Stereoselective Reactions in Continuous Flow Systems

Alkylations Chiral amino alcohol-functionaUzed polystyrene resins [36, 37] were useful catalysts in the enantioselective alkylation of aldehydes with diethyl zinc in continuous-flow columns. Another continuous flow system for the synthesis of enantioenriched diarylmethanols from aldehydes used transmetallation of tti-arylboroxins with diethyl zinc over amino alcohol-functionaUzed polystyrene resin catalyst [38]. 

Acylations Carbon-coated magnetic Co-nanoparticles tagged with azabis (oxazoline)-Cu(II) complexes were utilized in a Cu(I)-catalyzed azide/alkyne cycloaddition (CuAAC), and the efficacy of the resulting nanomagnetic catalyst was tested in the kinetic resolution (KR) of racemic l,2-diphenylethane-l,2-diol via asymmetric monobenzoylation under batch conditions and in a continuous flow-type reactor [39]. 

Hydrolytic reactions In a continuous flow process, polystyrene- and silica-bound chiral Co(salen) complexes were applied as efficient and highly enantioselective catalysts for the hydrolytic KR of terminal epoxides [40]. 

Ketone reduction The enantioselective hydrogenation of vicinal diketones such as l-phenylpropane-l,2-dione over cinchonidine-modifled Pt gave the (R)-enantiomers in excess. Faster reaction of (S)-hydroxyketone to the diols, contributed to an increase in enantioselective excess (ee) [41]. 

Transamination On-column 1,3-proton shift reactions-a key transformation in the biomimetic reductive amination process - were investigated using simple silica-adsorbed l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) catalyst [42, 43]. This process enabled chirality transfer from (H)-l-phenylethylamine to perfluoroalky-lated ketones. The continuous-flow process proved to be far superior to the batch processes, both from an economical and synthetic point of view. 

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Enzyme-Catalyzed Stereoselective Reactions in Continuous-Flow Systems... 

Figure 9.2 Stereoselective enzyme reactions in continuous-flow systems.

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