic response). Three regions can be identified as chemically meaningful domains exhibiting an internally strong electron delocalization (ELF = 0.823). Borazine may be described as a π aromatic compound, but it is not a globally aromatic species, as the electronic system is not as delocalized as in benzene. Dedicated to the 70th birthday of Prof. Tadeusz Marek Krygowski.     

Borazine Based Molecular Single Source Precursors for Si/B/N/C Ceramics

The family of borazines [(SiCl₃)NB(Cl_n(CH₃)_1–n]₃ was synthesized and characterized by NMR and IR spectroscopy, as wellas by single crystal structure analysis. After cross‐linking with methylamine, a soluble polymer was obtained that was transformed to anall‐inorganic random network of nominal composition Si_1.0B_1.0N_3.0C_0.9O_0.1 (idealized SiBN₃C) at 1400 °C. With an on‐set of weight loss at 1840 °C and a resistance against rapid oxidation in pure oxygen of up to 1300 °C, the new Si/B/N/C ceramic shows a high temperature performance similar to those of previously reported Si/B/N/C materials.     

Ab initio studies of the low-lying π-states of borazine

Self-consistent-field and configuration-interaction studies were performed on borazine, using a double-zeta basis set augmented by six diffuse -functions. Low-lying singlet and triplet states of the A ₁ , A ₂ and E species were calculated, corresponding to ^* excited valence and Rydberg states. A selection out of singly and doubly excited configurations relative to a set of reference configurations was made for each species. Calculated vertical excitation energies (in eV) are 7.12 for ¹ A ₂(V), 8.45 for ¹ E (3p-R), 8.57 for ¹ A ₁(V), 8.9 for ¹ E(V-R), 9.55 for ¹ E(3d-R), 6.98 for ³ A ₁(_V), 7.27 for ³ E(V), 7.82 for ³ A ₂(V), 8.30 for ³ E(3p-R), and 9.5 for ³ E(3d-R), where V and R refer to valence or Rydberg character. The results are compared with experimental excitation energies, previous ab initio studies of borazine, and with recent ab initio studies of benzene.Dedicated to Professor Dr. H. Hartmann on the occasion of his 65th birthday.