LongLarf

Functionalization of renewables: This Minireview summarizes the known reports on the catalytic functionalization of non‐reactive aromatic C−H bonds in promising furanic platform chemicals. Various approaches are considered, such as the well‐known directing‐group strategy and rare examples of undirected C−H functionalization. Implementation of such transformations will provide new materials, potential drugs, and fine chemicals.

Abstract

C−H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the “Holy Grail” of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C−H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass‐derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C−H functionalization for their modification. New methods of catalytic functionalization of non‐reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.

Synlett
DOI: 10.1055/s-1294-0158

Alkylated heteroarenes are widely found in bioactive molecules and pharmaceuticals. Therefore, there is great interest in developing a chemoselective alkylation of heteroarenes under mild conditions, particularly during a late-stage functionalization step for the purpose of rapid derivatization. Herein, we introduce an efficient visible-light-promoted C–H alkylation of nitrogen-containing heteroarenes by using C4-alkyl 1,4-dihydropyridines (DHPs) as radical precursors at ambient temperatures. A broad scope of heteroarenes, such as 4-hydroxyquinazoline and its derivatives, including those bearing electron-donating or electron-withdrawing groups, can be successfully alkylated in good yields by using various C4-alkyl DHPs.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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

Ba metal was activated by evaporation and cocondensation with heptane. This black powder is a highly active hydrogenation catalyst for the reduction of a variety of unactivated (non‐conjugated) mono‐, di‐ and tri‐substituted alkenes, tetraphenylethylene, benzene, a number of polycyclic aromatic hydrocarbons, aldimines, ketimines and various pyridines. The performance of metallic Ba in hydrogenation catalysis tops that of the hitherto most active molecular group 2 metal catalysts. Depending on the substrate, two different catalytic cycles are proposed. A: a classical metal hydride cycle and B: the Ba metal cycle. The latter is proposed for substrates that are easily reduced by Ba0 , i.e. conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba0 and BaH2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH2 ) confirm that the presence of metallic Ba has an accelerating effect.

A new diterpene synthase (CaCS) from Catenulispora acidiphila and its products were identified. The enzyme mechanism was studied by isotopic labelling experiments and usage of substrate analogues with blocked reactivity, resulting in a series of derailment products. Their chemistry was studied, leading to the biomimetic synthesis of a diterpenoid analogue of a brominated sesquiterpene known from the red seaweed Laurencia microcaldia.

Abstract

A new diterpene synthase from the actinomycete Catenulispora acidiphila was identified and the structures of its products were elucidated, including the absolute configurations by an enantioselective deuteration approach. The mechanism of the cationic terpene cyclisation cascade was deeply studied through the use of isotopically labelled substrates and of substrate analogues with partially blocked reactivity, resulting in derailment products that gave further insights into the intermediates along the cascade. Their chemistry was studied, leading to the biomimetic synthesis of a diterpenoid analogue of a brominated sesquiterpene known from the red seaweed Laurencia microcladia.

Sulfur and brimstone: Photocatalytic desulfurization is the most promising deep desulfurization method. This Review summarizes recent progress of photocatalytic desulfurization and emphasizes the roles of defect engineering, hybrid coupling, and structure modifications toward the enhancement of photocatalytic performance.

Abstract

Fuel oil, the most important strategic resource, has been widely used in industrial applications. However, the sulfur‐containing compounds in fuel oil also present humanity with huge environmental issues and health concerns due to the hazardous combustion waste. To address this problem, the low vulcanization of fuel production technology has been intensively explored. Compared with traditional hydrodesulfurization technology, the newly emerged photocatalytic desulfurization has the advantages of milder operating conditions, lower energy consumption, and higher efficiency, holding great prospect to achieve deep desulfurization. Though great efforts have been made, the desulfurization catalysts still suffer from inferior light absorption, fast recombination of photocarriers, and poor structure modification. This Review summarizes recent development of photocatalytic desulfurization, including the desulfurization principle, current desulfurization challenges, and corresponding solutions. Particularly, the roles of defect engineering, hybrid coupling, and structure modifications in the enhancement of photocatalytic performance are emphasized. In addition, the photocatalytic desulfurization mechanism is also introduced with the ^.OH and ^.O₂ ⁻ radicals as main active species. Finally, some perspectives on the photocatalytic desulfurization are provided, which can further optimize the desulfurization efficiency and guide future photocatalyst design.

Turning flue gas into fuel: CO₂ capture from flue gas (10 % CO₂) is demonstrated and the captured species were directly hydrogenated to methanol, a value‐added carbon feedstock using a ruthenium pincer catalyst. The integrated process comprises stable and high boiling tertiary amines in ethylene glycol solvent, mediating both the capture and conversion steps. This study is key in developing a sustainable and carbon‐neutral economy.

Abstract

Carbon dioxide capture using tertiary amines in ethylene glycol solvent was performed under ambient conditions. Subsequently, the CO₂ captured as alkyl carbonate salts was successfully hydrogenated to methanol, in the presence of H₂ gas and Ru‐Macho‐BH catalyst. A comprehensive series of tertiary amines were selected for the integrated capture and conversion process. While most of these amines were effective for CO₂ capture, tetramethylethylenediamine (TMEDA) and tetramethylbutanediamine (TMBDA) provided the best CH₃OH yields. Deactivation of the base due to side reactions was significantly minimized and substantial base regeneration was observed. The proposed system was also highly efficient for CO₂ capture from a gas mixture containing 10 % CO₂, as found in flue gases, followed by tandem conversion to CH₃OH. We postulate that such high boiling tertiary amine‐glycol systems as dual capture and hydrogenation solvents are promising for the realization of a sustainable and carbon‐neutral methanol economy in a scalable process.

Carbamates from CO₂ : Transition metal‐free approaches for the generation of carbamates using CO₂ as a C1 synthon are summarized. This Minireview focuses on the benefits of the recent advances of transition metal‐free methodologies and mechanistic insights in the pursuit of carbamate generation for the synthesis of fine chemicals and pharmaceuticals as a CO₂‐fixation strategy.

Abstract

Utilization of carbon dioxide as a C1 synthon is highly attractive for the synthesis of valuable chemicals. However, activation of CO₂ is highly challenging, owing to its thermodynamic stability and kinetic inertness. With this in mind, several strategies have been developed for the generation of carbon–heteroatom bonds. Among these, formation of C−N bonds is highly attractive, especially, when carbamates can be synthesized directly from CO₂. This Minireview focuses on transition metal‐free approaches for the fixation of CO₂ to generate carbamates for the production of fine chemicals and pharmaceuticals. Within the past decade, transition metal‐free approaches have gained increasing attention, but traditional reviews have rarely focused on these approaches. Direct comparisons between such methods have been even more scarce. This Minireview seeks to address this discrepancy.

Green and organic: The chemical reduction of carbon dioxide with hydrosilanes to various valuable chemicals, such as formamides and N‐methylated amines (upon the introduction of amines) also methanol and formic acid, is now possible using highly efficient, safe, and environmentally friendly heterogeneous organocatalysts. This Minireview discusses recent development of these heterogeneous catalysts and the possible determining factors in their activities.

Abstract

The utilization of carbon dioxide (CO₂) as feedstock for chemical industries is gaining interest as a sustainable alternative to nonrenewable fossil resources. However, CO₂ reduction is necessary to increase its energy content. Hydrosilane is a potential reducing agent that exhibits excellent reactivity under ambient conditions. CO₂ hydrosilylation yields versatile products such as silylformate and methoxysilane, whereas formamides and N‐methylated products are obtained in the presence of amines. In these transformations, organocatalysts are considered as the more sustainable choice of catalyst. In particular, heterogeneous organocatalysts featuring precisely designed active sites offer higher efficiency due to their recyclability. Herein, an overview is presented of the current development of basic organocatalysts immobilized on various supports for application in the chemical reduction of CO₂ with hydrosilanes, and the potential active species parameters that might affect the catalytic activity are identified.

A novel approach towards the efficient assembly of chiral α‐arylbenzamides is presented herein based on an enantioselective, nickel‐catalyzed reductive hydroarylation protocol. The use of neutral reagents and mild reaction conditions enabled the synthesis of α‐arylbenzamides in high enantiomeric purity providing an alternative access to pharmacologically relevant motifs present in anticancer, SARS‐CoV PLpro inhibitors, and KCNQ channel openers.

Abstract

A nickel‐catalyzed asymmetric reductive hydroarylation of vinyl amides to produce enantioenriched α‐arylbenzamides is reported. The use of a chiral bisimidazoline (BIm) ligand, in combination with diethoxymethylsilane and aryl halides, enables the regioselective introduction of aryl groups to the internal position of the olefin, forging a new stereogenic center α to the N atom. The use of neutral reagents and mild reaction conditions provides simple access to pharmacologically relevant motifs present in anticancer, SARS‐CoV PLpro inhibitors, and KCNQ channel openers.

This Minireview provides a broad overview of the advancements made in the synthesis and modification of amino acid derivatives using radical chemistry during the last decade. The overview is divided in two sections: methods for the de novo synthesis of α‐, β‐, and γ‐amino acids, and methods for the selective derivatisation of canonical and non‐canonical α‐amino acids.

Abstract

Amino acids (AAs) are key structural motifs with widespread applications in organic synthesis, biochemistry, and material sciences. Recently, with the development of milder and more versatile radical‐based procedures, the use of strategies relying on radical chemistry for the synthesis and modification of AAs has gained increased attention, as they allow rapid access to libraries of novel unnatural AAs containing a wide range of structural motifs. In this Minireview, we provide a broad overview of the advancements made in this field during the last decade, focusing on methods for the de novo synthesis of α‐, β‐, and γ‐AAs, as well as for the selective derivatisation of canonical and non‐canonical α‐AAs.

Publication date: 5 November 2020

Source: Chem, Volume 6, Issue 11

Author(s): Charlie T. McTernan, Guillaume De Bo, David A. Leigh

A novel bioorthogonal activation approach based on Pd(II)‐mediated transmetallation was developed to controllably activate stable organometallic NHC–gold(I)–phenylacetylide complex. The gold(I) complex, upon transmetallation activation, can catalyze alkyne π‐bond activations and can meanwhile potently inhibit thioredoxin reductase and induce cytotoxicity towards cancer cells in vitro and in vivo with high spatiotemporal selectivity.

Abstract

Controllably activating the bio‐reactivity of metal complexes in living systems is challenging but highly desirable because it can minimize off‐target bindings and improve spatiotemporal specificity. Herein, we report a new bioorthogonal activation approach by employing Pd(II)‐triggered transmetallation reactions to conditionally activate the bio‐reactivity of NHC–Au(I)–phenylacetylide complexes (1 a) in vitro and in vivo. A combination of ¹H NMR, LC‐MS, DFT calculation and fluorescence screening assays reveals that 1 a displays a reasonable stability against biological thiols, but its phenylacetylide ligand can be efficiently transferred to Pd(II), leading to in situ formation of labile NHC–Au(I) species that is catalytically active inside living cells and zebrafish, and can meanwhile effectively suppress the activity of thioredoxin reductase, potently inhibit the proliferation of cancer cells and efficiently suppress angiogenesis in zebrafish models.

Phosphine carboxylate, H₂PCO₂ ⁻, has been prepared for the first time both by the reaction of dihydrogen phosphide with carbon dioxide and by hydrolysis of phosphaethynolate, PCO⁻. Acidification of the salt yielded phosphine carboxylic acid that has a surprising kinetic stability compared to carbamic and carbonic acids. The mechanism of phosphaethynolate hydrolysis was investigated.

Abstract

We present a new adduct of carbon dioxide with dihydrogenphosphide, that may be prepared either by direct reaction of NaPH₂ with carbon dioxide or by hydrolysis of the phosphaethynolate ion (PCO⁻). In this hydrolysis transformation, a new mechanism is proposed for the electrophilic reactivity of the phosphaethynolate ion. Protonation to form phosphine carboxylic acid (PH₂COOH) and functionalization to form esters is shown to increase the strength of the P–C interaction, allowing for comparisons to be drawn between this species and the analogous carbamic (NH₂COOH) and carbonic acids (H₂CO₃). Functionalization of the oxygen atom is found to stabilize the phosphine carboxylate while also allowing solubility in organic solvents whereas phosphorus functionalization is shown to facilitate decarboxylation. Substituent migration occurs in some cases.

Nature, Published online: 03 November 2020; doi:10.1038/s41586-020-2919-z