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Möbius aromaticity and antiaromaticity in expanded porphyrins

Abstract

Aromaticity is a key concept in chemistry, dating back to Faraday's discovery of benzene in 1825 and Kekulé's famous alternating-double-bond structure of 1865. In 1858, the Möbius strip was discovered by Möbius and Listing. The Hückel rules for predicting aromaticity, stating that [4n + 2] π electrons result in an aromatic system, work for planar molecules. Although molecules with Möbius geometry are not found in nature, chemists have tried to synthesize such molecules since the first theoretical prediction by Heilbronner in 1964 and the prediction of Möbius aromaticity for suitable compounds with [4n] π electrons. However, Möbius-aromatic molecules have proved difficult to synthesize, and sometimes even to identify. Here we summarize recent contributions of several research groups that have succeeded in synthesizing Möbius-type molecules. The results of this survey lead us to suggest that the generation of Möbius topologies in expanded porphyrins is easier than hitherto appreciated.

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Figure 1: Representation of the topologies of π-conjugated electron systems.
Figure 2: The first reported Möbius-aromatic molecule, (CH)9+.
Figure 3: Möbius-aromatic [n]annulenes.
Figure 4: Strategy to stabilize the Möbius structure of annulenes.
Figure 5: Syntheses of Möbius-type expanded porphyrin molecules.
Figure 6: The metal-coordinated Möbius-aromatic expanded porphyrins.
Figure 7: Hückel and Möbius topologies of [28]hexaphyrins.

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Acknowledgements

This work was financially supported by the Star Faculty and WCU programmes (no. 2008-8-1955) from the Ministry of Education, Science and Technology of Korea, AFSOR/AOARD (grant no. FA4869-08-1-4097) and a Grant-in-Aid for Scientific Research from MEXT (grant no. 19205006). Z.S.Y. acknowledges a fellowship of the BK21 programme from the Ministry of Education, Science and Technology of Korea. We would like to express our gratitude to Sihyun Ham at Sookmyung Women's University for critical discussion and Jonathan. L. Sessler at University of Texas (Austin) for valuable suggestions.

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Yoon, Z., Osuka, A. & Kim, D. Möbius aromaticity and antiaromaticity in expanded porphyrins. Nature Chem 1, 113–122 (2009). https://doi.org/10.1038/nchem.172

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