Aromaticity and stability of azaborines.

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)πzz index and by visualizing the π-electron (de)shielding pattern as a response of the π 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 π-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 π-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 relationship reduces aromaticity to a greater extent by making the π-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.