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Synthetic and Mechanistic Aspects of Free-Radical Reactions in Solution

Cai, Yudong; (1999) Synthetic and Mechanistic Aspects of Free-Radical Reactions in Solution. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

This thesis is divided into three chapters. Chapter 1. Radical cyclisation of ortho-(2-propenyloxy)benzenediazonium tetrafluoroborate in aqueous solution in the presence of a reducing reagent and a hydrogen-atom donor gives a mixture products containing 3-methyl-2,3-dihydrobenzofuran and uncyclised allyloxybenzene. Enantioselective radical cyclisation of the diazonium salt was found in the presence of cyclodextrins. The enantiomeric excess of the resulting 3-methyl-2,3-dihydrobenzofuran is ca. 7% in the presence of hydroxypropyl-β-cyclodextrin and ca. 13% in the presence of hydroxypropyl-α-cyclodextrin. The results suggest that higher enantioselectivities for radical reactions in aqueous solution might be achieved by modifying the structures of the guest radicals and cyclodextrins. Chapter 2. Thiols act as polarity-reversal catalysts and promote the radical-chain cyclisation of alkenyloxysilanes at 60-65 °C, in the presence of di-tert-butyl hyponitrite as initiator. Allyloxysilanes give five-membered-ring products via 5-endo-trig cyclisation of the intermediate allyloxysilyl radical. Homoallyloxysilanes give mixtures of five-and six-membered heterocycles, but the intermediate silyl radicals undergo predominantly 6-endo cyclisation, in contrast to the corresponding carbon-centred radicals which cyclise preferentially in the 5-exo mode. An analogous pentenyloxysilane gives only the seven-membered-ring product via a 7-endo radical cyclisation. Steric effects play an important part in influencing the final-product stereochemistry when this is determined in the hydrogen-atom transfer reaction between the cyclic adduct radical and the thiol catalyst. An unsuccessful thiol-catalysed tandem cyclisation shows that it is important for the addition of the thiyl radical to the C=CH2 group to be reversible under the reaction conditions. Complementary EPR spectroscopic studies of the short-lived intermediate cyclic adduct radicals have been carried out in the absence of thiol and the structures and conformations of these species have been determined. Chapter 3. Alkanethiols with electron withdrawing S-alkyl groups and silanethiols act as polarity-reversal catalysts to promote the radical-chain racemisation of (R)-tetrahydrofurfuryl acetate and the cis-trans-isomerisation of 2,5-dimethyltetrahydro- furan at 60 °C, while simple alkanethiols are ineffective. The a-alkoxyalkyl radical derived from (R)-tetrahydrofurfuryl acetate has been studied by EPR spectroscopy and its conformation has been determined. The rate constant for hydrogen-atom abstraction by tert-butoxyl radicals from the tertiary CH group in 2,5-dimethyltetrahydrofuran is ca. 7.5 times greater than that for abstraction from the tertiary CH group in tetrahydrofurfuryl acetate at -30 °C

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Synthetic and Mechanistic Aspects of Free-Radical Reactions in Solution
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
URI: https://discovery.ucl.ac.uk/id/eprint/10122668
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