Transition-metal complexes and purified ketoreductases work together in eutectic mixtures for the synthesis of enantiopure secondary alcohols.

Phenolic acid decarboxylase from Bacillus subtilis (BsPAD) converts several p-hydroxycinnamic acid derivatives into the corresponding phydroxystyrenes, which are considered to be promising bio-based aromatic chemicals. Despite the enzyme's high activity and stability, the low solubility of its substrates presents severe limitations for the establishment of industrial processes. Accordingly, deep eutectic solvents (DESs) have emerged as interesting alternatives to aqueous or organic solvents and biphasic systems as they offer unique reaction conditions while remaining biocompatible and biodegradable. Herein, we show that BsPAD could tolerate choline chloride (ChCl)-based eutectic solvents with 0−50% water content, which allowed conversion to the corresponding p-hydroxystyrene derivatives (>99%) at substrate loadings of up to 300 mM due to the exceptional solubilizing properties of this solvent. As the enzyme showed some remarkable reactivity differences in DES and water, we further explored the substrate scope of the enzyme and a mutant with increased space in the active site. The comparison of substrates with different substituents on the aryl group indicated that the substrate preference is determined by steric, rather than electronic effects. Furthermore, we report that the choice of the solvent influences the acceptance of different substrates as evidenced by the fact that DES strongly favored the conversion of caffeic acid, which is only poorly converted in aqueous media.

The development of cell-free and self-sufficient biocatalytic systems represents an emerging approach to address more complex synthetic schemes under nonphysiological conditions. Herein, we report the development of a self-sufficient heterogeneous biocatalyst for the synthesis of chiral alcohols without the need to add an exogenous cofactor. In this work, an NADPH-dependent ketoreductase was primarily stabilized and further co-immobilized with NADPH to catalyze asymmetric reductions without the addition of an exogenous cofactor. As a result, the immobilized cofactor is accessible, and thus, it is recycled inside the porous structure without diffusing out into the bulk, as demonstrated by single-particle in operando studies. This self-sufficient heterogeneous biocatalyst was used and recycled for the asymmetric reduction of eleven carbonyl compounds in a batch reactor without the addition of exogenous NADPH to achieve the corresponding alcohols in 100 % yield and >99 % ee; this high performance was maintained over five consecutive reaction cycles. Likewise, the self-sufficient heterogeneous biocatalyst was integrated into a plug flow reactor for the continuous synthesis of one model secondary alcohol, which gave rise to a space-time yield of 97-112 g L day ; additionally, the immobilized cofactor accumulated a total turnover number of 1076 for 120 h. This is one of the few examples of the successful implementation of continuous reactions in aqueous media catalyzed by cell-free and immobilized systems that integrate both enzymes and cofactors into the solid phase.

Amine transaminases are frequently used for the production of chiral amines starting from prochiral ketones. These amines can be applied as active pharmaceutical ingredients or drug precursors. However, there are still limitations to the use of amine transaminases when it comes to bulky ketone substrates, such as biaryl ketones. Using data mining, an (R)-selective amine transaminase from Exophiala xenobiotica was identified which naturally converts biaryl ketone substrates to the corresponding amines with up to 85% conversion and excellent enantioselectivity (>99% ee). Its protein crystal structure was obtained with a resolution of 1.52 Å, which enabled us to explain this interesting substrate acceptance. Structure-guided protein engineering resulted in a quintuple variant with increased stability. Moreover, the amino acid exchange T273S increased the activity and broadened the substrate scope enabling conversions of various biaryl ketones with up to >99%. A preparative biotransformation of 1-(4-(pyridin-3-yl)phenyl)ethenone at 75 mM (15 g/L) resulted in 96% of isolated yield of the respective amine.

During the last decade, the combination of different metal-and bio-catalyzed organic reactions in aqueous media has permitted the flourishing of a variety of one-pot asymmetric multi-catalytic reactions devoted to the construction of enantiopure and high added-value chemicals under mild reaction conditions (usually room temperature) and in the presence of air. Herein, a comprehensive account of the state-of-the-art in the development of catalytic networks by combining metallic and biological catalysts in aqueous media (the natural environment of enzymes) is presented. Among others, the combination of metal-catalyzed isomerizations, cycloadditions, hydrations, olefin metathesis, oxidations, C-C cross-coupling and hydrogenation reactions, with several biocatalyzed transformations of organic groups (enzymatic reduction, epoxidation, halogenation or ester hydrolysis), are discussed.

The ruthenium-catalyzed redox isomerization of allylic alcohols was successfully coupled with the enantioselective enzymatic ketone reduction (mediated by KREDs) in a concurrent process in aqueous medium. The overall transformation, formally the asymmetric reduction of allylic alcohols, took place with excellent conversions and enantioselectivities, under mild reaction conditions, employing commercially and readily available catalytic systems, and without external coenzymes or cofactors. Optimization resulted in a multistep approach and a genuine cascade reaction where the metal catalyst and biocatalyst coexist from the beginning.

The ruthenium-catalysed isomerisation of allylic alcohols was coupled, for the first time, with asymmetric bioamination in a one-pot process in an aqueous medium. In the cases involving prochiral ketones, the ω-TA exhibited excellent enantioselectivity, identical to that observed in the single step. As a result, amines were obtained from allylic alcohols with high overall yields and excellent enantiomeric excesses.

A hybrid organo-and biocatalytic system for the asymmetric conversion of racemic alcohols into amines was developed. Combining an organocatalyst, AZADO, an oxidant, NaOCl, and an enzyme, transaminase, we implemented a one-pot oxidation-transamination sequential process in aqueous medium.The method showed broad substrate scope and was successfully applied to conventional secondary alcohols and sterically hindered -substituted cycloalkanols, where a highly stereoselective dynamic asymmetric bioamination enabled to set up both contiguous stereocenters with very high enantio-and diastereomeric ratio (>90% yield, >99% ee and up to 49:1 dr). KEYWORDS: One-pot reaction, organocatalysis, biocatalysis, asymmetric synthesis, amino alcohols, transaminase, AZADO. yield. On the other hand, treatment of 3a,b with di-tert-butyl dicarbonate led to the orthogonally protected derivatives (1R,2R)-5a,b. Actually, orthogonally protected amino alcohols are highly valuable molecules in both asymmetric catalysis and medicinal chemistry. 17 Debenzylation of 5a,b gives entry to N-protected amino alcohols (1R,2R)-6a,b, thus completing the practical syntheses of an interesting set of derivatives by means of quantitative and simple processes. Scheme 2. Organo-enzymatic platform towards valuable optically active -aminocycloalkanol derivatives. i) H2, Pd-C 10%, MeOH; ii) (Boc)2O, CH2Cl2, iii) CF3CO2H, CH2Cl2; iv) H2, Pd-C 10%, (Boc)2O, MeOH. CONCLUSIONSIn conclusion, we disclosed an expedient, stereoselective and operationally simple protocol for the synthesis of a variety of optically active amines, through the unprecedented one-pot combination of: i) organocatalyzed oxidation of secondary alcohols; ii) subsequent enantioselective bioamination of the transiently formed ketones. In all cases, the desired final amines were isolated in high yields and with excellent diastereo-and enantiomeric excesses. Likewise, it is worth noting the use of a one-pot methodology, in which the aqueous reaction medium from the organocatalyzed reaction feeds the 15 enzymatic amination. This methodology, which exploits the advantages of merging two catalytic worlds such as organo-and enzymatic catalysis, represents one of the few contributions of this kind of combination in water. Moreover, it is a robust alternative to the existing methodologies, with proven efficacy for a broad variety of secondary alcohols.