Kim, Kyoung Mahn; (1999) Novel radical reactions involving sulfur-containing compounds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

1. Thiol-catalysed Radical-chain Reduction of Organic halides by Hexabutylditin in the Presence of Malonic acid: Thiol-catalyzed radical-chain reduction using hexabutylditin and malonic acid has been developed and found to be effective for the reduction of organic halides. Thiols effectively catalyse the radical-chain reduction in conjunction with malonic acid. The reduction of tertiary α-haloesters with silanes in the presence of thiols was found to be inefficient because of steric congestion and unfavourable polar effects. The thiol-catalysed reduction of alkyl halides with hexabutylditin together with malonic acid proved to be an effective replacement for tributyitin hydride and the reduction of primary, secondary and tertiary alkyl halides and of 1-bromoacetophenone was successfully accomplished in excellent yields. Radicals generated from ethyl 2- bromohexanoate, 4-bromobenzyl bromide, methyl 2-bromobenzoate and methyl 2- iodobenzoate abstract hydrogen-atoms relatively slowly from thiols, because such radicals are either electrophilic or stabilised. However, the reduction of these halides could be carried out successfully under optimised conditions. Study of the neophyl rearrangement showed that the concentration of the thiol can be controlled so as to promote the formation of the rearranged product (isobutylbenzene) without the need to work under conditions of high dilution. 2. Radical-chain Desulfurization of 2-(Alkylthiomethyl) Acrylates with Triphenylphosphine: a New Route to 2-AlkyIacrylates. A study of radical-chain desulfurization of 2-(alkylthiomethyl)acrylates with triphenylphosphine has been carried out. Carbon-carbon bond-forming reactions using allylic sulfides mediated by phosphites or phosphines were based on the ready ?-scission of alkylthiophosphoranyl radical adducts to produce the alkyl radical. The radical reactions of simple allylic sulfides, without electron-withdrawing ester substituents, in the presence of phosphorus(III) compounds gave poor yields. However, 2-methylene-3-alkylthiopropanoate esters react with triphenylphosphine in refluxing octane, in the presence of initiator, to give 2-methylenealkanoates in moderate yields, together with triphenylphosphine sulfide. Success in this case is related to polar effects, because nucleophilic alkyl radicals add easily to electron-deficient alkenes bearing an electron- withdrawing group. 3. Radical-chain Reductive Alkylation of Electron-rich Alkenes: Carbon-carbon Bond Formation using Silanes in the Presence of Thiols as Polarity-reversal Catalysts. Radical-chain reductive alkylation of electron-rich alkenes with electrophilic radicals has been carried out. The use of triorganosilanes in the presence of thiols as polarity- reversal catalyst can serve as an effective replacement for trialkyltin hydride. The S-H group of a thiol provides an electron-deficient hydrogen, which favours hydrogen atom transfer to nucleophilic alkyl radicals, while polar effects discriminate against the abstraction of hydrogen from thiols by electrophilic radicals. The thiol-catalyzed reductive alkylation of unsubstituted terminal electron-rich alkenes by α-halogenoesters in the presence of triphenylsilane under free-radical conditions gave good yields of carboxyalkylated products. Under the same conditions, the radical-chain addition to prochiral alkenes of the type H2C=CR1R2, catalysed by small amounts of homochiral thiols, afforded optically-active adducts in moderate enantiomeric purity.

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