Oxazol-2-ylidenes. A new class of stable carbenes?

Kelemen, Zsolt; Hollóczki, Oldamur; Oláh, Julianna; Nyulászi, László

doi:10.1039/c3ra41177j

Cited by 33 publications

(38 citation statements)

References 92 publications

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“…There are several reports about employing solvent effects to tune carbene catalysis in terms of selectivity, yields, and reaction rates . For instance, the rate of an NHC catalytic reaction in 1,3‐dialkylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids has been shown to be significantly slower than in THF . This effect was attributed to the presence of C−H⋅⋅⋅C‐type hydrogen bonds between the carbene and the imidazolium cation, although the interaction between the ionic liquid cation and the base used in the reaction may also have played an important role.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen Bonding of N‐Heterocyclic Carbenes in Solution: Mechanisms of Solvent Reorganization

Gehrke

Hollóczki

2018

Chemistry A European J

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The hydrogen-bond dynamics of N-heterocyclic carbenes plays a central role in their proton-transfer reactions, and the effects of hydrogen bonding are also often invoked in corresponding organocatalytic applications. In the present study, the structures and lifetimes of hydrogen bonds have been investigated for several carbenes in alcohol-containing solutions by classical molecular dynamics simulations. The basicity of the carbene was found to be of major importance; while the least basic carbenes are often in their free form in the solvent, by increasing the basicity the simulations show increased hydrogen bonding, often even with two alcohol molecules at a time. In the latter structure the single lone pair of the carbene is in interplay with two hydrogen bond donors. The exchange mechanism is different for carbenes with different basicities, with different substituents, and in different solvents, occurring through the free carbene for the least basic compounds, and through the aforementioned doubly-hydrogen-bonded structure in case of the most basic derivatives. Since this process plays a central role also in H/D exchange reactions, we argue that the pK values calculated from the related measurements have a varying physical meaning for the different carbenes. The lifetimes of the hydrogen bonds are apparently also clearly related to the basicity of the carbene, with gradually increasing lifetimes for the most basic compounds.

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Section: Introductionmentioning

confidence: 99%

Hydrogen Bonding of N‐Heterocyclic Carbenes in Solution: Mechanisms of Solvent Reorganization

Gehrke

Hollóczki

2018

Chemistry A European J

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“…Es konnte bereits fürD iphenylcarbene -d ie sich strukturell allerdings sehr von NHCs unterscheiden -g ezeigt werden, dass Solvatation sehr starke Effekte auf die Reaktivitätvon Carbenen haben kann. [24] [25] Da Azolium-Kationen ebenfalls einen aromatischen Charakter aufweisen, [26,27] ist es angemessen, den assoziativen Mechanismus als eine elektrophile aromatische Substitutionsreaktion zu beschreiben.…”

Section: Angewandte Chemieunclassified

Treten in der N‐heterozyklischen Carben‐Organokatalyse wirklich Carbene auf?

Gehrke

Hollóczki

2017

Angewandte Chemie

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In der Organokatalyse werden häufig Azolium-Kationen in Anwesenheit einer Base eingesetzt, um wertvolle Syntheseschritte zu realisieren. Aufgrund der Annahme,d ass durch Deprotonierung des Kations ein freies Carben entsteht, welches mit dem Substrat reagiert und somit dessen Reaktivität entscheidend beeinflusst, werden Reaktionen dieses Types als N-heterozyklische Carben-Katalyse bezeichnet. Wirk çnnen hier jedochz eigen, dass ein Mechanismus,i nd em das Azolium-Kation analog der elektrophilen aromatischen Substitution direkt mit dem Substrat reagiert und somit eine freie Carbenspezies vermieden wird,e nergetischd eutlichgünstiger ist.

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“…24 Sulfur is also more electropositive than oxygen, hence thiazole-2ylidene 4 is a stronger donor. 24 Sulfur is also more electropositive than oxygen, hence thiazole-2ylidene 4 is a stronger donor.…”

Section: Different Heteroatomsmentioning

confidence: 99%

Quantifying N-heterocyclic carbenes as umpolung catalysts in the benzoin reaction: balance between nucleophilicity and electrophilicity

Schumacher

Goldfuß

2015

New J. Chem.

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Different types of N-heterocyclic carbenes, i.e. normal, reduced heteroatom stabilized, remote and abnormal carbenes, are computationally studied (THF-CPCM-B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)) in the benzoin reaction to assess their characteristics as umpolung catalysts. Two general trends are apparent with an increasing number of nitrogen atoms in the heterocycles: (a) the tendency for the initial addition of the carbenes to the aldehyde decreases, due to decreased nucleophilicity; (b) during the umpolung step, the developing negative charge in the Breslow intermediates becomes better stabilized by conjugation with the adjacent, more electrophilic carbene carbon atoms. Relative to the starting reactants, the free energies for the intermediates and the transition structures decrease with more remote positions of the nitrogen atoms to the carbene carbon atoms. Among the normal N-heterocyclic carbenes oxazole-2-ylidene shows the lowest activation barrier for the rate determining umpolung step (E a = 25.2 kcal mol À1 ) in the benzoin reaction. The comparison of all computed carbenes points to remote and abnormal pyridyl-3-ylidene as catalyst with the lowest activation energy (E a = 24.0 kcal mol À1 ). This abnormal carbene exhibits the best balance between nucleophilic and electrophilic characteristics in the benzoin reaction.

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Oxazol-2-ylidenes. A new class of stable carbenes?

Abstract: Oxazol-2-ylidenes. A new class of stable carbenes?3

Cited by 33 publications

References 92 publications

Hydrogen Bonding of N‐Heterocyclic Carbenes in Solution: Mechanisms of Solvent Reorganization

Hydrogen Bonding of N‐Heterocyclic Carbenes in Solution: Mechanisms of Solvent Reorganization

Treten in der N‐heterozyklischen Carben‐Organokatalyse wirklich Carbene auf?

Quantifying N-heterocyclic carbenes as umpolung catalysts in the benzoin reaction: balance between nucleophilicity and electrophilicity

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