SummaryThis review highlights the state of the art in the field of coupled chemo(enzymatic) reactions in continuous flow. Three different approaches to such reaction systems are presented herein and discussed in view of their advantages and disadvantages as well as trends for their future development.

Hydroxynitrile lyase from Hevea brasiliensis (HbHNL) is a promiscuous biocatalyst that, besides the native cyanohydrin reaction, also catalyzes the asymmetric Henry reaction yielding (S)‐β‐nitroalcohols with high enantiomeric excess. Since the Henry reaction is reversible, the enzyme can be also utilized for the production (R)‐enantiomers by means of resolution of racemic β‐nitroalcohols. Herein the biocatalytic retro‐Henry reaction is studied using the cleavage of 2‐nitro‐1‐phenylethanol as a model system. The main problem that prevents high levels of conversion or high ee values during the cleavage of the β‐nitroalcohol is the formation of benzaldehyde, which is known to be a strong enzyme inhibitor. The product inhibition is overcome by performing the biocatalytic retro‐Henry reaction in the presence of HCN, which reacts in situ with benzaldehyde and converts it to the less‐inhibitive mandelonitrile. By using such a reaction cascade, it was possible to conduct the resolution practically to completion (95 % ee, 49 % conversion). Furthermore, the catalyst productivity achieved during the resolution was ten times higher than that in the HbHNL‐catalyzed synthesis of (S)‐2‐nitro‐1‐phenylethanol by condensation of benzaldehyde and nitromethane.

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The asymmetric Henry reaction catalyzed by hydroxynitrile lyase from Hevea brasiliensis is an example of enzymatic catalytic promiscuity. It could be an attractive method to produce optically active β -nitroalcohols, but unfortunately the enzyme has very low activity in this unnatural reaction. To get an insight into the reaction mechanism, the enzyme kinetics of this promiscuous biotransformation were studied using the cleavage and synthesis of 2-nitro-1-phenylethanol as a model system. The results indicate that the kinetic behavior of the enzyme in the Henry reaction fi ts the classical Rapid Equilibrium Random Bi Uni/Uni Bi mechanistic model with independent substrate binding. The measured kinetic parameters imply that the bottleneck for this biotransformation is the very low turnover number of the enzyme, not the binding of the substrates.