Biocatalytic transformations in ionic

Kragl U. et al. Biocatalytic transformations in ionic liquids, Ionic Liquids in Synthesis, Wiley-VCH, Verlag GmbH Co. KGaA, Weinheim, 2003. 

Here, we will review the various issues related to biocatalytic reactions in ionic liquids. Biocatalyst tested in ionic liquids will be discussed firstly, and then the effect of ionic liquids on the activity, selectivity as well as on the stability of biocatalyst in ionic liquids will be surveyed. Finally, various applications of ionic liquids as reaction medium for biocatalytic transformations will be reviewed. 

The viscosity of ionic liquids is high compared with molecular solvents and increases with the chain length. Consequently, diffusion is bound to be slow in ionic liquids. The effects on biocatalytic transformations seem to be insignificant, however, except in extreme cases, presumably because the reaction times are measured in hours rather than minutes. 

Despite many efforts the use of classical organic solvents for biocatalytic transformations using whole cells remains limited. Water as a natural reaction medium limits the number of applications, since only a small number of substrates are sufficiently water soluble. As an alternative the use of an ionic liquid may be considered. A very early example for such a reaction has been reported by Cull in 2000 [360]. 

On the basis of encouraging work in the development of L-proline-DMSO and L-proline-ionic liquid systems for practical asymmetric aldol reactions, an aldolase antibody 38C2 was evaluated in the ionic liquid [BMIM]PF6 as a reusable aldolase-ionic liquid catalytic system for the aldol synthesis of oc-chloro- 3-hydroxy compounds (288). The biocatalytic process was followed by chemical catalysis using Et3N in the ionic liquid [BMIM]TfO at room temperature, which transformed the oc-chloro-(3-hydroxy compounds to the optically active (70% ee) oc, (3-epoxy carbonyl compounds. The aldolase antibody 38C2-ionic liquid system was also shown to be reusable for Michael additions and the reaction of fluoromethylated imines. 

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