Heterocyclic Carbenes

Hahn, F. Ekkehardt

doi:10.1002/anie.200503858

Cited by 457 publications

(137 citation statements)

References 84 publications

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“…[41] In the 13 C NMR spectra, chemical shifts due to the C-2 carbon are observed in the range ca 140.4-145.5 ppm, which is in agreement with reported data for similar benzimidazolium salts. [42][43][44] The signals for alkyl chain (-CH 2 -) are observed at ca 10.00-49.87 ppm. As expected, in the 1 H NMR spectra of the complexes, the signal corresponding to the NCHN proton resonance is absent due to the loss of this acidic proton after reacting with Ag 2 O.…”

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confidence: 97%

Benzimidazole‐based silver(I)–N‐heterocyclic carbene complexes as anti‐bacterials: synthesis, crystal structures and nucleic acids interaction studies

Asekunowo

Haque

Razali

et al. 2014

Applied Organom Chemis

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A series of new benzimidazolium salts as N-heterocyclic carbene (NHC) precursors has been synthesized. Reactions of these salts with Ag 2 O with varying metal-to-salt ratio facilitate the formation of a series of new binuclear and mononuclear Ag(I)-NHC complexes. All compounds were characterized using physicochemical and spectroscopic techniques. Single-crystal X-ray diffraction study reveals a binuclear structure for one of the complexes and a mononuclear one for two others. These complexes exist as cationic Ag(I)-NHC complexes with the chelation of carbene carbons to the silver centre in an almost linear manner. The compounds were screened for their anti-bacterial activities against Staphylococcus aureus (ATCC 12600) as a Gram-positive bacterium and Escherichia coli (ATCC 25922) as a Gram-negative bacterium. The results show that both bacteria appear markedly inhibited. Furthermore, the results suggest the possibility of steric variation as a modulation of the anti-bacterial activities. The nuclease activities of the compounds were assessed using gel electrophoresis and the results indicate that these complexes can cleave or degrade DNA and RNA via a non-oxidative mechanism.

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confidence: 97%

Benzimidazole‐based silver(I)–N‐heterocyclic carbene complexes as anti‐bacterials: synthesis, crystal structures and nucleic acids interaction studies

Asekunowo

Haque

Razali

et al. 2014

Applied Organom Chemis

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“…During the past two decades, carbene chemistry has undergone a profound revolution. Persistent triplet carbenes have been observed (3), and singlet carbenes have been isolated (4)(5)(6) and even become powerful tools for synthetic chemists (7,8). However, it is generally believed that singlet carbenes can be isolated only if their electron deficiency is reduced by the presence of at least one π-donor heteroatom directly bonded to the carbene center (9).…”

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Cyclopropenylidenes: From Interstellar Space to an Isolated Derivative in the Laboratory

Lavallo

Canac

Donnadieu

et al. 2006

Science

251

180

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Like many of the molecular species that have been detected in the interstellar medium, the singlet carbene cyclopropenylidene (C 3 H 2 ) has been presumed to be too unstable to isolate in the laboratory. However, by appending π-electron-donating amino groups to the triangular skeleton, we prepared a cyclopropenylidene derivative that is stable at room temperature. In contrast to previously isolated carbenes, this compound does not require a heteroatom adjacent to the electron-deficient carbon to confer stability. Despite the presence of amino groups, the geometric parameters of the cyclic skeleton, revealed by x-ray crystallography, are only slightly perturbed relative to those of the calculated structure of unsubstituted cyclopropenylidene. Stable cyclopropenylidene derivatives might thus serve as models for a better understanding of the formation of carbon-bearing molecules in the interstellar medium.

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“…We have shown that six--electron NHCs B (6), isoelectronic with borazenes (7), are highly thermally stable. Even more strikingly, the two--electron cyclopropenylidene C is stable enough to be isolated at room temperature (8), despite the absence of -donor heteroatoms directly bonded to the carbene center, a necessary criterion for previously isolated carbenes (9)(10)(11)(12)(13)(14).…”

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confidence: 99%

Stable four-π-electron, four-membered heterocyclic cations and carbenes

Ishida

Donnadieu

Bertrand

2006

Proc. Natl. Acad. Sci. U.S.A.

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. Subsequent deprotonation of the B-amino derivative affords the corresponding N-heterocyclic carbene, which has been isolated as yellow crystals. According to singlecrystal x-ray diffraction studies, both the carbene and its conjugate acid precursor have a planar structure, with no pyramidalization of the ring atoms. The structural parameters indicate that the four electrons are mostly distributed over the N-C-N fragment with little involvement of the boron center, and therefore both types of heterocycles escape from antiaromaticity. However, considering the ring strain and the presence of the Lewis acid center, the thermal stability of the carbene (mp 98°C without decomposition) is rather surprising. These results clearly suggest that the backbone of N-heterocyclic carbenes can be modified at will, without preventing their isolation.aromaticity ͉ boron C arbenes are compounds with a neutral, dicoordinate carbon atom with only six electrons in its valence shell. They are fundamentally important species that for a long time were considered as prototypical reactive intermediates, too unstable to isolate under usual experimental conditions (1). According to structural, thermodynamic, and magnetic criteria, from the populations, ionization potentials, and inner shell electron energy loss spectroscopy, it has been recognized that the first isolated N-heterocyclic carbenes (NHCs), the imidazol-2-ylidenes A (2) benefit from their aromatic character (3-5) (Fig. 1). Indeed, in addition to the stabilizing effect of the -donating, -attracting amino substituents, it has been concluded that aromaticity provides an additional stabilization of Ϸ25 kcal͞mol (3). We have shown that six--electron NHCs B (6), isoelectronic with borazenes (7), are highly thermally stable. Even more strikingly, the two--electron cyclopropenylidene C is stable enough to be isolated at room temperature (8), despite the absence of -donor heteroatoms directly bonded to the carbene center, a necessary criterion for previously isolated carbenes (9-14).As recently written by Mo and Schleyer (15): ''Although aromaticity is a virtual (not directly measurable) quantity, its usefulness as a central chemical concept is undiminished after two centuries.'' Clearly, abnormal energies govern the chemical behavior of (4n ϩ 2) and (4n) -electron-containing conjugated ring systems, which are more and less stable thermodynamically, respectively, than ''they ought to be' ' (16-18).At that point, the question was raised: is it possible to isolate formally antiaromatic heterocyclic carbenes? This endeavor requires the preparation of the stable conjugate acid precursors, which would also feature a (4n)--electron system. Following the analogy between borazenes and NHCs of type B, we chose a BNCN skeleton, and here we report the synthesis and single crystal x-ray diffraction study of cationic, planar, cyclic four--electron four-membered heterocycles D and a corresponding NHC E. Results and Discussion Treatment of N,N'-2,6-diisopropylphenyl-N-trimethylsilylfor-mamidine with...

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Heterocyclic Carbenes

Cited by 457 publications

References 84 publications

Benzimidazole‐based silver(I)–N‐heterocyclic carbene complexes as anti‐bacterials: synthesis, crystal structures and nucleic acids interaction studies

Benzimidazole‐based silver(I)–N‐heterocyclic carbene complexes as anti‐bacterials: synthesis, crystal structures and nucleic acids interaction studies

Cyclopropenylidenes: From Interstellar Space to an Isolated Derivative in the Laboratory

Stable four-π-electron, four-membered heterocyclic cations and carbenes

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