Lemieux–Johnson oxidation

Lemieux–Johnson Oxidation is a chemical reaction that involves the oxidative cleavage of olefins to produce carbonyl compounds using osmium tetroxide (OsO4) and sodium periodate (NaIO4) as reagents. This reaction is named after Raymond Urgel Lemieux and W. S. Johnson, who independently developed the method. The Lemieux–Johnson oxidation is particularly useful in organic synthesis for the preparation of aldehydes, ketones, and carboxylic acids from alkenes.

Mechanism

The mechanism of the Lemieux–Johnson oxidation involves two key steps. First, the alkene undergoes cis-dihydroxylation by osmium tetroxide, forming a cyclic osmate ester intermediate. This step is followed by the cleavage of the osmate ester by sodium periodate, resulting in the formation of two carbonyl compounds and regenerating the osmium tetroxide catalyst.

1. Cis-Dihydroxylation: The alkene reacts with osmium tetroxide to form a cyclic osmate ester. This step introduces two hydroxyl groups across the double bond in a cis configuration.

2. Oxidative Cleavage: Sodium periodate cleaves the cyclic osmate ester, leading to the formation of two carbonyl compounds. The osmium tetroxide is regenerated in this step and can be reused in subsequent reactions.

Applications

The Lemieux–Johnson oxidation is widely used in organic chemistry for the synthesis of complex molecules. It is particularly valuable in the synthesis of natural products and pharmaceuticals, where the selective oxidation of alkenes to carbonyl compounds is required. The reaction's ability to proceed under mild conditions without the need for harsh oxidizing agents makes it a preferred method for the oxidation of sensitive molecules.

Advantages

• Selectivity: The reaction provides a high degree of selectivity for the cis-dihydroxylation of alkenes.
• Mild Conditions: The reaction conditions are mild, reducing the risk of decomposing sensitive substrates.
• Catalytic Cycle: The osmium tetroxide is regenerated, making the process more economical and environmentally friendly.

Limitations

• Cost: Osmium tetroxide is expensive and toxic, which may limit its use on a large scale.
• Toxicity: The toxicity of osmium tetroxide requires careful handling and appropriate safety measures.

Safety

Due to the toxic nature of osmium tetroxide, it is imperative to conduct the Lemieux–Johnson oxidation in a well-ventilated area or under a fume hood. Personal protective equipment, such as gloves and safety glasses, should be worn to minimize exposure.

See Also

• Oxidation reactions
• Organic synthesis
• Osmium tetroxide
• Sodium periodate