A stable carbene as π-acceptor electrochemical reduction to the radical anion

Enders, Dieter; Breuer, K.; Raabe, Gerhard; Simonet, Jacques; Ghanimi, Ahmed; Stegmann, H. B.; Teles, J. Henrique

doi:10.1016/s0040-4039(97)00493-0

Cited by 49 publications

(40 citation statements)

References 13 publications

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“…DFT calculations show that the maximum spin density is located in the NBO-LUMO of the N 2 = C 3 double bond and at the C 3 -bound phenyl group. [90] Bertrand and co-workers succeeded with the preparation of the salt 27 with a 1,2,4-triazolium dication.…”

Section: Triazolin-5-ylidenesmentioning

confidence: 99%

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Heterocyclic Carbenes: Synthesis and Coordination Chemistry

2008

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The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

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Section: Triazolin-5-ylidenesmentioning

confidence: 99%

“…Phosphines (90), [178] secondary amines, [156] pyridyl (92), [179] or phenoxylato groups (93) [180] can function as additional donors. Again, the free ligands were not usually isolated but generated in situ.…”

Section: Angewandte Chemiementioning

confidence: 99%

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

2008

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“…Enders et al have described the reversible electrochemical reduction of a stable triazol-5-ylidene and characterised the resulting radical anion by EPR spectroscopy. [8] We recently reported the preparation, isolation and structural characterisation of 2c (R = 2-adamantyl) and its complexes [RhCl(2c)(cod)] (cod = 1,5-cyclooctadiene) and [Mo(2c)(CO) 4 ]. [9] We have now obtained the slightly less bulky neopentyl analogue [4j] 2d by deprotonation of 1d with lithium diisopropylamide in 54 % yield after standard workup (for experimental details, see the Supporting Information).…”

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confidence: 99%

Six‐Membered N‐Heterocyclic Carbenes with a 1,1′‐Ferrocenediyl Backbone: Bulky Ligands with Strong Electron‐Donor Capacity and Unusual Non‐Innocent Character

et al. 2009

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The chemistry of N-heterocyclic carbenes (NHCs) [1] has developed rapidly since Arduengo et al. described the first stable crystalline NHC, 1,3-di-1-adamantylimidazolin-2-ylidene (IAd), [2] in 1991. Based on pioneering work by the groups of Herrmann and Enders, [3] they are widely applied as ligands in transition metal-catalysed reactions.[1c,1e] Their steric and electronic properties can be modified by the exocyclic substituents at the nitrogen atoms as well as by the backbone which connects the two nitrogen atoms. While five-membered NHCs based on imidazol, imidazolin and 1,2,4-triazol have received most attention so far, there is great current interest in "non-standard" NHCs, i.e. those with ring sizes other than five and/or with heteroatoms in the backbone.[4] With a view to redox-tunable catalysts, [5] we have focused our efforts on [3]ferrocenophane-type NHCs, where formally a d-block metal atom constitutes an integral part of a six-membered ring (Figure 1). Figure 1. Structures of the ferrocene-based N-heterocyclic carbenes 2 and their formamidinium precursors 1. 2 is drawn in a way that highlights the six-membered ring structure.Independently from us, Bielawski and co-workers have also been addressing this system. They recently communicated the first two examples of such NHCs, generated in situ by deprotonation of the corresponding formamidinium tetrafluoroborates 1 (Figure 1).[6] 2a (R = iBu) and 2b (R = Ph) proved to be too unstable for isolation, and only 2a was sufficiently stable for NMR spectroscopic characterisation in solution. However, both could be trapped by coordination to [RhCl(L) 2 ] fragments. Electrochemical data proved to be in support of an electronic communication between the two metal centres in these Rh complexes, and throughspace π-backbonding between the Fe atom and the carbene C atom was invoked to be responsible for this.By utilising bulkier substituents, we have been able to synthesise stable, isolable NHCs of this kind. This gives us the opportunity for a detailed investigation of the redox

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“…As regards N-heterocyclic carbenes, their electrodic activity was first pointed out by Enders and Simonet [24]. Cyclic voltammetric curves of a triaryl-triazol-5-ylidene (Scheme 3) in DMF-0.1 M solution (Hg cathode; room temperature (rT); ν = 0.5 V·s −1 ; c = 2×10 −3 M) showed a reversible reduction peak (Ep = −1.8 V vs. Ag/AgI) related to the monoelectronic reduction of the carbene to the corresponding radical anion, which was characterized by electron spin resonance (ESR) measurements.…”

Section: Electrodic Activity Of the System Nhch + /Nhcmentioning

confidence: 99%

Advances in the Knowledge of N-Heterocyclic Carbenes Properties. The Backing of the Electrochemical Investigation

2016

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In the last twenty years, N-heterocyclic carbenes (NHCs) have acquired considerable popularity as ligands for transition metals, organocatalysts and in metal-free polymer synthesis. NHCs are generally derived from azolium based salts NHCH+X− by deprotonation or reduction (chemical or electrochemical) of NHCH+. The extensive knowledge of the physicochemical properties of NHCH+/NHC system could help to select the conditions (scaffold of NHC, nature of the counter-ion X−, solvent, etc.) to enhance the catalytic power of NHC in a synthesis. The electrochemical behavior of NHCH+/NHC system, in the absence and in the presence of solvent, was extensively discussed. The cathodic reduction of NHCH+ to NHC and the anodic oxidation of NHC, and the related effect of the scaffold, solvent, and electrodic material were emphasized. The electrochemical investigations allow acquiring further knowledge as regards the stability of NHC, the acidic and nucleophilic properties of NHCH+/NHC system, the reactivity of NHC versus carbon dioxide and the effect of the hydrogen bond on the catalytic efficiency of NHC. The question of the spontaneous or induced formation of NHC from particular ionic liquids was reconsidered via voltammetric analysis. The results suggested by the classical and the electrochemical methodologies were compared and discussed.

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A stable carbene as π-acceptor electrochemical reduction to the radical anion

Cited by 49 publications

References 13 publications

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

Six‐Membered N‐Heterocyclic Carbenes with a 1,1′‐Ferrocenediyl Backbone: Bulky Ligands with Strong Electron‐Donor Capacity and Unusual Non‐Innocent Character

Advances in the Knowledge of N-Heterocyclic Carbenes Properties. The Backing of the Electrochemical Investigation

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