Woodward Reaction: Mechanism & Hydroxylation

Woodward reaction is an addition reaction that involves the synthesis of syn-diols or cis-diols from alkenes. In the presence of water, an alkene treated with iodine and silver acetate will undergo nucleophilic displacement with acetate. 

• Woodward reaction is also known as Woodward cis-hydroxylation.
• The reaction of an anion with a double bond can be compared to an SN₂ displacement.
• It involves the displacement of two double bonds by a backside attack with stereochemistry, which is difficult in some situations.
• On hydrolysis, the intermediate product yields the required diols. 
• Robert Burns Woodward, an American organic scientist, was the inspiration for Woodward's reaction.
• The reaction is used in the synthesis of steroids.

Key Terms: Woodward Reaction, Iodine, Silver Acetate, Hydrolysis, Diols, Hydroxylation, Alkenes, Oxygen, Glycol, Hydroxy Group, SN₂ Displacement, Acetic Acid

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What is Woodward Reaction?

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Woodward reaction or Woodward cis-hydroxylation involves the treatment of olefins with iodine and silver acetate in wet acetic acid. The reaction will form syn-diols or cis-diols.

• In 1954, Woodward was the first to report on this reaction. 
• The reaction modifies the prevost reaction to form cis-glycols.
• As a result, it's known as the Woodward cis-hydroxylation.
• It is also known as Woodward oxidation.

This reaction is commonly done in two steps: first, the olefin is treated with iodine and silver acetate in dry acetic acid at room temperature, then water is added, and the mixture is heated to refluxing temperature. 

• Unlike the trans-hydroxylation in the Prevost Reaction, the Woodward reaction is always used to produce cis-glycols. 
• Furthermore, cyclic olefins frequently provide the dominant endo glycols.
• In this, acetate will undergo nucleophilic displacement in the presence of water.
• Although the Sharpless Dihydroxylation also produces cis-glycols.
• It should be noted that the Woodward cis hydroxylation produces the cis-glycol stereoisomer.
• The compound is the polar opposite of that produced by osmium tetroxide hydroxylation.

Woodward Reaction 

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Woodward Reaction Mechanism

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Woodward Reaction Mechanism facilitates the formation of syn-diols from alkenes. The mechanism involves the addition of iodine, which is followed by a nucleophilic displacement reaction with acetate in water. 

The steps involved in the Woodward Reaction Mechanism are as follows:

• Firstly, the addition of iodine produces a cyclic iodonium ion as an intermediate product.
• The reaction is carried out by nucleophilic substitution by acetate anion, similar to the Prévost Reaction:

Formation of Cyclic acetoxonium ion

• After that, a cyclic acetoxonium ion is formed which is shown as follows:

Cyclic acetoxonium ion

• The cyclic intermediate will undergo an SN₂ displacement reaction with acetate ion to form a five-membered ring compound.
• The next step involves the hydrolysis of a five-membered ring compound.
• Water appears to add readily as a nucleophile to the partly positive carbon atom of the intermediate.
• After that, protonation of the cyclic orthoacetate is done to yield a monoacylated diol.

Diols 

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Hydroxylation of Alkenes

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A hydroxy group is added to each end of the double bond to convert an alkene to a glycol. The double bond is dihydroxylated (or hydroxylated) as a result of this addition. 

• The oxidation of an organic substance is called hydroxylation of alkenes. 
• The production of -OH groups occurs when oxygen is introduced to the C-H group.
• It forms a bond, resulting in COH. 
• The process is initiated when oxygen reacts slowly, and catalysts or enzymes, also known as Hydroxylases, are required.

Hydroxylation of Alkenes 

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Different Methods of Hydroxylation

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The most common procedures for cis-hydroxylation are reactions with potassium permanganate, osmium tetroxide alone or as a catalyst, or the Woodward approach with silver iodoacetate. 

• The reaction with peracids is undoubtedly the most fundamental technique of trans-hydroxylation. 
• The Prevost reaction, as well as oxidation with hydrogen peroxide in an alkaline solution, is one such method.
• Another method involves the presence of particular oxide catalysts, which are helpful methods. 

The following are several methods for hydroxylation of alkenes.

• Potassium permanganate is used for hydroxylation.
• Using osmium tetroxide for hydroxylation.
• Using ester complexes for non-catalytic cis-hydroxylation of olefins.
• Hydrogen peroxide catalytic cis-hydroxylation of olefins.
• Metal chlorates catalyzed cis-hydroxylation of olefins.
• Sodium hypochlorite catalytic cis-hydroxylation of olefins.
• Cis-Hydroxylation of olefins with tert-butyl hydroperoxide as a catalyst.
• N-methyl morpholine N-oxide catalytic cis-hydroxylation of olefins.
• Cis-Hydroxylation of olefins by catalytic cis-hydroxylation with oxygen or Air.
• Organic peroxy acids are used for hydroxylation.
• Using hydrogen peroxide for hydroxylation
• Using halogens and silver carboxylates for hydration.

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Application of Woodward Reaction

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The application of woodward reaction are as follows:

• Woodward reaction is used in organic chemistry and steroid chemistry.
• The reaction is utilized to make several kinds of alkenes.
• It is utilized to make olefinic compounds with a lengthy chain.
• Woodward reaction is used for the production of long-chain olefinic chemicals.
• It is used for synthesis of natural compounds, colors, medications, and other things.

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Things to Remember

• Woodward reaction is a modified version of the Prevost hydroxylation process.
• In the presence of water, treatment of alkene with iodine and silver acetate to yields the required diols.
• Woodward reagent is an equimolar solution of iodine and silver acetate or silver benzoate in wet acetic acid.
• In moist conditions, the acetoxonium ion retains water and reacts to form a cis-hydroxyacetate.
• Woodward reaction involves reaction of olefin with iodine to produce an iodonium ion.
• The ion are later displaced by acetate ion to produce trans-iodoacetate.
• In moist conditions, the acetoxonium ion retains water and reacts to form a cis-hydroxyacetate, which when saponified yields Cis-diols.

Read More: Relation between Molarity and Molality