The effects of fluorine and chlorine substituents across the fjords of bifluorenylidenes: Overcrowding and stereochemistry

The bistricyclic aromatic enes (BAEs) (E)- and (Z)-1,1′- difluorobifluorenylidene, 1,8,1′,8′-tetrafluorobifluorenylidene, (E)-and (Z)-3,3′-difluorobifluorenylidene, 3,6,3′,6′- tetrafluorobifluorenylidene, and their chlorinated analogues were subjected to a DFT study of overcrowding in their fjord regions. The B3LYP hybrid functional was employed to calculate energies and geometries of the twisted conformations of these BAEs. The diastereomers E11′F2 and Z11′F2 have identical twist angles (ω = 37.1°) and similar degrees of overcrowding, but differ in the degree and mode of pyramidalization, χ. In E11′F2, χ(C⁹) = +χ(C^9′) = 7.0° (syn-pyramidalization), while in Z11′F2, χ(C⁹) = -χ(C^9′) = 1.0° (anti-pyramidalization). By contrast, in E11′C12 and Z11′C12, ω = 40.6° and 42.7°, respectively. Introducing four halogen substituents results in higher twist angles: ω = 40.3° in 181′8′F4 and 52.6° in 181′8′C14. Surprisingly, Z11′F2 is more stable than E11′F2 (ΔH₂₉₈ = -1.9 kJ/mol), whereas Z11′C12 is less stable than E11′C12 (ΔH₂₉₈ = 2.2 kJ/mol). Both results are consistent with the experimental relative stabilities of these diastereomers. The unexpected stability of Z11′F2 is explained by a combination of steric and electronic effects. Calculations of Coulomb energies for point charge systems of atoms C, F, and H in the fjord regions shows stabilization of the (Z) diastereomer by -45.5 kJ/mol. The dipole-dipole interactions in the fjord region destabilize Z11′F2 by 6.4 kJ/mol relative to E11′F2. Careful examination of the NMR spectra of E11′F2 and Z11′F2 shows, in the latter, evidence of long-range fluorine-fluorine coupling over seven bonds (11.4 Hz) and carbon-fluorine coupling over six bonds (4.8 Hz).