Unlike its lightercongeners, thehalogeniodine forms a number of stableorganic compounds, in which iodine exhibits higherformaloxidation states than −1 orcoordination number exceeding 1. These are thehypervalent organoiodines, often callediodanes after theIUPAC rule used to name them.
These iodine compounds arehypervalent because the iodine atom formally contains in itsvalence shell more than the 8 electrons required for theoctet rule. Hypervalent iodineoxyanions are known for oxidation states +1, +3, +5, and +7; organic analogues of these moieties are known for each oxidation state except +7.
In terms of chemical behavior, λ3‑ and λ5‑iodanes are generally oxidizing and/or electrophilic species. They have been widely applied towards those ends inorganic synthesis.[1]
Several different naming conventions are in use for the hypervalent organoiodines.
All begin with nonstandard formal charge assignments. In iodane chemistry, carbon is consideredmore electronegative than iodine, despite thePauling electronegativities of those respective atoms.[2] Thusiodobenzene (C6H5I) is an iodine(I) compound,(dichloroiodo)benzene (C6H5ICl2) andiodosobenzene (C6H5IO) iodine(III) compounds, andiodoxybenzene (C6H5IO2) an iodine(V) compound.
With that convention in place, IUPAC names assume complete electron transfer. Thus when iodine is ligated to an organic residue and twoLewis acids, it is in the +3oxidation state and the corresponding compound is aλ3‑iodane. A compound with iodine(V) would be aλ5‑iodane, and a hypothetical iodine(VII)‑containing compound would be aλ7‑iodane. Organyl-iodineethers, a kind ofλ3‑iodane, are sometimes calledorganic hypoiodites.
Alternatively, the hypervalent iodines can be classified usingneutral electron counting. Iodine itself contains 7 valence electrons, and, in a monovalent iodane such as iodobenzene (C6H5I), thephenyl ligand donates one additional electron to give a completed octet. In a λ3‑iodane, eachX-type ligand donates an additional electron, for 10 in total; the result is adecet structure. Similarly, many λ5‑iodanes aredodecet molecules, and hypothetical λ7‑iodanes aretetradecet molecules. As with other hypervalent compounds, N‑X‑L notation can be used to describe the formal electron count of iodanes, in which N stands for the number of electrons around the central atom X (in this case iodine), and L is the total number of ligand bonds with X. Thus, λ3‑iodanes can be described as 10‑I‑3 compounds, λ5‑iodanes as 12‑I‑5 compounds, and hypothetical λ7‑iodanes as 14‑I‑7 compounds.
As with other hypervalent compounds, iodanes bonding was formerly described usingd-orbital participation.3-center-4-electron bonding is now believed to be the primary bonding mode. This paradigm was developed by J.J. Musher in 1969.
One such bond exists in iodine(III) compounds, two such bonds reside in iodine(V) compounds and three such bonds would reside in the hypothetical iodine(VII) compounds.
Hypervalent organoiodine compounds are prepared by the oxidation of anorganyl iodide.
In 1886, German chemistConrad Willgerodt prepared the first hypervalent iodine compound,iodobenzene dichloride (PhICl2), by passingchlorine gas throughiodobenzene in a cooled solution ofchloroform:[3]
This preparation can be varied to produce iodobenzenepseudohalides. Cleaner preparations[4] begin withsolutions ofperacetic acid inglacial acetic acid, also due to Willgerodt:[5]
Theiodobenzene diacetate product hydrolyzes to thepolymericiodosobenzene (PhIO), which is stable in coolalkaline solution.[6] In hot water (or, in Willgerodt's original preparation,steam distillation), iodosobenzene instead disproportionates toiodoxybenzene andiodobenzene:[7]
2-Iodobenzoic acid reacts withoxone[8] or a combination of potassiumbromate andsulfuric acid to produce theinsoluble λ5‑iodane2-iodoxybenzoic (IBX) acid.[9] IBX acid is unstable and explosive, butacetylation tempers it to the stablerDess-Martin periodinane.[10]
Aliphatichypoiodites can be synthesized through a variant on theWilliamson ether synthesis: analkoxide reacts withiodine monochloride, releasing the alkyl hypoiodite andchloride.[11] Alternatively, theMeyer-Hartmann reaction applies: a silver alkoxide reacts withelemental iodine to give the hypoiodite andsilver iodide. They areunstable to visible light, cleaving intoalkoxyl andiodine radicals.[12]
The synthesis of organyl periodyl derivatives (λ7-iodanes) has been attempted since the early 20th century.[13] Efforts so far have met with failure, althougharyl λ7‑chloranes are known. Organic diesters of iodine(VII) are presumed intermediates in the periodate cleavage of diols (Malaprade reaction), although no carbon-iodine(VII) bond is present in this process.
Diaryliodonium salts are compounds of the type[Ar−I+−Ar]X−.[14] They are formally composed of a diaryliodonium cation[15] paired with acounteranion, but crystal structures show a long, weak, partially-covalent bond between the iodine and the counterion. Some authors have described this interaction as an example ofhalogen bonding,[16] but the interaction exists even with traditionallynoncoordinating ions, such asperchlorate,triflate, ortetrafluoroborate.[17] As a result, other authors regard the diaryliodonia as λ3-iodanes.[18]
The salts are generally T-shaped, with the counteranion occupying an apical position.[18] The overall geometry at the iodine atom ispseudotrigonal bipyramidal. The placement of ligands exhibitsapicophilicity: thephenyl group and chlorine group attain apical positions, while the other phenyl group and alone pair of electrons hold equatorial ones.
Salts with a halide counterion are poorly soluble in many organic solvents, possibly because the halides bridgedimers. Solubility improves withtriflate andtetrafluoroborate counterions.[17]
In general, the salts can be prepared from preformed hypervalent iodines such asiodic acid,iodosyl sulfate oriodosyl triflate. The first such compound was synthesised in 1894, via thesilver hydroxide-catalyzed coupling of two aryl iodides (theMeyer–Hartmann reaction):[19][20][21]
Alternatively, the iodane may be formedin situ: an aryl iodide is oxidized to an aryliodine(III) compound (such as ArIO), followed by aligand exchange. The latter can occur with organometallized arenes such as anarylstannane or-silane (anucleophilic aromatic substitution reaction) or unfunctionalized arenes in the presence of a Brønsted or Lewis acid (anelectrophilic aromatic substitution reaction).
Diaryliodonium salts react withnucleophiles at iodine, replacing one ligand to form the substituted arene ArNu and iodobenzene ArI. Diaryliodonium salts also react with metals M through ArMX intermediates incross-coupling reactions.
Hypervalent iodine compounds are predominantly used asoxidizing reagents, although they are specialized and expensive. In some cases they replace more toxic oxidants.[23]
Iodobenzene diacetate (PhIAc2) andiodobenzene di(trifluoroacetate) are both strong oxidizing agents used inorganic oxidations, as well as precursors for further organoiodine compounds. Ahypervalent iodine (III) reagent was used as oxidant, together with ammonium acetate as nitrogen source, to provide2-Furonitrile, a pharmaceutical intermediate and potential artificial sweetener.[24]
Current research focuses on the use of iodanes incarbon-carbon and carbon-heteroatombond-forming reactions. In one study, anintramolecular C-N coupling of analkoxyhydroxylamine to itsanisole group is accomplished with a catalytic amount of aryliodide intrifluoroethanol.[25]
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