Smits, Teika Marija; (2002) EPR spectroscopic and synthetic aspects of thiyl radical chemistry. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The reactions of the sulfide 1 with the nucleophilic radicals A and B have been studied in solution between 180 and 240 K using EPR spectroscopy. With the amine- boryl radical A abstraction of chlorine is the dominant reaction, while with the triethylsilyl radical B, competing SH2 dealkylation at sulfur takes place more rapidly. The relative reactivities of the chlorides 1 - 3 towards chlorine-atom abstraction by the radicals A and B increase along the series 3 < 2 < 1 and the high reactivity of the sulfide 1 is attributed to a combination of favourable polar and enthalpic factors. In contrast, the relative rates of hydrogen abstraction from MeSCH2CH2SMe and MeSCH2CH3 by the radical C indicate that the effect of the β-sulfur substituent is to slightly retard hydrogen-atom transfer from an SCH2 group to this electrophilic radical. Ab initio molecular orbital calculations have been carried out to aid the interpretation of these results. No evidence was found to support the proposed rapid rearrangement of β- alkylthioalkyl radicals to the more stable isomeric β-alkylthioalkyl radicals. Cyclic benzylidene acetals derived from 1, 2- and 1, 3-diols undergo an efficient redox rearrangement in the presence of a catalytic amount of thiol and peroxide initiator, to give benzoate esters. The thiol acts as a protic polarity-reversal catalyst and the derived thiyl radical specifically seeks out the benzylic C-H group in this radical-chain reaction. The monocyclic 2-phenyl-l, 3-dioxan-2-yl radical so formed then undergoes β-scission to give preferentially the more stabilised alkyl radical. However, analogous bicyclic radicals derived from 4, 6-O-benzylidene glucopyranosides afford primary radicals in preference to secondary radicals and density functional calculations have assisted in the interpretation of this surprising result. A report by Barton and Subramanian that the more stable secondary radical is formed in the tributyltin hydride-mediated reduction of a corresponding glucosidic thionocarbonate has been shown to be in error.

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