Homogeneous metal catalysed isotope exchange and oxidation of aromatic and aliphatic compounds in aqueous acetic acid containing platinum(IV) halide

Abstract

PtCl₆^2- reacts with benzene and substituted benzenes in aqueous acetic acid to give PtCl₄^2-, chlorinated aromatic compound and a smaller yield of dimer. In deuterated solvent extensive hydrogen isotope exchange occurs in both substrate and chlorinated product. PtCl₄^2- is shown to be an active catalyst for both exchange and oxidation reactions. Chlorobenzene is chlorinated predominantly in the meta and para positions with approximately equal activity. These data together with results from the effect of substituents indicate that metal catalysed chlorination is not determined by electronic factors but by steric considerations, in marked contrast to electrophilic chlorination. Similarly, PtBr₄^2- is a catalyst for bromination of benzene when the compound is treated with PtBr₆^2-. Exchange and chlorination occur via common intermediates and it is proposed that -complexes between aromatic and PtCl₄^2- are involved. Analogous chlorination, with PtCl₆^2-, of cyclohexane and the side chain of certain alkylbenzenes has also been observed. The structures of possible intermediates in these latter reactions are discussed.