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Enantioselective hydrogenation with self-assembling rhodium phosphane catalysts: influence of ligand structure and
Mandy-Nicole Birkholz1, Natalia V Dubrovina, Haijun Jiao
1Institut für Chemie der Universität Rostock, A.-Einstein-Strasse 3 a, 18059 Rostock, Germany. armin.boerner@catalysis.de
Abstract:
Three sets of new and related chiral phospholane and phosphepine ligands have been prepared for Rh-catalyzed enantioselective hydrogenation. The size and substitution pattern of the cyclic monophosphanes were varied. More importantly, the ligands differ in the nature of the heterocyclic group linked to the trivalent phosphorus atom: 2-pyridone or 2-alkoxypyridine. In the corresponding Rh complexes, the pyridone units of two monodentate P ligands can assemble by hydrogen bonding and form chelates. In contrast, synthetic precursors bearing alkoxypyridine appendages are not able to aggregate via intramolecular hydrogen bonds. The nature of self-assembly is dependent on the nature of the P ligand and the solvent used for the hydrogenation (CH2Cl2 vs. MeOH). These features affect the rate of the reaction as well as the enantioselectivity, which varied in the range of 0-99 % ee Complexation studies and DFT calculations were performed to explain these differences.
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