1.6.2 Lead Tetraacetate(Criegee Reagent)
M. L. Mihailovic, Z. Cekovic, Encylcopedia of Reagents for Organic Synthesis, John Wiley and Sons, Inc., L. A. Paquette, Ed., New York, 1995, 5, 2949. |
1.6.2.1 Introduction
Lead tetraacetate (LTA) is one of the powerful common oxidizing reagents available with wide applications for organic synthesis. The reagent is very toxic, hygroscopic and turns brown due to lead dioxide formation on exposure to air. Therefore, the reagent is to be handled with extreme care in a chemical hood. It can be prepared by adding red lead oxide, Pb3O4, to a mixture of acetic acid and acetic anhydride at ~70°C. Recrystallization of the crude LTA from AcOH can give colorless white crystals.
1.6.2.2 Reaction with Alcohols
LTA oxidizes alcohols to aldehydes and ketones in the presence of pyridine at ambient temperature. The reactions are efficient and over oxidation to carboxylic acids are not observed. For example, pentanol and cinnamyl alcohol can be oxidized to give aldehydes in the presence of pyridine with high yield (Scheme 4).
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Scheme 4 |
In the case of 1,2-diols, oxidative cleavage is observed to give aldehydes, ketones or both depending on the structure of the diols (Scheme 5). The reaction involves a cyclic intermediate and cis-1,2-diols exhibit greater reactivity compared to trans-1,2-diols. The reactions are performed in organic solvent such as benzene, toluene, dichloromethane and THF. When three or more hydroxyl groups are present on adjacent carbon atoms, then the middle one is converted into formic acid.
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Scheme 5 |