| Title
| - Polyethylated Aromatic Rings: Conformation and RotationalBarriers of 1,2,3,4,5,6,7,8-Octaethylanthracene,1,2,3,4,6,7,8-Heptaethylfluorene, and1,2,3,4,5,6,7,8-Octaethylfluorene
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| Abstract
| - 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-dihydroanthraceneand fluorene. MM3 calculations indicate that the two ethylated six-membered rings of 5 and 7 areconformationally independent. According to the calculations, two low-energy conformers of eachcompound are possible with the ethyl groups attached to the external aryl rings arranged in analternated “up−down” orientation. MM3 calculations indicate that in the lowest energy conformationthe central fluorene core of 8 adopts a twisted conformation to avoid repulsive steric interactionsbetween the ethyls at the bay region. Two fully alternated up−down conformations are possiblefor 8, differing in the orientation (“in” or “out”) of the ethyls in the bay region. MM3 calculationspredict that the lowest energy conformer is the fully alternated “out” form of C2 symmetry. Therotational barriers of 5, 7, and 8 are in the 8.7−11.3 kcal mol-1 range, the largest barriercorresponding to the more crowded octaethylfluorene 8. Anthracene 5 adopts in the crystal aconformation of approximate C2h symmetry with pairs of peri groups on the same edge of themolecule oriented syn. The conformations adopted in the crystal by 7 and 8 do not correspond tothe calculated lowest energy form. In the conformation of 7 in the crystal the ethyl groups on thetrisubstituted 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 ethylgroups.
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