The stereochemistry of overcrowded homomerous bistricyclic aromatic enes with alkylidene bridges

Abstract

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 4–6 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 Ci-anti-folded conformations with 53.0° and 28.8° folding dihedrals between pairs of benzene rings of tricyclic moieties. The central C9C9′ bond in 2 was essentially planar. A short C9⋯C10 distance of 2.81 Å in 2 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 ΔGc‡(E⇌Z) = 99.6 kJ mol−1 (CDBr3) and ΔGc# (inversion) = 97.9 kJ mol−1 (hexachlorobutadiene) in 5 and ΔGc‡ (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.