Selenium- and tellurium-bridged heteromerous overcrowded bistricyclic aromatic enes with central six-member and seven-member rings

A series of selenium-bridged and tellurium-bridged heteromerous bistricyclic aromatic enes (BAEs) 15-25 with six-member and seven-member central rings have been synthesized by Barton-Kellogg olefination, twofold extrusion coupling method between tricyclic aromatic thiones and tricyclic aromatic diazo derivatives. The crystal structures of thioxanthenylidene-anthracenone (15), selenoxanthenylidene-anthracenone (20), and telluroxanthenylidene-anthracenone (21) were determined. The ordered crystal structure of 21 showed a very high degree of folding of the telluroxanthenylidene moiety (58.5°) and very short intramolecular Te···C^9′ contact distance, indicating >20 % penetration which may reflect secondary nonbonding interactions and lack of aromatic telluroxanthenylium-anthracenolate contribution. The BAEs were subjected to a ¹H-, ¹³C-, ⁷⁷Se-, and ¹²⁵Te-NMR spectroscopic study. The ¹H-NMR spectra indicated anti-folded conformations in solution. The measured pronounced downfield ⁷⁷Se- and ¹²⁵Te-NMR shifts in 20 and 21, relative to the corresponding spectra of homomerous BAEs diselenoxanthylene (7) and ditelluroxanthylene (8), may be due to the proximity of the Se and Te bridges to the carbons of the central enes in the anti-folded conformations. This deshielding effect was larger in 21 than in 20, δ(¹²⁵Te)/δ(⁷⁷Se) = 1.53, a ratio higher than in any other member of the series.