Independent and combined Lewis acid-Lewis base catalysis with N-heterocyclic carbenes and rare-earth ions

Abstract

The research in this thesis describes the results of a collaborative project between Professor Polly Arnold and Professor Andrew Smith using independent and combined Lewis acid-Lewis base catalysis with N-heterocyclic carbenes (NHCs) and rare-earth ions.

Chapter 1 gives a brief overview of the historical and current literature on the discovery of NHCs detailing their fundamental properties giving context to the aims and objectives within this collaborative research project.

Chapter 2 describes the principles and use of NHCs as effective organocatalysts historically through to state-of-the-art. The development of a novel NHC-catalysed enantioselective [3+2] formal cycloaddition of α-aroyloxyaldehydes and oxaziridines is described. This methodology has given access to a range of anti-oxazolidin-4-ones with excellent levels of stereocontrol (up to
>95:5 dr, up to >99:1 er). Additionally, α,α-disubstituted oxaziridines proved to be suitable substrates for this process, giving access to novel tricyclic heterocycles. Temporal analysis of the reaction mixture by 1H NMR spectroscopy has allowed a catalytic cycle to be proposed.

Chapter 3 describes the historical and current literature on NHC-rare earth Lewis pair complexes, giving context to the promising applications which are investigated in this work. The synthesis of novel tris-aryloxide NHC-rare earth complexes Ln(Lᴿ)₃ (Lᴿ = 2-O-3,5-ᵗBu₂-C₆H₂(C{NCHCHNR}), R = ⁱPr, ᵗBu, Mes; Ln = Ce, Sm, Eu) is described. Cerium analogues Ce(Lᴿ)₃ react cleanly with carbon dioxide to generate tris-imidazolium carboxylate complexes, Ce(Lᴿ·CO₂)₃. While reactions of Ce(Lⁱᴾʳ)₃ with a range of heteroallenes such as isocyanates and isothiocyanates offer insight into the electronic and steric profile of these complexes. It is also demonstrated that sterically encumbered Ce(Lᴹᵉˢ·CO₂)₃ is a proficient catalyst in the synthesis of cyclic carbonates with epoxides and low pressures of carbon dioxide (1 atm). Furthermore, the functionalisation of carbon dioxide from Ce(Lⁱᴾʳ·CO₂)₃ was attempted using boranes and silanes.

Chapter 4 describes the historical and modern day literature on NHC-catalysed ring-opening polymerisation (ROP) of lactones. The development of a ROP of lactone monomers (±)-lactide and (±)-β-butyrolactone using Ce(LiPr)₃ as a catalyst is detailed. The former giving exceedingly high turnover frequencies (up to 270000 h⁻¹).

Chapter 5 gives a summary of the work undertaken within this research project and offers potential future developments of this research.

Chapter 6 gives experimental details of the work undertaken within this research project.

Date of Award	26 Jun 2019
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Andrew David Smith (Supervisor) & Polly L. Arnold (Supervisor)