Hypervalent Iodine Compounds [Synthetic Reagents]

　Iodine is atomic number 53, in the 5th period, Group 17, halogens of the periodic table. Iodine is a large-sized halogen element, easily polarizable, and low in electronegativity. It forms hypervalent organoiodanes beyond the octet theory by readily extending its valence. For example, hypervalent organoiodane with three ligands is a 10-I-3 type compound as shown in the figure. The central iodine atom forms a plane with two lone pairs and one σ-bond, and furthermore, this iodine atom coordinates two ligands with larger electronegativity in an apical position orthogonal to the plane resulting in the formation of a linear three center-four electron bond. The I-L bond in the apical position is weaker and longer than the covalent bond. For example, the I-O bond length in PhI(OAc)₂ is longer than the sum of their covalent bond radii (1.99 Å) and these bond lengths have a range of 2.15~2.16 Å.

　The stability and reactivity of hypervalent organoiodanes exhibit a greater dependence on the character of hypervalent bonds in the apical position. The hypervalent bonds in the apical position are easily cleaved, and the cleavage causes trivalent iodines with 10 electrons to be reduced to monovalent iodines of a more stable octet structure. For this reason, it exhibits good elimination and oxidation rate and finds application in organic syntheses.^1~3) (Diacetoxyiodo)benzene, the most typical trivalent iodine compounds, which is attempted to support the polymers. For example, Togo and coworkers have demonstrated several oxidative functional group conversion reactions using poly[4-(diacetoxyiodo)styrene](1).⁴⁾ After reaction, this polymer-supported reagent can be recovered by filtration and reused after re-oxidation with peracetic acid. Therefore, it is expected to be used as environmentallyfriendly chemical conversion reagent in the near future.