Solvent dependent asymmetric hydrogenation with self-assembled catalysts: a combined catalytic, NMR- and IR-study

摘要

For the first time the hydrogen bond based structure of self-aggregated Rh-phosphine complexes in fluorinated alcohols was directly determined, which gives a rationale for the high enantioselectivity observed in the asymmetric hydrogenation.rnThe effect of monodentate versus bidentate phosphorus ligands in metal-catalyzed asymmetric hydrogenation is a matter of controversial discussions in the literature. In general chelating ligands are reputed to display higher stereodiscriminating properties. However, catalysts bearing bulky monodentate ligands have also been shown as highly stereoselective. Unfortunately, the comparison of ligands differing strongly in steric and/or electronic properties does not really contribute to the final clarification of this issue. Some more clarity can be expected from the comparison of similar ligands under similar reaction conditions. In this connection we are interested in the solvent dependent structure of self-assembling catalysts. Due to their particular construction originally monodentate phosphorus ligands are able to aggregate through hydrogen bonds in the coordination sphere of a metal and thus "pseudo"-chelating ligands are formed.