Synthesis, redox activity and chemical utility of second-generation verdazyl radicals




Nowak, Bartosz Michal

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Protonation and hydrogen bonding effects on the redox properties of verdazyl radicals were investigated. Verdazyls with various substituents at the C3 position were synthesized, including species with no basic site, a basic site distant from the verdazyl heterocycle, a basic site proximal to the verdazyl heterocycle but with strained hydrogen bonding geometry, and a basic site proximal to the verdazyl heterocycle with favourable hydrogen bonding geometry. Treatment of these verdazyls with trifluoroacetic acid resulted in several general changes to their cyclic voltammograms: the oxidation and reduction potentials shifted to higher potentials, with concurrent loss of reversibility of the reduction process due to protonation of the verdazylate anion. These changes were likely a result of intermolecular interactions between the substrate and acidic medium rather than specific protonation or intramolecular hydrogen bonding effects. Acid treatment of the verdazyl with favourable hydrogen bonding geometry resulted in emergence of a new reversible process in the CV that was found to match the oxidation process of the corresponding leuco verdazyl. Attempts to synthesize this leuco verdazyl were successful, however purification was not completed. Spectroscopy of the crude product suggests that it may be stable to air, an uncommon feature in leuco species due to their propensity to be easily oxidized by O2. Preliminary investigations of the potential of verdazyls as alcohol oxidation catalysts were made. Several previously reported and novel species were investigated as stoichiometric and catalytic oxidants akin to nitroxides and phenoxyls. It was found that the verdazyls, verdazylium cations and verdazyl-metal complexes tested did not stoichiometrically or catalytically oxidize alcohols.



Verdazyls, Radicals, Hydrogen bonding, Alcohol oxidation