Periodinane

From Wikipedia, the free encyclopedia

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.

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

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.

[edit] References

↑ C. Willgerodt, Tageblatt der 58. Vers. deutscher Naturforscher u. Aertzte, Strassburg 1885.
↑ Benzene, iodoso-, diacetate J. G. Sharefkin and H. Saltzman Organic Syntheses, Coll. Vol. 5, p.660; Vol. 43, p.62 Online Article
↑ Benzene, iodoxy- J. G. Sharefkin and H. Saltzman Organic Syntheses, Coll. Vol. 5, p.665; Vol. 43, p.65 Online Article
↑ Benzene, iodoso- H. Saltzman and J. G. Sharefkin Organic Syntheses, Coll. Vol. 5, p.658; Vol. 43, p.60 Online Article
↑ 1,2-Benziodoxol-3(1H)-one, 1,1,1-tris(acetyloxy)-1,1-dihydro- Robert K. Boeckman, Jr., Pengcheng Shao, and Joseph J. Mullins Organic Syntheses, Coll. Vol. 10, p.696; Vol. 77, p.141 Online Article
↑ Hypervalent iodine(V) reagents in organic synthesis Uladzimir Ladziata and Viktor V. Zhdankin Arkivoc 05-1784CR pp 26-58 2006 Article