Thermoregulated Phase-Transfer Catalysis via PEG-Armed Ru(II)-Bearing Microgel Core Star Polymers: Efficient and Reusable Ru(II) Catalysts for Aqueous Transfer Hydrogenation of Ketones

Thermoregulated phase-transfer catalysis for the transfer hydrogenation of 2-octanone in 2-propanol/H(2)O biphasic media was achieved with ruthenium-bearing microgel-core star polymers with amphiphilic, thermosensitive poly(ethylene glycol) (PEG) arms [Ru(II)-PEG star], which were directly prepared by the ruthenium-catalyzed living radical polymerization in conjunction with a phosphine ligand-carrying styrene derivative. The star polymers were first placed in the aqueous (lower) layer at room temperature and immediately moved into the organic (upper) layer at 100 degrees C, and once again, moved down to the aqueous layer (lower) upon cooling the solution to room temperature. The Ru(II)-PEG star catalyst was clearly superior to the original Ru(II) catalyst and related non-microgel catalysts [Ru(II)-PEG block] in terms of activity and recovery/recycle, due to the unique designer structure of the microgel-core star polymers. Other substrates (less hydrophobic alkyl ketones and aromatic ketone) were also efficiently hydrogenated into the corresponding sec-alcohols with the star catalyst in aqueous media. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 373-379, 2010