Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Chiral tandem modifiers: highly efficient cinchonidine-derivative modifiers at low concentrations for the enantioselective hydrogenation of (E)-2,3-diphenylpropenoic acid over Pd/C

Chiral tandem modifiers linking two or three cinchonidine molecules at the 2′-position of the quinoline ring are highly effective at low concentrations for the enantioselective hydrogenation of PCA over Pd/C.

Encapsulating Cobalt into N-Doping Hollow Frameworks for Efficient Cascade Catalysis

The development of nonprecious catalysts for hydrogenation of organic molecules is of great importance in heterogeneous catalysis. Herein, we report a series of N-doped hollow carbon frameworks encompassing cobalt nanoparticles (denoted as Co@NHF-900) constructed as a new kind of reusable catalyst for this purpose by pyrolysis of ZIF-8@Co-dopamine under Ar atmospheres. Notably, the framework of ZIF-8 is essential for efficient catalyst by providing a carbon framework to support Co-dopamine. The experimental results reveal that the ZIF-8 renders a large hollow place within the catalysts, allowing the enrichment of the substrate and windows of the hollow structure and the ease of mass transfer of products during the reaction. All of the virtues made Co@NHF-900 a good candidate for hydrogenation of quinolines with high activity (TOF value of 119 h^-1, which is several times than that of akin catalysts) and chemoselectivity.

New Members of the Cinchona Alkaloids Family: Assembly of the Triazole Heterocycle at the 6' Position

The substance class of the well-known Cinchona alkaloids is widened by 6′-Amino-cinchonine and 6′-Amino-cinchonidine, novel compounds which incorporate a primary amino function in the quinolinic ring system. These key intermediates open the field for a range of fruitful chemistry. Here is described a short and direct pathway for the synthesis of triazole containing derivatives of the above-mentioned substances using the [3 + 2] Huisgen cycloaddition. For this purpose, the amines were first converted into the corresponding azides. Based on this, non-substituted and silyl-protected triazoles were synthesized as examples. Furthermore, didehydrated derivatives of quincorine and quincoridine were used as addition partners, resulting in compounds that carry the quinuclidine ring of the cinchona alkaloids at both ends. Some of these compounds were examined radiographically to investigate the position of the quinuclidine ring to the triazole. The solid-state structures of compounds 10, 11 and 28 were determined by X-ray diffraction analyses.