Phenyls with a Twist

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Science  05 Nov 2004:
Vol. 306, Issue 5698, pp. 945
DOI: 10.1126/science.306.5698.945a

Planar arrays of fused aromatic rings are stabilized by electron delocalization in their extended π-network, which leads to the bright colors that make these molecules such useful dyes. However, chemists interested in the competing interplay of steric and electronic factors have long tried to coax such molecules out of planarity by appending bulky substituents. Dai et al. have managed to crowd four pendant phenyls along a 10-carbon stretch of a diindenophenanthrene framework. The compound was formed in 20% yield from a precursor with four alkyne spacers, which were fused into the heptacyclic core through a series of intramolecular rearrangements. Crystallography revealed a severely twisted, nonplanar core geometry, particularly in the central phenanthrene ring. The dangling phenyl substituents are stacked parallel to one another in a partial helix, with the outer phenyls oriented 184° apart, presumably the result of both steric and π-stacking interactions. Nuclear magnetic resonance spectroscopy revealed a rotational barrier of ∼14 kcal/mol for the outer phenyls and a higher barrier for the two rings between them. — JSY

Org. Lett. 10.1021/ol0481434 (2004).

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