Amalia Levy

Selenium and Tellurium Tricyclics. Conformational Effects on 77Se and 125Te NMR Spectra

A series of selenium and tellurium tricyclics (1,2) have been synthesized and their 77Se and 125Te NMR chemical shifts determined. Semiempirical MNDO-PM3 calculations on representative 1 and 2 have been used to characterize their folded and planar conformations. The results indicate a pronounced effect of the conformations on these chemical shifts.

Zinc-mediated reductive dimerizations of telluroxanthone and selenoxanthone. Tellurium–selenium selectivity

Abstract Reductive dimerizations of 9 H -selenoxanthen-9-one ( 3 ) and 9 H -telluroxanthen-9-one ( 4 ) (1:1) with zinc in boiling AcOH and HCl for 1 hour gave the bistricyclic ethanes 9,9′-bi(9 H -selenoxanthene ( 10 ), 9,9′-bi(9 H -telluroxanthene) ( 11 ) and 9-(9′ H -telluroxanthen-9′-yl)-9 H -selenoxanthene ( 12 ) in the ratios 21 ( 10 ):35 ( 11 ):44 ( 12 ) (in addition to 9 H -selenoxanthene ( 13 ) and 9 H -telluroxanthene ( 14 )). Analogous reactions of 4 and 9 H -thioxanthen-9-one ( 9 ) (1:1) and 3 and 9 (1:1) gave the corresponding bistricyclic ethanes. The reactions were chalcogen selective with a preference towards the tellurium-bridged bistricyclic ethanes.

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 Bartons two-fold extrusion diazo–thione 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 C9′ 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 ΔGc‡ (conformational inversion) = 14.4 (14) and 19.4 kcal mol−1 (15) and ΔGc‡ (E,Z-topomerizations) > 21.6 kcal mol−1, indicating an increase of ΔGc‡ 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 unevenly anti-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.

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 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 A). 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.