Aromaticity Goes Flat Out

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Science  24 Aug 2001:
Vol. 293, Issue 5534, pp. 1405
DOI: 10.1126/science.293.5534.1405a

In principle, large cyclic molecules containing p-bonded subunits, such as pyrroles and thiophenes, should exhibit aromaticity if they follow the Huckel rule and contain 4n + 2 electrons. In practice, rings with a large number of subunits tend to twist over to form a figure eight, so that the orbital overlap needed to induce aromaticity is destroyed. Anand et al. now show that derivatives of octaphyrin, a 34p electron (n = 8) system, can be formed as a flat molecule if the two halves, each containing four subunits, are brought together in a single coupling step. The crystal structure of the sulfur derivative shows that one thiophene of each bithiophene unit is inverted to form a flat molecule. Nuclear magnetic resonance and ultraviolet visible spectroscopy confirm their aromatic nature. — PDS

J. Am. Chem. Soc., 10.1021/ja011265w.

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