Dependence of the Rate of an Interfacial Diels−Alder Reaction on the Steric Environment of the Immobilized Dienophile: An Example of Enthalpy−Entropy Compensation
作者:Youngeun Kwon、Milan Mrksich
DOI:10.1021/ja010740n
日期:2002.2.1
This paper describes a physical organic study of the relationship between the rate for an interfacial Diels-Alder reaction and the steric environment around the reacting molecules. The study used as a model reaction the cycloaddition of cyclopentadiene with a self-assembled monolayer (SAM) presenting benzoquinone groups surrounded by hydroxyl-terminated alkanethiolates. The accessibility of the quinone
本文描述了界面 Diels-Alder 反应速率与反应分子周围空间环境之间关系的物理有机研究。该研究使用环戊二烯与自组装单分子层 (SAM) 的环加成反应作为模型反应,该单分子层 (SAM) 呈现被羟基封端的链烷硫醇酯包围的苯醌基团。通过从不同长度的氢醌封端的烷硫醇 [HS(CH(2))(n)-HQ, n = 6-14] 和羟基封端的烷硫醇 [HS(CH(2) ))(11)-OH] 的恒定长度。循环伏安法用于通过监测具有氧化还原活性的醌的衰变来测量反应速率。随着醌的位置相对于单层羟基的变化,二级速率常数显示出适度的变化。对于其中醌组从界面扩展的单层,速率常数在 0.20 M(-1) s(-1) 附近振荡,对链烷硫醇的长度具有偶数依赖性。对于将醌基团置于周围羟基下方的单层,速率常数降低了大约 8 倍。对活化参数的检查表明,位于界面下方(并且在拥挤的环境中)的醌基团以比在界面处可接近的醌大 4