Design of Environmentally Benign Lewis Acid Catalysts Derived from Main Group Elements as an Active Center

刘易斯酸催化剂用于有价值的有机转化。尽管人们已经开展了许多开发金属-刘易斯酸催化剂的方法，但很少有人注意探索主族元素作为刘易斯酸中心。三苯甲基阳离子仅是前述实例，其中稳定的碳正离子充当活性中心。由主族元素组成的刘易斯酸催化剂可以通过共价键连接在催化剂分子上，有利于催化剂的循环使用。在此，我们希望报道作为新型刘易斯酸催化剂的鏻盐，其中鏻盐通过超价键与底物相互作用。合成了几种具有单环或双环结构的鏻盐。我们还引入氧官能团作为吸电子基团来稳定高价键合。在分子筛（MS）4A的存在下，在室温下1小时，由相应的羟基次膦酸酯和三氟甲磺酸酐制备鏻盐。我们尝试了用磷盐催化刘易斯酸催化Diels-Alder反应。使用α，β-不饱和酰胺作为亲二烯体，因为NMR实验强烈表明鳞盐用作酰胺官能度的活化剂。事实上，通过使用鏻盐观察到显著的速率加速。在0℃条件下，Diels-Alder产物在2 h内即可定量得到。综上所述，我们通过磷盐进行了新的刘易斯酸催化，其中二氧磷杂环己烷是通过高价键合作为刘易斯酸催化剂的关键结构。

Lewis acid catalysts are utilized to valuable organic transformations. Although a number of approaches to develop metallic-Lewis acid catalysts were performed, little attention has been paid to explore main group elements as Lewis acidic center. A trityl cation is only the precedent example, in which stabilized carbonium cation function as an active center. The Lewis acids catalyst composed of main group elements can be constructed the catalyst molecule from covalent bonding leading to an advantage in catalyst recycling. Herein we wish to report phosphonium salts as novel Lewis acid catalysts wherein phosphonium salts interact with substrate via hypervalence bonding. Several phosphonium salts with mono or bicyclic ring structure were synthesized. We also introduce oxygen functionality as an electron-withdrawing group to stabilize hypervalence bonding. Phosphonium salts were prepared from the corresponding hydroxy phosphinate and trifluoromethanesulfonic anhydride in the presence of molecular sieves (MS) 4A at room temperature for 1 h. We attempted Lewis acid catalysis by phosphonium salts in Diels-Alder reaction. α,β-unsaturated amide was employed as a dienophile, because NMR experiment strongly suggested that the phosphonium salts function as an activator for amide functionality. In fact, remarkable rate acceleration was observed by using phosphonium salts. Diels-Alder products were obtained quantitatively within 2 h at 0℃. In summary, we performed novel Lewis acid catalysis by phosphonium salts in which dioxaphosphacycle is the key structure to function as Lewis acid catalysts through hypervalence bonding.