Periodinane

 Periodinanes are chemical compounds containing hypervalent iodine. These iodine compounds are hypervalent because the iodine atom in it contains more than the 8 electrons in the valence shell required for the octet rule. When iodine is complexed with a monodentate electronegative ligand such as chlorine, iodine compounds occur with a +3 oxidation number as iodine(III) or λ³-iodanes or as a +5 oxidation number as iodine(V) or λ⁵-iodanes. Iodine itself contains 7 valence electrons and in a λ³-iodane three more are donated by the ligands making it a decet structure. λ⁵-iodanes are dodecet molecules. In an ordinary iodine compound such as iodobenzene the number of valence electrons is eight as expected. In order to get from iodine to a hypervalent iodine compound it gets oxidized with removal of first 3 electrons and then 5 electrons. The ligands in turn contribute electrons pairs and form coordinate covalent bonds by adding a total of 6 or 10 electrons back to iodine. In the L-I-N notation L stand for the number of electrons donated by ligands and N the number of ligands.

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Periodinane compounds

The concept of hypervalent iodine was developed by J.J. Musher in 1969. In order to accommodate the excess of electrons in hypervalent compounds the 3-center-4-electron bond was introduced in analogy with the 3-center-2-electron bond observed in electron deficient compounds. One such bond exists in iodine(III) compounds and two such bonds reside in iodine(V) compounds.

The first hypervalent iodine compound, dichloroiodobenzene (C₆H₅Cl₂I) was prepared in 1886 by the German chemist C. Willgerodt ^[1] by passing chlorine gas through iodobenzene in a cooled solution of chloroform.

C₆H₅I + ICl₃ → C₆H₅ICl₂ + ICl

λ³-iodanes such as diarylchloroiodanes have a pseudotrigonal bipyramidal geometry displaying apicophilicity with a phenyl group and a chlorine group at the apical positions and other phenyl group with two lone pair electrons in the equatorial positions. The λ⁵-iodanes such as the Dess-Martin periodinane have square pyramidal geometries with 4 heteroatoms in basal positions and one apical phenyl group.

Classical organic procedures exist for the preparation of iodosobenzene diacetate from peracetic acid and acetic acid ^[2].

C₆H₅I + CH₃COOOH → C₆H₅I(OOCCH₃)₂

Phenyliodine bis(trifluoroacetate), PIFA, or (bis(trifluoroacetoxy)iodo)benzene is a related compound based on trifluoroacetic acid

The acetate can be hydrolysed with water to iodoxybenzene or iodylbenzene C₆H₅O₂I ^[3]

This compound was first prepared by Willgerodt by disproportionation of iodosylbenzene under steam distillation to iodylbenzene and iodobenzene

2 PhIO → PhIO₂ + PhI

is a known oxidizing agent. Iodosobenzene diacetate can also be hydrolyzed to iodosylbenzene with sodium hydroxide which is actually a polymer with the molecular formula (C₆H₅OI)_n ^[4]. Iodosylbenzene is used in organic oxidations. Dess-Martin periodinane (1983) is another powerful oxidant and an improvement of the IBX acid already in existence in 1893. The IBX acid is prepared from 2-iodobenzoic acid and potassium bromate and sulfuric acid ^[5] and is insoluble in most solvents whereas the Dess-Martin reagent prepared from reaction of the IBX acid with acetic anhydride is very soluble. The oxidation mechanism ordinarily consists of a ligand exchange reaction followed by a reductive elimination.

Periodinane uses

The predominant use of hypervalent iodine compounds is that of oxidizing reagent replacing many toxic reagents based on heavy metals ^[6].

Current research focuses on their use in carbon-carbon and carbon-heteroatom bond forming reactions. In one study such reaction, an intramolecular C-N coupling of an alkoxyhydroxylamine to its anisole group is accomplished with a catalytic amount of aryliodide in trifluoroethanol ^[7]:

In this reaction the periodinane (depicted as intermediate A) is formed by oxidation of the aryliodide with the sacrificial catalyst mCPBA which in turn converts the hydroxylamine group to a nitrenium ion B. This ion is the electrophile in ipso addition to the aromatic ring forming a lactam with a enone group.

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