The complex synergy of water in metal/bromide autoxidations. Part II. Effect of water and catalyst on the aerobic oxidation of benzaldehydes and the effect of water on the elementary catalytic pathways

All of the rates of the elementary steps in the Co/Br and Co/Mn/Br homogeneous, liquid-phase catalyzed reactions decrease with increasing water concentration in acetic acid. The step-wise replacement of the acetic acid ligands by water ligands in the coordination sphere of the catalyst metals may be responsible for this behavior. The non-catalyzed and metal-catalyzed (Co, Co/Mn/Br and Co/Mn) aerobic oxidations of benzaldehyde and 4-methylbenzaldehyde are reported. ne non-catalyzed autoxidations are quite vigorous reactions In acetic acid/water mixtures but by-products from the Baeyer-Villiger reaction, the thermal decomposition of the peroxy acid, and over-oxidation to carbon dioxide limit the yield to the aromatic carboxylic acids. As the concentration of a Co or Co/Mn/Br catalyst increases these by-products are first reduced and then eliminated probably due to the very fast, selective reaction Of [CO(II)](2) with the peroxy acid. A Co/Mn catalyst completely inhibits the autoxidation of the benzaldehydes. There is a gradual change in the yield of terephthaldicarboxaldehyde from 4-methylbenzaldehyde with increasing Co/Mn/Br concentration suggesting that the non-catalyzed steps are being replaced by catalyzed ones. The autoxidation of heptaldehyde generates about 500 times more carbon monoxide than does benzaldehyde using a Co/Mn/Br catalyst and gives only a 50% yield to heptanoic acid consistent with excessive amounts of decarbonylation with aliphatic aldehydes.