Abstract for presentation at Chemeca 2007

Enantiopure molecules have applications in many industries, including pharmaceuticals, agrochemicals and food. Enantiopure molecules can be obtained using a dynamic kinetic resolution (DKR), which combines a kinetic resolution (KR) with an in-situ racemisation to eliminate one enantiomer of a racemic substrate. One of the most common DKRs is the conversion of (R,S)-1-phenylethanol to (R)-1-phenylethylacetate. Recent work suggests that the racemisation rate can be increased by using the common ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) as the reaction solvent. However, this IL is very viscous (153 mPa s at 31^oC), so obtaining good mixing and heat transfer would be challenging in industrial reactors. This paper therefore extends this result to mixtures of ILs with toluene to evaluate whether a less viscous IL mixture can be used to improve the overall yield and enantiomeric excess of the reaction.
With toluene as solvent, racemisation of (S)-1-phenylethanol by ruthenium p-cymene and triethylamine at 30^oC was incomplete. However, using [BMIM][PF₆]:toluene (50:50 v/v) as the solvent, the racemisation rate improved considerably: enantiomeric excess was reduced from 100% to 2.1% after 48 hours. This improvement could be attributed to a decrease in the amount of catalyst deactivation in [BMIM][PF₆]:toluene. In contrast, when combined with an enzymatic-resolution (in a DKR), the racemisation did not occur in [BMIM][PF₆]:toluene. This is possibly due to catalyst deactivation by acetaldehyde and acetic acid, which can accumulate in [BMIM][PF₆]. Consequently, [BMIM][PF₆]:toluene cannot be used directly as a new solvent for this DKR; some modification of the reactants and reaction conditions is required.