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 theconformational 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 thecorresponding bianthrones. The structures of 2–6 were established by 1H- and 13C-NMR spectroscopy and in thecases of 2 and 3, also by X-ray analysis. Compounds 2 and 3 adopted Ci-anti-folded conformations with 53.0 and 28.8 folding dihedrals between pairs of benzene rings of tricyclic moieties. The central C9 C9 bond in 2 wasessentially planar. A short C9 C10 distance of 2.81 Å in 2 indicated an intramolecular overcrowding effect inthe 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 stableconformations with folding dihedrals of 48.7 and 45.0 , respectively at AM1. A DNMR spectroscopic study of E,Z-isomerizations and conformational inversions gave ∆Gc‡(EZ)=99.6 kJ mol 1 (CDBr3) and ∆Gc# (inversion)=97.9 kJ mol 1 (hexachlorobutadiene) in 5 and ∆Gc‡ (inversion) > 108 kJ mol 1 (benzophenone) in 3. These highenergy barriers were interpreted in terms of less overcrowded fjord regions in the anti-folded ground-stateconformations