Exploration of novel carbon(II) and carbon(0) catalyst systems for organic synthesis
This PhD thesis is focused on the development of novel carbon(II) and carbon(0) catalysis for organic synthesis. More specifically, the major objective has been to explore and design non-toxic and effective catalysts based on: an unusual Bertrand carbene type, a so-called bis(dialkylamino)cyclopropenylidene (BAC), and the carbodicarbene (CDC) framework; the central carbon atom in these molecules is in the formal low-oxidation state ‘+II’ and ‘0’, respectively. 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. Prior to catalysis studies, the Lewis basicity of such carbon-based compounds has been assessed with 11B NMR analysis using various boron-based Lewis acids. Boron binding has been detected in all cases with a BAC, thereby confirming its strongly nucleophilic character and decreased steric demand. In contrast, only few ate complexes have been identified with CDCs (or precursors thereof), which means that CDCs may be more suitable for FLP catalysis. A preliminary electrophile binding study with a BAC has provided interesting data, based on which unprecedented aldimine Umpolung may be developed in the future. In the context of organocatalysis, BAC-mediated C–C bond formations between various Michael acceptors and N-tosyl imines have been developed (aza-Morita–Baylis–Hillman chemistry). In addition, C–N or C–Hal bond formations between various Michael acceptors and azodicarboxylates or electrophilic halogen reagents have been developed. The characteristic features of these unprecedented BAC catalyses include low catalyst loading, mild reaction conditions, and broad substrate scopes. Importantly, several novel chiral BACs have been synthesized and characterized, and excellent results have been achieved in BAC-catalysed asymmetric aza-MBH reactions (ee up to 97%). To the best of our knowledge, these data represent the first highly enantioselective BAC catalysis; chiral N-heterocyclic carbenes (NHCs) have proved to be substantially less effective in this context (ee up to 38%). In the same line, BAC-catalysed asymmetric borylations and silylations of Michael acceptors have been developed (preliminary ee up to 69%). These results demonstrate the high potential of the newly developed chiral BACs in asymmetric organocatalysis. Meanwhile, several BAC–gallium and BAC–iron complexes have been synthesized and characterized. These novel complexes may be used in Lewis acid catalysis after appropriate activation of the corresponding metal sites. Finally, the exploration of the catalysis potential of various C(0) compounds, namely CDCs, is still under investigation.