Oxidation of sec-alcohols with Ru(II)-bearing microgel star polymer catalysts via hydrogen transfer reaction: Unique microgel-core catalysis

Abstract

Oxidation of sec-alcohols was investigated with ruthenium-bearing microgel core star polymer catalysts [Ru(II)-Star]. The star polymer catalysts were directly prepared via RuCl2(PPh3)3-catalyzed living radical polymerization of methyl methacrylate (MMA), followed by the arm-linking reaction with ethylene glycol dimethacrylate (1) in the presence of diphenylphosphinostyrene (2). The Ru(II)-Star efficiently and homogeneously catalyzed the oxidation of 1-phenylethanol (S1) to give a corresponding ketone (acetophenone) in higher yield (92%) than the analogs of polymer-supported ruthenium complexes. Importantly, the star catalyst afforded high recycling efficiency in the oxidation. They held catalytic activity against three times catalysis even though they were recovered under air-exposure, whereas the conventional RuCl2(PPh3)3 lost the activity for same recycling procedure due to the deactivation by oxygen. The stability of the star catalysts during the recycle experiment was confirmed by detailed spectroscopic characterization. The star polymers also catalyzed oxidation for a wide range of sec-alcohols with aromatic and aliphatic groups. The substrate affinity was different from that with RuCl2(PPh3)3, suggesting the unique selectivity caused by the specific structure.