Simple and Effective Catalyst Separation by New CO₂‐Induced Switchable Organocatalysts

Abstract

CO -induced switchable tertiary amine-based organocatalysts were investigated for an efficient catalyst and product separation by its different partitioning between an organic and carbonated water phase. In this case study, the switching ability of eight tertiary amine-based catalysts between the organic and water phase by addition or removal of CO was investigated. Here, the catalyst switched both nearly completely (99.9 %) into the aqueous phase by addition of CO and effectively back into the organic phase (99.3 %) by expelling CO . With this technique, the organocatalyst was successfully recovered and reused twelve times without significant loss of activity (up to 90 % enantiomeric excess) for the asymmetric nitroaldol (Henry) reaction. After the first catalyst switch, evaporation of the solvent affords the product in 98 % purity without any further purification steps.

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

“…In methanol, which is a protic solvent, we got only lower yields (up to 82% vs. up to 100%) and enantioselectivities (up to 51% ee vs. up to 93% ee). This observation is in agreement with similar cinchona-based organocatalysis [48]. In methanol, hydrogen bonds could form between the solvent and the catalysts/reactants.…”

A Continuous‐Flow Route to Enantioenriched 3‐Substituted‐3‐Hydroxyoxindoles: Organocatalytic Aldol Reactions of Isatin with Acetone

“…A literature review of the performance of various organocatalysed cross aldol reactions of isatin with enolisable ketones suggested that the choice of solvent is crucial to the yield and enantioselectivity of the reaction [29,32,39] . In catalytic processes, solvent properties such as hydrogen‐bond donating ability (proticity or acidity), hydrogen‐bond accepting ability (basicity) and dipolarity (polarity) have been reported to strongly influence the rate and stereochemical outcome [38,40,41] . These properties have been used by several researchers to rationalise and identify ideal solvents for API solubilisation [42,43] and asymmetric reactions [38,40] as bulk properties like dielectric constant, dipole moments etc, often prove ineffective in understanding such correlations [31,44] .…”

“…In catalytic processes, solvent properties such as hydrogen‐bond donating ability (proticity or acidity), hydrogen‐bond accepting ability (basicity) and dipolarity (polarity) have been reported to strongly influence the rate and stereochemical outcome [38,40,41] . These properties have been used by several researchers to rationalise and identify ideal solvents for API solubilisation [42,43] and asymmetric reactions [38,40] as bulk properties like dielectric constant, dipole moments etc, often prove ineffective in understanding such correlations [31,44] . We therefore adopted the Kamlet‐Taft solvent parameters as obtained in literature [45] to rationally understand the variation of enantiomeric excess in different reaction media as observed in Table 1.…”

Recent advances utilized in artificial switchable catalysis

“…The catalyst switched both nearly completely (99.9%) into the aqueous phase by addition of CO 2 and effectively back into the organic phase (99.3%) by expelling CO 2 . 40 Another method that can be used to recovery of a catalyst from a reaction, including a light-controlled phase tag to separate homogeneous catalysts that can be switched between a neutral (lipophilic) phase and a charged (lipophobic) phase through the use of a tag-centered photoreaction. 41 The photoreaction results in drastic changes in the polarity and solubility of the catalyst.…”