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26.1.2.1.3 Variation 3: Using Rhodium Salts with Oxygen

DOI: 10.1055/sos-SD-026-00062

Balaban, T. S.Science of Synthesis, (200526131.

It has been shown that rhodium(I) complexes such as Wilkinson's catalyst [RhCl(PPh3)3] promote the oxygenation of terminal alkenes to methyl ketones and at the same time transform triphenylphosphine into triphenylphosphine oxide.[‌39‌‌41‌] Five-membered peroxometallocycles are proposed as reactive intermediate species which subsequently complex both the alkene and dioxygen to the rhodium(I) center. The mechanism of this activation of molecular oxygen mimics cytochrome P450 biochemical oxidations. The dioxygen and the substrate are bonded to the catalytic center, which has at its core the metal in a low oxidation state. The oxygen molecule is split so that one atom becomes incorporated into the substrate (thus explaining the denomination of monooxygenase) while the other atom remains bound to the metal, which is thus oxidized. The reaction becomes catalytic if a coreducing agent, for example triphenylphosphine, reduces the metal to the initial oxidation state. This mechanism differs from that of the Wacker process (see Section 26.1.2.1.2) in which water (or a hydroxide anion) becomes bonded to the alkene that is coordinated to the palladium metal in a higher oxidation state. The ketone and palladium(0) are then produced, and a cooxidative agent, such as copper(II), reoxidizes the metal so that the reaction becomes catalytic.

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