Nanofiltration for Homogeneous Catalysis Separation: Soluble Polymer-Supported Palladium Catalysts for Heck, Sonogashira, and Suzuki Coupling of Aryl Halides

Organic Solvent Nanofiltration in Pharmaceutical Applications

Separation and purification in organic solvents are indispensable procedures in pharmaceutical manufacturing. However, they still heavily rely on the conventional separation technologies of distillation and chromatography, resulting in high energy and massive solvent consumption. As an alternative, organic solvent nanofiltration (OSN) offers the benefits of low energy consumption, low solid waste generation, and easy scale-up and incorporation into continuous processes. Thus, there is a growing interest in employing membrane technology in the pharmaceutical area to improve process sustainability and energy efficiency. This Review comprehensively summarizes the recent progress (especially the last 10 years) of organic solvent nanofiltration and its applications in the pharmaceutical industry, including the concentration and purification of active pharmaceutical ingredients, homogeneous catalyst recovery, solvent exchange and recovery, and OSN-assisted peptide/oligonucleotide synthesis. Furthermore, the challenges and future perspectives of membrane technology in pharmaceutical applications are discussed in detail.

Ligand and Solvent Selection for Enhanced Separation of Palladium Catalysts by Organic Solvent Nanofiltration

Organic solvent nanofiltration (OSN) has been widely applied to separate and recycle homogeneous catalysts, but the influence of ligand and solvent selection on the performance of OSN is not fully understood. Here we prepared four palladium (Pd) catalysts by combining palladium acetate with four ligands of different molecular weights. Morphological and functional properties of the Pd catalysts were characterized by TEM, FTIR, and NMR. OSN experiments were conducted in a lab-scale dead-end filtration rig. Two commercial OSN membranes, PuraMem S600 (PS600) and DuraMem 500 (D500), were used to separate the Pd catalysts from different organic solvents (toluene, isopropanol, butanol/water, and methanol) that are specified to be compatible with, respectively. For both membranes, the pure solvent permeance was positively related to the degree of membrane swelling induced by the solvent. The solvent permeance decreased significantly after the addition of a solute, as a result of membrane fouling and concentration polarization. For the PS600 membrane, the Pd rejection in any solvent was closely correlated to the molecular weight of the ligand, which agrees with the pore-flow model. For the D500 membrane, on the other hand, there was no conclusive link between the Pd rejection and the type of ligand. The one-way analysis of variance (ANOVA) confirmed that the separation processes in PS600 and D500 membranes were controlled by different transport models. The findings shed light on the selection of ligand and solvent in OSN in order to enhance the separation of homogeneous catalysts.

Bis-salophen palladium complex immobilized on Fe3O4@SiO2 nanoparticles as a highly active and durable phosphine-free catalyst for Heck and copper-free Sonogashira coupling reactions

A new Fe₃O₄@SiO₂ superparamagnetic nanoparticles functionalized by a bis-salophen Schiff base Pd(ii)-complex and used as efficient catalyst for Heck and Sonogashira reactions in aqueous media.

Size-Dependent Catalytic Activity and Fate of Palladium Nanoparticles in Suzuki-Miyaura Coupling Reactions

Stable, catalytically active palladium nanoparticles of various average diameters (1.9-7.4 nm) have been synthesized and characterized by X-ray diffraction, spectroscopy, and microscopy techniques to demonstrate remarkable size-dependent and renewed catalytic activity toward the Suzuki-Miyaura coupling reaction in green protocol. The catalytic activity is found to depend on the amount of the reducing agent, stabilizer-precursor ratio, solvent composition, and aryl halides used. The product obtained by this reaction is characterized by ¹H NMR, ¹³C NMR, and IR spectroscopy analyses. A newly developed kinetic equation illustrates that while the catalyst particles of the lowest dimension are gradually exposed to the reactants and hence activated due to partial removal of capping polymer from the catalyst surface, others are deactivated due to agglomeration during the progress of the reaction, as conformed by the microscopic profiles of the used and unused catalysts.

Microporous organic nanotube network supported acid and base catalyst system for one-pot cascade reactions

Acid/base functionalized microporous organic nanotube networks with excellent catalytic performance for one-pot cascade reactions were successfully prepared.

A novel polymer containing phosphorus–nitrogen ligands for stabilization of palladium nanoparticles: an efficient and recyclable catalyst for Suzuki and Sonogashira reactions in neat water

Highly monodispersed palladium nanoparticles supported on polymer containing phosphorus and nitrogen ligands was successfully synthesized, characterized and used in Suzuki and Sonogashira coupling reactions in water.

Palladium chloride catalyzed photochemical Heck reaction

PdCl2 catalyzed carbon–carbon bond formation (Heck reaction) between substituted aryl halides and olefins was carried out without a ligand, under irradiation with UV–visible light. The results demonstrated that UV–visible light accelerated the rate of the reaction, leading to an excellent yield of corresponding products. The recovered palladium nanoparticles could be thermally recycled several times. PdCl2 gave excellent conversion up to the fifth addition of substrate.

Batchwise and continuous nanofiltration of POSS-tagged Grubbs-Hoveyda-type olefin metathesis catalysts

New molecular-weight-enlarged metathesis catalysts, which bear polyhedral oligomeric silsesquioxane (POSS) tags, were synthesized and characterized. The catalysts can be recovered from the reaction mixture by using nanofiltration techniques and can be reused. It was found that the membranes Starmem 228 and PuraMem 280 successfully separate the catalyst from the post-reaction mixtures to below 3 ppm. The application of these POSS-tagged catalysts in a continuous metathesis reaction was also investigated.

A homogeneous, recyclable polymer support for Rh(I)-catalyzed C-C bond formation

A robust and practical polymer-supported, homogeneous, recyclable biphephos rhodium(I) catalyst has been developed for C-C bond formation reactions. Control of polymer molecular weight allowed tuning of the polymer solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and insoluble in polar solvents. Using the supported rhodium catalysts, addition of aryl and vinylboronic acids to the electrophiles such as enones, aldehydes, N-sulfonyl aldimines, and alkynes occurs smoothly to provide products in high yields. Additions of terminal alkynes to enones and industrially relevant hydroformylation reactions have also been successfully carried out. Studies show that the leaching of Rh from the polymer support is low and catalyst recycle can be achieved by simple precipitation and filtration.