Ligand modification for the tuning of activity and selectivity in the chemoselective transfer hydrogenation of α,β-unsaturated carbonyls using EtOH as a hydrogen source

The selective reduction of α,β-unsaturated ketones, either at the olefinic or the carbonyl site, offers attractive synthetic opportunities. While carbonyl reduction is well established, selective olefin reduction is less common, particularly when using environmentally friendly ethanol as a hydrogen source. Recently, we reported a coordinatively unsaturated ruthenium complex containing an <i>N</i>,<i>N</i>'-bidentate coordinating pyridinium amidate (PYA) ligand as an efficient catalyst for ethanol-based transfer hydrogenation of α,β-unsaturated ketones; however, there was over-reduction and thus loss of selectivity in reactions over an extended period of time. Capitalizing on the facile synthetic modulation of PYA ligands, we herein report on a series of operationally unsaturated two-legged piano-stool ruthenium cymene complexes [Ru(N^N')(cym)](PF₆) 3a-e with modifications on the PYA-appended aroyl unit. Spectroscopic analysis of these complexes suggests a higher contribution of the π-basic zwitterionic resonance structure of the PYA unit in CD₂Cl₂ and a larger contribution of the π-acidic quinoidal structure in polar and more coordinating CD₃OD. The latter also allows for stabilization of the catalytically relevant alkoxide intermediate [Ru(OEt)(N^N')(cym)] 4. Application of complexes 3a-3e in transfer hydrogenation of <i>trans</i>-chalcone indicates generally good transfer hydrogenation activity and good selectivity towards olefin hydrogenation for all complexes. The variant with a <i>p</i>-CF₃-C₆H₄ substituted PYA ligand, complex 3c, combined high activity and very high selectivity, affording almost exclusively the desired saturated ketone product with only traces of the saturated alcohol even after prolonged reaction times, underpinning the effectiveness of PYA ligand modulation in tailoring activity and selectivity.