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

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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.