Preparation and Conformation of Octaethylbiphenylene

Abstract

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.

Conformational Studies by Dynamic NMR. 94.1 Cogwheel Pathway for the Stereomutations of Durene Derivatives Containing the Mesityl Ring

“…In both compounds 1 and 2 we have been able to observe, by X-ray diffraction, the crystal structure of the less stable of the two rotamers (syn and anti, respectively) detected in solution by low-temperature NMR spectroscopy. Although other examples of this type have been reported, such a feature is not that usually observed. MM calculations confirm that the structures of the more and of the less stable rotamers correspond to those determined by NMR in solution.…”

Stereochemistry of Polyethylated Aromatic Systems

“…It seems likely that the formation of the second σ-bond is a slower process in the polyethylated diradical than in the corresponding polymethylated diradical (probably because of the increase in steric strain) and hydrogen transfer reactions successfully compete with the formation of the four-membered ring. [31] (6)…”

“…Three different systems (1,2,3,4,5,6,7,8-octaethylanthracene, [30b] octaethylbiphenylene [31] and octaethylfluorene [30b] ) were studied containing two tetraethylphenylene units fused in a coplanar (or nearly coplanar) fashion and differing in the distance between peri ethyl groups, the shortest being present in the fluorene derivative (Scheme 7).…”

Octasubstituted biphenylenes: is there a favoured conformation?

“…We were interested to know whether this design concept could be extended to larger aromatic systems, such as biphenylenes. The X-ray crystal structure of octaethylbiphenylene revealed that this compound adopts an ababbaba conformation in the solid state rather than the fully alternating abababab conformation that was calculated to be the most stable (Taha et al, 2000;Marks et al, 2003). However, we previously prepared the new eight armed ligand octakis(2-pyridylmethylsulfanyl)biphenylene and were encouraged to find that in the solid state it was preorganized into the fully alternating abababab conformation (McMorran & Steel, 2003).…”