Polymer-bound, amphiphilic Hoveyda-Grubbs-type catalyst for ring-closing metathesis in water

We report on the synthesis of a new amphiphilic, polymer-bound variant of the Hoveyda-Grubbs catalyst via the coupling reaction of a carboxylic acid-functionalized poly (2-oxazoline) block copolymer with 2-isopropoxy-5-hydroxystyrene and subsequent reaction of the resulting macroligand with a second generation Grubbs catalyst. For the benchmark, the substrate diethyl diallylmalonate was studied in the ring-closing metathesis (RCM) reaction and a turn-over number (TON) of up to 390 in water was achieved. To the best of our knowledge, this is the highest value for any aqueous RCM reaction to date. For the first time, recycling of a ruthenium initiator in an aqueous RCM reaction has been successful to some extent. In addition, the micellar conditions accelerate the conversion of the hydrophobic diene and at the same time stabilize the active alkylidene species, although competing decomposition of the catalyst in water still impairs the catalyst performance. Residual ruthenium content was determined to be below 1 ppm in the product suggesting a very low leaching of the polymeric catalyst system.