Synthesis and application of thiourea-S,S-dioxide derivatives

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Abstract

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

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