Oxidation of alkynes by cobalt acetate bromide: A new mode of action for an important industrial catalyst

The catalytic oxidation of a variety of alkyne substrates has been studied using the well-known catalyst system, cobalt acetate bromide, under an O2atmosphere. Aryl-substituted alkynes generally gave diketones which cleaved to carboxylic acids under prolonged reaction. In some cases the diketone intermediates could be isolated in good yield. For example, reaction of diphenylacetylene with Co(OAc)2/NaBr and 0.02 eq. Mn(OAc)2as a promoter in 95% AcOH/H2O at 70°C for 7 h gave benzil in 78% selectivity. Whilst [Co(OAc)Br] oxidation catalysis normally involves H-atom abstraction, many of the alkynes used in our study could not proceed by this mechanism. The electron-richness of the alkyne was important in determining reaction rates. Thus, slower reaction was found for PhC≡C-C≡C-Ph than diphenylacetylene itself, and no reaction at all was observed for [PhC≡CC6H4-NO2]. On the other hand, the internal aliphatic acetylene, 4-octyne, did proceed via H- atom abstraction, giving the corresponding acetylenic ketone [CH3CH2CH2C≡CCOCH2CH3] in 80% yield.