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Abstract

The use of hydrogen peroxide as enviromentally benign oxidant for the oxidative iodination of organic molecules with iodine in the presence of halide ions as catalysts for the formation of iodine(I) species was investigated. The study of the reaction system iodine/ hydrogen peroxide/ acid revealed the formation of iodine(I) compound when HCl or HBr were used as acids. In the presence of 4 equivalents of HCl iodine was quantitatively oxidazed with hydrogen peroxide in MeCN, TFE or under solvent-free conditions. Quantitative oxidation of I2 was achieved by various forms of H2O2 while the most effective one was 50% aqueous solution in equimolar amount to I2.
This method was applied for the synthesis of tetraalkylammonium or pyridinium dichloroiodates(I) by the oxidation of I2 with 50% H2O2 in the presence of 4 equivalents of 37% HCl followed by the addition of quaternary ammonium chlorides or pyridine. Study of the reactivity has shown that dichloroiodates(I) in the presence of an activator, such as an acid or a fluorinated alcohol become effective reagents for the iodination of aromatic compounds as well as iodochlorination of alkenes. Reactivity and stability is depending on the structure of the cation, as dichloroiodates with smaller cations were more reactive and less stable while PyH+ICl2- was the most effective one.
In addition, properties and reactivity of dichloroiodic acid HICl2, which is formed during the oxidation of iodine with H2O2 in the presence of 4 equivalents of HCl, was studied. HICl2 was proven to be more powerful iodinating agent than Q+ICl2-, while the use of this reagent is more economical, as the only reaction by-product is HCl. Since HICl2 is stable in the solution, it can be used directly for iodination. Thus, aromatic compounds, such as anisoles, dimethoxybenzenes, phenols and anilines were efficiently iodinated. Reactivity was manipulated by the choice of the solvent using MeCN for more reactive substrates, while for less activated fluorinated solvents were used. Moreover, HICl2 was shown to be excellent iodinating reagent toward unsaturated compounds. Reactions in TFE proceeded fast and regioselectively with the formation of iodo-chloro adducts. The study of the mechanism suggested that the nature of the intermediate depends on the type of the substrate.
Furthermore, the reaction system I2/H2O2/4HCl, where HICl2 is generated, was upgraded toward environmentally benign method by using catalytic amounts of HCl for oxidative iodination of aromatics. In this system, reactions with reactive substrates proceeded via formation of ICl as reactive species, while with less reactive compounds ICl reacted preferable with HCl to form HICl2, which, however, is less reactive iodine(I) reagent. Catalytic system 0.5 I2/ 0.5 H2O2/ 0.1 HCl was used for iodination of aromatic compounds and its reactivity was again modulated by the choice of the solvent.
Finally, the use of H2O2 for the synthesis of other mixed trihalides was investigated. Quaternary ammonium dibromoiodates(I) were synthesized by using HBr instead of HCl for oxidation of iodine with H2O2. In the study of their reactivity for electrophilic aromatic substitution dual halogenating nature was revealed since mixture of bromo and iodo products were obtained. On the other hand, difluoroiodates(I) and dihalobromates(I) could not be prepared by employing H2O2 for oxidation of hydrogen halides due to high reduction potential and formation of non stable products.