The Stereochemistry of Overcrowded Homemerous Bistrucyclic Aromatic Enes With Alkylidene Bridges
The objective of the research was to study the e ff ects 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 TiCl 4 –Zn–pyridine–THF. The methylene-bridged bistricyclic ethenes 4 – 6 were synthesized by LiAlH 4 –AlCl 3 –Et 2 O reductions of the corresponding bianthrones. The structures of 2 – 6 were established by 1 H- and 13 C-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 C 9 C 9 bond in 2 was essentially planar. A short C 9 C 10 distance of 2.81 Å in 2 indicated an intramolecular overcrowding e ff ect 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 ∆ G c ‡ ( E Z ) = 99.6 kJ mol 1 (CDBr 3 ) and ∆ G c # (inversion) = 97.9 kJ mol 1 (hexachlorobutadiene) in 5 and ∆ G 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.
Suissa, Michal Rachel
Biedermann, Paul Ulrich