取代基對下列分子做分子內環化反應，影響位向選擇性之理論計算研究： 1. α-Sulfenyl-, α-Sulfinyl-, α-Sulfonyl-5-(R)-5-Hexenyl Radicals 2. 2,5-Hexadienyl Radicals
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2007
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本論文藉由理論計算的方法，在UB3LYP/6-31G(d)的計算層級下對各穩定點的結構做幾何優選，探討取代基對於α-sulfenyl-, α-sulfinyl-, and α-sulfonyl-5-(R)-5-hexenyl radicals以及2,5-hexadienyl radicals分子內環化反應的影響，以下將分為兩部分進行討論:
第一部分:
α-sulfinyl-5-hexenyl radicals在形成環狀產物時顯示了高位向選擇性，其生成五員環與六員環的產物比例為94.0/6.0，而在α-sulfenyl-, and α-sulfonyl-5-hexenyl radicals中環化形成六員環的比例較高。相反的，在C5接上甲基後產物則變成六員環，而其他的取代基如拉電子基的CN、NO2或者是推電子基的NH2等反應的主產物也是形成六員環，其中當反應物為α-sulfonyl-5-amine-5-hexenyl radicals時，經由計算後得到六員環的比例為100，為選擇性最高的反應物。
第二部分:
當不同種類的取代基如CN、NO2、CH3、NH2以及tert-butyl等取代在2,5-hexadienyl radicals的C1、C5以及C6位置上時，其對於分子內環化反應有不同效應的影響。當推電子基在C1上時，自由基的SOMO軌域能量會上升，進而增加與LUMO軌域之間的作用力，相反的拉電子基則會使SOMO軌域能量下降，進而增加與HOMO軌域之間的作用力。而上述的兩種作用力使得反應的活化能降低了0.9〜10.2 kcal/mol，使反應速率增加了3〜2.7 × 107倍。相似的作用力也發生在C6的位置，而反應的活化能降低了0.2-4.8 kcal/mol，反應速率則增加了2〜2800倍。而當取代基接在C5時則因為立障效應使產物變為六員環。最後，我們嘗試將雙取代基取代在C1以及C6的位置，並討論capto-dative effect對環化反應的影響，結果顯示拉電子基雙取代使活化能下降的較多。
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.
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.
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Keywords
自由基, 環化反應, 分子內