Effect of gas–liquid mass transfer on enantioselectivity in asymmetric hydrogenations

The enantiomeric excess (ee) in asymmetric hydrogenation may vary with the concentration of hydrogen dissolved in the liquid phase. This concentration is proportional to hydrogen pressure when the reactor operates in chemical regime, but depend on the stirring rate if mass transfer is limiting. For example, decrease of ee from 61 to 51% ee has been observed upon varying the stirring rate from 100 to 2000 rpm. A good characterisation of the reactor used is recommended to quantify the influence of hydrogen on ee.

The enantiomeric excess (ee) in asymmetric hydrogenation may vary with the concentration of dissolved hydrogen. Decrease of ee from 73 to less than 40% has been observed in the case of the Rh/diop catalysed hydrogenation of methyl-Z  -(α)-acetamidocinnamate. The graphic sketches the importance of reactor type on the ee, which can lead to misleading interpretations on the structure–enantioselectivity relationship.Figure optionsDownload high-quality image (117 K)Download as PowerPoint slide