Das „Wanzlick-Gleichgewicht“

Böhm, Volker P. W.; Herrmann, Wolfgang A.

doi:10.1002/1521-3757(20001117)112:22<4200::aid-ange4200>3.0.co;2-m

Cited by 32 publications

(4 citation statements)

References 79 publications

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“…Wanzlick Equilibrium [32] In his early papers, [33][34][35] Wanzlick proposed that tetraaminoethenes dissociated reversibly to generate diaminocarbenes to account for the reaction of the former with electrophiles to give products apparently derived from the carbene (his examples were all dihydroimidazol-2-ylidenes). However in 1964 Lemal et al [36] showed that typical tetraaminoethenes did not dissociate, even under much more drastic conditions than used by Wanzlick et al, by carrying out a negative crossover experiment (Scheme 2 a, Ar = p-tolyl).…”

Section: The Controversy Concerning the Kinetics Of Thementioning

confidence: 99%

When and How Do Diaminocarbenes Dimerize?

et al. 2004

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No example of a simple uncatalyzed dimerization of a diaminocarbene has been clearly established, so it is timely to ask what factors control the thermodynamics of this reaction, and what mechanisms are responsible for the observed dimerizations? In agreement with qualitative experimental observations, the dimerizations of simple five- and six-membered-ring diaminocarbenes are calculated to be 100 kJ mol(-1) less favorable than those of acyclic counterparts. This large difference is semiquantitatively accounted for by bond and torsional angle changes around the carbene centers. Carbenes such as (Et(2)N)(2)C are kinetically stable in THF at 25 degrees C in agreement with calculated energy barriers, but they rapidly dimerize in the presence of the corresponding formamidinium ion. This proton-catalyzed process is probably the most common mechanism for dimer formation, and involves formation of C-protonated dimers, which can be observed in suitable cases. The possibility of alkali-metal-promoted dimerization is raised, and circumstantial evidence for this is presented.

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Section: The Controversy Concerning the Kinetics Of Thementioning

confidence: 99%

When and How Do Diaminocarbenes Dimerize?

et al. 2004

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“…NMR experiments revealed decomposition of [(NHC)(PPh 3 )PdI 2 )] to [(NHC) 2 PdI 2 )] and trans [Pd(PPh 3 ) 2 I 2 ] over the course of several days (Scheme 1). [16] In contrast to this result, the reaction of [NiX 2 (PPR 3 ) 2 ] (X=Cl, Br) with NHCs showed the substitution of the sterically demanding phosphine ligands by carbene ligands [17,18] …”

Section: Introductionmentioning

confidence: 96%

“…[16] In contrast to this result, the reaction of [NiX 2 (PPR 3 ) 2 ] (X=Cl, Br) with NHCs showed the substitution of the sterically demanding phosphine ligands by carbene ligands. [17,18] We have recently reported the synthesis of 1-(n-butyl)-3-N-(2-Ar)acetamido-1, 3-imidazolium chloride (Ar = furylmethyl,phenylmethyl) and their reactivity towards [NiCl 2 (PÀ P)] (LÀ L=2 PPh 3 , dppf). [5] The interesting outcomes of our previous study prompted us to further explore the chemistry of these imidazolium salts towards palladium metal precursors and study their promising catalytic activities for Suzuki coupling reactions.…”

Section: Introductionmentioning

confidence: 99%

Synthesis of Palladium complexes derived from Amido linked N‐Heterocyclic Carbenes and their use in Suzuki cross coupling reactions

Chauhan¹,

Nagar²,

Chatterjee

et al. 2021

Zeitschrift anorg allge chemie

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Treatment of 1-(n-butyl)-3-N-(2-Ar)acetamido-1, 3-imidazolium chloride (Ar = furylmethyl,phenylmethyl) with excess K 2 CO 3 and [PdCl 2 (LÀ L)] (LÀ L=2 PPh 3 , dppf) afforded orange compounds of composition [(1-(n-butyl)-3-N-(2-Ar)acetamido-1,3-imidazol-2ylidene)] 2 Pd (Ar = furylmethyl; phenylmethyl). These complexes were characterized by NMR ( 1 H and 13 C{ 1 H} NMR), IR and microanalysis data. Subsequently, the catalytic efficiency of these complexes for cross coupling reactions between 4-haloarenens (halo = Br, I) and phenylboronic acid was studied under different solvents (acetonitrile, THF and DMF), temperatures with different catalyst loadings. The molecular structure of [(1-(nbutyl)-3-N-(2-furylmethyl)acetamido-1, 3-imidazol-2-ylidene)] 2 Pd was established by single crystal X-ray diffraction analysis.

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“…[3,4] Nevertheless, NHC molecules are known to dimerize in solution, [5] thereby reducing the effectiveness of the ligand (in its monomeric form) to coordinate with metal centers. Dimerization of NHCs, first proposed by Wanzlick, [6] has been a subject of interest both experimentally and theoretically. [7] A direct approach mechanism of NHC monomers to form a dimer, though highly exothermic, has been found to have prohibitively large activation energy.…”

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confidence: 99%

Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes

Karmakar

Datta

2014

Angew Chem Int Ed

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At room temperature, 1,2-hydrogen-transfer reactions of N-heterocyclic carbenes, like the imidazol-2-ylidene to give imidazole is shown to occurr almost entirely (>90 %) by quantum mechanical tunneling (QMT). At 60 K in an Ar matrix, for the 2, 3-dihydrothiazol-2-ylidene→thiazole transformation, QMT is shown to increase the rate about 10(5) times. Calculations including small-curvature tunneling show that the barrier for intermolecular 1,2-hydrogen-transfer reaction is small, and QMT leads to a reduced rate of the forward reaction because of nonclassical reflections even at room temperature. A small barrier also leads to smaller kinetic isotope effects because of efficient QMT by both H and D. QMT does not always lead to faster reactions or larger KIE values, particularly when the barrier is small.

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Das „Wanzlick-Gleichgewicht“

Cited by 32 publications

References 79 publications

When and How Do Diaminocarbenes Dimerize?

When and How Do Diaminocarbenes Dimerize?

Synthesis of Palladium complexes derived from Amido linked N‐Heterocyclic Carbenes and their use in Suzuki cross coupling reactions

Tunneling Assists the 1,2‐Hydrogen Shift in N‐Heterocyclic Carbenes

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