Special Issue Reviews and Accounts ARKIVOC 2009 (i) 1-62
terminology, structural types 1-7 are commonly referred to as “hypervalent iodine reagents”, while iodonium salts 8, ylides 9 and imides 10 usually are classified separately. In the present review, the preparation and chemistry of structural classes 1-5 will be discussed.
General aspects of structure and bonding in hypervalent iodine(III) organic compounds, or .3-iodanes according to the IUPAC nomenclature, have been summarized by Ochiai in a chapter in the volume on Hypervalent Iodine Chemistry in Topics in Current Chemistry.1b A brief summary of the key structural features of hypervalent iodine(III) compounds is provided below. The iodine atom in .3-iodanes, RIX2, has a total 10 electrons and the overall geometry of a distorted trigonal bipyramid with two heteroatom ligands X occupying the axial positions, and the least electronegative carbon ligand R and both electron pairs residing in equatorial positions. In the hypervalent model, bonding in RIX2 uses the non-hybridized 5p orbital of iodine in the linear X–I–X bond. Such a linear three-center, four-electron (3c–4e) bond is highly polarized and is longer and weaker compared to a regular covalent bond. This bond is termed “hypervalent” and the presence of this bond in .3-iodanes is responsible for their high electrophilic reactivity. Structural aspects of hypervalent iodine compounds have been investigated by numerous research groups,1c,3,4 most notably by Katritzky and coauthors in a series of papers published in 1989-1990.3 Several areas of structural research on hypervalent organoiodine have recently attracted especially active interest. These areas include the preparation and structural study of complexes of hypervalent iodine compounds with crown ethers4a or nitrogen ligands,4b,c selfassembly of hypervalent iodine compounds into various supramolecular structures,4d-f and the intramolecular secondary bonding in hypervalent iodine derivatives.4g-i
Typical coordination patterns in various organic derivatives of iodine(III) in the solid state with consideration of primary and secondary bonding have been summarized by Sawyer and coauthors5a in 1986 and updated in recent publications.5b-e
Several important spectroscopic structural studies of polyvalent iodine compounds in the solution have been published.6 Hiller and coauthors reported NMR and LC-MS study on the structure and stability of 1-iodosyl-4-methoxybenzene and 1-iodosyl-4-nitrobenzene in methanol solution.6a Interestingly, LC-MS analyses provided evidence that unlike the parent iodosylbenzene, which has a polymeric structure, the 4-substituted iodosylarenes exist in the monomeric form. Both iodosylarenes are soluble in methanol and provide acceptable 1H and 13C NMR spectra; however, gradual oxidation of the solvent was observed after several hours.6a Cerioni, Mocci and coauthors investigated the structure of bis(acyloxy)iodoarenes and benzoiodoxolones in chloroform solution by 17O NMR spectroscopy and also by DFT calculations.6b This investigation provided substantial evidence that the T-shaped structure of iodine(III) compounds observed in the solid state is also adopted in solution. Furthermore, the “free” carboxylic groups of bis(acyloxy)iodoarenes show a dynamic behavior, observable only in the 17O NMR. This behavior is ascribed to a [1,3] sigmatropic shift of the iodine atom between the two oxygen atoms of the carboxylic groups, and the energy involved in this process varies significantly between bis(acyloxy)iodoarenes and benzoiodoxolones.6b Silva and coauthors analyzed solutions of bis(acyloxy)iodobenzenes in acetonitrile, acetic acid, aqueous methanol
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