The stereochemistry of overcrowded homomerous bistricyclic aromatic enes with alkylidene bridges

The objective of the research was to study the effects of alkylidene bridges on the conformations and the conformational behaviour of overcrowded homomerous bistricyclic aromatic ethenes (1). The isopropylidene-bridged bistricyclic ethene 2 and 3 were synthesized by a reductive "dimerization" of 7, using TiCl₄-Zn-pyridine-THF. The methylene-bridged bistricyclic ethenes 4-6 were synthesized by LiAlH₄-AlCl₃-Et₂O reductions of the corresponding bianthrones. The structures of 2-6 were established by ¹H- and ¹³C-NMR spectroscopy and in the cases of 2 and 3, also by X-ray analysis. Compounds 2 and 3 adopted C_i-anti-folded conformations with 53.0° and 28.8° folding dihedrals between pairs of benzene rings of tricyclic moieties. The central C₉=C_9′ bond in 2 was essentially planar. A short C₉...C₁₀ distance of 2.81 Å indicated an intramolecular overcrowding effect in the highly folded bistricyclic ethene. Semiempirical PM3 and AM1 calculations of the anti-folded, syn-folded, twisted and orthogonally twisted conformations of 2 and 4 indicated that anti-folded 2 and 4 were the most stable conformations with folding dihedrals of 48.7° and 45.0°, respectively at AM1. A DNMR spectroscopic study of E, Z-isomerizations and conformational inversions gave ΔG_c^‡(E⇆Z) = 99.6 kJ mol^-1 (CDBr₃) and ΔG_c^# (inversion) = 97.9 kJ mol^-1 (hexachlorobutadiene) in 5 and ΔG_c^‡ (inversion) > 108 kJ mol^-1 (benzophenone) in 3. These high energy barriers were interpreted in terms of less overcrowded fjord regions in the anti-folded ground-state conformations.