Experimental Evidence For ‘Hyperaromaticity’

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.