Bismuth-Salt Oxidations

Abstract

Compounds of bismuth were first proposed as useful oxidants for organic synthesis by Rigby1–3 in 1949. Both the pentavalent and the trivalent states of bismuth were found to display oxidizing power. Bismuth(V) in the form of sodium bismuthate is an oxidant analogous to lead tetraacetate, being fairly specific for the fission of 1,2-diols. During the study of sodium bismuthate it was discovered that bismuth(III) salts can oxidize α-hydroxyketones to the corresponding diketone.3 Following the discovery, bismuth trioxide was developed as a highly specific reagent for this purpose. The reduction of bismuth trioxide in this reaction leads to the formation of metallic bismuth. Neither sodium bismuthate nor bismuth trioxide has been used extensively in the years that followed their introduction. Both the reagents are heterogeneous oxidants and are normally employed in a medium of acetic acid. The rather harsh reaction conditions and uncertainty about the composition of commercial sodium bismuthate are probable reasons for the relative neglect of the reagents. The yields are generally good and separation of the products from the used reagent is easy in most cases. Sodium bismuthate was adopted for the direct oxidation and analysis of corticosteroids present in urine, tolerance of the reagent to water being essential in this application (in contrast to lead tetraacetate).4