Mechanism of oxidation of saturated hydrocarbons by cobalt(III), manganese(III), and lead(IV) trifluoroacetates

Abstract

Oxidation of adamantane with lead tetra-acetate or manganese triacetate in acetic acid gives poor yields of primary products (based on the oxidant). However, oxidation of adamantane by manganese(III), cobalt(III), or lead(IV) acetates in trifluoroacetic acid gives high yields of 1-adamantyl trifluoroacetate as primary product. Other hydrocarbons are also oxidised by attack at a bridgehead carbon–hydrogen bond. Thus bicyclo[3.3.1]nonane with Pb⁴⁺ gives the trifluoroacetate of bicyclo[3.3.1 ]nonan-1-ol in 74% yield and similarly diamantane gives the two bridgehead trifluoroacetates in 93% yield. The generality of these oxidations is discussed. A product analysis shows that in contrast to anodic oxidation which gives products of fragmentation with substituted adamantanes, oxidation by Pb^IV, Co^III, or Mn^III leads only to products by attack at a bridgehead tertiary carbon–hydrogen bond. In further contrast, anodic oxidation of 1,3,5-trimethyl-7-t-butyladamantane (30) leads to fragmentation products in high yield, but oxidation of tetrasubstituted adamantanes such as (30) by metal salts is very sluggish and gives, in low yield, complex product mixtures. Comparison of the electrochemical oxidations with those by Co^III. Mn^III, and Pb^IV suggests that, in contrast to current views, oxidation of saturated hydrocarbons by metal salts does not proceed via cation radical intermediates. A mechanism proceeding by localised attack at a carbon–hydrogen bond is suggested. Adamantane with potassium permanganate in trifluoroacetic acid gives protoadamantanone (37) in 30% yield based on hydrocarbon consumed. A novel mechanism for this oxidation is tentatively proposed.