Stable non-N-heterocyclic carbenes (non-NHC): recent progress

Canac, Yves; Soleilhavoup, Michèle; Conejero, Salvador; Bertrand, Guy

doi:10.1016/j.jorganchem.2004.02.005

Cited by 173 publications

(63 citation statements)

References 57 publications

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“…Indeed, addition of a potassium complexing agent such as N,N,NЈ,NЈ-tetramethylethylenediamine to the toluene solution of Ea, followed by filtration, evaporation of the solvent, and extraction with hexane gave rise to a light yellow solution. The low field 13 C NMR signal for the free carbene Ea appeared shifted further downfield at 312.6 ppm. It is the most deshielded carbene carbon nucleus of any NHC reported so far (19,20).…”

Section: Resultsmentioning

confidence: 98%

“…The solid was sparingly soluble in toluene, but a signal at 295.1 ppm was observed in the 13 C NMR spectrum. The poor solubility and the low field chemical shift were indicative of a carbene-potassium complex Ea (24).…”

Section: Resultsmentioning

confidence: 99%

“…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%

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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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Section: Resultsmentioning

confidence: 98%

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

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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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“…Cyclic Alkyl(amino)carbenes (CAACs) Some years before the preparation of the first N-heterocyclic carbene by Arduengo et al [30] Bertrand et al described the stable (trimethylsilyl)[bis(diisopropylamino)phosphanyl]carbene 35 [8] and shortly thereafter the molecular structure of a crystalline phosphanyl(silyl)carbene [125] The stable noncyclic heteroatom-stabilized carbenes 36, [9] 37, [126] 38, [127] 39, [128] and 40 [129] have also been prepared (Scheme 43). [130] The synthesis of the phosphanyl(aryl)carbene 41 [131] marked the entrance into the chemistry of stable singlet carbenes which are stabilized by only one heteroatom. Shortly thereafter, the first amino(aryl)carbene 42 [132] and the first amino(alkyl)carbene 43 [133] (Scheme 43) were prepared.…”

Section: Heterocyclic Carbenesmentioning

confidence: 99%

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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“…In den Jahren zwischen 2000 und 2004 erweiterten wir das Repertoire stabiler Carbene um die Aminophosphanylcarbene wie J [13] und führten darüber hinaus auch monoheteroatomsubstituierte Carbene K, [14] L [15] und M [16] ein, die erhebliche Unterschiede in der Art der Stabilisierung aufweisen. Auch diese Ergebnisse wurden in einem früheren Übersichtsartikel zusammengefasst, [17] wobei wir unter anderem aufzeigten, welche sterischen und elek- Der Erfolg der homogenen Katalyse kann hauptsächlich der Entwicklung einer breiten Palette von Ligandengerüsten zugeschrieben werden, die vielfach genutzt wurden, um das Verhalten von Katalysatorsystemen gezielt abzustimmen. Spektakuläre Ergebnisse auf diesem Gebiet wurden mit cyclischen Diaminocarbenen, den so genannten N-heterocyclischen Carbenen (NHCs), erzielt, die sich vor allem durch ihr starkes s-Donorverhalten auszeichnen.…”

Section: Introductionunclassified

Stabile cyclische Carbene und verwandte Spezies jenseits der Diaminocarbene

2010

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Der Erfolg der homogenen Katalyse kann hauptsächlich der Entwicklung einer breiten Palette von Ligandengerüsten zugeschrieben werden, die vielfach genutzt wurden, um das Verhalten von Katalysatorsystemen gezielt abzustimmen. Spektakuläre Ergebnisse auf diesem Gebiet wurden mit cyclischen Diaminocarbenen, den so genannten N‐heterocyclischen Carbenen (NHCs), erzielt, die sich vor allem durch ihr starkes σ‐Donorverhalten auszeichnen. Obwohl sich die Strukturen der NHCs recht einfach modifizieren lassen, erreichen sie bei weitem nicht die Vielfalt etwa der Phosphorliganden. Auch eine große Zahl an stabilen acyclischen Carbenen ist bekannt, diese koordinieren aber entweder nur schlecht an Metalle oder führen zur Bildung sehr empfindlicher Metallkomplexe. In den letzten fünf Jahren wurden neue Arten stabiler Carbene und verwandter C‐Liganden entwickelt – bei denen es sich nicht um NHCs handelt –, die noch stärkere σ‐Donoreigenschaften aufweisen. In diesem Aufsatz beschreiben wir die Synthese, die Charakterisierung, die Stabilität, die elektronischen Eigenschaften, das Koordinationsverhalten und die katalytische Aktivität der daraus hergestellten Komplexe und stellen Vergleiche mit verwandten NHC‐Spezies an.

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Stable non-N-heterocyclic carbenes (non-NHC): recent progress

Cited by 173 publications

References 57 publications

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

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

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

Stabile cyclische Carbene und verwandte Spezies jenseits der Diaminocarbene

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