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Hypervalent Iodine Reagents
• Iodine most commonly found in monovalent oxidation state (-1) in many organic compounds because it is most
polarizable and electropositive element of the 17th group of the periodic table.
• It also forms stable poly-coordinate & multivalent compounds as listed below scheme.
• The first multivalent organic compound dichloroiodo benzene i.e. phICl2 was prepared by German chemist C. Willgerodt in
1886.
• Although its oxidizing properties were known but past 20 years polyvalent iodine has been used in organic synthesis as
good oxidizing reagents.
• Hypervalent iodine is a main group element that breaks the octet rule & has more than 8 electrons in its valence shell.
• Iodine most probable oxidation states are asiodine (I), Iodine (III), Iodine (V) & Iodine (VII).
• There are many hypervalent iodine reagent has been discovered but most frequently used are as follows.
1. 2-Idoxybezoic acid.(IBX)
2. Dess-Martin Reagent.
3. Difluroipdpbenzene.
4. Dichloroiodobenzene
5. Diacetoxyiodobenzene.
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In this presentation we shall discuss about about 2-idoxybenzoic acid(IBX).
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2-Idoxybenzoic Acid(IBX)
• IBX (2-Iodoxy benzoic acid) is an oxidizing reagent which specially oxidizes primary and secondary alcohols to
aldehyde and ketone respectively.
• It is prepared by the oxidation of 2-iodobenzoic acid by potassium bromated (KBrO3) or commercially available
oxone as reagent.
• IBX is crystalline but highly explosive when it is dry.
Preparation of IBX
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Mechanism of IBX
• Primary and Secondary alcohol on reacting with IBX converts to aldehyde and ketone respectively.
• Initially H from IBX and OH alcohol are eliminated as water.
• An I-O bond is formed between Oxygen from alcohol and iodine.
• This forms an unstable intermediate.
• This intermediate rearranges itself where O transfer its electrons to iodine which open pi bond on double
bonded O.
• Because of this O gets negative charge meanwhile C-H bond opens heterolytically and H bonds with O and
• C-O forms double bong forming Ketone.
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Examples of IBX Oxidation
Oxidative Cleavage through IBX
• IBX is a good reagent for the oxidative cleavage of vicinal diols (glycols) to aldehyde or
ketone in the presence of DMSO as co-oxidant.
Mechanism
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• Since O is more electronegative than S ,S-O bond undergoes heterolytic cleavage.
• This O attacks on I forming O-I bond and S becomes positively charged.
• When this new O-I bond is formed one of the double bond of O-S breaks and O becomes negatively charged.
• Now the O from alcohol gives its electrons to I which causes O-I bond to break and DMSO is formed.
• During above step deprotonation of diol takes place and O-I bond is formed.
• Now the O from OH on IBX donated its lone pain to C on diol making C-O bond which causes OH on diol to leave and form
water.
• The intermediate then rearranges itself so that two aldehydes molecules are formed.
• Example
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Selective Oxidation of Benzylic carbon
• The IBX is presence DMSO acts as useful oxidizing reagent and it oxidizes benzylic carbon to carbonyl compounds as
below
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Introduction of unsaturation through IBX oxidation as shown in the following examples
• Due to the hazardous condition and week solubility in organic solvent IBX has limitations in organic synthesis.
• Hence the better option in organic synthesis is available asDMP oxidation.
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Swern Oxidation
• Swern oxidation is a modern method normally preferred in organic synthesis.
• Where in 10 and 20 alcohols oxidises to aldehydes and ketones respectively using oxalyl chloride, DMSO and triethylamine
as organic base at low temperature.
• The volatile by-products are dimethylsulfide (Me2S), carbon monoxide (CO) and carbon dioxide (CO2).
• Due to the unpleasant odors of Me2S and CO is acutely toxic the reaction workup needs to be performed in a fume hood.
• The over oxidation of aldehyde to carboxylic acid will not occurred hence it is a selective oxidizing reagent for primary
alcohols.
• The oxalyl chloride (COCl)2 is a good effective electrophile reacts with alcohol at low temperature -78oC.
• There are two parts of the mechanism in first converting DMSO to dimethylsulfonium chloride by treating with oxalyl
chloride and in second part dimethylsulfonium chloride reacting to the alcohol which undergo oxidation to have desired
product as below.
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• Initially O from DMSC will attack C of oxalyl chloride and remove one chlorine forming C-O bond.
• This Chlorine will then attack S from behind and cause homolytic breaking in O-S bond.
• The resultant moiety rearrange and give out CO2,CO and Cl.
• This will activate DMSO for next step.
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• Now the lone pair of oxygen from alcohol will attack on S and remove Cl.
• This Cl will then pull H from intermediate forming HCl.
• Now trimethylamine will reduce methyl group from DMSO.
• Then resultant molecule will rearrange itself to give ketone.
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Examples of Swern Oxidation
• The reaction temperature must be kept low to avoid side reactions because the oxalyl choride is a good dehydrating agent
but temperature colder that -60oC acts as a source of chloride in the reaction. In some reactions the use of triethylamine as
the base can lead to epimerization at the carbon alpha to the newly formed carbonyl group.
• To avoid these side reaction bulky bases such as Lithiumdiisopropylethylamine (LDA) has been used.
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Corey-Kim Oxidation
• Corey-Kim oxidation name after American chemist Nobel laureates Elias James Corey and Korean American chemist
Choung Un Kim.
• It is a good oxidizing reagent for oxidation of 1o and 2o alcohols to aldehydes and ketones respectively by reacting
dimethyl sulfide (DMS) with N-chlorosuccinimide(NCS) in presence of triethylamine as organic base.
• Corey-Kim oxidation reaction does require temperature above -25oC whereas Swern oxidation requires very low
temperature up -780C to avoid the by-products formation.
• When Me2S react with NCS results in the formation of an active DMSO species which further reacts with alcohol to give
the oxidation product by proton abstraction using organic base Et3N.
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Mechanism
• Initially NCS is attacked by DMSO forming S-Cl bond making S positive and giving negative charge to N.
• Now positively charge S once again attacks N eliminating Cl forming N-S bond which is the intermediate.
• After this reaction is similar to swern oxidation where O from alcohol attack on S of intermediate forming S-O
bond.
• Then the methyl group is deprotonated by trimethylamine which after that undergoes rearrangement giving
ketone.
