The Synthesis and Reactivity of Highly Electrophilic Phosphonium Cations

Main group Lewis acids are used in a variety of Lewis acid-mediated transformations and as co-catalysts for polymerization reactions. In recent years, group XIII Lewis acids such as BF3, AlCl3, and B(C6F5)3 have been under active investigation for their catalytic applications. Extending this chemistry to group XV elements, our group recently showed that the phosphonium cation, [(C6F5)3PF][B(C6F5)4] displayed significant Lewis acidity. [(C6F5)3PF][B(C6F5)4] was shown to perform catalytic hydrodefluorination of fluoroalkanes; hydrosilylation of alkenes, ketones, imines and nitriles; and transfer hydrogenation of olefins. Similar reactivity was observed for [(SIMes)PFPh2][B(C6F5)4]2, which acquires its Lewis acidity from dicationic charge rather than perfluorinated substituents. Herein, we describe our efforts towards hydrogen activation by [(C6F5)3PF][B(C6F5)4], especially for hydrogenation of imines. Furthermore, the facile synthesis of a library of bis-phosphonium systems and their applications in Lewis acid-mediated catalysis are discussed. The synthesis of a chiral bis-phosphonium which may have applications in asymmetric catalysis is also described.M.Sc