Remote dipole effects as a means to accelerate [Ru(amino alcohol)]-catalyzed transfer hydrogenation of ketones

A new generation of 2-azanolbornyl amino alcohol ligands for the catalytic transfer hydrogenation reaction of aromatic ketones was synthesized. Extrem ely active catalysts were formed by introducing a ketal functionality at th e I car end of the ligand, Acetophenone was reduced in 96% cc at low cataly st loading, substrate: to catalyst ratio, S/C 5000, within 90 minutes with isopropyl alcohol as the hydrogen donor. It was round that the dioxolane su bstituent in the ligand increased the turnover frequency, TOF50 from 1050 h (-1) to 3000 h(-1) at an S/C ratio of 1000, Introduction of a methyl group at the carbinol carbon resulted in TOF50 as high as 8500 h(-1), Transfer hy drogenation of a range of aromatic ketones was evaluated and found to reach completion within 30 minutes at room temperature, and excellent enantiosel ectivity, up to 99% cc, was obtained. A possible explanation for the enhanc ed activity was provided by density functional calculations, which showed t hat the presence of a remote dipole in the ligand lowered the transition st ate energy.