Insight into the role of fluorinated dendrimers in ruthenium(II) catalyst for asymmetric transfer hydrogenation: The stabilizing effects from experimental and DFT approach

Abstract

A series of fluorinated dendritic chiral ligands have been designed and synthesized. These fluorinated dendrimers are capable of forming a well-defined semi-rigid structure, which was revealed to play a vital role in the ruthenium(II) bifunctional catalyst for asymmetric transfer hydrogenation of prochiral ketone substrates. In contrast to the classical non-fluorinated dendrimer carrier, both NMR and DFT study exhibit that the introduction of fluorine atoms leads to considerable intramolecular weak interactions such as π–π stacking and hydrogen bonding interactions, which make the dendritic backbone exist with a semi-rigid structure in the catalyst. This influences the performance of the catalytic center in terms of the stability and reusability. This concept was employed as a strategy to design a new Ru(II)-complex catalyst Ru-G-2′-F, which demonstrated obviously improved recycling ability up to fifteen times. Significantly enhanced activity, high enantioselectivity and outstanding recycling ability have also been achieved even at high reaction temperature with Ru-G-2-F.