Lipase-mediated dynamic kinetic resolution (DKR) of secondary alcohols in the presence of zeolite using an ionic liquid solvent system

Bienzymatic Dynamic Kinetic Resolution of Secondary Alcohols by Esterification/Racemization in Water

Dynamic kinetic resolution (DKR) is a key method used to prepare optically pure compounds in 100 % theoretical yield starting from racemic substrates by combining the interconversion of substrate enantiomers with an enantioselective transformation. Various chemoenzymatic DKR approaches have been developed to deracemize secondary alcohols, typically requiring an organic solvent to facilitate enantioselective acylation, primarily catalyzed by lipases, alongside racemization mediated by an achiral, non-enzymatic catalyst. Achieving both steps in an aqueous solution remained elusive. Herein, we report a DKR of racemic sec-alcohols in an aqueous solution requiring only two biocatalysts. The first key to success was to achieve fast racemization in a buffer employing a non-stereoselective variant of an alcohol dehydrogenase (Lk-ADH-Prince) via a hydrogen-borrowing oxidation-reduction sequence. Engineered variants of the acyltransferase from Mycobacterium smegmatis (MsAcT) enabled enantioselective acyl transfer in water. Besides the appropriate choice of the enzymes, identifying a suitable acyl donor was a second key to the success. The DKR was successfully demonstrated using (R)-selective MsAcT variants for a broad range of racemic (hetero)benzylic alcohols with 2,2,2-trifluoroethyl acetate as the acyl donor, yielding (R)-acetates with up to >99 % conv. and high-to-excellent optical purity (83-99.9 % ee). The (S)-acetates were accessible using a stereocomplementary (S)-selective MsAcT variant. Notably, substrate concentrations of up to 400 mM were tolerated in selected cases.

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Enzymatic Reactions using Ionic Liquids for Green Sustainable Chemical Process; Stabilization and Activation of Lipases

The enzymatic reaction is highly respected from an environmentally-friendly point-of-view. Optimization of the reaction media and supporting materials of enzymes must be investigated in parallel with the effort to develop new enzymes. Lipases are frequently used for organic syntheses as synthetic tools even industry because of their acceptance of having a broad range of substrates, stability, and availability. We have investigated the possibility of ILs as both a solvent and activating or stabilization agent of enzymes, in particular, lipase as a model enzyme. ILs allowed recyclable use of a lipase and significant acceleration of transesterification, and also enhanced the stability and reaction activity of a lipase by immobilization through a lyophilization process. We discuss how we enhanced the enzyme capability using the IL engineering focusing on lipase-catalyzed reactions, i. e., realization of the recyclable use of an enzyme, how ILs mediated the enhanced reaction rate, and improved the stability of the enzyme.

The Enhanced Intramolecular Energy Transfer and Strengthened ff Luminescence of a Stable Helical Eu Complex in Ionic Liquids

The luminescence of a Eu complex (EuL) is enhanced by stabilization of the coordination structure in highly viscous ionic liquids. The EuL was found to maintain a stable single helical structure both in organic solvents and in the ionic liquids [BMIM][PF₆] and [EMIM][PF₆]. A colorless solution of EuL dissolved in [BMIM][PF₆] exhibits bright red luminescence with a quantum yield of 32.3%, a value that is much higher than that in acetonitrile (12%). Estimated rate constants for the energy relaxation pathway indicate that the energy transfer efficiency is enhanced in [BMIM][PF₆] as a result of the suppression of molecular fluctuations in the ligands. Additionally, a highly luminescent helical structure is preserved in [EMIM][PF₆] up to 120 °C.

Activation of Lipase-Catalyzed Reactions Using Ionic Liquids for Organic Synthesis

The use of ionic liquids to replace organic or aqueous solvents in biocatalysis processes has recently received great attention, and much progress has been made in this area; the lipase-catalyzed reactions are the most successful. Recent developments in the application of ionic liquids as solvents in lipase-catalyzed reactions for organic synthesis are reviewed, focusing on the ionic liquid mediated activation method of lipase-catalyzed reactions.

Spatial effects of oxovanadium-immobilized mesoporous silica on racemization of alcohols and application in lipase-catalyzed dynamic kinetic resolution

The nano-scale pores of mesoporous silica and their polar environment accelerate the racemization to make the lipase/oxovanadium combo-catalysed DKR applicable to a wider range of alcohols.

Active biopolymers in green non-conventional media: a sustainable tool for developing clean chemical processes

By understanding structure–function relationships of active biopolymers (e.g. enzymes and nucleic acids) in green non-conventional media, sustainable chemical processes may be developed.