METAL CATALYSED TRANSFER HYDROGENATION, DEHYDROGENATION AND RACEMISATION OF AMINES

This thesis describes the study of the fundamental mechanisms relating to asymmetric transfer hydrogenation (ATH) of imines and the transfer dehydrogenation and racemisation of secondary amines by Cp*Rh and Cp*Ir complexes. A series of 16-electron complex of the type Cp*M(XNC6H4NX’) (where M = Rh or Ir and X, X’ = H or Ts) have been synthesised, fully characterised and demonstrated to be active catalysts for the transfer hydrogenation of imines. The crystal structures of the complexes reveal structural features that are important in understanding their unique ability to act as transfer hydrogenation catalysts including a remarkable trend in the M-N bond lengths, indicative of M-NH π-bonding and the absence of M-NTs π-bonding which has several implications for catalysis. The complexes allowed catalytic studies without the need for 18-electron precursors, as is the case for the more widely used Noyori-type Cp*M(Cl)(TsNCHPhCHPhNH2) (where M = Rh or Ir) ATH complexes. We observed key intermediates in the mechanism by NMR spectroscopy such as an 18-electron formate complex, Cp*Rh(OCHO)(TsNC6H4NH2) and the 18-electron Rh(III) hydride complex, Cp*Rh(H)(TsNC6H4NH2). An 18-electron chloride precursor complex, Cp*Rh(Cl)(TsNC6H4NH2) was also characterised by X-ray crystallography. DFT calculations were used to support our observations and an “outer-sphere” mechanism for the transfer hydrogenation of imines is proposed. We also studied the mechanism of a dimeric Cp*Ir diiodide complex, [Cp*Ir(I)2]2 that is capable of amine racemisation under mild conditions and characterised several key intermediates including an ammine-coordinated complex, Cp*Ir(I)2(NH3) which itself was demonstrated to be equally active for the racemisation process under identical conditions; amine bound complexes of the type Cp*M(X)2(PhCH2NHMe) (where M = Rh, Ir and X = Cl, I) proposed to be the first step in the mechanism; an imine-coordinated Cp*Ir diiodide complex characterised by 1H/15N HSQC and the reversible hydrogenation of this complex by addition of molecular hydrogen to form the corresponding amine. A mechanism based on the current evidence for the racemisation of amines using the [Cp*Ir(I)2]2 catalyst is proposed. Finally, we describe a method for producing hyperpolarised carbon dioxide gas by using the Cp*Rh(Cl)(TsNCHPhCHPhNH2) ATH catalyst to catalytically decompose a pre-hyperpolarised solution of natural abundance formic acid, which was observed by 13C NMR spectroscopy.