ceverelst

Nature, Published online: 26 October 2022; doi:10.1038/d41586-022-03338-8

Single atoms of platinum-group metals are excellent catalysts, but they tend to aggregate and lose their activity during chemical reactions. We used nanoislands of cerium oxide, peppered through the interior surfaces of high-surface-area porous oxide supports, to confine and stabilize single atoms of these expensive metals, turning them into efficient and long-lasting catalysts.

Water is utilized to prepare self-blown non-isocyanate polyurethane foams by in situ generating the blowing agent (CO₂) during the polymer network formation. The partial hydrolysis of one of the comonomers, the cyclic carbonate, generates the blowing agent. The process is facile, versatile, up scalable, uses low cost readily available reagents, can be complete in 30 minutes and is compatible with existing infrastructures for polymer foaming.

Abstract

For 80 years, polyisocyanates and polyols were central building blocks for the industrial fabrication of polyurethane (PU) foams. By their partial hydrolysis, isocyanates release CO₂ that expands the PU network. Substituting this toxic isocyanate-based chemistry by a more sustainable variant—that in situ forms CO₂ by hydrolysis of a comonomer—is urgently needed for producing greener cellular materials. Herein, we report a facile, up-scalable process, potentially compatible to existing infrastructures, to rapidly prepare water-induced self-blown non-isocyanate polyurethane (NIPU) foams. We show that formulations composed of poly(cyclic carbonate)s and polyamines furnish rigid or flexible NIPU foams by partial hydrolysis of cyclic carbonates in the presence of a catalyst. By utilizing readily available low cost starting materials, this simple but robust process gives access to greener PU foams, expectedly responding to the sustainability demands of many sectors.

Is single-atom catalysis under flow a technically and economically viable organic process technology? Early results suggest valuable insight en route to the industrial uptake of single-atom catalysis in the production of fine and specialty chemicals.

Abstract

Heterogeneous catalysis under continuous flow conditions is increasingly used by the chemical industry to synthesize fine chemicals. Is single-atom catalysis under flow a technically and economically viable organic process technology? Early results suggest valuable insight en route to the industrial uptake of single-atom catalysis in the production of fine and specialty chemicals.

An Fe^IV imide of a tetracarbene macrocycle active in nitrene transfer reactions can be generated from phenyltosyliodinane (PhI=NTs) either from the FeII precursor by oxidative addition (right) or from the FeIII precursor by a comproportionation reaction (left) (AN=acetonitrile, bold horizontal bars=macrocycle).

Abstract

Nitrene transfer reactions have emerged as one of the most powerful and versatile ways to insert an amine function to various kinds of hydrocarbon substrates. However, the mechanisms of nitrene generation have not been studied in depth albeit their formation is taken for granted in most cases without definitive evidence of their occurrence. In the present work, we compare the generation of tosylimido iron species and NTs transfer from Fe^II and Fe^III precursors where the metal is embedded in a tetracarbene macrocycle. Catalytic nitrene transfer to reference substrates (thioanisole, styrene, ethylbenzene and cyclohexane) revealed that the same active species was at play, irrespective of the ferrous versus ferric nature of the precursor. Through combination of spectroscopic (UV-visible, Mössbauer), ESI-MS and DFT studies, an Fe^IV tosylimido species was identified as the catalytically active species and was characterized spectroscopically and computationally. Whereas its formation from the Fe^II precursor was expected by a two-electron oxidative addition, its formation from an Fe^III precursor was unprecedented. Thanks to a combination of spectroscopic (UV-visible, EPR, Hyscore and Mössbauer), ESI-MS and DFT studies, we found that, when starting from the Fe^III precursor, an Fe^III tosyliodinane adduct was formed and decomposed into an Fe^V tosylimido species which generated the catalytically active Fe^IV tosylimide through a comproportionation process with the Fe^III precursor.

Synthesis
DOI: 10.1055/s-0042-1751367

This review summarises the rarely used method of alternating current electrolysis for the synthesis of organic products. Different waveforms have been investigated which opens the possibility for further influence the outcome of the electrolysis by variation of the frequency as well as the highest peak current. In recent years alternating current electrolysis has been applied in increasingly more complex transformations. Especially the functionalisation of (hetero)arenes, functional group manipulation, metathesis reactions, and transition-metal-catalysed cross-coupling reactions were reported in recent years and the results of these and some other investigations are summarized in this review article.1 Introduction1.1 Waveforms1.2 Objectives1.3 Early Examples of the Optimisation of Alternating Current Electrolysis2 Recent Applications of Alternating Current Electrolysis for Organic Synthesis2.1 Substitution Reaction on Arenes2.2 Nitrogen–Sulfur Bond Formation and Sulfur–Sulfur Bond Metathesis2.3 Oxidation and Reduction2.4 Cross-Coupling Reactions2.5 Frequency Optimisation3 Conclusion
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
Table of contents  |  Abstract  |  Full text

Convergent paired metal electrocatalysis enables the direct use of simple primary and secondary aliphatic acids as coupling partners for C(sp²)−C(sp³) bond formation by radical decarboxylation. This new electrophotocatalytic method features a broad substrate scope, wide functional-group tolerance, and could be used for late-stage functionalization of complex molecules.

Abstract

An electrophotochemical dual metal-catalyzed protocol for decarboxylative arylation of simple aliphatic carboxylic acids with aryl halides is reported. The relative stabilities of catalytically active metal complexes simultaneously generated at anode and cathode are the key design elements for the success of this convergent paired electrolysis. This new electrophotocatalytic method is mild, robust, and most importantly, capable of accommodating simple primary aliphatic acids including acetic acid – ubiquitous and variegated structural motifs yet remain oddly challenging substrates – directly as native functional groups for decarboxylative C(sp²)−C(sp³) bond formation.

Levulinic Acid Valorization: A simple impregnation-mixing-pyrolysis strategy was used to construct a highly efficient catalyst Co@mSiO₂−CN₃, with improved metal dispersion and enhanced Lewis acidity benefiting from the introduction of nitrogen-doped carbon materials, which could completely convert levulinic acid to γ-valerolactone under mild conditions.

Abstract

The design of robust catalyst was of great significance for the valorization of biomass platforms. Herein, an efficient Co@mSiO₂−CN catalyst was constructed via a simple impregnation-mixing-pyrolysis strategy, which could give 100 % yield of γ-valerolactone from levulinic acid for no less than five cycles under a relatively mild reaction conditions (160 °C, 1.5 MPa H₂,3 h).The results of catalyst characterizations and comparative experiments showed that the introduction of nitrogen-doped carbon materials promoted the dispersion of Co and enhanced the Lewis acidity of the catalyst, thereby promoting the dissociation of H₂ and the activation of C−O, respectively. The synergistic effect between the hydrogenation metal sites and the Lewis acidic site ensures excellent activity of the catalyst.

Nature, Published online: 16 September 2022; doi:10.1038/d41586-022-02952-w

The ozonolysis reaction is a classic of organic synthesis, but involves the formation of potentially explosive reaction intermediates. A modern, safer spin on this process makes use of previously overlooked chemistry.

230 million years ago, the earliest dinosaurs thrived in mild climates

2022 is a more appropriate year than 2015 to celebrate the 150^th anniversary of the Kekulé benzene structure, for in 1865 Kekulé did not propose the familiar hexagonal 1,3,5-cyclohexatriene formulation. Why did he not?

Abstract

Although the paper published by Kekulé in 1865 is generally accepted as the seminal document regarding the elucidation of the structure of benzene, it was not for another seven years that Kekulé introduced a “collision theory” which, in his view, made the 1,3,5-cyclohexatriene formulation at last acceptable. 2022 is therefore an appropriate year to celebrate the 150^th anniversary of his proposal. The present Perspective focusses in particular on how Kekulé’s mind has evolved during that short period of time. After a short introduction, followed by a historical note about the emergence of aromatic chemistry and the role of Kekulé as founder of structural organic chemistry, this Perspective discloses in three chapters the step-by-step unveiling of the 1,3,5-cyclohexatriene structure and focusses on Kekulé’s own dubiety about this formula. Eventually, in 1872 Kekulé’s benzene saga is completed in the article in which he discloses his final theory on the structure of benzene. In the last section, a short review is presented on the later quest for a fully symmetric single-formula representation of benzene.

A high-valent Cu^III−CF₃ compound enables direct C−H trifluoromethylation of arenes for the first time. Thermolysis of Cu^III−CF₃ bond provides concurrently CF₃ radical and a Cu^II oxidant, which synergistically engage in a S_EAr type reaction with arenes. This study establishes an unprecedented fundamental reactivity of Cu^III−CF₃ compounds, and thus represents a significant progress for Cu^III chemistry.

Abstract

Direct C−H trifluoromethylation of arenes and heteroarenes poses an important synthetic challenge that is highly desirable. High-valent Cu^III−CF₃ compounds have often been invoked in copper-mediated trifluoromethylation reactions, but the fundamental reactivity toward arenes is elusive. Herein, direct C−H trifluoromethylation of arenes/heteroarenes by a high-valent Cu^III−CF₃ compound is disclosed for the first time. The Cu^III−CF₃ compound serves CF₃ radical and a Cu^II oxidant by homolytic cleavage of a Cu^III−CF₃ bond, which engage synergistically in a S_EAr type reaction with arenes. The presence of K₂S₂O₈ co-oxidant can significantly improve the reaction yields. This reaction shows good efficiency, broad functional group tolerance, and the potential in late-stage functionalization. The reactivity of high-valent Cu^III−CF₃ compounds disclosed in this study represents an important progress in organofluorine and Cu^III chemistry.

Lignin valorization: Technical lignin is a large-volume and low-cost industrial resource for making renewable aromatic chemicals. Oxidative depolymerization with heterogeneous catalysts allows viable upgrading to value-added aromatic monomers and more sustainable biorefineries. This Review summarizes and discusses performance–reactivity patterns and product analytics and recovery for the efficient oxidative valorization of technical lignin.

Abstract

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

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