Selenium- and tellurium-bridged overcrowded homomerous bistricyclic aromatic enes

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 antifolded conformation with a folding dihedral of 49.2° and short Se₁₀ ⋯ H_9′ and Se_10′, ⋯ 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 C_9′. 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 ⋯ Te_10′ 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.