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 insitu 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.