Abstract
Poly(ethylene glycol) (PEG)-armed Ru(II)-bearing microgel-core star polymer catalysts were used for the transfer hydrogenation of ketones. The star catalysts (Ru(II)-PEG Star) were one-pot synthesized by ruthenium-catalyzed living radical polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and a sequential linking reaction with ethylene glycol dimethacrylate (1) and diphenylphosphinostyrene (2). The polymers efficiently and homogeneously reduced acetophenone into 1-phenylethanol in 2-propanol coupled with K2CO3 at a high yield, despite a low catalyst feed ratio to the substrate (Ru(II)/substrate=1/1000). Importantly, the catalytic activity was higher than that of the original RuCl2(PPh3)3, as well as that of similar polymer-supported Ru(II) catalysts, such as poly(methyl methacrylate)-armed star-, polystyrene gel- and random polymer-supported catalysts. Ru(II)-PEG Star is applicable to various substrates, including para-substituted aromatic, aliphatic and bulky ketones, where the activity of Ru(II)-PEG Star is is generally higher than that of RuCl2(PPh3)3. For example, the turnover frequency for 4-chloroacetophenone and cyclohexanone reached ∼1000 h−1, and the reduction rate of cyclopentanone and 3-methyl-5-heptanone was twice as high as that of RuCl2(PPh3)3. The star catalyst also showed high catalyst recyclability, independent of the substrate species. These features most likely arise from its unique reaction space, which consists of a ruthenium-embedded, hydrophobic microgel core surrounded by amphiphilic and polar PEGMA arms. Poly(ethylene glycol)-armed Ru(II)-bearing microgel star polymers, directly obtained from ruthenium-catalyzed living radical polymerization, were used as catalysts for transfer hydrogenation of ketones in 2-propanol. Owing to the good affinity of the core-reaction pocket and the surrounding arms to substrates and products, respectively, the star polymer catalysts homogeneously hydrogenated various ketones into the corresponding alcohols more efficiently than the other polymer-supported catalysts and the original ruthenium counterpart. The catalyst encapsulation into the unique microgel core further afforded efficient catalyst recycle and facile product recovery.
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