| Phosphorus compounds have a rich chemical history as Lewis donor ligands in transition metal and organometallic chemistry. In this chemistry, they have typically played auxiliary roles, permitting critical breakthroughs in catalysis centred at transition metal active sites. Phosphorus based Lewis acids, which can themselves serve as a primary locus of activity, have been studied to a lesser extent. Previously, the Stephan Group reported the preparation and Lewis acidity and consequent reactivity of the fluorophosphonium cations [(C6F5)2PhPF]+ and [(C 6F5)3PF]+. This reactivity has been attributed to their energetically accessible sigma*(P-F) acceptor orbitals. This original system requires strongly electron-withdrawing perfluoroaryl substituents, thus limiting potential structural variations.;The present work focuses on maintaining potent Lewis acidity at a fluorophosphonium centre while avoiding perfluoroarenes. In this dissertation, the preparation of several dicationic phosphonium salts is discussed. In addition, the versatility of this synthetic approach is investigated. To this end, phosphenium cations supported with triazole, chiral, and cAAC-carbenes were prepared. These dicationic phosphonium salts exhibit remarkable Lewis acidity in stoichiometric reactions and act as effective Lewis acid catalysts. These systems effect the hydrodefluorination of fluoroalkanes, hydrosilylation of olefins, deoxygenation of ketones, and the reduction of phosphine oxides and amides. Attempts to perform the Michaelis-Arbuzov rearrangements and subsequent reductions led to the catalytic generation of PH3, as well as primary and secondary phosphines from air stable phosphoethers and phosphoesters.;Finally, the preparation of tricationic and tetracationic phosphonium salts was investigated. Three synthetic strategies were explored, viz. preparation of a phosphenium dication, a 4,5-phosphinoimidazolium cation, as well as two linked carbene stabilized phosphenium cations. Subsequent oxidation of these species led to unexpected results, which are further discussed in Chapter 5. |
