MRV

Synthesis
DOI: 10.1055/a-1581-0934

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. However, most electrochemical transformations only employ one electrode (anodic oxidation or cathodic reduction) to afford the desired products, while the chemistry that occurs at the counter electrode yields stoichiometric waste. In contrast, paired electrochemical reactions can synchronously utilize the anodic and cathodic reactions to deliver the desired product, thus improving the atom economy and energy efficiency of the electrolytic process. This review gives an overview of recent advances in nickel-catalyzed paired electrochemical cross-coupling reactions of aryl/alkenyl halides with different nucleophiles.1 Introduction2 Nickel-Catalyzed Cross-Coupling Reactions2.1 C–C Bond Formation2.2 C–N Bond Formation2.3 C–S/O Bond Formation2.4 C–P Bond Formation3 Conclusion
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Synthesis
DOI: 10.1055/a-1577-7638

In recent decades, transition-metal-catalyzed nucleophilic dearomatization of electron-deficient heteroarenes, such as pyridines, quinolines, isoquinolines and nitroindoles, has become a powerful method for accessing unsaturated heterocycles. This short review summarizes nucleophilic dearomatizations of electron-deficient hetero­arenes with carbon- and heteroatom-based nucleophiles via transition-metal catalysis. A significant number of functionalized heterocycles are obtained via this transformation. Importantly, many of these reactions are carried out in an enantioselective manner by means of asymmetric catalysis, providing a unique method for the construction of enantio­enriched heterocycles.1 Introduction2 Transition-Metal-Catalyzed Nucleophilic Dearomatization of Hetero­arenes via Alkynylation3 Transition-Metal-Catalyzed Nucleophilic Dearomatization of Heteroarenes­ via Arylation4 Transition-Metal-Catalyzed Nucleophilic Dearomatization of Heteroarenes­ with Other Nucleophiles5 Transition-Metal-Catalyzed Nucleophilic Dearomatization with Nucleophiles Formed In Situ6 Conclusion and Outlook
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Decarboxylative coupling of aliphatic N-hydroxyphthalimide esters with polyfluoroaryl zinc reagents was achieved by dual photoredox and copper catalysis. This method allows the installation of alkyl groups on polyfluoroaryls with a wide range of F-content (2F-5F) and variable F-substitution patterns.

Abstract

Polyfluoroarenes are an important class of compounds in medical and material chemistry. The synthesis of alkylated polyfluoroarenes remains challenging. Here we describe a decarboxylative coupling reaction of N-hydroxyphthalimide esters of aliphatic carboxylic acids with polyfluoroaryl zinc reagents (Zn-Ar_F) via synergetic photoredox and copper catalysis. This method readily converts primary and secondary alkyl carboxylic acids into the corresponding polyfluoroaryl compounds, which could have a wide range of F-content (2F-5F) and variable F-substitution patterns on the aryl groups. Broad scope and good functional group compatibility were achieved, including on substrates derived from natural products and pharmaceuticals. Mechanistic study revealed that a [Cu-(Ar_F)₂] species could be responsible for the transfer of polyfluoroaryl groups to the alkyl radicals.

Assessment of the rich co-complexation chemistry of Ni(COD)₂ and PhLi has led to a new family of structurally diverse lithium nickelates being uncovered. Combined stoichiometric, catalytic, and kinetic studies reveal that these hetero-bimetallic complexes may be the key intermediates that facilitate the smooth C−O bond cleavage and cross-coupling of aryl ethers under mild conditions.

Abstract

The Ni-catalysed cross-coupling of aryl ethers is a powerful method to forge new C−C and C−heteroatom bonds. However, the inert C(sp²)−O bond means that a canonical mechanism that relies on the oxidative addition of the aryl ether to a Ni⁰ centre is thermodynamically and kinetically unfavourable, which suggests that alternative mechanisms may be involved. Here, we provide spectroscopic and structural insights into the anionic pathway, which relies on the formation of electron-rich hetero-bimetallic nickelates by adding organometallic nucleophiles to a Ni⁰ centre. Assessing the rich co-complexation chemistry between Ni(COD)₂ and PhLi has led to the structures and solution-state chemistry of a diverse family of catalytically competent lithium nickelates being unveiled. In addition, we demonstrate dramatic solvent and donor effects, which suggest that the cooperative activation of the aryl ether substrate by Ni⁰-ate complexes plays a key role in the catalytic cycle.

Synlett
DOI: 10.1055/s-0040-1719829

Nickel-catalyzed cross-coupling and photoredox catalytic reactions has found widespread utilities in organic synthesis. Redox processes are key intermediate steps in many catalytic cycles. As a result, it is pertinent to measure and document the redox potentials of various nickel species as precatalysts, catalysts, and intermediates. The redox potentials of a transition-metal complex are governed by its oxidation state, ligand, and the solvent environment. This article tabulates experimentally measured redox potentials of nickel complexes supported on common ligands under various conditions. This review article serves as a versatile tool to help synthetic organic and organometallic chemists evaluate the feasibility and kinetics of redox events occurring at the nickel center, when designing catalytic reactions and preparing nickel complexes.1 Introduction1.1 Scope1.2 Measurement of Formal Redox Potentials1.3 Redox Potentials in Nonaqueous Solution2 Redox Potentials of Nickel Complexes2.1 Redox Potentials of (Phosphine)Ni Complexes2.2 Redox Potentials of (Nitrogen)Ni Complexes2.3 Redox Potentials of (NHC)Ni Complexes
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Profound pharmacological benefits can be provided by site-selective incorporation of the deuterated “magic methyl” group, which can be considered an important tool for drug optimization and development. This review highlights site-selective trideuteromethylation strategies according to the trideuteromethylation reagent used and describes the mechanism and scope of the reactions. As such, this review aims to be a guide for researchers, offering them the available C−CD₃ formation strategies for the preparation of new or modified drugs or materials. For more details see the Review by M. Rueping et al. on page 11751 ff.

Kinetic isotope effects (KIEs, k _H/k _D) above the typical limit of 7 are reviewed, for organometallic reaction types from protonation of clusters to catalytic C−H activation. The KIEs are as large as 105, or even so large that reactivity is unobserved for deuterium. Large KIEs in organometallics have been explained by a combination of (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step.

Abstract

The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, k _H/k _D, typically falls between 1–7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal–carbon bonds, and dihydrogen reactivity. C−H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.

This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Abstract

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.