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Prevost Method (Anti-Dihydroxylation)
Prevost Method (Anti-Dihydroxylation)
The Prevost reaction involves the conversion of an alkene into a vicinal anti-diol using iodine and a silver carboxylate (commonly silver benzoate).
The Prevost reaction involves the conversion of an alkene into a vicinal anti-diol using iodine and a silver carboxylate (commonly silver benzoate).
Reaction Conditions: Reagents: Iodine (I₂) and silver benzoate (AgO₂CPh) 1:2 Molar Ratio; Solvent: Non-aqueous medium (like dry benzene) Note: high yield with thallium(I) acetate and thallium(I )benzoate instead of the silver carboxylates.
Reaction Conditions: Reagents: Iodine (I₂) and silver benzoate (AgO₂CPh) 1:2 Molar Ratio; Solvent: Non-aqueous medium (like dry benzene) Note: high yield with thallium(I) acetate and thallium(I )benzoate instead of the silver carboxylates.
Mechanism:
Mechanism:
1. Formation of Cyclic Iodonium Ion: The alkene reacts with iodine, forming a cyclic iodonium ion intermediate.
1. Formation of Cyclic Iodonium Ion: The alkene reacts with iodine, forming a cyclic iodonium ion intermediate.
2. Nucleophilic Attack: The benzoate ion (from silver benzoate) attacks the more substituted carbon of the iodonium ion, leading to anti-opening due to the backside attack mechanism.
2. Nucleophilic Attack: The benzoate ion (from silver benzoate) attacks the more substituted carbon of the iodonium ion, leading to anti-opening due to the backside attack mechanism.
3. Neighboring Group Participation: The intermediate undergoes neighbouring group participation by the benzoate group, forming an iodonium intermediate.
3. Neighboring Group Participation: The intermediate undergoes neighbouring group participation by the benzoate group, forming an iodonium intermediate.
4. Second Nucleophilic Attack: A second benzoate ion attacks from the opposite side, forming trans-dibenzoate ester.
4. Second Nucleophilic Attack: A second benzoate ion attacks from the opposite side, forming trans-dibenzoate ester.
5. Hydrolysis: Acidic hydrolysis of the ester yields an anti-vicinal diol.
5. Hydrolysis: Acidic hydrolysis of the ester yields an anti-vicinal diol.
Key Points:
Key Points:
1. Produces anti-diols due to the trans addition of hydroxyl groups.
1. Produces anti-diols due to the trans addition of hydroxyl groups.
2. Involves neighbouring group participation by the benzoate ion.
2. Involves neighbouring group participation by the benzoate ion.
3. The reaction avoids water, maintaining anhydrous conditions to control stereochemistry.
3. The reaction avoids water, maintaining anhydrous conditions to control stereochemistry.
Woodward Modification (Syn-Dihydroxylation)
Woodward Modification (Syn-Dihydroxylation)
The Woodward modification is a variation of the Prevost method that uses the same reagents but in the presence of water to yield syn-diols.
The Woodward modification is a variation of the Prevost method that uses the same reagents but in the presence of water to yield syn-diols.
Reaction Conditions: Reagents: Iodine (I₂), silver benzoate (AgO₂CPh) 1:1 Molar Ratio, and water.
Reaction Conditions: Reagents: Iodine (I₂), silver benzoate (AgO₂CPh) 1:1 Molar Ratio, and water.
Note: high yield with thallium(I) acetate and thallium(I )benzoate instead of the silver carboxylates.
Note: high yield with thallium(I) acetate and thallium(I )benzoate instead of the silver carboxylates.
Mechanism:
Mechanism:
1. Formation of Cyclic Iodonium Ion: Similar to the Prevost method, the alkene first forms a cyclic iodonium ion with iodine.
1. Formation of Cyclic Iodonium Ion: Similar to the Prevost method, the alkene first forms a cyclic iodonium ion with iodine.
2. Nucleophilic Attack: The benzoate ion attacks, opening the iodonium ring.
2. Nucleophilic Attack: The benzoate ion attacks, opening the iodonium ring.
3. Water Attack: The intermediate is then attacked by water, leading to the syn-addition of hydroxyl groups.
3. Water Attack: The intermediate is then attacked by water, leading to the syn-addition of hydroxyl groups.
4. Hydrolysis: Finally, basic hydrolysis yields the syn-vicinal diol.
4. Hydrolysis: Finally, basic hydrolysis yields the syn-vicinal diol.
Key Points:
Key Points:
1. Produces syn-diols due to the participation of water in the reaction mechanism.
1. Produces syn-diols due to the participation of water in the reaction mechanism.
2. The presence of water alters the pathway.
2. The presence of water alters the pathway.
3. The stereochemistry is controlled by the attack of water, leading to cis-product formation.
3. The stereochemistry is controlled by the attack of water, leading to cis-product formation.
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