Hydrogenation catalytically competent intermediates

Scheme 4.8 Synthesis and characterization of catalytically competent intermediates in bis(imino)pyridine iron-catalyzed hydrogenation and hydrosilylation reactions.

The mechanism of hydrogenation of alkenes using Wilkinson s catalyst ( ) is shown below. The two other species outside the box have been either detected in, or isolated from, solutions undergoing catalysis (S = solvent). However, the actual catalytic cycle is given within the box. The species outside the box were found to be too sluggish to react as competent intermediates in the rapidly proceeding catalytic cycle. Hence, they are simply by-products of some of the catalytically active species. Their accumulation actually slows the rate of the catalytic reaction. 

The examples of direct observation of solvate dihydrides together with the computational data collected in the Table 1.1 illustrate the fact that these species are kinetically competent intermediates in the catalytic cycle of Rh-catalyzed asymmetric hydrogenation. 

Some Ni(II) complexes show catalytic activity for the electrocatalytic reduction of C02 in water, where an intermediate formation of Ni(I) species has been proposed. To obtain a useful electrocatalyst in the electroreduction of C02, the selectivity of the process is highly important. As many electrochemical systems available for reducing C02 require the presence of water, the reduction of molecular hydrogen is always a competing reaction that needs to be avoided. 

Halpern studied the rate behavior of the overall catalytic hydrogenation of cyclohexene by Wilkinson s catalyst, as well as the rate behavior of the individual steps of the proposed catalytic cycle. The core of the catalytic cycle was shown in Scheme 15.1 to illustrate the hydrogen-first pathway, but a more complete view of the pathway deduced from Halpern s studies is shown in Scheme 15.6. This overall scheme was deduced by isolating and determining the rate behavior of each individual stage of the catalytic cycle, demonstrating that these steps can account for the kinetic behavior of the overall catalytic reaction, and demonstrating that several previously isolated compounds were not kinetically competent to be intermediates in the catalytic process. 

Mechanistic study of asymmetric hydrogenation showed that the reaction of the major isomer of the alkene-metal complex gave the minor isomer of the product. This paradox was resolved when it was found that the minor isomer reacts far faster with H2. A useful general point emerges from this work catalysis is a kinetic phenomenon and so the activity of a system may rely on a minor, even miniscule component of a catalyst. This emphasizes the danger on relying too much on spectroscopic methods in studying catalysts. The fact that a series of plausible intermediates can all be seen by, say, NMR in the catalytic mixtures does not mean these are the true intermediates. What we need to do is to show that each of the proposed intermediates react sufficiently fast to account for the formation of products, that is, that they are kinetically competent to do the reaction. 

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