Mathias

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.

Mild, rapid, and efficient demethylation of aryl alkyl ethers is accomplished using tris(pentafluorophenyl)borane via the Piers-Rubinsztajn reaction. This two-step deprotection allows alkyl ethers to be used as effective phenol protecting groups without the need for harsh deprotection conditions.

Abstract

We report that the Piers-Rubinsztajn reaction enables rapid deprotection of aryl alkyl ethers under ambient conditions. This chemistry leverages tris(pentafluorophenyl)borane and silyl hydrides to convert aryl methyl ethers to siloxanes, which can then be cleaved using 1 % HCl in EtOH. We examined 26 derivatives and routinely obtained yields >85 %, even in the presence of sterically demanding groups and complex substrate structures. Other alkyl ethers including ethyl, propyl, isopropyl, tert-butyl, and benzyl groups were also easily removed.

Synlett
DOI: 10.1055/s-0041-1738384

The present article presents a personalized Account on the synthesis of nitrogen heterocycles by domino C–N coupling/hydroamination reactions and related processes. The starting materials, 2-alkynyl-1-halohetarenes, are regioselectively available by Sonogashira reactions of various 1,2-dihalogenated heterocycles, such as thiophenes, benzothiophenes, furans and benzofurans, pyridines, quinolines, pyrimidines, and other ring systems. More complex products are formed by domino C–N coupling/hydroamination/C–H arylation reactions of 2-alkynyl-1-halohetarenes with 2-bromoanilines. In these reactions, not only two, but three bonds are formed in one step. In many cases, the products constitute new heterocyclic core structures and show interesting pharmacological or fluorescence properties.1 Introduction2 Domino C–N Coupling/Hydroamination Reactions3 Domino C–N Coupling/Hydroamination/C–H Arylation Reactions4 Conclusions
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Synthesis
DOI: 10.1055/s-0040-1719922

Aromatic carboxylic acids are prone to decarboxylate in high-temperature water (HTW). While the decarboxylation kinetics of several aromatic carboxylic acids have been explored, studies on their compatibility with organic syntheses in HTW are scarce. Herein, we report the hydrothermal synthesis (HTS) of 2,3-diarylquinoxaline carboxylic acids from 1,2-diarylketones and 3,4-diaminobenzoic acid. A detailed study of the reaction parameters was performed to identify reaction conditions towards minimal decarboxylation. Thirteen 2,3-diarylquinoxaline-6-carboxylic acids are obtained at temperatures between 150–230 °C within 5–30 minutes. The reported conditions feature comparable performance to those of classic syntheses, avoiding volatile organic solvents, strong acids and toxic catalysts. Decarboxylated quinoxalines arise as side products in variable amounts via direct decarboxylation of the 3,4-diaminobenzoic acid. To completely inhibit the decarboxylation, we show that suitable structural analogues of 3,4-diaminobenzoic acid can act as starting compounds. Thus, ester hydrolysis of methyl 3,4-diaminobenzoate and deprotection of di-Boc-protected 3,4-diminobenzoic can be coupled with the HTS of quinoxaline towards quinoxaline carboxylic acids, while fully avoiding decarboxylated side products.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Synthesis
DOI: 10.1055/a-1807-3188

An environmentally friendly and safe synthesis of phloroglucinol and its derivatives through the flow hydrogenation of 1,3,5-trinitrobenzenes on heterogeneous copper catalysts is reported. It was found that hydrogenation of 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene, 2,4,6-trinitroxylene, and 2,4,6-trinitromesitylene in methanol over Cu–Al mixed oxides derived from layered double hydroxides led to selective formation of the corresponding triaminobenzenes, which were isolated from the reaction mixture in the form of double salts with sulfuric acid and were stable in storage. Subsequent hydrolysis in aqueous solution gave the phloroglucinol derivatives in good yields (75–82%).
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Castoff to bioactive molecules: The diversified functionalization of propenylcatechol, a direct depolymerized product from naturally occurring catechyl lignin, resulted in a series of functional molecular skeletons. Starting from castor seed coats, annuloline (natural product) and CC-5079 (antitumor) were synthesized with a superior step-efficiency to that of reported synthetic routes.

Abstract

Catechyl lignin (C-lignin) is a naturally occurring linear homogeneous biopolymer composed solely of caffeyl alcohol subunits with cleavable benzodioxane linkages. The inherent structural features of propenylcatechol, a direct depolymerized product of castor seed coats C-lignin, render it a sustainable and promising platform for the synthesis of bioactive molecules. Herein, diversified transformations of propenylcatechol, including C=C bond difunctionalization, β-modification, β,γ-rearrangement, and γ-methyl derivatization, were reported based on known or developed methods. A series of functional molecular skeletons involved in the current synthetic routes for the preparation of pharmaceuticals and bioactive molecules were obtained. Starting from castor seed coats, annuloline (natural product) and CC-5079 (antitumor) were synthesized using facile and inexpensive reagents in only four- and five-sequence reactions, respectively, thereby demonstrating a superior step-efficiency to that of reported synthetic routes. Almost all atoms in the C-lignin biopolymer were incorporated into the final products owing to the intrinsic structures of naturally occurring C-lignin. Bioactive molecules produced from C-lignin integrate a low-carbon footprint with high-quality and economical manufacture of pharmaceuticals.

Synthesis
DOI: 10.1055/s-0040-1719905

Nitro compounds are vital raw chemicals that are widely used in academic laboratories and industries for the preparation of various drugs, agrochemicals, and materials. Thus, nitrating reactions are of great importance for chemists and are even taught in schools as one of the fundamental transformations in organic synthesis. Since the discovery of the first nitrating reactions in the 19th century, progress in this field has been constant. Yet, for many years the classical electrophilic nitration approach using a mixture of strong mineral acids dominated the field. However, in recent decades, the attention of researchers has focused on new reactivity and new reagents that can provide access to nitro compounds in a practical and straightforward way under mild reaction conditions. Organic nitrating reagents have played a special role in this field since they have enhanced reactivity. They also allow nitration to be carried out in an ecofriendly and sustainable manner. This review examines the development and application of organic nitrating reagents.1 Introduction2 Organic Nitrating Reagents2.1 Alkyl Nitrites2.2 Nitroalkanes2.3 Alkyl Nitrates2.4 N-Nitroamides2.5 N-Nitropyrazole2.6 N-Nitropyridinium Salts3 Organic Nitrating Reagents Generated In Situ3.1 Acyl Nitrates3.2 Trimethylsilyl Nitrate3.3 Nitro Onium Salts4 Organic Nitronium Salts5 Organic Nitrates and Nitrites5.1 Ammonium Nitrates5.2 Heteroarylium Nitrates5.3 Other Organic Nitrates5.4 Organic Nitrites6 Conclusion and Outlook
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Synthesis
DOI: 10.1055/a-1752-5471

The introduction of the Green Chemistry Principles in the late 1990s formed the basis for a transition to a greener environment. These Principles have become an integral part in the work on designing chemical processes, especially for large-scale manufacture. The ultimate target is the achievement of a sustainable production method allowing hundreds of tons of valuable materials to be prepared. For this purpose, a holistic view must be applied to the elements constituting a fully-fledged process encompassing layout of the synthetic route, defining starting materials and their origin, output of product and quality features, quantity of effluent streams and waste, recovery and recycling of chemicals involved, and energy consumption. These parameters form a complex matrix where the individual components are in a complicated relationship with each other. This short review addresses these issues and the benefits of life-cycle assessment and metrics commonly used to measure the performance of chemical manufacturing – all from a pharmaceutical industry perspective as experienced by the author.1 Introduction: Facing Severe Challenges2 The Historical Context: Addressing an Image Problem3 Prospects, Drivers and Roadmap for the Green Future4 Living by the Principles: Industrial Perspectives5 Taking the Green Route – Catalysis Leading the Way: Case Stories6 State of the Art: How Green Are We?7 Sending Signals, Creating Impressions: Focus on Communication8 Conclusions
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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