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dc.contributor.authorCollett, Christopher J.
dc.coverage.spatial454en_US
dc.date.accessioned2013-07-24T13:10:44Z
dc.date.available2013-07-24T13:10:44Z
dc.date.issued2013
dc.identifieruk.bl.ethos.577288
dc.identifier.urihttp://hdl.handle.net/10023/3868
dc.description.abstractThis thesis describes our physical organic and mechanistic investigations into N Heterocyclic Carbene (NHC) mediated organocatalytic transformations, through a collaboration with the research group of Dr AnnMarie O’Donoghue and PhD student Richard Massey at Durham University. Initial research focused upon the determination of kinetic acidities and associated pKₐ values for a range of triazolium salts using C(3) H/D exchange, monitored by ¹H NMR spectroscopy. Estimates for pKₐ values in the range 16.6 17.4 were obtained, which are some ~2 and ~3 5 pK units lower than analogous imidazolium and thiazolium species respectively, with modest N substituent (0.3 pK units) effects observed. At lower pD values, an altered pD dependence indicates a dicationic triazolium species is formed (through N(1) protonation) with an estimated pKₐᴺ¹ of -0.2-0.5 and C(3) H pKₐ values at least 2 units lower than their monocationic analogues. This methodology was subsequently extended to mesoionic NHCs, where pKa values of 23.0 27.1 for a range of triazolium and 30.2 31.0 for a range of imidazolium salts were estimated. A detailed study of the NHC catalysed intramolecular Stetter reaction was also undertaken using ¹H NMR spectroscopy. A range of 3 (hydroxybenzyl)azolium salts (adducts), formed from the addition of NHC to aldehyde were isolated, enabling the generation of reaction profiles and the determination of rate constants. The reaction proceeds via rapid and reversible adduct generation, followed by rate limiting Breslow intermediate formation, with electron withdrawing N aryl substituents increasing the rate of product formation. Consistent with rate limiting deprotonation, deuterium exchange studies of O methylated adduct analogues found electron withdrawing N-aryl units gave faster exchange. Examination of the equilibrium constants for adduct formation revealed that both in the case of NHCs bearing 2,6 disubstituted N aryl units and aldehydes bearing a 2 ether substituent, the equilibrium position is significantly shifted towards adduct. Finally, studies at sub-stoichiometric NHC concentrations, monitored by HPLC, imply the reaction is first order with respect to NHC precursor, but zero order in aldehyde, again indicative of rate limiting deprotonation.en_US
dc.language.isoenen_US
dc.publisherUniversity of St Andrews
dc.subjectOrganocatalysisen_US
dc.subjectN-Heterocyclic carbeneen_US
dc.subjectNHCen_US
dc.subjectPhysical organicen_US
dc.subjectMechanisticen_US
dc.subjectNMRen_US
dc.subjectBenzoinen_US
dc.subjectStetteren_US
dc.subjectLewis baseen_US
dc.subject.lccQD262.C766
dc.subject.lcshCatalysisen_US
dc.subject.lcshOrganonitrogen compoundsen_US
dc.subject.lcshCarbenes (Methylene compounds)en_US
dc.subject.lcshBenzoinen_US
dc.subject.lcshLewis acidsen_US
dc.titleMechanistic studies of azolium ions and their role in organocatalysisen_US
dc.typeThesisen_US
dc.type.qualificationlevelDoctoralen_US
dc.type.qualificationnamePhD Doctor of Philosophyen_US
dc.publisher.institutionThe University of St Andrewsen_US
dc.rights.embargodate2019-02-21en_US
dc.rights.embargoreasonThesis restricted in accordance with University regulations. Print and electronic copy restricted until 21st February 2019en_US


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