Multiphase Homogeneous Catalysis: Common Procedures and Recent Applications

Abstract

A top priority in the development of new catalytic processes is the challenging full separation of the catalyst components from the products. Only extremely small residues of metals or ligands are often tolerated in the final product, particularly in the case of active pharmaceutical ingredients; additionally, environmental and cost considerations urge the development of processes that enable the separation and reuse of the catalyst. Multiphase homogeneous catalysis provides technical solutions to these needs, as witnessed by the rapidly accelerating development of this field. Through the analysis of recent examples, the creativity in devising technical solutions for multiphase homogeneous reactions is outlined and the potential rewards are illustrated.

Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions

“…Likewise, three-liquid-phase MEIC systems based on dodecyldimethylammonium tungstate/water/organic solvent and dimethyldioctylammonium (DiC 8 ) acid salts (dihydrogen phosphate and hydrogen sulfate) have been reported for epoxidation and sulfide oxidation reactions under mild reaction conditions. For instance, Nardello-Rataj and coworkers reported a high catalytic activity and selectivity in the epoxidation of alkenes with H 2 [35] POMs are anionic clusters generally soluble in the aqueous phase, but showing low affinity for organic reagents.…”

“…In addition to the aforementioned studies, Zhu et al also proposed a possible catalytic mechanism for biphasic oxidation in emulsion by relying on a complete collection of Raman spectra of tungsten and molybdenum peroxo species. [38] The authors suggested a possible conversion of the dimeric Q 2 The versatility and ease of the assembly between cationic surfactants and anionic catalysts to generate tunable processes have resulted in outstanding developments in oxidation/epoxidation reactions, allowing easy product separation and catalyst recovery. [38,39] The second strategy relies on the synthesis of organicinorganic hybrid amphiphilic surfactants where the POM acts as a polar head group.…”

“…Convenient solvents can also favor the separation of the reaction products and catalyst recovery. [2] However, the use of solvents artificially increases the complexity of a reaction system and requires further extraction/purification steps for recovery and recycling. As a matter of fact, about 6.4 Mt of alcohol-based solvents were used worldwide in 2011.…”

A Liquid–Liquid Biphasic Homogeneous Organocatalytic Aldol Protocol Based on the Use of a Silica Gel Bound Multilayered Ionic Liquid Phase

“…These include superlative activities and selectivities under mild homogeneous conditions, simple operations for product-catalyst separation with minimum cross-contamination, and catalyst recycling. [4] A biphasic system consisting of two mutually insoluble solvents is proposed. In one phase, the solvent entraps the catalyst; in the other phase, reactants are delivered to and products removed from the catalyst-containing solvent, in which the reaction takes place.…”

A New Robust and Efficient Ion‐Tagged Proline Catalyst Carrying an Amide Spacer for the Asymmetric Aldol Reaction

“…Indeed, the new substituent is supposed to increase catalyst lipophilicity, but at the same time it must contain an ionic group. The ionic group plays two roles: (i) it opens us the opportunity to apply homogeneous biphasic liquid‐liquid catalytic protocols, for example, working in ionic liquids as solvents, under solvent‐less conditions in the presence of stoichiometric amounts of water, or under aqueous emulsions,7 and ii) it can improve catalyst activity if a stabilizing electrosteric effect is in action 8…”