Mechanism of Laccase-TEMPO-Catalyzed Oxidation of Benzyl Alcohol

The oxidation of benzyl alcohol by air, catalyzed by the organocatalyst TEMPO and the enzyme laccase has been investigated. To establish the kinetically significant pathways and corresponding kinetic parameters, a series of experiments is conducted with synthesized stable oxidized and reduced forms of the organocatalyst, the oxoammonium cation, and hydroxylamine. The time course of TEMPO and its oxidized and reduced derivatives is monitored off line by a combination of GC analysis, UV/Vis spectroscopy, EPR spectroscopy, and FTIR spectroscopy. TEMPO is found to be regenerated through noncatalyzed comproportionation of the oxoammonium cation with hydroxylamine. A kinetic model is presented based on the experimentally determined kinetically significant pathways. The time dependences of the concentrations of the three redox states of TEMPO and benzyl alcohol are adequately described by the model. The results provide new leads for the development of a practical process for a combined laccase–TEMPO‐catalyzed selective oxidation of alcohols. A change of TEMPO: The mechanism of benzyl alcohol conversion to benzaldehyde in air catalyzed by the o rganocatalyst 2,2,6,6‐tetramethylpiperidinyl‐1‐oxyl (TEMPO) and the enzyme laccase is elucidated. In the organocatalytic cycle, TEMPO is regenerated through noncatalyzed comproportionation of the oxoammonium cation with hydroxylamine. Based on this mechanism, a kinetic model has been developed and validated.