Kaushal, Parveen; (1990) Investigations of the structures and reactions of neutral free radicals in solution. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Before this work, no direct evidence for cyclisation of the 2-(isocyanatocarbonyl)ethyl radical to give the succinimidyl radical existed, because the latter radical had never been generated from acyclic reagents. Using a combination of e.s.r. spectroscopic techniques and product analysis, the cyclisation of ω-(isocyanatocarbonyl)alkyl radicals derived from the co-bromoalkanoyl isocyanates has been studied. Imidyl radicals, generated by photolysis of, or halogen-atom abstraction from, N-halogenoimides, are efficiently trapped by But2C=CH2 to give relatively persistent adducts which have been characterised by e.s.r. spectroscopy. Bromine-atom abstraction from BrCH2CH2C(O)NCO yields H2CCH2C(O)NCO which undergoes rapid 1,5-endo-cyclisation to give the succinimidyl radical. This cyclisation has been investigated using e.s.r. spectroscopy in conjunction with spin-trapping by But2C=CH2 and ButN=O. The rate coefficient for cyclisation of H2CCH2C(O)NCO has been estimated to be 3.7 x 10 6 s-1 at 328 K in cyclohexane from analysis of the products fromthe radical-chain reaction between BrCH2CH2C(O)NCO and triethylgermane. E.s.r. and product-analysis studies show that H2CCMe2C(O)NCO cyclises, more rapidly than H2CCH2C(O)NCO, to give the 2,2-dimethylsuccinimidyl radical, which subsequently undergoes ring opening to yield Me2CCH2C(O)NCO. The overall rearrangement of H2CCMe2C(O)NCO to Me2CCH2C(O)NCO represents a 1,2-shift of the -C(O)NCO group via an intermediate imidyl radical. The glutarimidyl radical is formed by 1,6-endo-cyclisation of H2CCH2CH2C(O)NCO. It is proposed that the rapid cyclisation of ω-isocyanatoalkyl radicals provides strong evidence that the unpaired electron occupies a σ-orbital in the product imidyl radicals. Section B Polarity reversal catalysis (PRC) of hydrogen-atom abstraction reactions, in particular when the slow abstraction of electron deficient hydrogen by electrophilic t-butoxyl radicals was studied. A number of amine-alkyIboryl radicals have been generated in fluid solution, by hydrogen-atom abstraction from the corresponding amine-alkylborane complexes, using photochemically-produced t-butoxyl radicals, and studied by e.s.r. spectroscopy. The air-stabilities of the parent complexes have been investigated. Amine-alkylboryl radicals abstract halogen rapidly from both alkyl bromides and chlorides. These radicals are highly nucleophilic and rapidly abstract an electron deficient α-hydrogen atom from esters, ketones, lactones, anhydrides, and imides. Because of this property of the derived boron-centred radicals, amine-alkylboranes act as polarity reversal catalysts for the net abstraction of electron deficient hydrogen atoms by the electrophilic t-butoxyl radical. Relative reactivities of alkyl bromides and of esters have been measured by e.s.r. spectroscopy using competition techniques.

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