Aromaticity and Stability of Azaborines
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The influence of the relative boron and nitrogen positions on aromaticity of the three isomeric 1,2-, 1,3-, and 1,4-azaborines has been investigated by computing the extra cyclic resonance energy, NICS(0)(pi zz) index and by visualizing the pi-electron (de) shielding pattern as a response of the pi system to a perpendicular magnetic field. The origin of the known stability trend, in which the 1,2-/1,3-isomer is the most/least stable, was examined by using an isomerization energy decomposition analysis. The 1,3-arrangement of B and N atoms creates a charge separation in the pi-electron system, which was found to be responsible for the lowest stability of 1,3-azaborine. This charge separation can, in turn, be considered as a driving force for the strongest cyclic pi-electron delocalization, making this same isomer the most aromatic. Despite the well-known fact that the B-N bond attenuates electron delocalization due to large electronegativity difference between the atoms, the 1,4-B, N re...lationship reduces aromaticity to a greater extent by making the pi-electron delocalization more one-directional (from N to B) than cyclic. Thus, 1,4-azaborine was found to be the least aromatic. Its lower stability with respect to the 1,2-isomer was explained by the larger exchange repulsion.
Keywords:aromaticity / conjugation / density functional calculations / heterocycles / bond theory
Source:Chemistry. A European Journal, 2014, 20, 50, 16558-16565
- Wiley-V C H Verlag Gmbh, Weinheim