Reaction of metal-ion complexes with hydrocarbons. Part III. Palladium acetate in the oxidation and autoxidation reactions of cyclohexene, cyclohexa-1,3-diene, and cyclohexa-1,4-diene in acetic acid solution

Abstract

Cyclohexene, cyclohexa-1,3-diene, and cyclohexa-1,4-diene react rapidly with palladium acetate in acetic acid solution to yield benzene. The reaction has a heterogeneous catalytic step since it depends on palladium metal formed in the system. The metal is slightly hydrided and is a very efficient disproportionation catalyst. Both HClO₄ and NaOAc promote oxidation at the expense of disproportionation. Similar oxidations of cyclohexanone or hydroxy-cyclohexenes give phenol while acetoxy-cyclohexenes yield mixtures of phenyl acetate and benzene. In 0·5M-HClO₄ there is a competing very fast oxidation that yields cyclohexanone, cyclohex-1-en-1-yl acetate, and cyclohex-3-en-1-yl acetate from cyclohexene. In the presence of oxygen, cyclohexene undergoes a slow palladdium acetate-catalysed homogeneous oxidation to yield cyclohex-2-en-1-yl acetate. A concurrent free-radical process yields cyclohex-2-en-1-ol and cyclohex-2-en-1-one. Cyclohexa-1,3-diene is oxidised rapidly by oxygen in the presence of Pd(OAc)₂ giving a complex mixture of products, the main one being cyclohex-2-ene-1,4-diol diacetate. Under similar homogeneous conditions, cyclohexa-1,4-diene gives benzene, cyclohex-4-ene-1,3-diol diacetate, and Pd₂(C₆H₈)(OAc)₂ as a by-product. The palladium acetate-catalysed oxidation of cyclohexene, cyclohexa-1,3-diene, and cyclohexa-1,4-diene by benzoquinone yields cyclohex-2-en-1-yl acetate, cyclohex-2-ene-1,4-diol diacetate, and cyclohex-4-ene-1,3-diol diacetate respectively with benzene, also, from cyclohexa-1,4-diene. The roles of oxypalladation adducts and π-cyclohexenylpalladium complexes in these reactions are discussed.