Theoretical studies of substitutions on the regioselectivity of intramolecular cyclization reactions for the following molecules

• Main Author: 吳靜雯
• Other Authors: 何嘉仁
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/65937251881081851880

博士 === 國立臺灣師範大學 === 化學系 === 95 === We carried out the DFT calculation of intramolecular cyclization reaction of α-sulfenyl-, α-sulfinyl-, α-sulfonyl-5-(R)-5-hexenyl radicals and 2,5-hexadienyl radicals by density functional theory. All of the local minimum structures are optimized with 6-31G(d) basis set at the levels of UB3LYP. There are two sections rendered here. Section 1: The α-sulfinyl-5-hexenyl radical exhibits unexpected regioselectivity (94.0:6.0) via the 5-exo mode, whereas the α-sulfenyl- and α-sulfonyl-5-hexenyl radicals show increasing branching ratios of the 6-endo product. In contrast, the cyclization of the α-sulfur-based 5-methyl-substituted counterparts yields essentially the 6-endo products in all cases; in particular, the α-SO2-5-CH3-5-hexenyl radical gives high regioselectivity (98.8:1.2) via the 6-endo mode. Several other 5-substituted moieties, including the electron-withdrawing – CN and NO2 – or electron-donating substituents – NH2 – also proceed preferentially to 6-endo closure. The α-sulfonyl- 5-amine-5-hexenyl radical is calculated to proceed exclusively the 6-endo product, a demonstration of the high synthetic value of the reaction. Section 2: Various substituents – CN, NO2, CH3, NH2, and tert-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 to 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 2800 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.