accelerations in the relative rates are more dependent on the thiourea substituents than on the reactants or solvent. Although the catalytic effectiveness is the strongest in noncoordinating, nonpolar solvents, such as cyclohexane, it is also present in highly coordinating polar solvents, such as water. In 1,3-dipolar cycloadditions, the thiourea catalysts demonstrate only very moderate selectivity for reactions
我们检查了一系列的狄尔斯-阿尔德反应和1,3-偶极环加成反应中取代的
硫脲的催化活性。动力学数据表明,观察到的相对速率的加速更依赖于
硫脲取代基,而不是反应物或溶剂。尽管在非配位的非极性溶剂(例如
环己烷)中催化效果最强,但在高配位的极性溶剂(例如
水)中也有催化作用。在1,3-偶极环加成反应中,
硫脲催化剂对具有反电子需求的反应仅表现出非常中等的选择性。我们的实验强调,疏
水和极性相互作用都可以共存,即使在高度配位的溶剂中,这些催化剂也具有活性。