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

Abstract

BN-doping, i.e., replacing a nonpolar CC bond with the polar BN bond, in organic compounds imparts desirable properties, for instance, hydrogen storage and adsorption, and such compounds could be used as precursors to bigger compounds and for smart material design. Pentafulvenes, nonbenzenoid aromatic compounds, are intermediates for the synthesis of a wide range of compounds owing to their ability to participate in multiple cycloaddition reactions. Herein, we doped BN into a pentafulvene at its exocylic position that led to two isomeric BN-fulvenes: 1-pyrroloborane (P-1H) and 1-aminoborole (B-1H). The 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 BN bond results in 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. Similarly, for ring substitutions, substitution on the B-side of the BN bond results in more stabilization: isomers B-2Y and B-3Z compared to P-2Y and P-3Z, respectively, are predicted to exhibit small to moderate stabilization. In this stability trend, several factors–electronegativity, geometry, natural population analysis charges, aromaticity, and frontier orbital energies–play crucial roles. The current study could be beneficial for designing BN-fulvenes of particular stabilities and electronic properties.