Mechanistic Insights into the Oxidation of Veratryl Alcohol with Co(salen) and Oxygen in Aqueous Media: An in‐situ Spectroscopic Study

Many transition‐metal complexes can perform catalytic oxidations, but their corresponding reaction pathways are still not clear. In this study, the mechanism of Co(salen)‐catalyzed [salen = N,N'‐bis(salicylidene)ethylenediamine] oxidization of veratryl alcohol (3,4‐dimethoxybenzyl alcohol) by dioxygen in alkaline aqueous solution was elucidated with in‐situ ATR‐IR, Raman and UV/Vis spectroscopy. The mechanism of this reaction seems to start by formation of a bis‐μ‐hydroxo[(Co(salen)]2 species, which explains the dramatic effect of pH on the reaction rate. Substrate coordination to this species leads to formation of a cobalt‐bound veratryl alkoxo intermediate, to which oxygen molecule can bind. Formation of a μ‐peroxo bridge between two such Co(salen) substrate units is observed in the UV/Vis spectra. Transfer of a hydrogen atom from the substrate to the peroxo bridge results in detachment of the product aldehyde and regeneration of the initial bis‐μ‐hydroxo[(Co(salen)]2 species. In the overall cycle two substrate molecules are oxidized to aldehyde and molecular oxygen is reduced to water. The rate‐limiting step is the detachment of the product molecule, which is aided by the methoxy substituents in the aromatic ring of the benzylic alcohol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)