Novel N-heterocyclic carbene ligands for use in asymmetric catalysis
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This investigation sought to establish new and adaptable methods for incorporating an enantiomeric component into NHC ligands and to develop novel precatalyst prototypes capable of asymmetric induction for use in transition metal mediated catalysis. This thesis details the fulfilment of these objectives through the design, synthesis and characterisation of forty‐eight previously unreported compounds including twenty‐six new NHC proligands and fourteen NHC complexes.
NHC derivatives of the chiral mono‐terpene camphor are explored in Chapter 2, primarily through the synthesis of a series of bornyl‐acetamide linked NHC proligands, demonstrating expedient incorporation of an enantiomeric moiety by acetamide linkage. Bornyl‐acetamide NHC complexes of Ag(I), Pd(II), Pt(II) and Ru(II) are studied and structural elucidation supported by comparison with the Pd(II) and Pt(II) complexes of an achiral cyclohexyl‐acetamide analogue (Chapter 3). Chapter 4 introduces metallo‐ NHC proligands derived from imidazo[4,5‐f][1,10]phenanthroline, capable of backbone coordination of a ruthenium‐polypyridine component. Alternatively, use of an acetamide‐tether provides metallo‐NHC proligands of 5‐acetamido‐1,10‐ phenanthroline and 4‐acetamido,2,2’‐bipyridine (Chapter 5). Also outlined is the synthesis of several remarkable hetero‐dinuclear NHC complexes in addition to the effective enantiomeric resolution of a metallo‐NHC proligand.
The performance of selected systems in an asymmetric Suzuki coupling is examined in Chapter 6. Several of the organic‐NHC ligands elicit comparable activity to reported systems, however, product formation is not observed when using an acetamide‐linked metallo‐NHC ligand. This is most likely related to amide‐group coordination which occurs readily for the metallo‐NHC ligands compared with the bornyl and cyclohexyl analogues. Such conclusions are supported by the comprehensive structural elucidation of all NHC complexes prepared in this thesis, providing evidence for the behaviour of catalytic intermediates involving these ligands.