Maria B. Ezhova

Catalytic oxidations using ruthenium porphyrins

The major goal of the Chapter is to review developments in the use of Ru-porphyrin complexes as homogeneous (or matrix-supported) catalysts for oxygenation and oxidation processes. The subject was given impetus with the discovery of a remarkable reaction in which a Ru(II) porphyrin complex reacted with O2 to give a trans-dioxo-Ru(VI) species. Such species, which can be formed from a wide range of O-atom donors, were shown subsequently to be capable of acting as a bis(monooxygenase) in transferring both the coordinated oxo ligands (as O-atoms) to olefinic substrates, saturated hydrocarbons, phosphines, and thioethers, and the processes become catalytic in the presence of excess of the O-atom donor. Further, the dioxo species can also exhibit oxidase-like activity, and effect stoichiometric or catalytic oxidative-dehydrogenation of phenols, alkoxyarenes, alcohols, and amines. Use of chiral porphyrins has led to catalytic, asymmetric epoxidation and hydroxylations, even though radical intermediates are invoked, as well as oxygenation of racemic substrates (phosphines and more interestingly tertiary alkanes) to yield chiral products by kinetic resolution processes. The reaction mechanisms invoked range from genuine O-atom transfer (from RuVI, RuV, or RuIV species, where the disproportionation reaction [2 O=RuIV⇋RuII+O=RuVI=O] is important), to free-radical induced processes, particularly when the porphyrin ligands are extensively halogenated, as Ru complexes generally of such porphyrins are extremely active in radical-type decomposition of hydroperoxides, often present as trace impurities in hydrocarbon substrates.