The Fe-catalyzed oxidation of aroyl hydrazones to aroyl hydrazines: mechanistic insight to a remarkable reaction

The FeIII-catalyzed oxidation of pyridinecarbaldehyde isonicotinoyl hydrazone to isonicotinoyl picolinoyl hydrazine in aqueous solution has been studied from a kinetico-mechanistic perspective. The reaction is more complex than one would anticipate from the known simple and reproducible preparative procedures. A series of very fast ligand coordination reactions to the FeIII center are followed by slower-N=CH-bond hydrolysis and rapid ligand-to-metal electron transfer coupled to Ḣ elimination. These processes lead to a partially oxidized ligand mixture with FeII coordinated to unreacted hydrazone. The slow decomposition of this FeII-hydrazone complex in aerobic alkaline solution produces ferric hydroxide as a new source of FeIII to sustain the hydrazone (-CO-NH-N=CH-) to hydrazine (CO-NH-NH-CO-) oxidation. © 2010 Taylor & Francis