N-heterocyclic carbenes: rare earth element incorporated bifunctional catalysts and organocatalysts
Ewing, Paul Michael David Andrew
The research in this thesis describes the synthesis of bifunctional N-heterocyclic carbene-rare earth element complexes. Their catalytic potential is demonstrated through the ring-expansion polymerisation of a variety of bio-renewable lactone monomers allowing for the generation of biodegradable cyclic polymers. In addition, the development of α-thiophenylaldehydes as precursors to azolium enolate and acylazolium intermediates in NHC catalysis is outlined and the subsequent reactivity with different nucleophilic and electrophilic species is demonstrated. CHAPTER ONE introduces NHCs and highlights the key milestones involved in their discovery and eventual isolation. The most important structural and electronic features of NHCs is outlined and a brief discussion regarding the successes of NHCs as ligands for transition metals is highlighted. CHAPTER TWO gives a detailed account of the use of functionalised NHCs as ligands for electropositive rare earth elements. The synthesis and characterisation of previously reported and new ortho-aryloxide NHC proligands, and their subsequent coordination chemistry with a variety of electropositive Mᶦᶦᶦ (M = Ce, La, Y, Sm) is shown. The new NHC-rare earth complexes are characterised by a combination of analytical techniques including solution-state NMR spectroscopies, mass spectrometry, elemental analysis, and X-ray diffraction studies. The NHC-Mᶦᶦᶦ interaction is compared for each complex and with the literature, revealing that NHC ligands with a saturated carbon backbone exhibit weaker bonding to rare earth elements than corresponding unsaturated NHCs due to differing stereo-electronic effects. CHAPTER THREE provides an overview of the synthesis of biodegradable polymers with a particular focus on the formation of cyclic polylactide. The reactivity of the NHC-Mᶦᶦᶦ fragments in the ortho-aryloxide tethered NHC-rare earth metal complexes synthesised in chapter two are tested towards the ring-expansion polymerisation of bio-renewable lactone monomers, primarily rac-lactide but also L-lactide, ß-butyrolactone and ɛ-caprolactone. It is found that homoleptic NHC-Mᶦᶦᶦ complexes of the form ML₃ᴿ are highly active bifunctional catalysts for the selective synthesis of high molecular weight cyclic polylactide, reaching turnover frequencies up to 864,000 h⁻¹ (M = Ce). The mechanism of initiation is investigated through control and DFT studies, revealing a bifunctional mechanism is in operation, in which synergy between the Lewis acidic rare earth element and Lewis basic NHC ligand gives rise to the formation of cyclic polymers with high catalytic rates. The relative Lewis acidity and size of the rare earth element ionic radii is also shown to have a significant effect on the catalyst activity where Ce~La>Sm>>Y. CHAPTER FOUR describes the development of α-thiophenylaldehydes to act as acylazolium and azolium enolate precursors when combined with NHCs. A novel NHC-catalysed redox rearrangement of the α-thiophenylaldehydes to a variety of thiolesters is demonstrated. Addition of alcohol and amine nucleophiles has also allowed access to a series of esters and amides respectively. The electronics of the chosen NHC catalyst was shown to have a major influence on both the rates and product selectivity in these redox reactions. Preliminary results are shown of an NHC-catalysed formal [4+2] cycloaddition between α-thiophenylaldehydes and α,ß-unsaturated tosyl imines that allowed for the isolation of enantio- and diastereo-enriched dihydropyridinones (98:2 er, 94:6 dr). CHAPTER FIVE gives experimental and characterising data for all compounds and reactions conducted in this body of work.