Fluorine’s Positive Side Revealed

Halonium ions are organic compounds in which a chlorine, bromine, or iodine atom bridges two carbons and is positively charged. They are important synthetic intermediates that have been critical in helping understand how substituent groups on neighboring carbons affect stereochemistry in organic substitution reactions. However, it has not been clear whether fluorine, the lightest of the halogens and the most electronegative element on the periodic table, can form a positive ion in solution in the same manner. But Thomas Lectka and coworkers at Johns Hopkins University have now reported “positive” evidence for the existence of a fluoronium ion (Science, DOI: 10.1126/science.1231247). The trick has been finding a suitable precursor molecule. Lectka’s group came up with a double norbornyl system in which two bridgehead carbons are in proximity to each other. By heating the molecule in aqueous trifluoroethanol, the researchers coaxed fluorine substituted on one carbon to attack the other carbon, forming a bridge. They confirmed the fluoronium ion’s existence via kinetics studies, deuterium-labeling experiments, and computational studies. Lectka says his group hasn’t spent much time yet exploring the reactivity of the short-lived fluoronium ion, but he thinks taming the ion could enable new synthetic methods for making valuable organofluorine compounds.