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Title: Sulfur-containing radicals : from EPR spectroscopic studies to synthetic methodology
Author: Fielding, Alistair John
ISNI:       0000 0001 3464 2157
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2002
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A variety of novel homochiral silanethiols have been prepared and investigated as polarity-reversal catalysts for the enantioselective hydrosilylation of a prochiral alkene. The enantiomeric excesses of the products were disappointingly small. A computer modelling procedure, based on molecular mechanics, was applied in order to understand the results and quantitative agreement with experiment was reasonably good. Silanethiols have been oxidised to the corresponding disulfides and these have been investigated as photochemical sources of silanethiyl radicals. Electron paramagnetic resonance (EPR) spectroscopy has been used to study the reactivity of these silanethiyl radicals and the results have been compared with the known chemistry of alkanethiyl radicals. Competitive hydrogen-atom abstraction from, and thiyl radical addition to, propene and cyclopentene, together with addition to phosphines, alkyl isocyanides and the SH2 dealkylation of trialkylboranes have all been studied. The radical-chain isomerisation of allyl silyl ethers takes place on heating in the presence of an initiator and an arenethiol as a protic polarity-reversal catalyst. Various thiols were tested and pentafluorothiophenol was found to be particularly effective in this role. It has been shown that thermodynamic equilibrium between isomers is established under the conditions used for these experiments. This methodology has also been applied to the isomerisation of allyl alkyl ethers to alkyl vinyl ethers. As a model for the corresponding reactions of thiyl radicals, EPR spectroscopy has been used to determine the relative molar rate constants for abstraction of allylic and benzylic hydrogens from cyclic and acyclic acetals by tert-butoxyl radicals using competition experiments. Absolute rate constants and Arrhenius activation parameters for β-scission of R10(R20)CR3 have been determined by a steady-state EPR method and the results can be understood in terms of angle-strain and stereoelectronic effects. Calculations using density functional theory have been carried out in support of this work and the agreement between computed and experimental results is remarkably good.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Organic chemistry