Elimination-Addition Mechanism for Nucleophilic Substitution Reaction of Cyclohexenyl Iodonium Salts and Regioselectivity of Nucleophilic Addition to the Cyclohexyne Intermediate

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• 作者：Morifumi Fujita ; Wan Hyeok Kim ; Yuichi Sakanishi ; Koji Fujiwara ; Sayaka Hirayama ; Tadashi Okuyama ; Yasuhiro Ohki ; Kazuyuki Tatsumi ; and Yasunori Yoshioka
• 刊名：Journal of the American Chemical Society
• 出版年：2004
• 出版时间：June 23, 2004
• 年：2004
• 卷：126
• 期：24
• 页码：7548 - 7558
• 全文大小：182K
• 年卷期：v.126,no.24(June 23, 2004)
• ISSN：1520-5126

文摘

The reaction of 4-substituted cyclohex-1-enyl(phenyl)iodonium tetrafluoroborate with tetrabutylammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. Theisomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. Thereaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as acommon intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping tolead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generatedin the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines areless effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases inducethe E2 elimination (k_H/k_D > 2), while the amines allow formation of a cyclohexenyl cation in chloroform tolead to E1 as well as S_N1 reactions (k_H/k_D 1). Bases are much less effective in methanol, and methoxidewas the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyneto give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivityof nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, whichare governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMOpopulation and is preferentially attacked by the nucleophile.