Combining biocatalysis and organocatalysis for the synthesis of piperidine alkaloids.

There is continued interest in developing cascade processes for the synthesis of key chiral building blocks and bioactive natural products (or analogues). Here, we report a hybrid bio-organocatalytic cascade for...

Spherical Silica Modified with Magnesium and Ruthenium—Synthesis, Characterization and Catalytic Properties

The design of different bimetallic catalysts is an important area of catalytic research in the context of their possible applications in the cascade processes, meeting the requirements of the so-called green chemistry. In this study, such catalysts were obtained by the incorporation of magnesium species into spherical silica, which was in the next step covered with porous silica and modified with ruthenium species. The structure and chemical composition of the materials obtained were determined by XRD measurements, low temperature N2 adsorption/desorption, SEM, ICP-OES and XPS methods. The catalytic activities of materials obtained were tested in 2-propanol decomposition and hydrogenation of levulinic acid. The results obtained confirmed the successful coverage of nanospheres with porous silica. A much higher concentration of ruthenium species was found on the surface of the catalysts than in their bulk. The opposite relationship was observed for magnesium species. The modification of nanospheres with silica had a positive effect on the catalytic activity of the materials obtained. For the most active sample, i.e., Ru/NS/3Mg/NS, 49% of levulinic acid conversion in its hydrogenation process was reported with γ-valerolactone as the only product.

Recent Advances on Piancatelli Reactions and Related Cascade Processes

The Piancatelli reaction, which is the rearrangement of 2-furylcarbinol to cyclopentenone, involves a key furanoxonium ion intermediate and a furan ring opening-4π electrocyclization process. In recent years, the original oxa-Piancatelli reaction has been extended to a large family of aza- and carbo-Piancatelli reactions and related cascade processes, providing a powerful platform for the construction of diverse functionalized cyclopentenones and polycyclic cyclopentanones. Meanwhile, chiral Brønsted/Lewis acid based catalytic asymmetric approaches to Piancatelli reactions have also been achieved for the assembly of highly valued chiral cyclopentenone scaffolds. In this short review, we present an overview of the recent developments in these areas and focus primarily on reports published in the last five years.1 Introduction2 Diastereoselective Oxa-, Aza- and Carbo-Piancatelli Reactions3 Diastereoselective Cascade Piancatelli Reactions4 Asymmetric Piancatelli Reactions and Related Cascade Processes5 Miscellaneous Furanoxonium Ion-Based Rearrangements6 Conclusion

A review on the halodefluorination of aliphatic fluorides

Halodefluorination of alkylfluorides using Group 13 metal halides has been known for quite some time (first reported by Newman in 1938) and is often utilised in its crude stoichiometric form to substitute fluorine with heavier halogens. However, recently halodefluorination has undergone many developments. The reaction can be effected with a range of metal halide sources (including s-block, f-block and p-block metals), and has been developed into a catalytic process. Further, methods for monoselective halodefluorination in polyfluorocarbons have been developed allowing exchange of only a single fluorine with a heavier halogen. The reaction has also found use in cascade processes where the final product may not even contain halide, yet the conversion of fluorine to more reactive halogens is a pivotal reaction step in the cascade. This review provides a summary of the developments in the reaction since its inception until now.

Selectivity descriptors for the direct hydrogenation of CO2 to hydrocarbons during zeolite-mediated bifunctional catalysis

Cascade processes are gaining momentum in heterogeneous catalysis. The combination of several catalytic solids within one reactor has shown great promise for the one-step valorization of C1-feedstocks. The combination of metal-based catalysts and zeolites in the gas phase hydrogenation of CO2 leads to a large degree of product selectivity control, defined mainly by zeolites. However, a great deal of mechanistic understanding remains unclear: metal-based catalysts usually lead to complex product compositions that may result in unexpected zeolite reactivity. Here we present an in-depth multivariate analysis of the chemistry involved in eight different zeolite topologies when combined with a highly active Fe-based catalyst in the hydrogenation of CO2 to olefins, aromatics, and paraffins. Solid-state NMR spectroscopy and computational analysis demonstrate that the hybrid nature of the active zeolite catalyst and its preferred CO2-derived reaction intermediates (CO/ester/ketone/hydrocarbons, i.e., inorganic-organic supramolecular reactive centers), along with 10 MR-zeolite topology, act as descriptors governing the ultimate product selectivity.

Biomass delignification with green solvents towards lignin valorisation: ionic liquids vs deep eutectic solvents

The use of renewable resources as feedstocks to ensure the production of goods and commodities for society has been explored in the last decades to switch off the overexploited and pollutant fossil-based economy. Today there is a strong movement to set bioeconomy as priority, but there are still challenges and technical limitations that must be overcome in the first place, particularly on biomass fractionation. For biomass to be an appellative raw material, an efficient and sustainable separation of its major components must be achieved. On the other hand, the technology development for biomass valorisation must follow green chemistry practices towards eco-friendly processes, otherwise no environmental leverage over traditional petrochemical technologies will be acquired. In this context, the application of green solvents, such as ionic liquids (ILs) and deep eutectic solvents (DES), in biomass fractionation is envisaged as promising technology that encompasses not only efficiency and environmental benefits, but also selectivity, which is a crucial demand to undertake cascade processes at biorefinery level. In particular, this article briefly discusses the disruptive achievements upon the application of ILs and DES in biomass delignification step towards an effective and selective separation of lignin from polysaccharides. The different physicochemical properties of these solvents, their interactions with lignin and their delignification capacity will be scrutinized, while some highlights will be given to the important characteristics of isolated lignin fractions for further valorisation. The advantages and disadvantages between ILs and DES in biomass delignification will be contrasted as well along the article.

When freeze-out occurs due to a non-Boltzmann suppression: a study of degenerate dark sector

Exponential suppression or commonly known as the Boltzmann suppression in the number density of dark matter is the key ingredient for creating chemical imbalance prior to the usual thermal freeze-out. A degenerate/quasi-degenerate dark sector can experience a different exponential suppression in the number density analogous to the radioactive decay law leading to a delayed freeze-out mechanism of dark matter known as the co-decaying dark matter. In this work, we study the dynamics of a multicomponent dark matter from thermally decoupled degenerate dark sector in a hidden U(1)X extension of the Standard Model. We compute the relic density of dark matter frozen-out through the co-decaying mechanism by solving four coupled Boltzmann equations. We demonstrate how temperature T′ of the dark sector changes due to all types of 3 → 2 and 2 → 2 interactions along with the eternal expansion of the Universe. We find that 3 → 2 interactions enhance T′ by producing energetic particles in the dark sector while the excess heat is transferred by 2 → 2 interactions to the entire dark sector. As the direct detection is possible only through the feeble portal couplings, we investigate the neutrino and γ-ray signals from dark matter annihilation via one step cascade processes and compare our results with the measured fluxes of atmospheric neutrinos by Super-Kamiokande and diffuse γ-rays by Fermi-LAT, EGRET, INTEGRAL collaborations. We find that the present scenario easily evades all the existing bounds from atmospheric neutrino and diffuse γ-ray observations for degenerate dark sector. However, the constraints are significant for quasi degenerate scenario.

Recent Advances in the Transition Metal Catalyzed Addition of Carboxylic Acids to Alkynes

: Recent advances in the metal-catalyzed hydrofunctionalization of alkynes with carboxylic acids are comprehensively reviewed. Both inter- and intramolecular processes, leading respectively to enol esters and lactones, are discussed, as well as the involvement of these transformations in the synthesis of natural products and biologically active molecules, and the assembly of elaborated heterocyclic compounds though cascade processes. Literature published since 2011 is covered.