“<i>Syn</i>-<i>Effect</i>” in the Isomerization of (<i>E</i>)-<i>α</i>-Fluorovinylic Sulfones to the Corresponding Allylic Sulfones under Basic Conditions
作者:Tetsuya Nakamura、Samar Kumar Guha、Yoshihiro Ohta、Daisuke Abe、Yutaka Ukaji、Katsuhiko Inomata
DOI:10.1246/bcsj.75.2031
日期:2002.9
Toward the elucidation of the origin of the “syn-effect,” the stereochemistry of the isomerization of (E)-α-fluorovinylic sulfones to the corresponding allylic sulfones under mild basic conditions was investigated. The ratio of (Z)-isomers of the resulting allylic sulfones decreased in the following order for the γ-substituents of the starting vinylic sulfones: F- > EtO- > CH3- > BnS- > CH3CH2- > (CH3)2CH- > (CH3)3C-, C6H5-. The fluorine atom showed the highest “syn-effect,” which is herein defined as an effect that stabilizes the syn-conformation in the transition state against a steric or nonbonded repulsion; the rest of the series was in accord with previous results found in the conversion of α-unsubstituted (E)-vinylic sulfones to the corresponding allylic sulfones under similar conditions. These results were rationalized based on the σ→π* interaction.
为了阐明“同分异构效应”的起源,研究人员研究了在温和碱性条件下(E)-α-氟乙烯基砜与相应的烯丙基砜的异构化立体化学。对于起始乙烯基砜的γ-取代基,所得烯丙基砜的(Z)-异构体的比例按以下顺序递减:F- > EtO- > CH3- > BnS- > CH3CH2- > (CH3)2CH- > (CH3)3C-, C6H5-。氟原子表现出最强的“同分异构效应”,该效应在此定义为在过渡态下稳定同分异构体构象,使其免受空间位阻或非键排斥力的影响;该系列其余部分与之前在类似条件下将α-未取代(E)-乙烯基砜转化为相应的烯丙基砜的结果一致。这些结果基于σ→π*相互作用得到了合理的解释。