A DFT study on pyridine-derived N-heterocyclic carbenes |
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Authors: | MZ Kassaee FA Shakib M Ghambarian |
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Institution: | a Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran b Department of Chemistry, Islamic Azad University, Shahre-Ray Branch, Tehran, Iran c Faculty of Chemistry, University of Tabriz, 5166616471 Tabriz, Iran |
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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. |
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Keywords: | N-Heterocyclic carbenes Thermodynamic and kinetic stability Tautomerization Nucleophilicity DFT calculations |
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