Tale of the Breslow intermediate, a central player in N-heterocyclic carbene organocatalysis: then and now

Pareek, Monika; Reddi, Yernaidu; Sunoj, Raghavan B.

doi:10.1039/d1sc01910d

Cited by 101 publications

(79 citation statements)

References 111 publications

(34 reference statements)

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“…Beyond this wall, an ovel chemistry of ketyl radical anions can be envisioned, in line with Rehbeins intriguing proposal of possible off-path radical mechanisms for classical benzoin-type reactions. [23a, 12] Breslowsm echanism had remained hypothetical for decades.The first observation and isolation of athiazolylidene C NS was reported half acentury later, [9,10] and the corresponding Breslow intermediates af ew years ago only. [11] Establishing ab etter vision for the redox capability of the intermediates is another step towards abetter comprehension of radical NHC-catalysis.O ur next efforts will be directed towards afiner comprehension of the kinetics and thermodynamics in Angewandte Chemie catalytic cycles.M eanwhile,w ehope that future works and methodological results from the research community will enrich and challenge further these rough mechanistic proposals.…”

Section: Discussionmentioning

confidence: 99%

“…[4][5][6][7][8] Thefirst step of the catalytic cycle is well-accepted:NHC catalysts react with aldehyde derivatives to yield enaminols A•H,s oc alled Breslow intermediates. [9][10][11] Early hypotheses involved formation of radicals through as ingle electron transfer (SET) from A•H to an oxidative substrate S.In2019, in collaboration with the Bertrand group,w ed emonstrated that even electron-enriched models for A•H were too weak reductants and proposed the corresponding enolates A À as more suitable reducing agents (Figure 1a). [12] Note that earlier catalytic studies considered triazolylidene (C TN )d erivatives,f or whom no electrochemical data were available.…”

Section: Introductionmentioning

confidence: 99%

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Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions**

et al. 2021

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We report the synthesis of acyl azolium salts stemming from thiazolylidenes C NS , triazolylidenes C TN, mesoionic carbenes C MIC and the generation of their corresponding radicals and enolates, covering about 60 Breslow-type derivatives. This study highlights the role of additives in the redox behavior of these compounds and unveils several critical misconceptions about radical transformations of aldehyde derivatives under N-heterocyclic carbene catalysis. In particular, the reducing ability of enolates has been dramatically underestimated in the case of biomimetic C NS . In contrast with previous electrochemical studies, we show that these catalytic intermediates can transfer electrons to iodobenzene within minutes at room temperature. Enols derived from C MIC are not the previously claimed super electron donors, although enolate derivatives of C NS and C MIC are powerful reducing agents.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions**

et al. 2021

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“…Evolved by nature [1], and eventually driven by refined fundamental understanding [2], N-Heterocyclic Carbene species (NHCs) have conquered a whole universe of practical applications in materials science [3] and especially in catalysis. As a pool of switchable, multifunctional, adaptable, or tunable ligands, they still gain growing relevance in the fields of organometallic chemistry [4] and organocatalysis [5].…”

Section: Introductionmentioning

confidence: 99%

A Fluoroponytailed NHC–Silver Complex Formed from Vinylimidazolium/AgNO3 under Aqueous–Ammoniacal Conditions

et al. 2022

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3-(1H,1H,2H,2H-Perfluorooctyl)-1-vinylimidazolium chloride [2126844–17–3], a strong fluorosurfactant with remarkably high solubility in water, was expediently converted into the respective doubly NHC-complexed silver salt with nitrate as counter ion in quantitative yield. Due to its vinyl substituents, [bis(3-(1H,1H,2H,2H-perfluorooctyl)-1-vinylimidazol-2-ylidene)silver(I)] nitrate, Ag(FNHC)2NO3, represents a polymerizable N-heterocyclic carbene transfer reagent, thus potentially offering simple and robust access to coordination polymers with crosslinking metal bridges. The compound was characterized by infrared and NMR spectroscopy, mass spectrometry as well as elemental analysis, and supplemented by X-ray single-crystal structure determination. It crystallizes in the monoclinic crystal system in the space group P21/c. With 173.3°, the geometry of the Ag-carbene bridge deviates slightly from linearity. The disordered perfluoroalkyl side chains exhibit a helical conformation.

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“…Carbenes and their heavier analogues, silylenes, germylenes, and stannylenes R 2 E (E = C, Si, Ge, Sn) are fundamental divalent species in group 14 element chemistry. , Although carbenes tended to be elusive species for decades, , they were efficiently generated in situ and used in multiple organic transformations − and transition-metal-mediated reactions. , It was a long journey seeking for stable carbenes. Eventually, Guy Bertrand reported the isolation of phosphinocarbene in 1988, and Antony Arduengo published about imidazole-2-ylidene in 1991 .…”

Section: Introductionmentioning

confidence: 99%

Tetrylenes: Electronic Structure, Stability, Reactivity, and Ligand Properties—A Comparative DFT Study

Nechaev

2021

Organometallics

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A comprehensive theoretical analysis of the electronic structure, reactivity, and ligand properties for various types of divalent derivatives of group 14 elements, tetrylenes, R 2 E (E = C, Si, Ge, Sn), was performed at the DFT level. For a deeper understanding of the stability and the origin of chemical properties of tetrylenes, molecules stable in the monomeric form in the solid state with real substituents were considered. Tetrylenes stabilized by various factors were studied: steric shielding, attachment of electron-withdrawing substituents to E atom, electron delocalization, and intramolecular coordination bonding. The effect of various stabilization factors on the chemical and ligand properties of tetrylenes was investigated using theoretical modeling. The obtained data can be used for the rational design of tetrylenes and their complexes with transition metals and can also serve as a comprehensive tutorial on the theoretical aspects of the structure and reactivity of tetrylenes within one approach and at one level of theory.

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Tale of the Breslow intermediate, a central player in N-heterocyclic carbene organocatalysis: then and now

Abstract: N-heterocyclic carbenes (NHCs) belong to the popular family of organocatalysts used in a wide range of reactions, including that for the synthesis of complex natural products and biologically active compounds....

Cited by 101 publications

References 111 publications

Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions**

Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions**

A Fluoroponytailed NHC–Silver Complex Formed from Vinylimidazolium/AgNO3 under Aqueous–Ammoniacal Conditions

Tetrylenes: Electronic Structure, Stability, Reactivity, and Ligand Properties—A Comparative DFT Study

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