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Synthesis

Experimental Evidence For ‘Hyperaromaticity’

Scientists report the first definitive evidence for aromaticity in benzenium and other arenium ions

by Stuart A. Borman
December 6, 2010 | A version of this story appeared in Volume 88, Issue 49

AROMATIC FLOW
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Credit: Courtesy of P. Fowler & D. Bean
The benzenium ion (left) and its ring current (yellow arrows, right), as calculated by Patrick W. Fowler and David Bean of the University of Sheffield. The current must divert around benzenium’s CH2 group, but it survives the detour.
Credit: Courtesy of P. Fowler & D. Bean
The benzenium ion (left) and its ring current (yellow arrows, right), as calculated by Patrick W. Fowler and David Bean of the University of Sheffield. The current must divert around benzenium’s CH2 group, but it survives the detour.

Measuring the rates of dehydration of cis and trans benzene dihydrodiols in aqueous acid solution has led researchers serendipitously to a long-sought goal: the first definitive experimental evidence for aromaticity of the benzenium ion and other arenium ions (Org. Lett., DOI: 10.1021/ol1014027). Robert S. Mulliken, a pioneer of molecular orbital theory, predicted that such ions would have aromatic character—unusually high stability owing to electron delocalization. But others, including theoretician Michael J. S. Dewar, opposed the idea of “hyperconjugation” in such ions—delocalization arising not only from π electrons but also from σ electrons. Recently, Paul von Ragué Schleyer of the University of Georgia has been a champion of arenium aromaticity, but he has had few disciples. Now, Rory A. More O’Ferrall of University College Dublin and coworkers provide kinetic evidence for arenium ion aromaticity and call the phenomenon “hyperaromaticity.” Theoreticians subsequently showed that the benzenium ion should have an electron ring current, a sign of aromaticity. Implications of the findings “include the origin of stereochemical control of products from dioxygenase enzymes,” which might generate arenium intermediates, More O’Ferrall says.

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