Generation and reactivity of carbon, sulphur and tellurium centred free radicals
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Using ESR dimer-radical equilibrium techniques the heat of formation, △Hf⁰, of the pentamethylcyclopentadienyl radical was determined as 7.4 kcal mol⁻¹, and hence SEMeH was calculated to be 37.9 kcal mol⁻¹. It was attempted to determine △Hf⁰ for the pentaethyl- and pentamethoxycarbonylcyclopentadienyl radicals, but this proved unsuccessful. From appearance energy measurements, △Hf⁰ (Pentadienyl) was determined to be 49.6 kcal mol⁻¹ Using photoacoustic calorimetry, the C₃-H bond dissociation energy of penta⁻¹,4-diene and several 3-substituted pentadienes were determined; i.e. for penta⁻¹,4-diene (DH° (C₃-H) = 76.6±0.6 kcal mol⁻¹), 3-methyl- penta⁻¹,4-diene (DH°(C₃-H) = 76.6±0.6 kcal mol⁻¹) and 3-hydroxypenta⁻¹,4-diene (DH°(C₃-H) = 69.0±0.6 kcal mol⁻¹). ESR exchange broadening methods were used to show that SEESR (3-methylpentadienyl) is virtually the same as that of the parent pentadienyl radical, (i.e. 25 kcal mol⁻¹). The 3-hydroxypentadienyl, l-hydroxy-3-methylpentadienyl and l-trimethylsilyloxy-3-methylpentadienyl radicals were generated, but exchange broadening was not observed in the accessible temperature range. Sulphonate esters were examined as a potential source of free radicals. The reaction of simple alkyl alkanesulphonates with photochemically generated trimethyltin radicals in the cavity of an ESR spectrometer gave the corresponding alkanesulphonyl radicals. Good ESR spectra were obtained when sulphonyl radicals, generated from sulphonate esters, were used to form adduct radicals with alkenes of the type CH₂=CHR where R is an electron releasing substituent, (R = OMe, OBuⁿ, SPh, SiME₃). Delocalised radicals were generated from sulphonate esters which contained a terminal alkenyl or alkynyl substituent. For example, the reaction of allyl methanesulphonate with photochemically generated trimethyltin radicals in the cavity of an ESR spectrometer gave good spectra of the allyl radical. The pent-4-ene-1-sulphonyl radical was generated from the corresponding sulphonyl chloride using several different radical initiator systems (tri-n-butyltin hydride, triphenylsilane, hexa-n-butylditin, 9-trimethylstannyl-9,10-dihydroanthracene, copper (II) chloride and tris (triphenylphosphine) ruthenium (II) chloride). The radical was found to cyclise in a predominantly endo-fashion to give 3-chlorotetrahydrothiopyran-1,1-dioxide, the exact proportion of endo- and exo-products depending on the temperature. The hex-5-ene-1-sulphonyl radical was found to give thiepane-1,1- dioxide, also the product of endo-cyclisation. The inclusion of a bulky ring substituent forced the cyclohexenylethanesulphonyl radical to cyclise in the exo-mode to give 2-chloro-9-thiabicyclo[4.3.0]nonane-9,9-dioxide. Dialkyltellurides and dialkylditellurides gave the corresponding alkyl radical on photolysis in the cavity of an ESR spectrometer. For example, diallyltelluride gave good spectra of the allyl radical. Under thermolysis conditions, a nitrosodurene spin trap was employed to observe the trapped alkyl radicals. It is believed that certain of the trapped radicals were tellurium-centred species.
Thesis, PhD Doctor of Philosophy
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