Enantiomerically pure cyanohydrins are of great importance in the chemical and pharmaceutical industries. Their synthesis is possible through the use of highly selective hydroxynitrile lyases. In this work, an R-selective hydroxynitrile lyase (AtHNL) from Arabidopsis thaliana was immobilized inside a silica microreactor with a tortuous and hierarchical pore structure. After immobilization, the enzyme activity was tested for benzaldehyde 1a, and its analogs 4-fluorobenzaldehyde 1b, 4-methoxybenzaldehyde 1c and 4-(trifluoromethyl)benzaldehyde 1d. With their different degrees of reactivity they also display a different susceptibility to the racemic chemical background reaction. It was shown that the use of a flow microreactor suppressed the background reaction even for the most susceptible substrate 1d. Furthermore, the use of a flow microreactor enabled high substrate conversion (90–95%) while maintaining a high enantiomeric excess (90–98%) with residence times of 3 to 30 min. The productivity, which depended on substrate reactivity and flow rate, was evaluated by space–time-yield (STY) and reached a value from 60 g L⁻¹ h⁻¹ to 1290 g L⁻¹ h⁻¹. Additionally it was demonstrated that the stability of the immobilized enzyme depends on the flow rates used and thus on the shear forces acting inside the microreactor and interfacial effects associated with them.

Dynamic kinetic resolution (DKR) is one of the most attractive methods for enantioselective synthesis. In the reported studies, lipase B from Candida antarctica (CALB) immobilized on siliceous mesoporous cellular foams (MCF) functionalized with different hydrophobic groups, and two ruthenium complexes with substituted cyclopentadienyl ligands were investigated as catalysts for the chemoenzymatic DKR of (rac)-1-phenylethanol, using Novozym 435 as a benchmark biocatalyst. Studies on the (rac)-1-phenylethanol transesterification reaction showed that CALB supported on MCFs grafted with methyl groups is a promising biocatalyst and isopropenyl acetate is a preferable acylation agent. Both Ru-complexes activated by K₃PO₄ or t-BuOK, proved to be effective catalysts of the racemization reaction. The final DKR experiments using all catalysts combinations singled out, gave 96% conversion, and (R)-1-phenylethyl acetate enantiomeric excess of 98% in 8 h using K₃PO₄ activator.

Dynamic kinetic resolution allows for the synthesis of enantiomerically pure asymmetric alcohols. Cyclopentadienyl-derived ruthenium catalysts were immobilized with an ionic liquid, [BMIM][NTf₂], on multiwall carbon nanotubes and used for the racemization of chiral secondary alcohols. This successful approach was combined with the enantioselective enzymatic acylation of secondary alcohols (1-phenylethanol and 1-(1-naphthyl)ethanol) using Novozyme^® 435. The resulting catalytic system of the ruthenium racemization catalysts and enzymatic acylation led to chiral esters being obtained by dynamic kinetic resolution. The immobilized catalytic system in the ionic liquid gave the same activity of >96% yield within 6 h and a selectivity of 99% enantiomeric excess as the homogeneous system, while allowing for the convenient separation of the desired products from the catalyst. Additionally, the process can be regarded as green, since the efficient reuse of the catalytic system was demonstrated.

Dynamic kinetic resolution (DKR) is one of the most attractive routes to enantioselective synthesis, and ruthenium complexes are often applied as racemization catalysts. Two substituted cyclopentadienyl ruthenium complexes were immobilized covalently and non-covalently on mesoporous silica of mesocellular foam (MCF) and Santa Barbara Amorphous (SBA)-15 type functionalized with a 3 carbon spacer and 4-(chloromethyl)-N-amidobenzoate moiety. The catalysts were studied in a model reaction of secondary alcohol racemization. The immobilization decreased catalyst activity, considerably more for SBA-15 than for MCFs, and complete racemization of 1-phenylethanol was achieved within 24 h with the MCF-supported catalyst. The catalyst could be recovered and reused, thus paving the way for further development of the DKR process. The synthesized materials were fully characterized by Fourier-transform infrared spectroscopy analysis, thermogravimetry analysis, inductively cou-pled plasma optical emission spectrometry, and nitrogen adsorption at 77 K.

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