LongLarf

Synthesis
DOI: 10.1055/a-1509-5954

Nickel-catalyzed reductive homocoupling of aryl ethers has been achieved with Mg(anthracene)(thf)3 as a readily available low-cost reductant. DFT calculations provided a rationale for the specific efficiency of the diorganomagnesium-type two-electron reducing agent. The calculations show that the dianionic anthracene-9,10-diyl ligand reduces the two aryl ether substrates, resulting in the homocoupling reaction through supply of electrons to the Ni-Mg bimetallic system to form organomagnesium nickel(0)-ate complexes, which cause two sequential C–O bond cleavage reactions. The calculations also showed cooperative actions of Lewis acidic magnesium atoms and electron-rich nickel atoms in the C–O cleavage reactions.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Visible-light-mediated coupling of acylsilanes with indoles is reported. This photoreaction occurs under mild conditions mostly in quantitative yield and provides stable silylated N,O-acetals. The potential of the method is documented by the successful coupling of carbohydrates with indole alkaloids and by the conjugation of polymers.

Abstract

Light-mediated coupling of acylsilanes with indoles is reported. This photo click reaction occurs under mild conditions (415 nm) mostly in quantitative yield and provides stable silylated N,O-acetals via light mediated siloxycarbene generation with subsequent indole-N-H insertion. We show that this very efficient and fully atom economic coupling process can be applied to conjugate complex systems, as documented by the clicking of carbohydrates with indole alkaloids. The method is also applicable to the conjugation of polymer chains. The linking acetal moiety can be readily cleaved and it is also shown that wavelength-selective coupling and cleavage with acyl silanes bearing a second photoactive moiety is possible. This is documented by a successful polymerization/depolymerization sequence and by a polymer folding/unfolding process.

Synlett
DOI: 10.1055/a-1503-6330

Chemoselective cross-coupling of phenol derivatives is valuable for generating products that retain halides. Here we discuss recent developments in selective cross-couplings of chloroaryl phenol derivatives, with a particular focus on reactions of chloroaryl tosylates. The first example of a C–O-selective Ni-catalyzed Suzuki–Miyaura coupling of chloroaryl tosylates is discussed in detail.1 Introduction2 Density Functional Theory Studies on Oxidative Addition at Nickel(0)3 Stoichiometric Oxidative Addition Studies4 Development of a Tosylate-Selective Suzuki Coupling5 Conclusion and Outlook
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Synlett
DOI: 10.1055/a-1520-9916

Coupled catalysis using a riboflavin-derived organocatalyst and molecular iodine successfully promoted the aerobic oxidation of thiols to disulfides under metal-free mild conditions. The activation of molecular oxygen occurred smoothly at room temperature through the transfer of electrons from the iodine catalyst to the biomimetic flavin catalyst, forming the basis for a green oxidative synthesis of disulfides from thiols.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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The application of earth abundant Mn, Fe, Co, and Ni complexes for various heterocycle synthesis using alcohol as a coupling partner is discussed in the present review.

Abstract

Development in the area of acceptorless dehydrogenation (AD) and borrowing hydrogen (BH) catalysis emerge as one of the potential tools for various C−C and C-heteroatom bond forming reactions. Alcohols, which are important lignocellulosic biomass products, act as pivotal electrophilic coupling partners in such processes and interestingly only H₂ or H₂O is eliminated as a byproduct. Initially, the area was developed by the use of noble metal catalysts. Recently, base metals such as Mn, Fe, Co, and Ni proved to be environmentally benign and inexpensive alternatives for the noble metals in the application of AD and BH methods. This transition metal catalyzed AD and BH approaches also allow access toward a plethora of structurally important heterocyclic molecules via environmentally benign and atom economical strategy. Herein, we summarize the current and rising expansion of base metal catalyzed heterocycles synthesis through acceptorless dehydrogenation and borrowing hydrogenation strategy.

Ammonia (NH₃) is a globally important commodity for fertilizer production, but its synthesis by the Haber-Bosch process causes substantial emissions of carbon dioxide. Alternative, zero-carbon emission NH₃ synthesis methods being explored include the promising electrochemical lithium-mediated nitrogen reduction reaction, which has nonetheless required sacrificial sources of protons. In this study, a phosphonium salt is introduced as a proton shuttle to help resolve this limitation. The salt also provides additional ionic conductivity, enabling high NH₃ production rates of 53 ± 1 nanomoles per second per square centimeter at 69 ± 1% faradaic efficiency in 20-hour experiments under 0.5-bar hydrogen and 19.5-bar nitrogen. Continuous operation for more than 3 days is demonstrated.

If I had a nickel: A nickel-catalyzed thiolation of aryl nitriles to access functionalized aryl thioethers is developed. The ligand dcype (1,2-bis(dicyclohexylphosphino)ethane) as well as the base KO ^t Bu (potassium tert-butoxide) are essential to achieve this transformation. This scalable and practical process involves both a C−C bond activation and a C−S bond formation.

Abstract

A nickel-catalyzed thiolation of aryl nitriles has been developed to access functionalized aryl thioethers. The ligand dcype (1,2-bis(dicyclohexylphosphino)ethane) as well as the base KO ^t Bu (potassium tert-butoxide) are essential to achieve this transformation. This scalable and practical process involves both a C−C bond activation and a C−S bond formation. Furthermore, this reaction shows a high functional-group tolerance and enables the late-stage functionalization of important molecules.

Nature Chemistry, Published online: 10 June 2021; doi:10.1038/s41557-021-00715-0

The desymmetrization of easily accessible malonic esters represents an attractive approach towards the formation of chiral quaternary stereocentres, but is largely limited to enzymatic hydrolysis. Now, a zinc-catalysed asymmetric hydrosilylation reaction—that works with a broad scope of substrates—has been shown to reduce one of the esters to a primary alcohol with excellent enantiocontrol.

Nature Communications, Published online: 10 June 2021; doi:10.1038/s41467-021-23887-2

Chiral α-aryl ketones are versatile building blocks, and represent important pharmacophores existing in many drug molecules such as ibuprofen and naproxen. Here the authors for such ketones but using nickel and photoredox dual catalysis in asymmetric benzylic C−H acylation of alkylarenes and employing carboxylic acids as acyl surrogates.

Synlett
DOI: 10.1055/a-1517-5895

The use of tertiary amines as surrogates for secondary amines has prominent advantages in terms of stabilization and ease of handling. A Ni-catalyzed transamidation of N-acylsaccharins with tertiary aromatic amines is reported. By using tert-butyl hydroperoxide as the terminal oxidant, this reaction permits selective cleavage of the C(sp3)–N bonds of unsymmetrical tertiary aromatic amines depending on the sizes of the alkyl substituents.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Synthesis
DOI: 10.1055/a-1504-8924

A series of 1-alkoxy-3-methyl- and 3,4-dimethyl-3-phospholene 1-oxides, as well as 1-alkoxy-3-methylphospholane 1-oxides were prepared in good yields by the microwave (MW)-assisted [bmim][PF6]-catalyzed transesterification of the corresponding methyl or ethyl esters. The alcoholyses studied represent another case, where MW irradiation has had a crucial role on the course of the reaction. The method developed is an alternative possibility to other esterifications starting from the corresponding phosphinic chlorides and acids.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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A mechanistic study of the dual-ligand-enabled directing-group-free C−H activation of arenes with palladium catalysts and the arene as limiting reagent is described. A detailed experimental and computational investigation supports a 1:1:1 complex of palladium and the two complementary ligands as the active species that enables a partially rate-limiting concerted C−H activation as part of a Pd⁰/Pd^II cycle.

Abstract

Recently palladium catalysts have been discovered that enable the directing-group-free C−H activation of arenes without requiring an excess of the arene substrate, thereby enabling methods for the late-stage modification of complex organic molecules. The key to success has been the use of two complementary ligands, an N-acyl amino acid and an N-heterocycle. Detailed experimental and computational mechanistic studies on the dual-ligand-enabled C−H activation of arenes have led us to identify the catalytically active species and a transition state model that explains the exceptional activity and selectivity of these catalysts. These findings are expected to be highly useful for further method development using this powerful class of catalysts.

Recent years have witnessed a rapid growth of biocatalytic methods in organic synthesis. This Minireview summarizes the developments in the current state of the art for practical enzyme-mediated assembly of carbocycles, drawing special attention to case studies that have been performed on preparative scale.

Abstract

The Pd-catalyzed carbon-carbon bond formation pioneered by Heck in 1969 has dominated medicinal chemistry development for the ensuing fifty years. As the demand for more complex three-dimensional active pharmaceuticals continues to increase, preparative enzyme-mediated assembly, by virtue of its exquisite selectivity and sustainable nature, is poised to provide a practical and affordable alternative for accessing such compounds. In this minireview, we summarize recent state-of-the-art developments in practical enzyme-mediated assembly of carbocycles. When appropriate, background information on the enzymatic transformation is provided and challenges and/or limitations are also highlighted.

Synlett
DOI: 10.1055/s-0040-1706042

Reduction-and-oxidation (redox) reactions are one of the most utilized approaches for the synthesis of value-added compounds. With the growing awareness of green chemistry, researchers have searched for new and sustainable pathways for performing redox reactions. From this, a new field has gained tremendous attention, namely photoredox catalysis. Here, molecules can be easily oxidized or reduced with the use of one of Nature’s biggest resources: visible light. This tutorial paper gives the basics of photoredox catalysis along with limited examples to encourage further research in this blooming research area.1 Introduction2 Redox Chemistry3 Photochemistry3.1 Laws of Photochemistry3.2 Principles3.3 Examples4 Photoredox Catalysis4.1 General Principles4.2 Classification of Redox Processes4.3 Other Mechanistic Considerations4.4 Stern–Volmer Plots4.5 Photophysical Properties4.6 Redox Potentials5 Photocatalysts5.1 Metal-Based Photocatalysts5.2 Organic Dyes5.3 Semiconductors6 Dual Catalysis7 Conclusions
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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