Probing stability of exocyclic BN-doped fulvenes in ground state: 1-aminoboroles vs 1-pyrroloboranes

Abstract

BN-doping, i.e., replacing a nonpolar C=C bond with the polar BN bond, in organic compounds, imparts desirable properties, for instance, hydrogen storage, adsorption, precursors to the bigger compounds, and smart material design. Pentafulvenes, the nonbenzenoid aromatics, are the intermediates for the synthesis of a wide range of compounds owing to their multiple cycloaddition behaviour. Herein, we BN-doped pentafulvene at its exocylic position that led to two isomeric BN-fulvenes: 1-pyrroloborane (P-1H) and 1-aminoborole (B-1H). Thermodynamic stability of these isomers was probed by symmetrically placing various substituents at the available exocyclic and ring positions. Results reveal that the direct substitution on the B-side of the B=N bond brings significant stabilization to the parent B-1H and its derivative P-1X fulvenes compared to the N-side substituted analogues P-1H and B-1X, respectively. Further, considering ring substitutions, again B-side of the B=N bond is stabilized: isomers B-2Y and B-3Z compared to P-2Y and P-3Z, respectively, are predicted with small to moderate stabilization. In this stability trend, several factors–electronegativity, geometry, natural population analysis charges, aromaticity, and frontier orbital energies–play a cruicial role. The current study could be beneficial for designing BN-fulvenes of particular stability and electronic properties.