A DFT study on pyridine-derived N-heterocyclic carbenes

M. Z. Kassaee, F. A. Shakib, M. R. Momeni, M. Ghambarian, S. M. Musavi

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31 Scopus citations

Abstract

To appreciate the chemistry of N-heterocyclic carbenes (NHCs), eight carbenic tautomers of pyridine (azacyclohexadienylidenes) are studied at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G* and B3LYP/6-311++G**//B3LYP/6-31+G* levels of theory. Various thermodynamic parameters are calculated for these minima, along with a kinetic focus on carbene-pyridine tautomerization. Appropriate isodesmic reactions show stabilization energies of 2-azacyclohexa-3,5-dienylidene (1) and 4-azacyclohexa-2,5-dienylidene (6) as 119.4 and 104.1 kcal/mol, rather close to that of the synthesized 1,3-dimethylimidazol-2-ylidene (129.2 kcal/mol). Three different mechanisms are suggested for the tautomerizations including: [1,2]-H shift, [1,4]-H shift, and three sequential [1,2]-H shifts. The calculated energy barrier for [1,2]-H shift of 1 is 44.6 kcal/mol, while the first [1,2]-H shift for the proposed sequential mechanism of 6 requires 65.1 kcal/mol. Three preliminary minimum templates are introduced, which may possess the potential of synthetic consideration: 2,6-di(X)-3,5-dichloro-4-azacyclohexa-2,5-dienylidene for X=Mes, t-Bu, and Ad.

Original languageEnglish (US)
Pages (from-to)10093-10098
Number of pages6
JournalTetrahedron
Volume65
Issue number48
DOIs
StatePublished - Nov 28 2009

Keywords

  • DFT calculations
  • N-Heterocyclic carbenes
  • Nucleophilicity
  • Tautomerization
  • Thermodynamic and kinetic stability

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