Oxidative cleavage of β-O-4 bonds in lignin model compounds with polymer-supported Ni–Salen catalysts†

Transition metal-catalyzed lignin oxidative cleavage reactions have attracted considerable attention. In this work, polymerized ionic liquid-tagged Salen ligands have been initially synthesized, followed by anion exchange, and then coordination with Ni(II) via a –N₂O₂– tetradentate structure. Finally, the as-obtained Ni–Salen complexes were polymerized to give a Ni–Salen polymer catalyst (poly-Ni-[Salen-Vim][OAc]₂). The resulting catalyst showed 99% conversion and 88% selectivity to oxidative cleavage products for the oxidative cleavage of a lignin model compound (2-phenoxy-1-phenylethanone) without any base additive at 110 °C. The polymeric ionic liquid-tagged Salen(Ni) catalysts can be separated easily by centrifugation after the reaction and recycled for five runs with a slight loss of activity. Additionally, studies on birch lignin depolymerization indicated that polymer-supported Ni Salen catalysts were able to cleave β-O-4 linkages to produce dimeric products. Further investigation suggests that the oxidative cleavage reaction was proceeded via a radical pathway.