TY - JOUR
T1 - A DFT study on pyridine-derived N-heterocyclic carbenes
AU - Kassaee, M. Z.
AU - Shakib, F. A.
AU - Momeni, M. R.
AU - Ghambarian, M.
AU - Musavi, S. M.
PY - 2009/11/28
Y1 - 2009/11/28
N2 - 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.
AB - 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.
KW - DFT calculations
KW - N-Heterocyclic carbenes
KW - Nucleophilicity
KW - Tautomerization
KW - Thermodynamic and kinetic stability
UR - http://www.scopus.com/inward/record.url?scp=70350433063&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=70350433063&partnerID=8YFLogxK
U2 - 10.1016/j.tet.2009.09.043
DO - 10.1016/j.tet.2009.09.043
M3 - Article
AN - SCOPUS:70350433063
SN - 0040-4020
VL - 65
SP - 10093
EP - 10098
JO - Tetrahedron
JF - Tetrahedron
IS - 48
ER -