Fe(TSPc)-catalysed benzylic oxidation and subsequent dealkylation of a non-phenolic lignin model

The mechanism by which the phthalocyanine complex trisodium tetra-C sulfonatophthalocyanineiron(III) (Fe(TSPc)) promotes oxidation and dealkylation of non-phenolic benzyl alcohol units in lignin under alkaline conditions was investigated using simple lignin model compounds. The iron complex effectively oxidised benzylic hydroxyl groups in 2-hydroxyethyl apocynol (5), with the extent of oxidation increasing with the amount of Fe(TSPc) added. To a limited extent, Fe(TSPc) also catalysed the oxidation of 5 by small amounts of oxygen in the reaction mixture. However, the extent of oxidation did not increase on adding excess air or oxygen, as greater amounts of oxygen also increased complex degradation. Phenolic products were formed in these reactions in the presence of Fe(TSPc), but at a rate slower than that of the benzylic oxidation. Further experiments indicated that the phenolic products were formed by base-promoted loss of the ether group at C4 and that such dealkylation occurred more readily in oxidised products.