Introducing boranorcaradienes with more stability than their corresponding borepins: Reversal of tautomerization via substituents at theoretical levels

Abstract

In contrast to typical borepins, which appear about 40.0 kcalmol−1 more stable than their corresponding tautomeric boranorcaradienes, we have found 2 species, which have reversed this trend and pushed the equilibrium in favor of their corresponding boranorcaradienes. They are namely 1a,9b-dihydro-1H-borireno[2,3-h]pyridazino[4,3-f]cinnoline and 1a,9b-dihydro-1H-borireno[2,3-h]pyrimido[5,4-f]quinazoline which stand up among 14 isomeric systems probed, at B3LYP/AUG-cc-pVTZ, M06-2X/AUG-cc-pVTZ, MP2/AUG-cc-pVTZ, and HF/AUG-cc-pVTZ. Energy barriers are calculated in gas-phase, where the possibility of a rapid interconversion of the isomers is ruled out. Generally, dibenzoboranorcaradienes assume Cs symmetry with planar geometry and dihedral angle of zero degree. In contrast, their corresponding borepins show a high tendency for puckering with dihedral angle of ~66°. The preference of the latter for puckered non-planar geometries is evidenced by natural bonding orbitals calculations and visually through their frontier molecular orbitals. Main interactions appear to be hyperconjugations of σ and π bonds across the rings. Position and number of nitrogen atoms on the fused rings seem to affect the energy gap, dipole moment, symmetry, dihedral angle, the chemical shift, NICS, bond lengths, and charge distribution.