Mansø Mads, Petersen Anne Ugleholdt, Moth-Poulsen Kasper, Nielsen Mogens Brøndsted
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
Org Biomol Chem. 2020 Mar 18;18(11):2113-2119. doi: 10.1039/d0ob00147c.
The kinetics of the thermal quadricyclane-to-norbornadiene (QC-to-NBD) isomerization reaction was studied for a large selection of derivatives where the one NBD double bond contains a cyano and aryl substituent of either electron-withdrawing or -donating character. While the kinetics data did not satisfy a linear-free-energy-relationship for all the derivatives based on Hammett σ values, we found individual linear relationships for derivatives containing either electron-withdrawing or electron-donating para substituents on the aryl group; with the most electron-witdrawing substituent in the one series and with the most electron-donating substituent in the other providing the fastest reaction (corresponding to opposite slopes of the Hammett plots). All data were well described, however, by a linear relationship when using Creary radical values; the correlation could be slightly improved by using a combination of σ and values (used in ratio of 0.104 : 1). The results imply a combination of polar and free radical effects for the isomerization reaction of this specific class of derivatives, with the latter playing the most significant role. The NBD derivatives were prepared by Diels-Alder cycloaddition reactions between cyclopentadiene and 3-arylpropiolonitriles, and in the case of bromophenyl derivatives further cyanation and Sonogashira coupling reactions were performed.
针对大量衍生物研究了热双环[2.2.1]庚烷-降冰片二烯（QC-到-NBD）异构化反应的动力学，其中一个降冰片二烯双键含有吸电子或供电子性质的氰基和芳基取代基。虽然基于哈米特σ值的动力学数据对所有衍生物并不满足线性自由能关系，但我们发现对于芳基上含有吸电子或供电子对位取代基的衍生物存在单独的线性关系；在一个系列中具有最强吸电子取代基且在另一个系列中具有最强供电子取代基时反应最快（对应于哈米特图的相反斜率）。然而，当使用克里里自由基值时，所有数据都能很好地用线性关系描述；通过使用σ和值的组合（以0.104 : 1的比例使用），相关性可以略有改善。结果表明，对于这类特定衍生物的异构化反应，存在极性和自由基效应的组合，其中后者起最重要的作用。降冰片二烯衍生物通过环戊二烯与3-芳基丙炔腈之间的狄尔斯-阿尔德环加成反应制备，对于溴苯基衍生物，还进行了进一步的氰化和Sonogashira偶联反应。
