Continuous‐Flow Di‐N‐Alkylation of 1H‐Benzimidazole in a Fixed‐Bed Reactor

A new process for a continuous‐flow di‐N‐alkylation of 1H‐benzimidazole to 1H‐benzimidazole‐3‐ium iodide by methylene iodide in the presence of potassium carbonate in a fixed‐bed reactor is presented. The synthesis was transferred from batch to continuous operation with similar yields and conversion rates. Moreover, the influence of temperature and residence time in the continuous flow setup was characterized; optimized conditions led to a doubling of yield. In addition, the continuous flow allowed for a better control of the two‐step reaction by adding an additional tube reactor after the fixed bed that further enhanced the overall performance. With this, the continuous‐flow system presented itself as superior due to higher available temperatures and a better controllability.     

Magnetic Nanoparticles‐Supported Palladium: A Highly Efficient and Reusable Catalyst for the Suzuki,Sonogashira, and Heck Reactions

A highly efficient, air‐ and moisture‐stable and easily recoverable magnetic nanoparticle‐supported palladium catalyst has been developed for the Suzuki, Sonogashira and Heck reactions. A wide range of substrates was coupled successfully under aerobic conditions. In particular, the performance of the magnetic separation of the catalyst was very efficient, and it is possible to recover and reuse it at least eight times without significant loss of its catalytic activity.     

Solid Acids as Heterogeneous Support for Primary Amino Acid‐Derived Diamines in Direct Asymmetric Aldol Reactions

We have achieved the non‐covalent immobilization of chiral primary amino acid‐derived diamines on organic and inorganic sulfonated solid acids through acid‐base interaction. With the commercial sulfonated fluoropolymer nafion® NR50 as support an optimal balance was found between activity and stereoselectivity of the supported catalyst in direct asymmetric aldol reactions of linear ketones and aromatic aldehydes. Under optimized conditions aldol products were obtained in high yields and with excellent enantioselectivities for the syn‐product (up to 98% ee). Furthermore, catalysis with the supported diamine was demonstrated to occur truly heterogeneously and the loaded nafion® NR50 beads could be reused several times. Ultimately, the immobilized catalyst/nafion® NR50 system was successfully implemented in a fixed‐bed reactor set‐up under continuous flow conditions.     

Palladium Supported on an Acidic Resin: A Unique Bifunctional Catalyst for the Continuous Catalytic Hydrogenation of Organic Compounds in Supercritical Carbon Dioxide

1% Palladium‐doped acidic resin (Amberlyst^® 15; styrene‐divinylbenzene matrix with sulfonic acid groups) is shown to be a highly active catalyst for the continuous catalytic hydrogenation of CC bonds in supercritical carbon dioxide (scCO₂) without affecting CO bonds. This 1% Pd/Amberlyst‐15 catalyst promotes the industrially important selective formation of 2‐ethylhexanal from crotonaldehyde in a “one‐pot” pathway involving hydrogenation and aldol condensation with a number of merits. The selectivity behavior of 1% Pd/Amberlyst‐15 is strikingly different compared to that of 1% Pd/C and 1% Pd/Al₂O₃ due to its prominent bifunctional nature based on sulfonic acid groups adjacent to metallic Pd sites. Hybrid “[Pd_n–H]⁺” sites are suggested to act as both metal and acid sites promoting the bifunctional catalysis.     

Hydrophilic CNC‐Pincer Palladium Complexes: A Source for Highly Efficient,Recyclable Homogeneous Catalysts in Suzuki–Miyaura Cross‐Coupling

The Suzuki biaryl coupling of a range of electronically dissimilar arylboronic acids and aryl bromides is performed in neat water with excellent to quantitative yields by means of a new CNC‐pincer palladium catalyst that is soluble in water due to its para‐carboxy group. Extremely high turnover numbers and frequencies combined with a remarkable robustness allow an effective catalyst reuse in sustainable conditions.     

Enhancement Factor for Gas Absorption in a Finite Liquid Layer. Part 2: First‐ and Second‐Order Reactions in a Liquid in Plug Flow

Gas absorption accompanied by an irreversible chemical reaction of first‐order or second‐order in a liquid layer of finite thickness in plug flow has been investigated. The analytical solution to the enhancement factor has been derived for the case of a first‐order reaction, and the exact solution to the enhancement factor has been obtained via numerical simulation for the case of a second‐order reaction. The enhancement factor in both cases is presented as a function of the Fourier number and tends to deviate from the prediction of the existing enhancement factor expressions based on the penetration theory at Fourier numbers above 0.1 due to the absence of a well‐mixed bulk region in the liquid layer. Approximate enhancement factor expressions that describe the analytical and exact solutions with an accuracy of 5 % and 9 %, respectively, have been proposed.