Stereochemistry of selenium- and tellurium-bridged heteromerous bistricyclic aromatic enes. The fluorenylidenechalcoxanthene series

The effects of selenium and tellurium bridges on the conformations and dynamic stereochemical behavior of heteromerous bistricyclic aromatic enes (1) were studied. 9-(9′H-Fluoren-9′-ylidene)-9H-selenoxanthene (9) and 9-(9′H-fluoren-9′-ylidene)-9H-telluroxanthene (10) were synthesized, applying Barton's two-fold extrusion diazothione coupling method, which is especially suited for heteromerous 1. The isopropyl derivatives 14, 15 and the benzannulated derivatives 16, 17, and 18 were prepared analogously. The structures of 9, 10, 14, 15, and 16-18 were established by ¹H, ¹³C, ⁷⁷Se, and ¹²⁵Te NMR spectroscopy and in the cases of 9 and 10 also by X-ray analysis. The molecules of 9 and 10 adopted anti-folded and folded conformations with 56.3/62.0° and 10.2/8.0° (9) and 63.6 and 2.2° (10) folding dihedrals, higher than in 7 and 8. The degrees of pyramidalization of C⁹ and C^9′ were 2.8/3.9° and 0.9/2.1° (9) and 8 and 15° (10). Considerable overcrowding was evident in the short Se¹⁰ ··· C⁹ and Te¹⁰ ··· C⁹ contact distances in 9 and 10. The crystal structures of 10 indicated relatively short intermolecular Te ··· Te distances (408 pm). The ¹³C NMR chemical shifts of 9,10, 9-(9′H-fluoren-9′-ylidene)-9H-xanthene (12) and 9-(9′H-fluoren-9′-ylidene)-9H-thioxanthene (13) indicated a variation in C⁹ of the chalcoxanthenylidene moiety, ascribed to through space interactions of Se, Te and S with C⁹. The ⁷⁷Se and ¹²⁵Te NMR signals of 9-10 and 14-17 were shifted downfield relative to the homomerous 7 and 8. A DNMR study of 14 and 15 gave ΔG_c^‡ (conformational inversion) = 14.4 (14) and 19.4 kcal mol^-1 (15) and ΔG_c^‡ (E,Z-topomerizations) > 21.6 kcal mol^-1, indicating an increase of ΔG_c^‡ in the fluorenylidenechalcoxanthenes series (11): O < S < Se < Te. The fluorenylidene-derived 1 were found to show distinct behavior for conformational inversions and E,Z-isomerizations. Semiempirical PM3 calculations of 9 and 10 indicated that unevenlyanti-folded conformations were most stable. Conformational inversions of 9 and 10 proceed via the twisted transition states corresponding to calculated barriers of 14.8 and 21.6 kcal mol^-1 in excellent agreement with experiment. The E,Z-isomerizations proceed via orthogonally twisted biradical transition states with predicted barriers of 27.0 and 34.0 kcal mol^-1 for 9 and 10, respectively.