A set of 16 new and closely related bisphospholane ligands have been prepared by using a highly flexible and convergent approach. Each synthesis can be performed on an industrially relevant scale. The bisphosphines differ in the nature of the bridge connecting both phospholane units. Bridges are formed by three-, four-, five- and six-membered heterocyclic or alicyclic rings. Bisphospholanes and their Rh-precatalysts have been investigated by using results of theoretical calculations (DFT) and analytic measurements ((31)P and (103)Rh NMR spectroscopy, X-ray structure analysis). The studies showed that catalysts based on ligands with maleic anhydride or maleimide bridges give constantly superior enantioselectivities in methanol as the solvent. This may account for optimised steric and electronic effects. However, by changing the solvent catalysts with other backbones can give rise to excellent results. This gives proof that simple correlations between steric and electronic properties and results in the enantioselective hydrogenation frequently claimed in literature are not general.

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Abstract:The synthesis of chiral 3-hydroxy-2-methylpropanoic acid esters (e.g., "Roche ester" 3a) based on the rhodium-catalyzed stereoselective hydrogenation of Baylis-Hillman reaction products was investigated. Full conversions and enantioselectivities of up to 99% at a substrate/catalyst ratio of up to 500/1 were achieved by application of bisphospholanes of the catASium M series as ancillary ligands. An interesting kinetic resolution was observed by the diastereoselective hydroxy-directed hydrogenation of related racemic b-branched precursors affording mainly anti-isomers with up to 96%ee.

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