Liquid-phase metal-centred autoxidation of styrene catalysed by rhodium species

Abstract

During the autoxidation of styrene, catalysed by [{RhCl(C₂H₄)₂}₂] at 110 °C in the presence of radical inhibitor, acetophenone and benzaldehyde arise by a path avoiding both radical chains and a Wacker cycle. Only about half of the oxygen and styrene consumed appear as measured products, and the nature of the side-reactions is unknown. The various activities of several rhodium(I) complexes suggest that co-ordination of both the styrene and oxygen to the metal is essential for reaction, and the co-ordination of each of these ligands to rhodium has been observed under conditions approaching those of reaction. The initial rate of acetophenone formation accords with the expression: Rate =C[styrene][catalyst][O₂]//_{1 +C′[O2]} A sequential mechanism is proposed in which an initially formed catalyst-styrene adduct reacts with oxygen. In oxygen-saturated solution, the activation energy is 70 kJ mol^–1. The catalyst undergoes rapid oxidative deactivation, but is not regenerated by treatment with hydrogen. Styrenes having methyl or phenyl substituents at the olefinic positions are resistant to oxidation.