Iodine V Heterocycles

Within the broad field of hypervalent iodine chemistry, five-membered iodine(V) heterocycles occupy a special place. There has been significant interest in the cyclic X -iodanes, mainly 2-iodoxybenzoic acid (IBX) 

IBX in the solid state has a three-dimensional polymeric stmcture due to strong intermolecular secondary I - O contacts and hydrogen bonding. A detailed X-ray diffraction study of IBX samples, prepared by the oxidation of 2-iodobenzoic acid with potassium bromate, revealed the presence of the powder and the macrocrystalline forms of IBX [674]. It was also noticed that the powder form of IBX is more reactive in the reaction with acetic anhydride than the macrocrystalline form and thus is more useful as the Dess-Martin periodinane precursor (Section 2.23.2). Treatment of macrocrystalline IBX with aqueous sodium hydroxide and then with hydrochloric acid can be used to convert it into the more reactive powder form [674]. 

IBX is a potentially explosive compound and even the bromate-free samples of IBX are not safe. Santagostino and coworkers reported that as a rule pure IBX explodes at 233 °C [680]. The explosibility tests of analytically pure IBX samples (over 99% purity) confirmed the earlier observations by Plumb and Harper 

Theoretical and experimental studies of the value and proton affinity of IBX solutions in aqueous media and DMSO have been published. In particular, the aqueous pATa value of 2.40 for IBX was obtained by using standard potentiometric titration methods [686]. The relatively high acidity of IBX should be taken into consideration while using this important reagent in the oxidation of complex organic molecules. The gas-phase proton affinities of the anions of IBX (1300 25 kJ moH) and 2-iodosylbenzoic acid (1390 10 kJ mor ) using mass spectrometry-based experiments were reported [687], The experimental results were supported by theoretical calculations, which yielded proton affinities of 1336 and 1392 kJ mor for the anions of IBX and 2-iodosylbenzoic acid, respectively, at the B3LYP/aug-cc-PVDZ level of theory [687]. 

A pyridinium salt of IBX has been prepared, by treatment of IBX with pyridine, in the form of stable, non-explosive, colorless crystals [697]. Pyridinium 2-iodobenzoate has a similar reactivity pattern to that of IBX and can be used as a safe oxidant that is soluble in organic solvents. IBX can also form relatively stable complexes with A-oxides, sulfoxides (DMSO) and tetrahydrofuran [698,699]. Such ligand complexation allows us to fine-tune the reactivity profile of IBX [698]. 

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