TY - JOUR
T1 - The calculated effects of substitution on intramolecular cyclization of 2,5-hexadienyl radicals
AU - Wu, Ching Wen
AU - Chen, Hui Lung
AU - Ho, Jia Jen
N1 - Funding Information:
The National Science Council of Republic of China (NSC 95-2113-M-003-002) supported this research; the National Center for High-Performance Computing provided the Gaussian package and computer time. We thank Professors M. Hayashi, J.C. Jiang, and C.F. Yao for helpful discussion of calculations of rate coefficients and the interaction between molecular orbitals.
PY - 2007/8/1
Y1 - 2007/8/1
N2 - The effects of substituents on the rate of intramolecular cyclization of the 2,5-hexadienyl radicals have been investigated computationally with DFT theory, using the UB3LYP functional. Various substituents - CN, NO2, CH3, NH2, and t-butyl - at various positions - C1, C5 and C6 - were considered in the calculations. An electron-donating substituent on the C1 position raises the radical SOMO energies to increase the interaction with the alkene LUMO, whereas an electron-withdrawing counterpart lowers the SOMO and increases the interaction with the alkene HOMO. Both interactions decrease the activation energies, by 0.9-10.2 kcal/mol, and increase the rate of reaction rate, from 3 to 2.7 × 107 times. Similar results were obtained for the substituents at the C6 position, and the activation energies for the intramolecular cyclization were decreased by 0.2-4.8 kcal/mol and the reaction rate increased from 2 to 2.8 × 103 times. The substituent at the C5 position favors the formation of a 6-endo product because of a steric effect. The effects of disubstituents at both C1 and C6 positions were also investigated; the results showed that the electron-withdrawing groups decrease most effectively the activation energies. The so-called captodative effect was also investigated.
AB - The effects of substituents on the rate of intramolecular cyclization of the 2,5-hexadienyl radicals have been investigated computationally with DFT theory, using the UB3LYP functional. Various substituents - CN, NO2, CH3, NH2, and t-butyl - at various positions - C1, C5 and C6 - were considered in the calculations. An electron-donating substituent on the C1 position raises the radical SOMO energies to increase the interaction with the alkene LUMO, whereas an electron-withdrawing counterpart lowers the SOMO and increases the interaction with the alkene HOMO. Both interactions decrease the activation energies, by 0.9-10.2 kcal/mol, and increase the rate of reaction rate, from 3 to 2.7 × 107 times. Similar results were obtained for the substituents at the C6 position, and the activation energies for the intramolecular cyclization were decreased by 0.2-4.8 kcal/mol and the reaction rate increased from 2 to 2.8 × 103 times. The substituent at the C5 position favors the formation of a 6-endo product because of a steric effect. The effects of disubstituents at both C1 and C6 positions were also investigated; the results showed that the electron-withdrawing groups decrease most effectively the activation energies. The so-called captodative effect was also investigated.
KW - Cyclization
KW - Density functional theory
KW - Intramolecular
KW - Ring closure
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U2 - 10.1016/j.theochem.2007.03.013
DO - 10.1016/j.theochem.2007.03.013
M3 - Article
AN - SCOPUS:34250842519
SN - 0166-1280
VL - 815
SP - 11
EP - 20
JO - Journal of Molecular Structure: THEOCHEM
JF - Journal of Molecular Structure: THEOCHEM
IS - 1-3
ER -