[1304-76-3] · Bi2O3 · Bismuth(III) Oxide · (MW 465.96)
(oxidizing agent specific for conversion of a-hydroxy ketones to diketones1)
Physical Data: mp 820 °C; d 8.9 g cm-3.
Solubility: insol water and organic solvents; sol acids.
Form Supplied in: amorphous yellow powder.
Handling, Storage, and Precautions: no special precautions.
Bismuth(III) oxide is a specific reagent for the oxidation of a-hydroxy ketones to the corresponding diketones.2,3 The oxidation is generally carried out in acetic acid at about 100 °C. Bismuth(III) oxide is soluble in acetic acid under these conditions and forms bismuth triacetate, the active oxidizing agent, which becomes reduced to metallic bismuth. Oxidation of simple a-hydroxy ketones is rapid, but more highly substituted compounds may require several days for complete reaction. Reaction of bismuth(III) oxide with a-hydroxy ketones is confined to examples possessing an a-hydrogen atom. It has been suggested that the oxidation proceeds via an enediol intermediate. Bismuth triacetate may act as an acylating agent towards alcohols and amines. This reaction occurs only at high temperatures (200 °C no solvent) and would not normally interfere with the oxidation of a-hydroxy ketones.
In a typical procedure the a-hydroxy ketone, dissolved in acetic acid, is heated on a steam bath with 1.2 equiv of bismuth(III) oxide. During the course of the reaction black metallic bismuth precipitates. Unhindered substrates are normally oxidized in under an hour under these conditions. The reaction mixture is filtered to remove bismuth and excess reagent; water is added and the product is extracted into ether. Yields of greater than 90% may be obtained. Sugars are not oxidized by the reagent and neither is ascorbic acid. The presence in the reaction mixture of certain polyhydroxy compounds, such as glucose and catechol, inhibits the oxidation of a-hydroxy ketones owing to the formation of bismuth-containing compounds with these materials.1
Jonathan P. Kitchin
3M, St Paul, MN, USA