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

Abstract

The Fe III -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 Fe III center are followed by slower -N¼CH-bond hydrolysis and rapid ligand-to-metal electron transfer coupled to H . elimination. These processes lead to a partially oxidized ligand mixture with Fe II coordinated to unreacted hydrazone. The slow decomposition of this Fe IIhydrazone complex in aerobic alkaline solution produces ferric hydroxide as a new source of Fe III to sustain the hydrazone (-CO-NH-N¼CH-) to hydrazine (CO-NH-NH-CO-) oxidation.