IS METALLIC PALLADIUM FORMED IN WACKER OXIDATION OF ALKENES

The oxidation of cyclohexene with molecular oxygen catalyzed by solid Pd/C or Pd(II) acetate-hydroquinone-iron phthalocyanine gives cyclohex ane, benzene and oxygenated products. Oxygen pressure and solvent used influence significantly the distribution of the products and at the p ressure above 2 atm no cyclohexane is formed. Over Pd/C catalyst in th e absence of oxygen, disproportionation to cyclohexane and benzene (th e ratio is nearly 2:1) proceeds exclusively. Under comparable conditio ns 2-cyclohexenol is disproportionated to cyclohexanol and phenol and some of it rearranges to cyclohexanone. The explanation for the dispro portionation of cyclohexene under the Wacker conditions is that Pd0 ce ntres intermediately formed after stoichiometric oxidation of cyclohex ene by Pd(II) are not completely reoxidized, but depending on the reac tion conditions, they can partly aggregate and then, similarly to meta llic surfaces, dehydrogenate cyclohexene to benzene. The hydrogen spec ies formed migrate on the palladium surface and hydrogenate cyclohexen e or at sufficient oxygen pressure they are oxidized to water.