Aryl substituted N-hydroxyphthalimides as mediators in the laccase-catalysed oxidation of lignin model compounds and delignification of wood pulp

Aryl substituted N-hydroxyphthalimides (NHPIs) have been tested as mediators in the laccase-promoted oxidation of non-phenolic monomeric and dimeric lignin model compounds and in the delignification of kraft pulp samples. In the oxidation of the model compounds a significant increase in the product yields was observed upon increasing the electron donating properties of the NHPI ring substituent. Product yields also increased, but to a smaller extent, by increasing the electron donating properties of the aromatic substituent in the lignin models. These results suggest the contribution to the overall reactivity of both the oxidation of the aryl substituted NHPI to the N-oxyl radical (X-PINO) by laccase and the hydrogen atom transfer step from the substrate to the X-PINO, with a major contribution of the former process. When applied to the delignification of kraft pulps again the mediation efficiency increased by increasing the electron donating properties of the NHPI aryl substituent, the best mediators being 4-Me-NHPI and 4-MeO-NHPI. Thus, the use of non-phenolic lignin model compounds in the oxidation promoted by the laccase/X-NHPI system is well suited to mimic the behaviour of the lignin polymer.