A DFT study on pyridine-derived N-heterocyclic carbenes

作者：Kassaee M Z^*; Shakib F A; Momeni M R; Ghambarian M; Musavi S M

来源：Tetrahedron, 2009, 65(48): 10093-10098.

DOI：10.1016/j.tet.2009.09.043

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

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++GB3LYP/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-azacycloliexa-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-2ylidene (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 ate 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.

• 出版日期2009-11-28