The Strain of Fusing

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Science  14 Feb 2003:
Vol. 299, Issue 5609, pp. 981
DOI: 10.1126/science.299.5609.981c

One way to generate an “anti-aromatic” system, in which the overall π-bonding network is destabilizing, is to add two electrons to an aromatic network. Aprahamian et al. have done that in two strained pyrene systems; a large ring bridges the ends of pyrene and bends the π-bonded network out of a single plane without disrupting aromaticity. Nuclear magnetic resonance spectroscopy revealed that adding two electrons did not yield an anti-aromatic molecule. Instead, a new σ bond formed across one of the six-membered rings, creating fused cyclopropyl and cyclopentyl rings. This alternative not only diminishes the induced ring strain in the pyrene network, but it also allows the two added electrons to localize and distance themselves. Thus, although strain does not reduce aromaticity in these pyrenes, it does affect their reactivity. — PDS

J. Am. Chem. Soc. 10.1021/ja0291991 (2003).

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