Reactive intermediates, catalytic mechanism

A catalytic mechanism, which is supported by deuterium-labeling experiments in the corresponding Ru-catalyzed procedure [146], is shown in Scheme 47. Accordingly, the reactive Fe-hydride species is formed in situ by the reaction of the iron precatalyst with hydrosilane. Hydrosilylation of the carboxyl group affords the 0-silyl-A,0-acetal a, which is converted into the iminium intermediate b. Reduction of b by a second Fe-hydride species finally generates the corresponding amine and disiloxane. 

High-pressure photochemistry has been used very successfully in studying the mechanisms of catalytic reactions. Irradiation of a suitable precursor permits in-situ preparation of reactive intermediates such as coordinatively unsaturated complexes or radicals. It is thus possible to check whether these species are involved in the catalytic cycle. 

The approach proposed in this report is similar to those proposed independently by Satterfield and Gaube5 in a sense that the total product spectrum is a combination of two distinct sets of products produced as a result of either two different mechanisms, for instance, two different reactive intermediates,4 or two different catalytic surfaces,5 each producing a different product spectrum. 

Figures 19.6 through 19.11 detail the isotopic exchange rates during water-gas shift for the formate, C02, and Pt-CO bands, in switching from the 12C to 13C label. In all cases, the reactive exchange rates of formate and C02 were virtually identical, implicating the formate species as the likely intermediate to the water-gas shift catalytic mechanism over Pt/Zr02 and PtNa/Zr02 catalysts. The DRIFTS spectra at the top of each figure show the switching of these species from...

A reaction mechanism may involve one of two types of sequence, open or closed (Wilkinson, 1980, pp. 40,176). In an open sequence, each reactive intermediate is produced in only one step and disappears in another. In a closed sequence, in addition to steps in which a reactive intermediate is initially produced and ultimately consumed, there are steps in which it is consumed and reproduced in a cyclic sequence which gives rise to a chain reaction. We give examples to illustrate these in the next sections. Catalytic reactions are a special type of closed mechanism in which the catalyst species forms reaction intermediates. The catalyst is regenerated after product formation to participate in repeated (catalytic) cycles. Catalysts can be involved in both homogeneous and heterogeneous systems (Chapter 8). 

This section will describe the various applications of HP IR spectroscopy to determine reaction mechanisms of transition metal catalysed reactions. It will begin by looking at truly in situ studies, carried out under catalytic conditions, and then consider investigations of stoichiometric reaction steps and characterisation of reactive intermediates. 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

These complexes (205) were found to be good models for the reactive intermediates involved in the catalytic decomposition of alkyl hydroperoxides (Haber Weiss mechanism), and in the catalytic hydroxylation of hydrocarbons by ROOH. 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

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