MRV

A chemoselective electrooxidative heterocoupling of two different enamines is reported for the synthesis of unsymmetrically substituted NH-pyrroles. A “magic effect” of the additive trifluoroethanol is utilized to achieve the desired chemoselectivity by tuning the oxidation potentials and controlling the activation energy of the rate-determining step.

Abstract

An electrochemical method for the synthesis of unsymmetrically substituted NH-pyrroles is described. The synthetic strategy comprises a challenging heterocoupling between two structurally diverse enamines via sequential chemoselective oxidation, addition, and cyclization processes. A series of aryl- and alkyl-substituted enamines were effectively cross-coupled from an equimolar mixture to synthesize various unsymmetrical pyrrole derivatives up to 84 % yield. The desired cross-coupling was achieved by tuning the oxidation potential of the enamines by utilizing a “magic effect” of the additive trifluoroethanol (TFE). Additionally, extensive computational studies reveal the unique role of TFE in promoting the heterocoupling process by regulating the activation energies of the reaction steps through H-bonding and C−H⋅⋅⋅π interactions. Importantly, the developed electrochemical protocol was found to be equally efficient for the homocoupling of enamines to form symmetric pyrroles up to 92 % yield.

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

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

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.