However, all previously reported attempts to grow single crystals of IBS from methanol or acetonitrile resulted in reduction with the formation of 2-iodosylbenzenesulfonic acid as confirmed by X-ray diffraction analysis. Despite these problems, IBS was previously characterized by 1H and 13C NMR, IR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Moreover, IBS has high reactivity towards polar organic solvents (acetonitrile, DMSO, methanol) being readily reduced to 2-iodosylbenzenesulfonic acid upon contact with these solvents. The resulting IBS is insoluble in nonpolar solvents (dichloromethane, chloroform, etc.). Because of the high solubility of IBS in water, this mixture is difficult to separate. The direct oxidation of 2-iodobenzenesulfonic acid ( 2) with Oxone leads to the formation of the desired IBS ( 1, Scheme 1), however, contaminated with inorganic impurities. The hydrolysis of sulfonic ester 3 forms IBS as a mixture with methanol which is quickly oxidized by IBS in situ producing the corresponding iodine(III) heterocycle, 2-iodosylbenzenesulfonic acid ( 4) as main product. The first preparation and isolation of IBS ( 1) was attempted in 2006 using two different approaches: direct oxidation of 2-iodobenzenesulfonic acid ( 2) by Oxone or hydrolysis of methyl 2-iodoxybenzenesulfate ( 3, Scheme 1). Recent research has revealed the extreme activity of IBS as a catalyst in numerous other oxidations, such as: the oxidation of benzylic and alkane C–H bonds, the oxidation of phenols to 1,2-quinones, the cyclization and cross-coupling reactions, and the site-selective hydroxylative dearomatization of 2-substituted phenols to either 1,2-benzoquinols or their cyclodimers.
In 2009, Ishihara and co-workers have reported an extremely active catalytic system for oxidation of alcohols based on 2-iodoxybenzenesulfonic acid (IBS) as the active species. Moreover, these reagents are generally not suitable as active species in catalytic reactions due to the low reactivity and harsh conditions required for their in situ generation. IBX and DMP are mild oxidants with a relatively low reactivity towards some substrates. 2-Iodoxybenzoic acid (IBX) and the product of its acetylation Dess–Martin periodinane (DMP) are the most common oxidants used for selective oxidation of alcohols to carbonyl compounds as well as for a variety of other synthetically useful oxidative transformations.
Hypervalent iodine(V) compounds represent an important class of oxidative reagents extensively employed in organic synthesis. Recently, the interest in synthetic applications of hypervalent iodine compounds as stoichiometric reagents or catalysts has experienced an explosive growth.
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