Salen complexes with bulky substituents as useful tools for biomimetic phenol oxidation research.

The catalytic properties of bulky water-soluble Co-, Cu-, Fe- and Mn-salen complexes in the oxidation of phenolic lignin model compounds have been studied in aqueous water--dioxane solutions (pH 3--10). Mn catalysts were found to oxidize coniferyl alcohol in a same reaction time as horseradish peroxidase (HRP) enzyme and Mn and Co catalysts showed different regioselectivity suggesting a different substrate to catalyst interaction in the oxidative coupling. When the oxidation of material more relevant to plant polyphenolics was studied, the results indicated that the complexes catalyze one- and two-electron oxidations depending on the bulk of the substrate.