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Title: Synthetic and mechanistic aspects of iron-catalysed cross-coupling
Author: Cogswell, Paul
ISNI:       0000 0004 5914 5263
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2014
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An investigation into the active species in iron-catalysed Kumada-type reactions of mesitylmagnesium bromide in the presence of stoichiometric TMEDA revealed that the anion [Fe{meshr is the only observable paramagnetic species under the reaction conditions. It had previously been reported that the neutral adduct [Fe{mesh{TMEDA)) was an on-cycle intermediate that reacted with the electrophile; however, the anionic species was shown to react faster with electrophiles allowing the catalytic cycle accepted in the literature to be revised. When the reactions of less-bulky aryl Grignard reagents were studied, oxidation states below Fe(lI) were accessed as soluble Fe(l) complexes or suspensions of Fe{O) nanoparticles. This suggests different manifolds are in operation depending on the steric properties of the nucleophile and a general catalytic cycle is likely invalid. A range of iron-phosphine complexes was investigated as alternative catalysts in the Suzukitype cross-coupling of tetraarylborates in the presence of sub-stoichiometric diaryl zinc species. The coupling reactions of more synthetically expedient aryl trialkylborates formed from commercially available starting materials with benzyl halides were developed. The iron-catalysed cross-coupling reactions of related aryl nucleophiles of aluminium, indium, gallium and thallium with alkyl, benzyl and allyl halides were achieved using a commercially available iron pre-catalyst. Fe(l) was shown to be a kinetically accessible oxidation state under catalytic conditions, and complexes of the form [FeX(dpbzhJ (X = CI, 4-tolyl) were formed using all the relevant nucleophiles. Iron-catalysed cross-coupling reactions using 'soft' nucleophiles have previously required the use of expensive or synthetically challenging bisphosphines based on the orthophenylene backbone. The development of "pre-reduction" conditions allowed a range of state-of-the-art coupling reactions to be performed using pre-catalysts based on the cheap and readily ava ilable bisphosphine 1,2-bis(diphenylphosphino)ethane. Mechanistic investigations revealed the importance of Fe(l) complexes as active species, with the loss of a ligand from the catalytic resting state [FeCI(dppehJ likely to occur prior to rate determining oxidative addition.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available