Investigations into the synthesis and properties of atropisomeric N-heterocyclic carbenes

Abstract

This thesis describes the development of chiral N heterocyclic carbenes (NHCs) substituted with N binaphthyl groups. Their catalytic utility is examined in homogenous organometallic reactions and organocatalysis. Chapter 1 provides an introduction to asymmetric catalysis. The use of chiral phosphines is described, followed by the properties, design and applications of chiral NHCs. Chapter 2 discusses the synthesis of NHC precursors with biaryl substituents and chirality at the 4- and 5-positions of the imidazole. Two pathways utilising Pd catalysed reactions were examined, however preparation of the target compound proved challenging. This resulted in the development of a new design, where chirality is moved from the backbone to the N substituents. This approach required access to atropisomeric binapthyl amines as the key starting materials. Chapter 3 describes the synthesis of atropisomeric binaphthyl amines using the asymmetric Pinhey Barton chemistry and the BINOL approach. Chiral oxazoline ligands were applied to the Pinhey Barton reaction for the first time, and their interactions with aryllead triacetates investigated using 1H NMR spectroscopy. The preparation of the required binaphthyl amine was achieved by selective functionalisation of (M) BINOL. Chapter 4 describes the transformation of the atropisomeric binaphthyl amine into imidazolium salt disubstituted with chiral binaphthyls. Complexes with transition metals were synthesised and their electronic properties and conformational stabilities examined using spectroscopic techniques. In catalysis, the new Rh(I) NHC complexes showed good reactivities in the asymmetric hydrosilylation and hydrogen transfer reactions, however only poor enantioselecties were observed. Efforts to improve reactivity did not result in a corresponding increase in enantioselectivity, highlighting the need to minimise exocyclic C N bond rotation. Expansion of the family of atropisomeric NHC precursors to include 1,2,4 triazolium and thiazolium salts and their application in organocatalysis is described in Chapter 5. The fluxionality of the heteroazolium salts was examined using 1H NMR spectroscopy. They showed interesting reactivities in the Stetter and Baylis Hillman reactions, giving an insight into the effect of the steric properties of the NHC on chemoselectivity. Chapter 6 provides a summary of the work described in this thesis and suggests future developments. Full experimental procedures are included in Chapter 7.