1,2,3,4,5,6,7,8-Octaethylanthracene (5), 1,2,3,4,6,7,8-heptaethylfluorene (7), and 1,2,3,4,5,6,7,8-octaethylfluorene (8) were synthesized by Friedel-Crafts ethylations of 9,10-dihydroanthracene and fluorene. MM3 calculations indicate that the two ethylated six-membered rings of 5 and 7 are conformationally independent. According to the calculations, two low-energy conformers of each compound are possible with the ethyl groups attached to the external aryl rings arranged in an alternated "up-down" orientation. MM3 calculations indicate that in the lowest energy conformation the central fluorene core of 8 adopts a twisted conformation to avoid repulsive steric interactions between the ethyls at the bay region. Two fully alternated up-down conformations are possible for 8, differing in the orientation ("in" or "out") of the ethyls in the bay region. MM3 calculations predict that the lowest energy conformer is the fully alternated "out" form of C(2)() symmetry. The rotational barriers of 5, 7, and 8 are in the 8.7-11.3 kcal mol(-1) range, the largest barrier corresponding to the more crowded octaethylfluorene 8. Anthracene 5 adopts in the crystal a conformation of approximate C(2)(h) symmetry with pairs of peri groups on the same edge of the molecule oriented syn. The conformations adopted in the crystal by 7 and 8 do not correspond to the calculated lowest energy form. In the conformation of 7 in the crystal the ethyl groups on the trisubstituted ring adopt the unusual all syn arrangement. Octaethylfluorene 8 adopts a conformation with a twisted central fluorene core but with a syn arrangement of a pair of vicinal ethyl groups.
Dimerization of tetraethylbenzyne (generated by reaction of 1, 2-dibromo-3,4,5,6-tetraethylbenzene (8) with 1 equiv of BuLi) afforded in low yield octaethylbiphenylene (3), together with a major product which was characterized as 2,3,4,5,3',4', 5'-heptaethyl-2'-vinylbiphenyl (9). X-ray diffraction indicates that biphenylene 3 adopts in the crystal a conformation of approximate C(2)(h )()symmetry with the ethyl groups within each phenylene ring arranged in an alternated up-down fashion. Notably, pairs of vicinal ethyl groups located at peri positions are oriented in a syn arrangement in the crystal. Low temperature NMR spectroscopy is consistent with the presence in solution of either the crystal conformation or a fully alternated conformation lacking any syn interaction. Molecular mechanics (MM3), semiempirical (AM1, PM3), and ab initio calculations indicate that the crystal conformation is a high energy form, and that the lowest energy conformation is the fully alternated form. The topomerization barrier of the methylene protons of the ethyl groups of 3 is 9.4 +/- 0.1 kcal mol(-)(1), which is between the rotational barriers of 8 and 1,2,3, 4-tetraethylbenzene 7 (9.9 +/- 0.1 and 8.2 +/- 0.1 kcal mol(-)(1), respectively). The similarity in rotational barriers suggests that a given tetraethylphenylene subunit does not markedly affect the rotational barrier of the ethyl groups of the other subunit.
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