Catalysis involving metal -complex intermediates

In contrast to catalytic oxidations involving metal peroxide intermediates, catalysis by metal-oxo complexes does not in theory require the presence of a coreducing agent. Indeed, the reduction... 

Several reports of catalysis of the decomposition of formic acid involving homogeneous transition metal complexes and proceeding by means of metalloformate intermediates have recently appeared in the literature. For example, Rh(C6H4PPh2)(PPh3)2 (8) catalyzes the decomposition of formic acid to C02 and H2 via the intermediacy of the product of oxidative-addition of HCOOH, Rh(HC02)(PPh3)3 (56). -Elimination of the hydride from the... 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

The classic method for carrying out catalytic oxidations of hydrocarbon substrates has involved the use of the hydrocarbon itself as solvent and trace amounts of hydrocarbon-soluble metal complexes, such as metal stearates, naph-thenates, or acetylacetonates, as catalysts. These reactions were generally carried out to low conversions to avoid excessive by-product formation. Catalysis involved redox reactions of the metal catalyst with intermediate hydroperoxides. 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

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