Indirect hydrogenation mechanism

If the number of catalytic species is three, then the reaction rate becomes more complicated, but still managable to derive using a steady-state approximation. However, this laborous excercise is not needed, as we can directly use the derivation presented for the three-step catalytic sequence with linear steps, i.e. equation (4.114). Examples of direct and indirect hydrogenation mechanisms when the reaction mechanism can be expressed by a cycle with three intermediates are presented in Figure 5.10 and Figure 5.11 respectively. 

Figure 5.11. Three step indirect hydrogenation mechanism...

The kinetic expression for the indirect hydrogenation mechanism can be written in a similar fashion. 

Fig. 5.11 Three-step (A) direct and (B) indirect hydrogenation mechanisms.

It is not possible for the indirect damping mechanism, as considered semiclassically by Robertson and Yarwood [84] and later quantum mechanically by Boulil et al. [90], to be the unique damping mechanism occurring in a hydrogen bond, because the quantum mechanism leads, at the opposite of the less rigorous semiclassical treatment, to a drastic collapse of the lineshapes. 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

There remains some doubt about the first step of the overall reaction in MeCN the final products are usually the A -alkylacetamides, as shown in Eq. (35). In neutral solution, at extreme anodic potentials, it is difficult to decide between direct [Eqs. (1), (2), and/or (3)] and indirect electron transfer [Eqs. (16), (17), and/or (18)]. For oxidation in MeCN-BF4 solutions the variation in potentials is best explained in terms of the direct mechanism. An indirect oxidation mechanism involving hydrogen abstraction by electrogenerated nitrate radical has recently been proposed for the electrolysis of linear alkanes in tert-BUOH/H2O mixtures containing HNO3 and saturated with O2 [25]. 

Hydrogenation mechanisms catalysed by transition metal complexes could be somewhat more complicated combining direact and indirect hydrogenation containing a single cycle with 4 catalytic intermediates (Figure 5.13)... 

Molecular emission cavity analysis (MECA) is a flame chemiluminescence technique based on the generation of excited molecules, radicals, or atoms within a hydrogen diffusion flame. The excited species are formed by direct or indirect chemiluminescence mechanisms and are confined within the inner space of a small cavity, which is positioned at a preselected point of the flame environment. The emission is monitored at the characteristic wavelength of... 

Any computational method that leads to Af/f of the participants in a hydrogenation reaction also gives Ahyd/f indirectly. Molecular mechanics is such a method. MM calculations are fast and not very demanding of computer resources. 

The concentration dependence of the indirect reduction mechanism for TCE can be understood from the data in Figure 9. At low TCE concentrations, there is sufficient atomic hydrogen produced from water reduction to measurably contribute to TCE dechlorination. For example. Figure 9 shows that at concentrations less than 1 mM, the current for water reduction was greater than the direct current for TCE reduction, whereas, at higher TCE concentrations, the water reduction current was small compared to the direct reduction current for TCE. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Cell suspensions of Geobacter sulfurreducens can conple the oxidation of hydrogen to the reduction of Tc(VII) to insolnble Tc(IV). An indirect mechanism involving Fe(II) was also observed, and was snbstantially increased in the presence of the redox mediator AQDS (Lloyd et al. 2000). 

Further mechanistic insights into hydrogenations catalyzed by HRuCl(PPh3)3 (7, p. 83) have been obtained indirectly, from studies on hydrogenation of some ruthenium(III) phosphine complexes (83). A frequently considered mechanism for hydrogen reduction of metal salts involves slow formation of an intermediate monohydride, followed by a faster reaction between the hydride and starting complex (/, p. 72), Eqs. (2) and (3) ... 

Since the first use of catalyzed hydrogen transfer, speculations about, and studies on, the mechanism(s) involved have been extensively published. Especially in recent years, several investigations have been conducted to elucidate the reaction pathways, and with better analytical methods and computational chemistry the catalytic cycles of many systems have now been clarified. The mechanism of transfer hydrogenations depends on the metal used and on the substrate. Here, attention is focused on the mechanisms of hydrogen transfer reactions with the most frequently used catalysts. Two main mechanisms can be distinguished (i) a direct transfer mechanism by which a hydride is transferred directly from the donor to the acceptor molecule and (ii) an indirect mechanism by which the hydride is transferred from the donor to the acceptor molecule via a metal hydride intermediate (Scheme 20.3). 

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