Highly efficient asymmetric transfer hydrogenation of ketones in emulsions

A biocatalytic approach for resolution of 3-hydroxy-3-phenylpropanonitrile with the use of immobilized enzymes stabilized with ionic liquids

Due to the growing importance of synthesizing active pharmaceutical ingredients (APIs) in enantiomerically pure form, new methods of asymmetric synthesis are being sought. Biocatalysis is a promising technique that can lead to enantiomerically pure products. In this study, lipase from Pseudomonas fluorescens, immobilized on modified silica nanoparticles, was used for the kinetic resolution (via transesterification) of a racemic mixture of 3-hydroxy-3-phenylpropanonitrile (3H3P), where the obtaining of a pure (S)-enantiomer of 3H3P is a crucial step in the fluoxetine synthesis pathway. For additional stabilization of the enzyme and enhanced process efficiency, ionic liquids (ILs) were used. It was found that the most suitable IL was [BMIM]Cl; a process efficiency of 97.4 % and an enantiomeric excess (ee%) of 79.5 % were obtained when 1 % (w/v) of that IL in hexane was applied and the process was catalyzed by lipase immobilized on amine-modified silica.

Asymmetric transfer hydrogenation of aryl ketoesters with a chiral double-chain surfactant-type catalyst in water

A chiral double-chain surfactant-type ligand was designed and synthesized. The rhodium catalyst formed from the ligand can self-assemble into chiral vesicular aggregates in water, which was applied to ATH of broad range of aromatic ketoesters in neat water.

Asymmetric transfer hydrogenation coupled with dynamic kinetic resolution in water: synthesis of anti-β-hydroxy-α-amino acid derivatives

The use of asymmetric transfer hydrogenation combined with dynamic kinetic resolution for the synthesis of β-hydroxy-α-(tert-butoxycarbonyl)amino esters in water is described. This procedure provides the desired amino alcohols in good yields, diastereoselectivities, and enantioselectivities. A surfactant is employed to achieve good yields due to the hydrophobic nature of both the catalyst and substrate. The reaction setup is operationally simple, and nondegassed water can be used as the solvent.

Opportunities offered by chiral η⁶-arene/N-arylsulfonyl-diamine-RuII catalysts in the asymmetric transfer hydrogenation of ketones and imines

Methods for the asymmetric transfer hydrogenation (ATH) of ketones and imines are still being intensively studied and developed. Of foremost interest is the use of Noyori's [RuCl(η⁶-arene)(N-TsDPEN)] complexes in the presence of a hydrogen donor (i-PrOH, formic acid). These complexes have found numerous practical applications and have been extensively modified. The resulting derivatives have been heterogenized, used in ATH in water or ionic liquids and even some attempts have been made to approach the properties of biocatalysts. Therefore, an appropriate modification of the catalyst that suits the specific requirements for the reaction conditions is very often readily available. The mechanism of the reaction has also been explored to a great extent. Model substrates, acetophenone (a ketone) and 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline (an imine), are both reduced by this Ru catalytic system with almost perfect selectivity. However, in each case the major product is a different enantiomer (S- for an alcohol, R- for an amine when the S,S-catalyst is used), which demanded an in-depth mechanistic investigation. Full-scale molecular modelling of this system enabled us to visualize the plausible 3D structures of the transition states, allowing the proposition of a viable explanation of previous experimental findings.