Direct Spectroscopic Identification of BN-Arynes and Subtle Steric Effects on Nitrogen Fixation.

1,2-Azaborinines are the BN analogues of arynes through exchange of the formal CC triple bond by an isoelectronic BN bond. The BN-arynes are an underexplored class of reactive intermediates. Dibenzo[c,e][1,2]azaborinine (10,9-BN-phenanthryne) 1 was inferred as reactive intermediate by trapping reactions. Here it is shown that 1 can be generated in the gas phase by thermolysis from the pyridine adduct of 9-azido-9-borafluorene by cleavage of the dative bond with pyridine and dinitrogen extrusion. The ionization potential of 1 is 8.2 eV with ionization resulting from the π HOMO. Under cryogenic matrix isolation conditions, 9-azido-9-borafluorene photolysis results in isomerization to the dinitrogen adduct of 1 without involvement of a triplet borylnitrene intermediate. Photochemical nitrogen extrusion from 1 ⋅ N2 is not possible and nitrogen fixation is irreversible under cryogenic conditions. In contrast, 2,4,7,9-tetra-tert-butyldibenzo[c,e][1,2]azaborinine can be photogenerated from the corresponding azidoborole precursor under cryogenic matrix isolation conditions, and nitrogen fixation is precluded due to steric hindrance. The BN stretching vibration at about 1750 cm-1 is much weaker than in typical linear diaryl iminoboranes.

Solution Phase Reactivity of Dibenzo[c,e][1,2]azaborinine: Activation and Insertion into Si-E Single Bonds (E=H, OSi(CH3 )3 , F, Cl) by a BN-Aryne.

The boron-nitrogen analogue of ortho-benzyne, 1,2-azaborinine, is a reactive intermediate that features a formal boron-nitrogen triple bond. We here show by combining experimental and computational techniques that the Lewis acidity of the boron center of dibenzo[c,e][1,2]azaborinine allows interaction with the silicon containing single bonds Si-E through the silicon bonding partner E (E=F, Cl, OR, H). The binding to boron activates the Si-E bonds for subsequent insertion reaction. This shows that the BN-aryne is a ferocious species that even can activate and insert into the very strong Si-F bond.

Photo electron transfer induced desilylation of N,N-bis(trimethylsilyl)aminodibenzoborole to aminodibenzoborole.

The synthesis of 9-amino-9-borafluorene is described using a photoinduced twofold desilylation of the N,N-bis(trimethylsilyl) derivative 2. The mechanistic analysis suggests an initial single electron transfer step from 2 to the halogen containing solvent. 9-Amino-9-borafluorene undergoes a photoinduced cyclooligomerization, most reasonably to the dimer.

Thermal dehydrochlorination in the 4-fluoroaniline-trichloroborane system: identification of reactive intermediates involved in the formation of B,B',B''-trichloro-N,N',N''-tri((4-fluoro)phenyl)borazine.

Borazines are used in chemical vapor deposition processes to produce hybrid graphene-boron nitride nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the mechanism of formation of B,B',B''-trichloro-N,N',N''-tri(p-fluorophenyl)borazine (3a) from p-fluoroaniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory. These studies suggest the initial formation of the 1 : 1 adduct 1a (ArNH2BCl3, Ar = 4-fluorophenyl) with a dative B-N bond that could be fully characterized including single crystal X-ray diffraction. Adduct 1a undergoes unimolecular hydrogen chloride elimination with a first-order rate constant of k1 = 3.03(7) × 10-2 min-1 in toluene at 100 °C. This rate constant is in very good agreement with the one derived (k1 = 3.18 × 10-2 min-1) from computed activation parameters (ΔH‡373.15 = 28.1 kcal mol-1, ΔS‡373.15 = 1.56 eu, ΔG‡373.15 = 27.6 kcal mol-1). The product of the first hydrogen chloride evolution is anilinodichloroborane ArNHBCl2 (2a). Compound 2a cannot be isolated in a pure form due to instability, but its presence as a transient reactive intermediate can be derived from NMR spectroscopy. Reactive intermediates other than anilinodichloroborane cannot be assigned by NMR spectroscopy. We propose that the mechanism of formation of borazine 3a involves the reaction of 2a with 4-fluoroaniline as the rate determining step.

Synthesis and Ring Strain of a Benzoborirene-N-Heterocyclic Carbene Adduct.

The reduction of an N-heterocyclic carbene (1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene, IiPr Me 2 ) adduct of dichloro(ortho-bromophenyl)borane by tert-butyl lithium at low temperature yields the IiPr Me 2 adduct A of parent benzoborirene, a highly strained boron-containing bicyclic compound. A is unstable at room temperature and dimerizes at low temperature to the bis-IiPr Me 2 adduct of 9,10-dihydro-9,10-diboraanthracene, characterized by single-crystal X-ray crystallography.