Enhanced proton loss from neutral free radicals: toward carbon-centered superacids

Radical centers close to protons are known to enhance their dissociation. Investigation of the generality of this radical enhanced deprotonation (RED-shift) phenomenon, and the kinds of structures in which it operates, are reported. The pK_as for sulfinic, sulfonic, pentan-2,4-dione, and Meldrum's acid species, with adjacent radicals centered on C-, N-, and O atoms, were computed by a DFT method from free energies of deprotonation. All series showed significant RED-shifts that increased with the electronegativity of the radical center. The hugely negative pK_a obtained for a Meldrum's acid with an alkoxyl radical substituent showed it to belong to the superacid class. The ethyne unit was found to be uniquely effective at enhancing acidity and conducting RED-shifts through chains up to and beyond 20 atoms. These connector units enable a radical center to alter the pK_a of a spatially remote acidic group. RED-shifted species were characterized by conjugate radical anions displaying site exchange of spin with electronic charge.