2-Iodoxybenzoic acid

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ImageFileL1 = 2-iodoxybenzoic-acid-3D-balls.png ImageFileR1 = 2-iodoxybenzoic-acid-3D-vdW.png IUPACName =
OtherNames = IBX
Section1 = Chembox Identifiers
CASNo = 61717-82-6
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Section2 = Chembox Properties
Formula = C₇H₅IO₄
MolarMass = 280.02 g/mol
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Section7 = Chembox Hazards
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RPhrases = R22 R34 R44

IBX acid or 2-Iodoxybenzoic acid is an organic compound used in organic synthesis as an oxidizing agent. This Periodinane is especially suited to oxidize alcohols to aldehydes. The IBX acid is prepared from 2-iodobenzoic acid, potassium bromate and sulfuric acid. [cite journal | title = Dess-Martin periodinane: 1,1,1-Triacetoxy-1,1-dihydro-1,2-benziodoxol-3(1"H")-one [1,2-Benziodoxol-3(1"H")-one, 1,1,1-tris(acetyloxy)-1,1-dihydro-] | author = Robert K. Boeckman, Jr., Pengcheng Shao, and Joseph J. Mullins | journal = Organic Syntheses | volume = 77 | pages = 141 | date = 2000 | url = http://www.orgsyn.org/orgsyn/prep.asp?prep=v77p0141 "(also in the [http://www.orgsyn.org/orgsyn/pdfs/v77p0141.pdf Collective Volume (2004) 10:696] (PDF))."] Frigerio and co-workers have also demonstrated, in 1999 that potassium bromate may be replaced by commercially available Oxone. One of the main drawbacks of IBX is its limited solubility; IBX is insoluble in many common organic solvents. In the past, it was believed that IBX was shock sensitive, but it was later determined that samples of IBX were shock sensitive due to the residual potassium bromate left from its preparation.Fact|date=February 2007 Commercial IBX is stabilized by carboxylic acids such as benzoic acid and isophthalic acid.

Reaction mechanism

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right|frame|Hypervalent_twisting_mechanism|The_hypervalent twisting mechanism during conversion of methanol to formaldehyde: a) ligand exchange reaction (activation energy 9.1 kcal/mol (38 kJ/mol), b) hypervalent twist 12.1 kcal/mol (51 kJ/mol), c) elimination 4.7 kcal/mol (20 kJ/mol)). There is steric repulsion between protons in red.] The reaction mechanism for an oxidation of an alcohol to an aldehyde according the so-called hypervalent twisting mechanism [cite journal | title = Enhancing 2-Iodoxybenzoic Acid Reactivity by Exploiting a Hypervalent Twist | author = Julius T. Su and William A. Goddard III | journal = J. Am. Chem. Soc. | date = 2005 | volume = 127 | issue = 41 | pages = 14146–14147 | doi = 10.1021/ja054446x | url = http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2005/127/i41/abs/ja054446x.html | format = abstract ] involves a ligand exchange reaction replacing the hydroxyl group by the alcohol followed by a twist and a elimination reaction. The twist is a requirement because the iodine to oxygen double bond is oriented out of plane with the alkoxy group and the concerted elimination would not be able to take place. This twist reaction is a rearrangement in which the oxygen atom is moved into a proper plane for a 5 membered cyclic transition state in the elimination reaction and is calculated by Computational chemistry to be the rate-determining step in the oxidation. The twist mechanism also explains why oxidation is faster for larger alcohols than for small alcohols. The twist is driven forward by the steric hindrance that exists between the ortho hydrogen atom and the protons from the alkoxy group and larger alkoxy groups create larger steric repulsion. The same computation predicts a much faster reacting IBX derivative with a 100 fold reaction rate when this ortho hydrogen atom is replaced by a methyl group thus facilitating the twist until the elimination reaction takes prevalence as the rate determining step.

IBX exists as two tautomers one of which is the carboxylic acid. The acidity of IBX which has been determined in water (pKa 2.4) and DMSO (pKa 6.65)cite journal | title = o-Iodoxybenzoic Acid (IBX): pK_a and Proton-Affinity Analysis | author = Michael J. Gallen, Régis Goumont, Timothy Clark, François Terrier, Craig M. Williams | journal = Angewandte Chemie International Edition | volume = 45 | issue = 18 | pages = 2929–2934 | date = 2006 | doi = 10.1002/anie.200504156] is known to affect organic reactions, for instance acid-catalyzed isomerization accompanying oxidations.

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IBX is also available as silica gel or polystyrene bound IBX. In many application IBX acid is replaced by Dess-Martin periodinane which is more soluble in common organic solvents. A sample reaction is a IBX oxidation used in the total synthesis of eicosanoid: [cite journal | title = Asymmetric total synthesis of eicosanoid | author = Debendra K. Mohapatra and Gorakhanath S. Yellol | journal = Arkivoc | url = http://www.arkat-usa.org/home.aspx?VIEW=MANUSCRIPT&MSID=1210 | volume = 2005 | issue = iii ]

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alcohol to aldehyde key data: a) IBX, DMSO, THF, 4h, 94% chemical yield (Mohapatra, 2005)] .

In 2001, K.C. Nicolaou and co-workers published a series of papers in the Journal of the American Chemical Society demonstrating, among other transformations, the use of IBX to oxidize benzylic carbons to conjugated aromatic carbonyl compounds.

Oxidative cleavage

The use of IBX in combination with dimethyl sulfoxide was demonstrated in oxidative cleavage of vicinal diols to ketones: [cite journal |title=Oxidative cleavage of vicinal diols: IBX can do what Dess–Martin periodinane (DMP) can't |journal=Org. Biomol. Chem. |year=2007 |volume=5 |pages=767–771 |doi=10.1039/b618135j |author=Jarugu Narasimha Moorthy, Nidhi Singhal and Kalyan Senapati]

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The reaction mechanism for this glycol cleavage is based on initial formation of an adduct between 10-I-4 IBX and DMSO to an 12-I-5 intermediate 3 in which DMSO acts as a leaving group for incoming alcohol 4 to intermediate 5. One equivalent of water is split off forming 12-I-5 spirobicyclic periodinane 6 setting the stage for fragmentation to 7. With hydroxyl alpha protons presents oxidation to the acyloin competes. Trifluoroacetic acid is found to facilitate the overall reaction.

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References