Development of low-oxidation state nitrogen, carbon and silicon catalysts
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Date
07/07/2017Item status
Restricted AccessAuthor
Papafilippou, Alexandros
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Abstract
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.