![]() Really the only reagent that comes close to the hypoiodite in terms of yield is NIS in the weaky acidic solvent hexafluoroisopropanol (HFIP). Figure 2: comparison of iodination methods Figure 2 shows a comparison of the sulfonyl hypoiodite with other iodinating reagents using tetrahydrobenzo furanone as a test substrate. ![]() In addition to the obvious safety benefits, the silver salt approach could be used to generate a range of hypoiodites with different reactivity depending on the nature of the sulfonate. ![]() The in situ generation from the silver salt gave comparable results to the peroxide mode of formation. Figure 1: generation of sulfonyl hypoiodites and reaction with arenes Thankfully for those involved in the project (and anyone who subsequently implements the chemistry) an alternative in situ method was developed using silver mesylate and molecular iodine (Figure 1). 3 Attempts to make the methanesulfonate reagent by reaction of iodide with bis(methanesulfonyl) peroxide were thwarted by the fact that the peroxide precursor is explosive. Tobias Ritter has succeeded in accessing previously unexplored electrophilic iodinating agents- sulfonyl hypoiodites (although they do claim the intermediate is putative). Elemental iodine (I 2) and electrophilic iodinating reagents such as N-iodosuccinimide (NIS) and 1,3-diiodo-5,5-dimethylhydantoin (DIH) are generally not reactive enough to iodinate electron-poor aromatic or heteroaromatic systems, and if they react at all the yields are low and the reactions non-selective. They are generally more difficult to access than the corresponding chlorides or bromides due largely to the limited availability of electrophilic iodinating reagents of comparable reactivity to their lighter halogen cousins. 2 As is common in synthetic science, the intermediates you really need are the ones that are most difficult to make and iodoarenes are no exception. I sure I don’t need to reiterate how useful iodinated intermediates are in synthesis- most significantly their heightened reactivity in metal catalysed cross coupling chemistry, 1 and, more recently their application as organocatalysts in hypervalent iodine chemistry. A couple of interesting papers published recently on aryl iodination.
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