Department of Chemistry, Fudan University , 220 Handan Road, Shanghai 200433, China.
The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute , 10550 North Torrey Pines Road, La Jolla, California 92037, United States.
J Am Chem Soc. 2016 Jun 22;138(24):7512-5. doi: 10.1021/jacs.6b04278. Epub 2016 Jun 8.
Cyclic processes involving medium-sized rings show low rates because internal strains-torsions and transannular interactions-are created during the reactions. High dilution is often used to slow the competing bi- and higher-molecular processes but cannot accelerate the desired cyclization reaction. Here we apply cavitands to the formation of medium- to large-sized rings through conversion of long-chain diisocyanates to cyclic ureas. The reactions take place in aqueous (D2O) solution, where hydrophobic forces drive the starting materials into the cavitands in folded conformations. The guest assumes the shape to fill the space properly, which brings the reacting ends closer together than they are in bulk solvent. Complexation overcomes some of the internal strains involved in precyclization shapes of the guest molecules and accelerates the cyclization. The results augur well for applications of water-soluble cavitands to related processes such as remote functionalization reactions.
涉及中等大小环的循环过程速率较低，因为在反应过程中会产生内部应变-扭曲和环间相互作用。通常采用高稀释度来减缓竞争的双分子和更高分子过程，但不能加速所需的环化反应。在这里，我们通过将长链二异氰酸酯转化为环状脲来应用主体分子（cavitands）形成中等至大环。反应在水（D2O）溶液中进行，其中疏水作用力将起始材料驱动到主体分子中，形成折叠构象。客体分子采用合适的形状来填充空间，从而使反应端彼此更靠近，而不是在体相溶剂中那样。配合作用克服了客体分子预环化形状中涉及的一些内部应变，并加速了环化反应。这些结果为水溶性主体分子在相关过程（如远程官能化反应）中的应用提供了良好的前景。
