Wittkopp Alexander, Schreiner Peter R
Institut für Organische Chemie, Georg-August-Universität Göttingen Tammannstrasse 2, 37077 Göttingen, Germany.
Chemistry. 2003 Jan 20;9(2):407-14. doi: 10.1002/chem.200390042.
We examined the catalytic activity of substituted thioureas in a series of Diels-Alder reactions and 1,3-dipolar cycloadditions. The kinetic data reveal that the observed 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 with inverse electron demand. Our experiments emphasize that both hydrophobic and polar interactions can co-exist, making these catalysts active, even in highly coordinating solvents. This class of catalysts increases the reaction rates and endo-selectivities of Diels-Alder reactions, in a similar manner to weak Lewis acids, without concomitant product inhibition.
我们研究了一系列狄尔斯-阿尔德反应和1,3-偶极环加成反应中取代硫脲的催化活性。动力学数据表明，观察到的相对速率加速更多地取决于硫脲取代基，而非反应物或溶剂。尽管在非配位、非极性溶剂（如环己烷）中催化效果最强，但在高配位极性溶剂（如水）中也存在这种效果。在1,3-偶极环加成反应中，硫脲催化剂对逆电子需求反应仅表现出非常适度的选择性。我们的实验强调，疏水相互作用和极性相互作用可以共存，使这些催化剂即使在高配位溶剂中也具有活性。这类催化剂以类似于弱路易斯酸的方式提高了狄尔斯-阿尔德反应的速率和内型选择性，且不会伴随产物抑制。
