Cooperative effects of ruthenium micellar catalysts and added surfactants in transfer hydrogenation of ketones in water

The effect of various added surfactants S on the activity of surface-active ruthenium complexes RuLn (n = 8, 16) in transfer hydrogenation of ketones in water was investigated. The catalysts RuLn capable of forming mixed micelles with S showed an increase in activity. RuL16 can form mixed micelles with all types of S, while the less surface-active counterpart RuL8 strongly interacts only with anionic and zwitterionic S. The mixtures of anionic surfactants with RuL8 demonstrated strong synergic enhancement of their activities by up to two orders of magnitude in hydrogenation of hydrophobic ketones. In mixed micelles, the added surfactant lowers the cmc of RuLn, resulting in better emulsification of the substrate. It was also shown that careful choice of surfactant S allowed for better and controlled hydration of the mixed micelles RuLn/S. Both aspects concurred to achieve higher catalytic performances in hydrophobic ketone reduction. For example, in RuLn/S, the better hydrated alkane sulfonate DSS always outperforms the less hydrated alkyl sulfate SDS.