Development of low-oxidation state nitrogen, carbon and silicon catalysts
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This PhD thesis is focused on the development of novel low-oxidation state main group catalysis for organic synthesis. More specifically, the major objective has been to explore and design non-toxic and effective catalysts based on the following isoelectronic species: nitreones [nitrogen(I)], carbones [carbon(0)], and silylones [silicon(0)]; the corresponding central nonmetal atom in these molecules is in the formal low-oxidation state ‘+I’ and ‘0’, respectively. These species have been calculated to be strong Lewis and Brønsted bases. In addition, compared with established base catalysts such as N-heterocyclic carbenes (NHCs), nitreones, carbones, and silylones formally possess an additional lone pair of electrons at the central atom. In turn, these species may be used in base catalysis or as ligands in metal catalysis, and in the context of frustrated Lewis pair (FLP) or dual catalysis. The Lewis basicity of these N(I), C(0), and Si(0) compounds has been assessed with 11B NMR analysis using a variety of boron Lewis acids. These boron binding data have been compared with results obtained using NHCs as a Lewis base. Nitreones –more specifically cyclopropen-imines– have been explored in base catalysis. These N(I) Lewis bases have been uncovered to catalytically activate a variety of silicon-based pro-nucleophiles for subsequent bond formation with carbonyl and imine derivatives as well as aziridines. Successfully used pro-nucleophiles include TMS–CN, TMS– CF3, TMS–N3, and TMS–Cl. The characteristic features of this unprecedented cyclopropenimine Lewis base catalysis include low catalyst loading, mild reaction conditions, and broad substrate scopes. Various “normal” imines have proved to be catalytically inactive under the same conditions. In a similar context, carbones and silylones have been used to develop novel catalytic umpolung reactions, which turned out to be too challenging at this stage. Importantly though, silylones have been shown to activate the B–H bond of suitable pro-nucleophiles. Finally, several carbone–metal complexes have been synthesized and characterized. These novel species may be used in Lewis acid or dual catalysis after appropriate activation of the corresponding metal site.