Stabilized carbenes do not dimerize

Nyulászi, László; Veszprémi, Tamás; Forro, Anna

doi:10.1039/b003588m

Cited by 51 publications

(71 citation statements)

References 18 publications

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“…Bertrand et al [44] have recently reported that free Me6, generated by reaction of 2-chloro-1,4,5,6tetrahydro-1,3-dimethylpyrimidinium chloride with Hg(SiMe 3 ) 2 is indefinitely stable and can be sublimed at 40 8C. We find that when carbene iPr5 is generated from the formamidinium tetraphenylborate precursor, using a slight excess of F in THF, it shows no sign of dimerization after 24 h in~1m solution in [D 8 ]THF; the 13 C NMR peak at d = 235.5 ppm suggests that the carbene is essentially free of complexation. Denk et al [17] reported that carbenes Me5-iPr5 persist in solution (in C 6 D 6 ), but slowly dimerize.…”

Section: Experimental Evidence For Dimer Formation From the Reaction mentioning

confidence: 59%

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: Experimental Evidence For Dimer Formation From the Reaction mentioning

confidence: 59%

When and How Do Diaminocarbenes Dimerize?

et al. 2004

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“…In previous theoretical studies, it has been demonstrated that CSEs are proportional to the dimerization barriers of NHCs [17,20]. Nyulászi et al [13] have studied a series of carbene species, including NHCs, and observed that CSE of carbenes correlates almost linearly with their dimerization reaction energies.…”

Section: Computational Approachesmentioning

confidence: 97%

“…Theoretical approaches have been applied to enhance our understanding about the bonding nature and the stability of NHCs and carbenes [10][11][12][13][14][15][16][17][18][19][20]. It is known from these studies that, the stability of free NHC is mainly attributed to the p p -p p delocalization of nitrogen atoms.…”

Section: Introductionmentioning

confidence: 99%

Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study

Lai

Guo

Lee

et al. 2005

Journal of Organometallic Chemistry

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“…1) have been reported and demonstrate the weakness of the C¼C bond. [8] Alder and co-workers have calculated that the activation barrier for head-to-head dimerization of two carbenes such as 7 (R = R' = Me) is over 100 kJ mol À1 , corresponding to a half-life of~100 h at a concentration of 1 M in THF. [9] Alternatively, dimerization assisted by some form of Lewis or Brønsted catalysis is substantially more favoured.…”

Section: Introductionmentioning

confidence: 99%

Proton transfer reactions of a bridged bis‐propyl bis‐imidazolium salt

Massey

Quinn

Zhou

et al. 2016

J of Physical Organic Chem

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Tetraazafulvalene 1 has found broad application in reduction and other related transformations and is conveniently generated from bis‐propyl bis‐imidazolium salt 4 with a strong base in a non‐protic solvent. The proposed mechanism for the formation of 1 involves initial deprotonation at C(2) to give a mono‐carbene 9 followed by intramolecular reaction at the second azolium centre. Herein, we report the second‐order rate constants for deuteroxide‐catalysed exchange in aqueous solution of the C(2)‐hydrogens of bis‐propyl bis‐imidazolium di‐iodide salt 4 and related monomeric dipropyl imidazolium iodide 10 of kDO = 1.37 × 104 and 1.79 × 102 M−1 s−1, respectively, and used these data to calculate pKa values of 21.2 and 23.1. The greater C(2)‐H acidity of the doubly bridged bis‐propyl bis‐imidazolium salt 4 relative to 10 may be attributed to the inductive or electrostatic destabilization of the conjugate acid dicationic azolium ion 4 relative to the monocationic carbene 9, which is enhanced by bis‐tethering. Formation of tetraazafulvalene 1 was not observed under the aqueous conditions employed highlighting that carbene reprotonation significantly outcompetes dimerization under these conditions. Copyright © 2016 John Wiley & Sons, Ltd.

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Stabilized carbenes do not dimerize

Cited by 51 publications

References 18 publications

When and How Do Diaminocarbenes Dimerize?

When and How Do Diaminocarbenes Dimerize?

Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study

Proton transfer reactions of a bridged bis‐propyl bis‐imidazolium salt

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