ISSN:
1434-1948
Keywords:
Asymmetric catalysis
;
Hydrogenations
;
Hemilabile ligand
;
Hydroxy phosphane
;
Methoxy phosphane
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The influence of two threo-arranged methoxy or hydroxy groups as hemilabile ligands in seven-membered (diphosphane)RhI chelates upon the asymmetric hydrogenation is studied. In comparison to the parent complex based on 1,4-bis(diphenylphosphanyl)butane (DPPB) a lowering of the reaction rate is caused by the hemilabile ligands. The catalyst bearing the hydroxy groups gives significantly higher enantioselectivities than the corresponding methoxy complex (by ca. 35 % ee). X-ray structural analysis reveals that the oxygen atom of the hydroxy group is by 0.9 Å closer to the metal center than that of the methyl ether. 31P-NMR studies give evidence that the dihydroxy diphosphane ligand binds at low temperature exclusively in the η3-coordination mode, whereas for the dimethoxy complex also an η2-coordinated isomer can be found. These differences in complexation could be responsible for the superior enantioselectivities achieved with this and other hydroxy catalysts.
Type of Medium:
Electronic Resource
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