The stereochemistry of overcrowded homomerous bistricyclic aromatic enes with alkylidene bridges

The objective of the research was to study the effects of alkylidene bridge s on the conformations and the conformational behaviour of overcrowded homo merous bistricyclic aromatic ethenes (1). The isopropylidene-bridged bistri cyclic 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 bian thrones. The structures of 2-6 were established by H-1- and C-13-NMR spectr oscopy 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 degrees and 28.8 degrees folding dihedrals between pairs of benzene rings of tricyclic moieties. Th e central C-9... C-9' bond in 2 was essentially planar. A short C-9... C-10 distance of 2.81 Angstrom in 2 indicated an intramolecular overcrowding ef fect in the highly folded bistricyclic ethene. Semiempirical PM3 and AM1 ca lculations 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 degrees and 45.0 degree s, respectively at AM1. A DNMR spectroscopic study of E, Z-isomerizations a nd conformational inversions gave Delta G(c)(double dagger)(E reversible ar row Z) = 99.6 kJ mol(-1) (CDBr3) and Delta G(c)(#) (inversion)= 97.9 kJ mol (-1) (hexachlorobutadiene) in 5 and Delta G(c)(double dagger) (inversion) > 108 kJ mol(-1) (benzophenone) in 3. These high energy barriers were interp reted in terms of less overcrowded fjord regions in the anti-folded ground- state conformations.