Iron-catalysed cross-coupling chemistry is undergoing a renaissance due advantages in cost, toxicity and abundance compared to traditional cross-coupling catalysts. In Chapter 2 it is shown that expensive, synthetically challenging bis-phosphines used in the iron-catalysed cross-coupling of alkyl halides and arylboronic acid pinacol esters can be replaced in many cases with simple iron salts in phosphine-free procedures. More challenging substrates require the use of cheap, widely available bis-phosphines such as dppe (l,2-bis(diphenylphosphino)ethane) or dppp (1,3- bis(diphenylphosphino)propane). An EPR spectrum of the reaction mixture shows the presence of previously reported iron(l) complex [FeBr(dppehL which is the likely resting state in the catalytic cycle. Kinetic studies indicate oxidative addition of the alkyl halide via single-electron transfer to a three-coordinate iron species to be the rate-determining step. Having demonstrated that the key feature for facilitating iron-catalysed Suzuki coupling is the activation of a boronic acid pinacol ester with an organometallic nucleophile, in Chapter 3 it was shown that this form of activation could be applied to bis(pinacolato)diboron, allowing the iron catalysed borylation of alkyl, allyl, benzyl and, to a lesser extent, aryl halides. A unique, low-valent, phosphine-supported iron-boryl complex, [Fe(Bpin)(dpbzhL (pin = pinacolato) was isolated from the reaction of [FeCI₂(dpbz)₂] (l,2-bis(diphenylphosphino)benzene) and Li[B₂pin₂(¹Bu)], however this complex failed to react in stoichiometric reactions with 1- bromobutane, reflecting the finding that excess phosphine greatly retards the rate of ironcatalysed borylation. The uniqueness of these low-valent iron-boryl intermediates provided an opportunity to investigate new forms of catalysis based on these complexes. Chapter 4 describes attempts to catalyse the boryl/electrophile difunctionalisation of styrene using Li[B₂pin₂(¹Bu)) and iron precatalysts. It was found that stoichiometric iron is capable of mediating the boryl protonation of styrene; however attempts to trap the organometallic intermediate proved fruitless, and deuterium labelling studies were unable to determine the origin of the a-hydrogen in the resultant product. In order to expand the utility of iron-catalysed cross-coupling, in Chapter 5 attempts to couple electrophiles ·derived from enantiomerically pure l,2-amino alcohols are described. A range of electrophiles including l,2-amino halides, cyclic sulfamidates, oxazolidinones, and oxazolinethiones were screened for activity in iron- and nickel-catalysed cross-coupling; however none of these furnished the desired products.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:687060 |
| Date | January 2014 |
| Creators | Pye, Dominic Richard |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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