Fluorine activates aromatic rings

Chemists have yet another use for fluorine, as a through-space activator for electrophilic aromatic substitutions. Thomas Lectka and coworkers at Johns Hopkins University used a Diels-Alder reaction to assemble a molecule (shown) in which a fluorine atom pokes into the π-electron cloud of an arene ring (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201601989). The team carried out a nitration of the molecule and observed a reaction rate more than 1,500 times as great as that of a similar molecule in which the fluorine points away from the ring. “Normally, fluorine has only a small effect on the rates of aromatic substitutions when covalently linked to the ring,” Lectka says. “Evidently, when it is positioned over the π-cloud of the ring, it becomes a strong activator.” The researchers suggest that lone electron pairs on the fluorine can stabilize positively charged intermediates, aiding nitration and other electrophilic substitution reactions. Dartmouth College fluorine chemist David Lemal says that, in addition to small-molecule applications, he could imagine using the effect in “a mutated enzyme containing a judiciously placed fluorinated amino acid, thus helping clarify or modify the enzyme’s action.”