Steffensmeier, Eric Michael (2016-12). Tetraphosphines with Tetraphenylelement Cores as Linkers in Catalysis and Surface Supported Synthesis. Doctoral Dissertation.
The principal directions of this thesis involve (1) the synthesis and characterization of tetraphosphine linkers with rigid tetraphenylelement cores and derivatives thereof, (2) the study of the tetraphosphines and their phosphonium salts with respect to their interactions with silica, (3) the creation, characterization and activity studies of immobilized Wilkinson-type hydrogenation catalysts, and (4) the synthesis, characterization, and performance investigation of a surface-supported Wittig reagent for alkene synthesis. Several different tetraphenylelement scaffolds with silicon and tin centers and phosphine groups in the para positions have been synthesized. These tetraphenylelement compounds have the advantage of incorporating a rigid backbone, making them ideal immobilized linkers that pevent interactions of coordinated metal centers with the reactive silica surface, and it diminishes the probability of deactivation of the metal complexes by dimerization. A range of tetraphosphines and their derivatives have been generated by using one of two synthetic routes. The phosphines feature various substituents, for example, phenyl, cyclohexyl, tert-butyl, or isopropyl groups. The scaffolds were equally accessible with either silicon or tin at the core. The tetraphosphines can be immobilized on silica via three phosphonium groups by using ethoxysilanes, which leaves one free phosphine at the top of the scaffold. The phosphines are covalently bound via the counteranions of the phosphonium groups and are not readily leached off of the surface. This was demonstrated by placing one representative immobilized scaffold in eight different solvents, where no leaching could be detected by ^31P NMR, even in very polar solvents like DMSO. Furthermore, impregnated phosphonium salts were mobile on the surface of silica in the presence of solvents, but their solubilities did not correlate with the mobilities, which indicates that surface detachment is not a crucial element in the mode of mobility. A new class of immobilized Wilkinson-type catalysts, consisting of a linker featuring four phosphine moieties with a rigid tetraphenylelement core, has been generated. These new catalysts can easily be recycled almost indefinitely, they display fast substrate conversion at very low temperatures, no active species leach into the supernatant, and they are resistant to oxidation in air. However, the catalyst was reduced during batchwise recycling, resulting in the formation of Rh nanoparticles. The presence of the latter was proven by split and poisoning tests, double bond migration studies, as well as electron microscopy. Using a chelating linker to bind to the Rh center did not prevent nanoparticle formation. Finally, one tetraphosphine scaffold was selected to create a surface-supported Wittig reagent. This was achieved in two different ways. Using the first method, the free phosphine of the scaffold was quaternized with benzyl bromide and subsequently transformed into the ylide by deprotonation. The ylide then underwent the Wittig reaction with benzaldehyde under optimized conditions. Approximately the same yield as for the reaction with Ph3P=CHPh in solution (73%) was obtained, but the E/Z ratio (84%) was substantially higher. The phosphine oxide byproduct of the Wittig reaction remained tethered to the surface with no phosphine oxide leaching into solution. The second method for generating a surface-bound Wittig reagent involved synthesizing the tetraylide first, then reacting three of the ylide groups with the silica surface. This method produced similar results with respect to the Wittig reaction to those when using the first method. However, the crucial difference was that the scaffold could easily be washed off of the silica support with THF.