Hypervalent organoiodine compounds

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Unlike its lighter congeners, the halogen iodine forms a number of stable organic compounds, in which iodine exhibits higher formal oxidation states than -1 or coordination number exceeding 1. These are the hypervalent organoiodines, often called iodanes after the IUPAC rule used to name them.

Contents

These iodine compounds are hypervalent because the iodine atom formally contains in its valence shell more than the 8 electrons required for the octet rule. Hypervalent iodine oxyanions 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 in organic synthesis. [1]

Nomenclature

Several different naming conventions are in use for the hypervalent organoiodines.

All begin with nonstandard formal charge assignments. In iodane chemistry, carbon is considered more electronegative than iodine, despite the Pauling electronegativities of those respective atoms. [2] Thus iodobenzene (C6H5I) is an iodine(I) compound, (dichloroiodo)benzene (C6H5ICl2) and iodosobenzene (C6H5IO) iodine(III) compounds, and iodoxybenzene (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 two Lewis acids, it is in the +3 oxidation state and the corresponding compound is a λ3‑iodane. A compound with iodine(V) would be a λ5‑iodane, and a hypothetical iodine(VII)‑containing compound would be a λ7‑iodane. Organyl-iodine ethers, a kind of λ3‑iodane, are sometimes called organic hypoiodites.

Alternatively, the hypervalent iodines can be classified using neutral electron counting. Iodine itself contains 7 valence electrons, and, in a monovalent iodane such as iodobenzene (C6H5I), the phenyl ligand donates one additional electron to give a completed octet. In a λ3‑iodane, each X-type ligand donates an additional electron, for 10 in total; the result is a decet structure. Similarly, many λ5‑iodanes are dodecet molecules, and hypothetical λ7‑iodanes are tetradecet 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.

Electron structure

As with other hypervalent compounds, iodanes bonding was formerly described using d-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.

Examples

Hypervalent organoiodine compounds are prepared by the oxidation of an organyl iodide.

In 1886, German chemist Conrad Willgerodt prepared the first hypervalent iodine compound, iodobenzene dichloride ( Ph I Cl 2), by passing chlorine gas through iodobenzene in a cooled solution of chloroform: [3]

Ph I + Cl 2 → PhICl2

This preparation can be varied to produce iodobenzene pseudohalides. Cleaner preparations [4] begin with solutions of peracetic acid in glacial acetic acid, also due to Willgerodt: [5]

C6H5I + CH3C(O)OOH + CH3COOH → C6H5I(OC(O)CH3)2 + H2O

The iodobenzene diacetate product hydrolyzes to the polymeric iodosobenzene (PhIO), which is stable in cool alkaline solution. [6] In hot water (or, in Willgerodt's original preparation, steam distillation), iodosobenzene instead disproportionates to iodoxybenzene and iodobenzene: [7]

2 PhIO → PhIO2 + PhI

2-Iodobenzoic acid reacts with oxone [8] or a combination of potassium bromate and sulfuric acid to produce the insoluble λ5‑iodane 2-iodoxybenzoic (IBX) acid. [9] IBX acid is unstable and explosive, but acetylation tempers it to the stabler Dess-Martin periodinane. [10]

Aliphatic hypoiodites can be synthesized through a variant on the Williamson ether synthesis: an alkoxide reacts with iodine monochloride, releasing the alkyl hypoiodite and chloride. [11] Alternatively, the Meyer-Hartmann reaction applies: a silver alkoxide reacts with elemental iodine to give the hypoiodite and silver iodide. They are unstable to visible light, cleaving into alkoxyl and iodine 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, although aryl 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

Diaryliodonium salts are compounds of the type [Ar−I+−Ar]X−. [14] They are formally composed of a diaryliodonium cation [15] paired with a counteranion, 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 of halogen bonding, [16] but the interaction exists even with traditionally noncoordinating ions, such as perchlorate, triflate, or tetrafluoroborate. [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 is pseudotrigonal bipyramidal. The placement of ligands exhibits apicophilicity: the phenyl group and chlorine group attain apical positions, while the other phenyl group and a lone pair of electrons hold equatorial ones.

Salts with a halide counterion are poorly soluble in many organic solvents, possibly because the halides bridge dimers. Solubility improves with triflate and tetrafluoroborate counterions. [17]

In general, the salts can be prepared from preformed hypervalent iodines such as iodic acid, iodosyl sulfate or iodosyl triflate. The first such compound was synthesised in 1894, via the silver hydroxide-catalyzed coupling of two aryl iodides (the Meyer–Hartmann reaction): [19] [20] [21]

Meyer-Hartmann-Reaktion Uebersichtsreaktion V1.svg

Alternatively, the iodane may be formed in situ: an aryl iodide is oxidized to an aryliodine(III) compound (such as ArIO), followed by a ligand exchange. The latter can occur with organometallized arenes such as an arylstannane or -silane (a nucleophilic aromatic substitution reaction) or unfunctionalized arenes in the presence of a Brønsted or Lewis acid (an electrophilic aromatic substitution reaction).

Diaryliodonium salts react with nucleophiles at iodine, replacing one ligand to form the substituted arene ArNu and iodobenzene ArI. Diaryliodonium salts also react with metals M through ArMX intermediates in cross-coupling reactions.

Uses

Sacrificial catalyst mCPBA oxidizes an aryliodide reagent to iodine(III) intermediate A. A in turn converts the hydroxylamine group to a nitrenium ion, B. The nitrenium is performs electrophilic ipso addition to the aromatic ring, forming an enonic lactam. HypervalentIodineCNbondFormation.png
Sacrificial catalyst mCPBA oxidizes an aryliodide reagent to iodine(III) intermediate A. A in turn converts the hydroxylamine group to a nitrenium ion, B. The nitrenium is performs electrophilic ipso addition to the aromatic ring, forming an enonic lactam.

Hypervalent iodine compounds are predominantly used as oxidizing reagents, although they are specialized and expensive. In some cases they replace more toxic oxidants. [23]

Iodobenzene diacetate (PhIAc2) and iodobenzene di(trifluoroacetate) are both strong oxidizing agents used in organic oxidations, as well as precursors for further organoiodine compounds. A hypervalent iodine (III) reagent was used as oxidant, together with ammonium acetate as nitrogen source, to provide 2-Furonitrile, a pharmaceutical intermediate and potential artificial sweetener. [24]

Current research focuses on the use of iodanes in carbon-carbon and carbon-heteroatom bond-forming reactions. In one study, an intramolecular C-N coupling of an alkoxyhydroxylamine to its anisole group is accomplished with a catalytic amount of aryliodide in trifluoroethanol. [25]

See also

Related Research Articles

In organic chemistry, an aryl halide is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. Haloarenes are different from haloalkanes because they exhibit many differences in methods of preparation and properties. The most important members are the aryl chlorides, but the class of compounds is so broad that there are many derivatives and applications.

<span class="mw-page-title-main">Dess–Martin periodinane</span> Chemical reagent

Dess–Martin periodinane (DMP) is a chemical reagent used in the Dess–Martin oxidation, oxidizing primary alcohols to aldehydes and secondary alcohols to ketones. This periodinane has several advantages over chromium- and DMSO-based oxidants that include milder conditions, shorter reaction times, higher yields, simplified workups, high chemoselectivity, tolerance of sensitive functional groups, and a long shelf life. However, use on an industrial scale is made difficult by its cost and its potentially explosive nature. It is named after the American chemists Daniel Benjamin Dess and James Cullen Martin who developed the reagent in 1983. It is based on IBX, but due to the acetate groups attached to the central iodine atom, DMP is much more reactive than IBX and is much more soluble in organic solvents.

<span class="mw-page-title-main">Periodinane</span>

Periodinanes also known as λ5-iodanes are organoiodine compounds with iodine in the +5 oxidation state. These compounds are described as hypervalent because the iodine center has more than 8 valence electrons.

The Ullmann condensation or Ullmann-type reaction is the copper-promoted conversion of aryl halides to aryl ethers, aryl thioethers, aryl nitriles, and aryl amines. These reactions are examples of cross-coupling reactions.

<span class="mw-page-title-main">Lithium iodide</span> Chemical compound

Lithium iodide, or LiI, is a compound of lithium and iodine. When exposed to air, it becomes yellow in color, due to the oxidation of iodide to iodine. It crystallizes in the NaCl motif. It can participate in various hydrates.

<span class="mw-page-title-main">2-Iodoxybenzoic acid</span> Chemical compound

2-Iodoxybenzoic acid (IBX) is an organic compound used in organic synthesis as an oxidizing agent. This periodinane is especially suited to oxidize alcohols to aldehydes. IBX is most often prepared from 2-iodobenzoic acid and a strong oxidant such as potassium bromate and sulfuric acid, or more commonly, oxone. One of the main drawbacks of IBX is its limited solubility; IBX is insoluble in many common organic solvents. IBX is an impact- and heat-sensitive explosive (>200°C). Commercial IBX is stabilized by carboxylic acids such as benzoic acid and isophthalic acid.

<span class="mw-page-title-main">(Bis(trifluoroacetoxy)iodo)benzene</span> Chemical compound

(Bis iodo)benzene, C
6H
5I(OCOCF
3)
2, is a hypervalent iodine compound used as a reagent in organic chemistry. It can be used to carry out the Hofmann rearrangement under acidic conditions.

Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective insecticides, although some are extremely toxic to humans, including sarin and VX nerve agents.

<span class="mw-page-title-main">Tetrafluoroborate</span> Anion

Tetrafluoroborate is the anion BF−
4. This tetrahedral species is isoelectronic with tetrafluoroberyllate (BeF2−
4), tetrafluoromethane (CF4), and tetrafluoroammonium (NF+
4) and is valence isoelectronic with many stable and important species including the perchlorate anion, ClO−
4, which is used in similar ways in the laboratory. It arises by the reaction of fluoride salts with the Lewis acid BF3, treatment of tetrafluoroboric acid with base, or by treatment of boric acid with hydrofluoric acid.

Iodine compounds are compounds containing the element iodine. Iodine can form compounds using multiple oxidation states. Iodine is quite reactive, but it is much less reactive than the other halogens. For example, while chlorine gas will halogenate carbon monoxide, nitric oxide, and sulfur dioxide, iodine will not do so. Furthermore, iodination of metals tends to result in lower oxidation states than chlorination or bromination; for example, rhenium metal reacts with chlorine to form rhenium hexachloride, but with bromine it forms only rhenium pentabromide and iodine can achieve only rhenium tetraiodide. By the same token, however, since iodine has the lowest ionisation energy among the halogens and is the most easily oxidised of them, it has a more significant cationic chemistry and its higher oxidation states are rather more stable than those of bromine and chlorine, for example in iodine heptafluoride.

<span class="mw-page-title-main">Iodosobenzene</span> Chemical compound

Iodosobenzene or iodosylbenzene is an organoiodine compound with the empirical formula C6H5IO. This colourless solid compound is used as an oxo transfer reagent in research laboratories examining organic and coordination chemistry.

<span class="mw-page-title-main">Organocopper chemistry</span> Compound with carbon to copper bonds

Organocopper chemistry is the study of the physical properties, reactions, and synthesis of organocopper compounds, which are organometallic compounds containing a carbon to copper chemical bond. They are reagents in organic chemistry.

Organoiodine chemistry is the study of the synthesis and properties of organoiodine compounds, or organoiodides, organic compounds that contain one or more carbon–iodine bonds. They occur widely in organic chemistry, but are relatively rare in nature. The thyroxine hormones are organoiodine compounds that are required for health and the reason for government-mandated iodization of salt.

<span class="mw-page-title-main">Organobismuth chemistry</span>

Organobismuth chemistry is the chemistry of organometallic compounds containing a carbon to bismuth chemical bond. Applications are few. The main bismuth oxidation states are Bi(III) and Bi(V) as in all higher group 15 elements. The energy of a bond to carbon in this group decreases in the order P > As > Sb > Bi. The first reported use of bismuth in organic chemistry was in oxidation of alcohols by Frederick Challenger in 1934 (using Ph3Bi(OH)2). Knowledge about methylated species of bismuth in environmental and biological media is limited.

Alcohol oxidation is a collection of oxidation reactions in organic chemistry that convert alcohols to aldehydes, ketones, carboxylic acids, and esters where the carbon carries a higher oxidation state. The reaction mainly applies to primary and secondary alcohols. Secondary alcohols form ketones, while primary alcohols form aldehydes or carboxylic acids.

Iodobenzene dichloride (PhICl2) is a complex of iodobenzene with chlorine. As a reagent for organic chemistry, it is used as an oxidant and chlorinating agent.

Carbonyl oxidation with hypervalent iodine reagents involves the functionalization of the α position of carbonyl compounds through the intermediacy of a hypervalent iodine(III) enolate species. This electrophilic intermediate may be attacked by a variety of nucleophiles or undergo rearrangement or elimination.

Phenol oxidation with hypervalent iodine reagents leads to the formation of quinone-type products or iodonium ylides, depending on the structure of the phenol. Trapping of either product is possible with a suitable reagent, and this method is often employed in tandem with a second process.

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

<span class="mw-page-title-main">(Diacetoxyiodo)benzene</span> Chemical compound

(Diacetoxyiodo)benzene, also known as phenyliodine(III) diacetate (PIDA) is a hypervalent iodine chemical with the formula C
6H
5I(OCOCH
3)
2. It is used as an oxidizing agent in organic chemistry.

References

  1. ↑ (Anastasios), Varvoglis, A. (1997). Hypervalent iodine in organic synthesis. London: Academic Press. ISBN   9780127149752. OCLC   162128812.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. ↑ However, iodanes usually feature bonds to carbon in its sp2- or sp-hybridized state. The hybridization-specific electronegativities of sp2 and sp carbon are estimated to be 3.0 and 3.3, respectively (Anslyn and Dougherty, Modern Physical Organic Chemistry, University Science Books, 2004).
  3. ↑ C. Willgerodt, Tageblatt der 58. Vers. deutscher Naturforscher u. Aertzte, Strassburg 1885.
  4. ↑ J. G. Sharefkin and H. Saltzman. "Benzene, iodoso-, diacetate". Organic Syntheses ; Collected Volumes, vol. 5, p. 660.
  5. ↑ Willgerodt, C. (1892). "Zur Kenntniss aromatischer Jodidchloride, des Jodoso- und Jodobenzols". Chem. Ber. (in German). 25 (2): 3494–3502. doi:10.1002/cber.189202502221.
  6. ↑ H. Saltzman and J. G. Sharefkin. "Benzene, iodoso-". Organic Syntheses ; Collected Volumes, vol. 5, p. 658.
  7. ↑ J. G. Sharefkin and H. Saltzman. "Benzene, iodoxy-". Organic Syntheses ; Collected Volumes, vol. 5, p. 665.
  8. ↑ Frigerio, M.; Santagostino, M.; Sputore, S. (1999). "A User-Friendly Entry to 2-Iodoxybenzoic Acid (IBX)". J. Org. Chem. 64 (12): 4537–4538. doi:10.1021/jo9824596.
  9. ↑ Robert K. Boeckman, Jr., Pengcheng Shao, and Joseph J. Mullins. "1,2-Benziodoxol-3(1H)-one, 1,1,1-tris(acetyloxy)-1,1-dihydro-". Organic Syntheses {{cite journal}}: CS1 maint: multiple names: authors list (link); Collected Volumes, vol. 10, p. 696.
  10. ↑ Dess, D. B.; Martin, J. C. (1983). "Readily accessible 12-I-5 oxidant for the conversion of primary and secondary alcohols to aldehydes and ketones". J. Org. Chem. 48 (22): 4155–4156. doi:10.1021/jo00170a070.
  11. ↑ Glover, Stephen A.; Goosen, André (January 1980). "Synthesis of β=iodo--butyl and methyl ethers from the reaction of alkenes with -butyl and methyl hypoiodites". Tetrahedron Letters. 21 (20): 2005–2008. doi:10.1016/S0040-4039(00)93669-4.
  12. ↑ Beebe, Thomas R.; Barnes, Beverly A.; Bender, Keith A.; Halbert, Allan D.; Miller, Robert D.; Ramsay, Martin L.; Ridenour, Michael W. (June 1975). "Oxidation of alcohols with acetyl hypoiodite". The Journal of Organic Chemistry. 40 (13): 1992–1994. doi:10.1021/jo00901a028. ISSN   0022-3263.
  13. ↑ Luliński, Piotr; Sosnowski, Maciej; Skulski, Lech; Luliński, Piotr; Sosnowski, Maciej; Skulski, Lech (2005-05-13). "A Novel Aromatic Iodination Method, with Sodium Periodate Used as the Only Iodinating Reagent". Molecules. 10 (3): 516–520. doi: 10.3390/10030516 . PMC   6147649 . PMID   18007324.
  14. ↑ Merritt, Eleanor A.; Olofsson, Berit (2009). "Diaryliodonium Salts: A Journey from Obscurity to Fame". Angew. Chem. Int. Ed. 48 (48): 9052–9070. doi:10.1002/anie.200904689. PMID   19876992.
  15. ↑ Note that in the diaryliodonium salt description, the compound is not hypervalent, and the bonding number is the standard one for iodine (λ1). It is a 8-I-2 species. In the other common description of these compounds as covalent iodanes, they are formally 10-I-3 and λ3.
  16. ↑ Resnati, G.; Ursini, M.; Pilati, T.; Politzer, P.; Murray, J. S.; Cavallo, G. (2017-07-01). "Halogen bonding in hypervalent iodine and bromine derivatives: halonium salts". IUCrJ. 4 (4): 411–419. doi:10.1107/S2052252517004262. ISSN   2052-2525. PMC   5571804 . PMID   28875028.
  17. 1 2 Neckers, Douglas C.; Pinkerton, A. Alan; Gu, Haiyan; Kaafarani, Bilal R. (2002-05-28). "The crystal and molecular structures of 1-naphthylphenyliodonium tetrafluoroborate and 1-naphthylphenyliodonium tetrakis(pentafluorophenyl)gallate". Journal of the Chemical Society, Dalton Transactions (11): 2318–2321. doi:10.1039/B202805K. ISSN   1364-5447.
  18. 1 2 "Iodonium salts in organic synthesis". www.arkat-usa.org. Retrieved 2018-12-30.
  19. ↑ Hartmann, Christoph; Meyer, Victor (1894). "Ueber die Jodoniumbasen". Berichte der Deutschen Chemischen Gesellschaft (in German). 27 (1): 502–509. doi:10.1002/cber.18940270199.
  20. ↑ Bothner-By, Aksel A.; Vaughan, C. Wheaton Jr. (1952). "The Gross Mechanism of the Victor Meyer and Hartmann Reaction". J. Am. Chem. Soc. 74 (17): 4400–4401. doi:10.1021/ja01137a048.
  21. ↑ Wang, Zerong (2010). "Meyer–Hartmann Reaction". Comprehensive Organic Name Reactions and Reagents. John Wiley & Sons, Inc. pp. 1910–1912. doi:10.1002/9780470638859.conrr429. ISBN   9780470638859.
  22. ↑ Dohi, T.; Maruyama, A.; Minamitsuji, Y.; Takenaga, N.; Kita, Y. (2007). "First hypervalent iodine(III)-catalyzed C-N bond forming reaction: catalytic spirocyclization of amides to N-fused spirolactams". Chemical Communications . 44 (12): 1224–1226. doi:10.1039/b616510a. PMID   17356763.
  23. ↑ Hypervalent iodine(V) reagents in organic synthesis Uladzimir Ladziata and Viktor V. Zhdankin Arkivoc 05-1784CR pp 26-58 2006 Article
  24. ↑ Chenjie Zhu; Sun, Chengguo; Wei, Yunyang (2010). "Direct oxidative conversion of alcohols, aldehydes and amines into nitriles using hypervalent iodine(III) reagent". Synthesis. 2010 (24): 4235–4241. doi:10.1055/s-0030-1258281.
  25. ↑ Dohi, T.; Maruyama, A.; Minamitsuji, Y.; Takenaga, N.; Kita, Y. (2007). "First hypervalent iodine(III)-catalyzed C-N bond forming reaction: catalytic spirocyclization of amides to N-fused spirolactams". Chemical Communications . 44 (12): 1224–1226. doi:10.1039/b616510a. PMID   17356763.
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