Oxygenation studies. Part 7. Catalytic dioxygenation of cyclo-octa-1,5-diene at a rhodium centre

Abstract

Studies on the oxygenation of cyclo-octa-1,5-diene by molecular oxygen using as catalysts [RhCl(PPh₃)₃(O₂)](1), and related dioxygen complexes in the presence of excess PPh₃are reported. A regioselective homoco-oxygenation of the diene is found to occur with (1) to give cyclo-octane-1,4-dione, in competition with some homoco-oxygenation of PPh₃. ¹⁸O-Labelling experiments establish that both oxygens in a molecule of (1) are transferred to a molecule of the diene and initial rate measurements indicate that the slow step in the catalytic cycle follows displacement of one phosphine ligand in (1) by the diene. An overall mechanistic cycle, involving a sequence of five-, seven-, and four-membered metallacyclic intermediates, is suggested.