The same experiments were conducted with both enantiomers of diol 2a to assess its rate of transfer. Ideally, the epoxide substrate should diffuse rapidly through the membrane while the diol should not cross it. Swelling of PDMS membranes in contact with some solvents changes their permeability properties . Four solvents were tested in these diffusion experiments: acetonitrile, methanol, tert-amyl alcohol and dichloromethane, with a 150 µm PDMS membrane at 1 M of 1a or 2a at room temperature. The last solvent allows to a good swelling and conducts to a fast diffusion of the epoxide 1a leading to a racemic in both compartment after 10 h. Enantiopure diol 2a in dichloromethane is also not kept in each compartment, leading to a decrease of the enantiomeric excess to 65% after 10 h. Tertamyl alcohol exhibits the same tendencies with a lower relative speed of diffusion compared to dichloromethane. A strong difference of permeability was observed in methanol and acetonitrile, in which the epoxide 1a can diffuse through the membrane (less than 30% ee for methanol and less than 50% ee for acetonitrile after 24 h), while the enantiopure diol 2a is retained with almost no transfer after 24 h. As methanol can play a role of nucleophile during the HKR process, acetonitrile was selected as the most convenient solvent .