Abstract
The effects of selenium and tellurium bridges on the conformations of overcrowded homomerous bistricyclic aromatic enes were studied. The structures of the target molecules 9,9′-bi(9H-selenoxanthen-9-ylidene) (7) and 9,9′-bi(9H-telluroxanthen-9-ylidene) (8) were established by 1H, 13C, 77Se, 125Te NMR spectroscopy, and by X-ray analysis. The molecules adopted anti-folded conformations with 53.6 (7) and 53.1° (8) folding dihedrals between pairs of benzene rings of the tricyclic moieties, whereas the corresponding folding dihedral in 9-methylene-9H-selenoxanthene 20 was considerably lower, 32.4°. An X-ray analysis of 9,9′-bi(9H-selenoxanthene) (9) indicated an anti-folded conformation with a folding dihedral of 49.2° and short Se10⋯H9′ and Se10′⋯H9 distances. Compounds 7 and 8 exhibited low degrees of overcrowding in the fjord regions. Considerable overcrowding was evident in the short Se10⋯C9 and Te10⋯C9 contact distances in 7 and 8. The high shielding of the protons in the fjord regions of 7 and 8 revealed anti-folded conformations in solution. The 13C NMR chemical shifts of 7 and 8 were characterized by low-field absorptions of C9 and C9′. Semi-empirical PM3 calculations of the anti-folded, syn-folded, and twisted conformations indicated that anti-folded-7 and syn-folded-8 were the most stable conformations, respectively. The special stability of syn-folded-8 was attributed to the short intramolecular Te10⋯Te10′ distance (3.06 Å). Compounds 7 and 8 were synthesized by reductive “dimerizations” of 9H-selenoxanthene-9-thione (13) and 9H-telluroxanthene-9-thione (17) with copper in boiling toluene. Compound 7 was also synthesized by diazo–thione coupling between 13 and 9-diazo-9H-selenoxanthene (14), followed by elimination of sulfur from the intermediate thiirane 15. 9,9′-Bi(9H-selenoxanthene) (9) and 9,9′-bi(9H-telluroxanthene) (10) were prepared by low valent titanium induced reductive “dimerizations” of 9H-selenoxanthen-9-one (11) and 9H-telluroxanthen-9-one (12), respectively, using TiCl4/Zn/pyridine–THF.
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