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Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Under aprotic conditions, the stoichiometric reaction of N‐heterocyclic carbenes (NHCs) such as imidazolidin‐2‐ylidenes with aldehydes affords Breslow Intermediates (BIs), involving a formal 1,2‐C‐to‐O proton shift. We herein report kinetic studies (NMR), complemented by DFT calculations, on the mechanism of this kinetically disfavored H‐translocation. Variable time normalization analysis (VTNA) revealed that the kinetic orders of the reactants vary for different NHC‐to‐aldehyde ratios, indicating different and ratio‐dependent mechanistic regimes. We propose that for high NHC‐to‐aldehyde ratios, the H‐shift takes place in the primary, zwitterionic NHC‐aldehyde adduct. With excess aldehyde, the zwitterion is in equilibrium with a hemiacetal, in which the H‐shift occurs. In both regimes, the critical H‐shift is auto‐catalyzed by the BI. Kinetic isotope effects observed for R‐CDO are in line with our proposal. Furthermore, we detected an H‐bonded complex of the BI with excess NHC (NMR). [ABSTRACT FROM AUTHOR]

Kinetic study of organic reactions, especially multistep catalytic reactions, is crucial to in-depth understanding of reaction mechanisms. Here we report our kinetic study on the chiral disulfonimide-catalyzed cyanosilylation of an aldehyde, which revealed that two molecules of TMSCN are involved in the rate-determining C–C bond-forming step. In addition, the apparent activation energy, enthalpy of activation, and entropy of activation were deduced through a study of the temperature dependence of the reaction rates. More importantly, a novel and efficient method that makes use of the progress rates was developed to treat kinetic data obtained by continuous monitoring of the progress of a reaction by in situ FTIR. [ABSTRACT FROM AUTHOR]

A difunctionalization of alkenes through sequential addition of a radical and a nucleophile has been developed, which is suggested to proceed by a radical chain mechanism not requiring a catalyst. An electron transfer step to the oxidant benzoyl peroxide is facilitated by protonation with a strong acid. [ABSTRACT FROM AUTHOR]

Common and non‐toxic aldehydes are proposed as reagents for alkylation of ketones instead of carcinogenic alkyl halides. The developed reductive alkylation reaction proceeds in the presence of the commercially available ruthenium catalyst [(cymene)RuCl2]2 (as low as 250 ppm) and carbon monoxide as the reducing agent. The reaction works well for a broad substrate scope, including aromatic and aliphatic aldehydes and ketones. It can be carried out without a solvent and often gives nearly quantitative yields of the products. This straightforward and cost‐effective method is promising not only for laboratory application but also for industry, which produces carbon monoxide as a large‐scale waste product. [ABSTRACT FROM AUTHOR]

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent‐related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ+) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions. [ABSTRACT FROM AUTHOR]

An overview of methods for the initiation of radical chain reactions by specific initiator compounds, which generate radicals, is given. These can be utilized to initiate any kind of radical chain reaction by transforming substrates into the desired radical intermediates. Azo initiators, peroxides, nitroxides, trialkylboranes, dialkyl zinc compounds, and type I photoinitiators are discussed, as well as methods of redox‐ and sonochemical initiation. Methods of direct radical formation from the substrates, such as photoredox catalysis or high‐energy irradiation, are not included. The focus of this review lies on rather "low" temperatures in the range of 50 °C down to −78 °C, which can be useful to achieve more selective reactions. Illustrative applications of such radical chain initiators in a variety of reactions are discussed, including stereoselective ones and polymerizations. Radical, steady, go: Molecular radical chain initiators can initiate radical chain reactions by transforming substrates into the desired radical intermediates. The focus of this review lies on initiators for applications at rather low temperatures between 50 °C and −78 °C, which can be useful to achieve more selective reactions. [ABSTRACT FROM AUTHOR]

Abstract: Alkenyl and aryl peroxides are a special class of organic peroxides. Under ambient conditions, they are usually short‐lived, rapidly decomposing into radicals by homolytic O−O bond cleavage. They play an important role in the chemistry of the lower atmosphere, where they are formed through ozonolysis of alkenes. In the dark, this pathway is considered the major source of hydroxyl radicals, the “detergent of the atmosphere”. In solution, alkenyl and aryl peroxides can be formed by various methods and their decomposition can be harnessed synthetically. For example, reactions involving alkenyl peroxides can be used to introduce ketones through radical additions, and nucleophilic aromatic substitution reactions generating aryl peroxides have been used to synthesize phenols. The radicals can also initiate radical polymerization reactions or chain reactions and mediate oxidative coupling by C−H bond functionalization. Knowledge of their chemistry could be helpful for projects generating or utilizing peroxides. [ABSTRACT FROM AUTHOR]

An oxidative variant of the thiol-ene reaction has been developed, achieving the direct addition of thiols to olefins to form sulfoxides. The reaction uses tert-butyl hydroperoxide as oxidant and methanesulfonic acid as catalyst. The latter is believed to catalyze the oxidation of the intermediate sulfide to the sulfoxide. No special precautions are necessary to exclude oxygen, yet the products are formed without oxidation at the β-position. Styrenes, acrylic acid derivatives, alkynes, and thiophenols gave the highest yields, while aliphatic olefins and thiols were less effective. [ABSTRACT FROM AUTHOR]

This account summarizes how scientific advances were made in the authors' research group by combining method development in organic synthesis with detailed mechanistic studies. The discovery of an unexpected autoxidative coupling reaction led, by virtue of an ever increased understanding of its mechanism, to a strategy for green C-H functionalization reactions, novel modes of radical generation, addition reactions of ketones to alkenes and new insights into an old reaction, the Baeyer-Villiger oxidation. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The acid-mediated reaction of ketones with hydroperoxides generates radicals, a process with reaction conditions similar to those of the Baeyer-Villiger oxidation but with an outcome resembling the formation of hydroxyl radicals via ozonolysis in the atmosphere. The Baeyer-Villiger oxidation forms esters from ketones, with the preferred use of peracids. In contrast, alkyl hydroperoxides and hydrogen peroxide react with ketones by condensation to form alkenyl peroxides, which rapidly undergo homolytic OO bond cleavage to form radicals. Both reactions are believed to proceed via Criegee adducts, but the electronic nature of the peroxide residue determines the subsequent reaction pathways. DFT calculations and experimental results support the idea that, unlike previously assumed, the Baeyer-Villiger reaction is not intrinsically difficult with alkyl hydroperoxides and hydrogen peroxide but rather that the alternative radical formation is increasingly favored. [ABSTRACT FROM AUTHOR]

Oxindoles bearing ketone side chains in the 3-position can be synthesized by Brønsted acid catalysis from N-aryl methacrylamides, ketones, and hydroperoxides. The cyclized products are presumably formed in a radical cascade reaction, initiated by decay of intermediate alkenyl peroxides. In the case of acrylic substrates that do not undergo cyclization, γ-peroxyketones were isolated instead, indicating that the final cyclization step of the cascade does not take place in these cases. [ABSTRACT FROM AUTHOR]

The acid-catalyzed reactions of photochemically generated tetrahydrocarbazole peroxides with anilines have been studied experimentally and computationally to identify the underlying reaction mechanism. The kinetic data indicate a reaction order of one in the hydroperoxide and zero in the aniline. Computational investigations using density functional theory support the experimental findings and predict an initial tautomerization between an imine and enamine substructure of the primarily generated tetrahydrocarbazole peroxide to be the rate controlling step. The enamine tautomer then loses hydrogen peroxide upon protonation, generating a stabilized allylic carbocation that is reversibly trapped by solvent or aniline to form the isolated products. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Based on a mechanistic study, we have discovered a Brønsted acid catalyzed formation of ketone radicals. This is believed to proceed via thermally labile alkenyl peroxides formed in situ from ketones and hydroperoxides. The discovery could be utilized to develop a multicomponent radical addition of unactivated ketones and tert-butyl hydroperoxide to olefins. The resulting γ-peroxyketones are synthetically useful intermediates that can be further transformed into 1,4-diketones, homoaldol products, and alkyl ketones. A one-pot reaction yielding a pharmaceutically active pyrrole is also described. [ABSTRACT FROM AUTHOR]

In einer kinetischen Studie zeigen Albrecht Berkessel et al. (e202117682), dass unter aprotischen Bedingungen das BI selbst diese kritische H-Verschiebung autokatalytisch bewirkt. Keywords: Autokatalyse; Breslow-Intermediat; Carbene; NMR-Spektroskopie; Reaktionsmechanismus DE Autokatalyse Breslow-Intermediat Carbene NMR-Spektroskopie Reaktionsmechanismus 1 1 1 06/01/22 20220607 NES 220607 B Breslow-Intermediate b (BIs) sind zentrale Spezies in der Umpolungs-Katalyse durch N-heterocyclische Carbene (NHCs). Bei ihrer Bildung aus Aldehyden und NHCs ist der Mechanismus, durch den das zwitterionische Primärintermediat in das BI umgewandelt wird, seit langem ein Rätsel, da die damit verbundene C-zu-O 1,2-H-Verschiebung kinetisch sehr ungünstig ist. [Extracted from the article]

Keywords: Autocatalysis; Breslow Intermediate; Carbenes; NMR; Reaction Mechanisms EN Autocatalysis Breslow Intermediate Carbenes NMR Reaction Mechanisms 1 1 1 06/01/22 20220607 NES 220607 B Breslow intermediates b (BIs) are pivotal species in Umpolung catalysis effected by N-heterocyclic carbenes (NHCs). Inside Back Cover: Formation of Breslow Intermediates from N-Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal (Angew. Autocatalysis, Breslow Intermediate, Carbenes, NMR, Reaction Mechanisms. [Extracted from the article]

Abfall nicht verschwendet: Eine mechanistische Studie der autoxidativen Kupplung von Xanthen mit Cyclopentanon zeigt einen autoinduktiven Effekt des Koppelprodukts Wasserstoffperoxid. Es erzeugt in Gegenwart von Säure und Keton Radikale, was die Reaktion beschleunigt, indem ein zusätzlicher Reaktionspfad zum reaktiven Peroxidintermediat eröffnet wird. Diese Entdeckung ermöglicht Brønsted ‐ Säure ‐ katalysierte oxidative Kupplungen. [ABSTRACT FROM AUTHOR]

Waste not wasted: A mechanistic study of the autoxidative coupling of xanthene with cyclopentanone uncovered an autoinductive effect of the waste product hydrogen peroxide. It generates radicals in the presence of acid and ketones, which accelerate the reaction by providing an additional pathway to the reactive hydroperoxide intermediate. This discovery could be applied to achieve other Brønsted acid catalyzed oxidative coupling reactions. [ABSTRACT FROM AUTHOR]

An organocatalytic oxidative synthesis of N-protected tetrahydroisoquinolines is described by C-H functionalization via intermediate peroxides. The peroxides were synthesized from tert-butylhydroperoxide under metal-free thermal conditions and were converted into the final products by Brønsted acid catalyzed substitution. The nucleophile scope was investigated in detail and proved to be broad; N-deprotection of the coupling products could also be achieved. [ABSTRACT FROM AUTHOR]

The article discusses the application of V-catalyzed oxidative coupling for C1-substituted N-alkyl tetrahydroisoquinoline derivatives. It reveals that tetrahydroisoquinoline derivatives comprise an extensive class of organic molecules including various natural products and pharmaceutically active compounds. A proposed catalytic cycle for the reaction is illustrated.

This article highlights the current scope of oxidative coupling reactions of tertiary amines. Emphasis has been placed on recent synthetic and mechanistic studies and suggests potential areas of improvement for the future. [ABSTRACT FROM AUTHOR]

Oxidative coupling reactions offer an elegant method to form a carbon-carbon bond from two carbon-hydrogen bonds. The ubiquity f carbon-hydrogen bonds in organic molecules offers the potential to couple simple starting materials in order to create products of igher complexity in a single step without the formation of unwanted waste products. This article reviews catalytic oxidative coupling reactions that proceed without the formation of intermediate organometallic species bearing carbon-metal bonds, focusing on key developments and recent publications. 1 Introduction 2 Arenes 2.1 Binaphthols 2.2 Biphenols and Bianilines 2.3 Other eactions with Arenes 3 Carbonyl and Related Compounds 4 Amines 4.1 Cyanation 4.2 Arylation 4.3 Nitroalkylation 4.4 Coupling ith Carbonyl Compounds 5 Amides 6 Ethers 7 Allylic Compounds 8 Hydrocarbons 8.1 Arylation 8.2 Coupling with Carbonyl ompounds 9 Conclusions and Outlook. [ABSTRACT FROM AUTHOR]

The chiral phosphoric acid TRIP, a useful Br?nsted acid catalyst, easily becomes contaminated with metal impurities in the form of phosphate salts during synthesis. This significantly reduces the content of free acid in the product which can hamper the catalytic activity. Methods to easily judge whether TRIP contains mainly the free acid or phosphate salts are presented, using ?H NMR spectroscopy or a simple pH test. An improved synthetic protocol for TRIP was established that reliably produces the free acid. [ABSTRACT FROM AUTHOR]

Cyclic tertiary amines can be oxidatively coupled with a variety of silyl enol ethers or ketene acetals to furnish tertiary Mannich bases. The reactions are catalyzed by simple copper salts employing elemental oxygen as the oxidant. [ABSTRACT FROM AUTHOR]

The article investigates the evolution of biomolecular homochirality. It highlights two models for the evolutionary routes that may have been taken by simple molecules in a racemic prebiotic world to arrive at the high levels of enantiopurity inherent in modern biological molecules. It illustrates the application of chemical reaction engineering and equilibrium thermodynamics, two basic intellectual building blocks of chemical engineering, to the development of plausible mechanisms for this fundamental process.

The combination of vanadium- and organocatalysis allows for the direct oxidative coupling of cyclic tertiary amines with non-activated ketones without the need for preformed leaving groups. [ABSTRACT FROM AUTHOR]

Copyright of Nachrichten aus der Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Ever since Pasteur noticed that tartrate crystals exist in two non-superimposable forms that are mirror images of one another—as are left and right hands—the phenomenon of chirality has intrigued scientists. On the molecular level, chirality often has a profound impact on recognition and interaction events and is thus important to biochemistry and pharmacology. In chemical synthesis, much effort has been directed towards developing asymmetric synthesis strategies that yield product molecules with a significant excess of either the left-handed or right-handed enantiomer. This is usually achieved by making use of chiral auxiliaries or catalysts that influence the course of a reaction, with the enantiomeric excess (ee) of the product linearly related to the ee of the auxiliary or catalyst used. In recent years, however, an increasing number of asymmetric reactions have been documented where this relationship is nonlinear, an effect that can lead to asymmetric amplification. Theoretical models have long suggested that autocatalytic processes can result in kinetically controlled asymmetric amplification, a prediction that has now been verified experimentally and rationalized mechanistically for an autocatalytic alkylation reaction. Here we show an alternative mechanism that gives rise to asymmetric amplification based on the equilibrium solid-liquid phase behaviour of amino acids in solution. This amplification mechanism is robust and can operate in aqueous systems, making it an appealing proposition for explaining one of the most tantalizing examples of asymmetric amplification—the development of high enantiomeric excess in biomolecules from a presumably racemic prebiotic world. [ABSTRACT FROM AUTHOR]

Copyright of Nachrichten aus der Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Nachrichten aus der Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)