1911-01-9Relevant articles and documents
Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation
Messelberger, Julian,Grünwald, Annette,Goodner, Stephen J.,Zeilinger, Florian,Pinter, Piermaria,Miehlich, Matthias E.,Heinemann, Frank W.,Hansmann, Max M.,Munz, Dominik
, p. 4138 - 4149 (2020)
We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridinylidene) form persistent radical cations, whereas others such as the dimer ofN,N'-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) withN,N'-dimethyl imidazolin-2-ylidene (NHC),N,N'-dimethyl imidazolidin-2-ylidene (saNHC) andN-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2' positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.
Steric stabilization of nucleophilic carbenes
Denk,Thadani,Hatano,Lough
, p. 2607 - 2609 (1997)
Two tert-butyl groups at the nitrogen atoms of the cyclic carbene 2, which is accessible from 1 by reduction with potassium, provide sufficient stability to enable it to be stored indefinitely under exclusion of air and moisture. In contrast, sterically less shielded carbenes of type 2 (R = Me, Et, iPr) dimerize slowly at room temperature to olefins 3.