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Title: Probing for catalysed arylboron and arylzinc coupling reactions
Author: Gower, Nicholas
ISNI:       0000 0004 2739 1574
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2012
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The effects on reactions of catalytically-capable trace metal impurities were investigated in the context of two separate reports of iron-catalysed Suzuki reactions. In both cases, the reported reactions were found to irreproducible and the likelihood of palladium contamination was discussed. In one of the cases, it was found that as little as 0.0001 mol% of Pd(OAc)2 could account for the observed activity. As a result of pressure from our group and others, both papers were retracted. The importance of testing for trace impurities and of conducting blank reactions is discussed. The roles of l,2-bis(diphenylphosphino)benzene (dpbz) and l,3-bis(diphenylphosphino)propane (dppp) ligands in the Fe-catalysed Negishi and Suzuki reactions were investigated through a structure/reactivity study. A series of modified dppp and dpbz analogues were prepared and characterised and some of their key structural differences analysed. The ligand library was then investigated in both the Fe-catalysed Suzuki and Negishi reactions. Despite the broad range of structurally-related but significantly different phosphine ligands tested, nearly all were shown to perform well in both of the test-bed reactions. Reasons for the unprecedented high activity of this class of catalysts are discussed and the importance of pre- forming iron complexes before their application to catalysis is highlighted. In parallel with a full kinetic study and the isolation of potential reactive intermediates conducted by other group members, the mechanism of the Fe-catalysed Negishi reaction was investigated. Kinetic analysis identified Fe(l) as the most likely lowest oxidation state in the catalytic cycle and efforts were made to isolate Fe(l) intermediates. A series of Fe(l) complexes had been isolated by other group members and these were tested in a range of representative Negishi reactions. An Fe(l) bromide complex was identified as a likely on-cycle intermediate and the implications of this in catalysis are discussed. The use of boron/zinc transmetallation for the generation of nucleophiles in Fe-catalysed coupling reactions was investigated. It was found that under forcing conditions, the aryl transfer reaction could be made to proceed without the need for additional catalysts. The mechanism of boron/zinc transmetallation was investigated using llB NMR spectroscopy and DFT calculations. It was demonstrated that the degree to which the alkyl groups from the zinc transferred to the boron was far higher than was previously thought and that the addition of stabilising solvents or ligands could dramatically alter the positions of the equilibria in transmetallation reactions. Two examples of 11 tetraarylborate salts formed in transmetallation reactions were isolated and structurally characterised. The implications of these observations on the catalyst-free aryl transfer and other reported aryl transfer reactions are discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available