MRV

Synlett
DOI: 10.1055/a-1665-9220

A red-light-mediated [3+2] annulation of cyclopropylamines with akenes or alkynes in the presence of N,N′-dipropyl-1,13-dimethoxyquinacridinium is reported. An array of cyclopentane or cyclopentene derivatives with diverse functional groups have been obtained in moderate to excellent yields under mild conditions.
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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Asymmetric photocatalysis: This review provides a current overview of visible-light-induced asymmetric photocatalysis enabled by chiral organocatalysts. Innovative approaches including mono- and dual-catalysis are discussed. Chiral organocatalysis such as enamine catalysis, iminium-ion catalysis, Brønsted acid/base catalysis, and N-heterocyclic carbene catalysis have been exploited to induce chirality transfer of photocatalytic reactions. We hope that this review will spark new ideas for designing novel strategies in the field of asymmetric photocatalysis.

Abstract

Visible-light photocatalysis has advanced as a versatile tool in organic synthesis. However, attaining precise stereocontrol in photocatalytic reactions has been a longstanding challenge due to undesired photochemical background reactions and the involvement of highly reactive radicals or radical ion intermediates generated under photocatalytic conditions. To address this problem and expand the synthetic utility of photocatalytic reactions, a number of innovative strategies, including mono- and dual-catalytic approaches, have recently emerged. Of these, exploiting chiral organocatalysis, such as enamine catalysis, iminium-ion catalysis, Brønsted acid/base catalysis, and N-heterocyclic carbene catalysis, to induce chirality transfer of photocatalytic reactions has been widely explored. This Review aims to provide a current, comprehensive overview of asymmetric photocatalytic reactions enabled by chiral organocatalysts published through June 2021. The substrate scope, advantages, limitations, and proposed reaction mechanisms of each reaction are discussed. This review should serve as a reference for the development of visible-light-induced asymmetric photocatalysis and promote the improvement of the chemical reactivity and stereoselectivity of these reactions.

Hydrogen atoms sometimes react far faster than deuterium atoms. Reactions can proceed over 100 times faster with hydrogen, or the deuterium reaction may not be observed at all. Examples of large kinetic isotope effects throughout organotransition metal chemistry are reviewed. Possible causes for these observed effects are discussed, including vibrational differences, multistep reactions, and proton tunneling. Artwork depicting a hydrogen cheetah and a deuterium sloth by Caitlyn Y. Tong. Read more in the Minireview by M. A. Bowring et al. on page 14800 ff.

The photoexcitable borate enabling Minisci alkylation of heteroarene has been described. This protocol using alkylborate only needed O₂ and visible light, which provides a considerably clean system. The direct photoexcitation of alkylborate allowed the generation of tertiary, secondary, and primary-alkyl radical including methyl radical.

Abstract

The photoexcitable borate enabling Minisci C−H alkylation of heteroarene has been described. This protocol using alkylborate only needed O₂ and visible light, which provides a considerably clean system. The direct photoexcitation of alkylborate allowed the generation of tertiary, secondary, and primary-alkyl radical including methyl radical. This light-driven Minisci reaction gave a choice for the functionalization of heteroarenes.

A novel Si-based Lewis superacid was synthesized by using highly electron-withdrawing perfluoropinacolato substituents. The formed silane was obtained as the acetonitrile mono-adduct and comprehensively characterized. The reaction with Et₃SiF led to fluoride abstraction yielding the anionic fluorosilicate and the acetonitrile stabilized silylium ion. Catalysis screenings demonstrated high activity in Lewis acid catalyzed reactions.

Abstract

Despite the earth abundance and easy availability of silicon, only few examples of isolable neutral silicon centered Lewis superacids are precedent in the literature. To approach the general drawbacks of limited solubility and unselective deactivation pathways, we introduce a Lewis superacid, based on perfluorinated pinacol substituents. The compound is easily synthesized on a gram-scale as the corresponding acetonitrile mono-adduct 1⋅(MeCN) and was fully characterized, including single crystal X-ray diffraction analysis (SC-XRD) and state-of-the-art computations. Lewis acidity investigations by the Gutmann-Beckett method and fluoride abstraction experiments indicate a Lewis superacidic nature. The challenging Si−F bond activation of Et₃SiF is realized and promising catalytic properties are demonstrated, consolidating the potential applicability of silicon centered Lewis acids in synthetic catalysis.

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.