Selective hydrogen transfer reduction of ketones by recyclable ruthenium complex catalysts containing a ‘ROMP polymer-attached’ ligand

Various pyridine and 2,2′-bipyridyl ligands attached to the polymer chain end of ring-opened poly(norbornene) were prepared by living ring-opening metathesis polymerization (ROMP) using Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2. The prepared pyridine and 2,2′-bipyridyl derivatives were examined for their use as ligands for catalytic hydrogen transfer reduction of cyclohexanone in the presence of Ru(acac)3 (in toluene/iPrOH in the presence of NaOiPr, at 50 °C, acac: acetylacetonato). The catalytic activity increased upon addition of the above polymer as the ligand, and the prepared catalyst could be recovered by filtration as the precipitate, by pouring the reaction mixture into methanol. The recovered catalyst could be reused without decrease in the activity. Since the olefinic double bond in the ROMP polymer was not hydrogenated under these reaction conditions, exclusive reduction of carbonyl group in various ketones, such as acetophenone, 5-hexen-2-one, 2-allylhexanone could be achieved in this catalysis.

Graphical abstractVarious pyridine and 2,2′-bipyridyl ligands attached to the polymer chain end were prepared by the living ring-opening metathesis polymerization. The activity for ruthenium catalyzed hydrogen transfer reduction of cyclohexanone increased upon addition of the ligand, and the catalyst could be reused without deactivation. The exclusive reduction of carbonyl group in various ketones could be achieved in this catalysis. Figure optionsDownload full-size imageDownload as PowerPoint slide