Reactions in non-conventional media for sustainable organic synthesis

The growing awareness of the pressing need for greener, more sustainable technologies has focused attention on the use of atom efficient catalytic methodologies for the manufacture of fine chemicals and pharmaceuticals. Another aspect which is receiving increasing attention is the use of alternative reaction media that circumvent the problems associated with many of the traditional volatile organic solvents. The use of nonconventional reaction media also provides opportunities for facilitating the recovery and recycling of the catalyst. Liquid-liquid biphasic catalysis provides an industrially attractive method for the recovery and recycling of catalysts as an alternative to the more traditional solid heterogeneous catalysts. Various approaches to liquid-liquid biphasic catalysis.- aqueous biphasic, fluorous biphasic, supercritical carbon dioxide, ionic liquids, and various combinations thereof - are presented and compared. "The best solvent is no solvent" but if a solvent is needed then water has a lot to recommend it and catalysis in aqueous biphasic systems is has found broad application in an industrial setting. Similarly, supercritical carbon dioxide is an interesting reaction medium in the context of green chemistry and catalysis in various mono- and biphasic systems involving this solvent are presented. Fluorous biphasic systems and ionic liquids also have advantages in certain situations and the advantages and limitations of these media are compared. The ultimate in clean catalytic technologies is to telescope multistep syntheses into one pot in the form of catalytic cascade processes. Examples of such catalytic cascades involving both chemo- and biocatalytic conversions are presented. Biocatalysis has a distinct advantage in this context in that the reactions all take place at or close to ambient temperature and pressure. A problem encountered in the design of catalytic cascade processes is the incompatibility of the various catalysts. The solution to this problem is compartmentalization which is achieved by immobilization of the various catalysts. In this context, a novel and effective method for the immobilisation of enzymes as cross-linked enzyme aggregates (CLEAs) is presented and the use of a combi CLEA, containing two enzymes, for the one-pot conversion of benzaldehyde to S-mandelic acid is reported.