Synthesis and application of thiourea-S,S-dioxide derivatives
Apps, James Frederick Shuan-Liang
experimentally and computationally. Hydrogen peroxide oxidation of acyclic and cyclic\ud N,N’-dialkylthioureas furnished S,S-dioxides in agreement with computational\ud predictions. Some S,S,S-trioxides were also isolated. An X-ray crystallographic study of\ud N,N’-diisopropylthiourea dioxide, first synthesised in this work, verified computational\ud models. The prediction of a stable N,N’-diarylthiourea dioxide derivative was supported\ud by successful isolation of the dioxide.\ud The application of N,N’-diisopropylthiourea dioxide as a reducing agent was investigated.\ud Removal of tosyl groups from N-tosylaziridines, deprotection of CBz-amines and\ud reduction of nitriles could not be realised. However, aldehydes and ketones were\ud successfully reduced to the respective alcohols in yields comparable with that of thiourea\ud dioxide. Disulfides and N-tosylsulfimides were reduced to a higher degree with N,N’-\ud diisopropylthiourea dioxide than thiourea dioxide under simple, mild conditions.\ud The mechanism of decomposition of N,N’-diisopropylthiourea dioxide to give radical\ud anions was investigated with N-tosylsulfimides and a cyclopropylketone. The study\ud revealed that at high pH, heterolysis of the C-S bond in N,N’-diisopropylthiourea dioxide\ud led to the formation of a sulfinate dianion SO2\ud 2-. The dianion was thought to rapidly\ud oxidise to a powerful reducing species, the radical anion SO2\ud ·- and subsequently effect\ud reduction via a single-electron transfer pathway. A full mechanism of decomposition and\ud reduction is proposed.\ud An investigation into the role of thiourea dioxides as N-heterocyclic carbene (NHC)\ud synthons was carried out. It was thought that decomposition of ethylenethiourea\ud dioxides, via the elimination of sulfur dioxide, would provide an alternative approach to\ud metal NHC complex synthesis. Oxaziridine oxidation of acyclic thioureas, successfully\ud established in this work, was applied to the preparation of ethylenethiourea dioxides. A\ud short study revealed considerable potential for thiourea dioxides as NHC precursors.
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