Stepwise Hydrogen Transfer

In a number of papers, Limbach et al. have proposed to use formal kinetics in order to describe the case of stepwise HH-transfer [18, 24], This method has been 

In Fig. 6.11 is depicted a general scheme of a stepwise HH-transfer reaction between the initial state A and the final state D. B and C are intermediates whose concentration is small. In each reaction step a single H is transferred, the other H is bound. Let us denote the formation of the intermediate as dissociation and the backward reaction as neutralization . The corresponding free energy reaction profile is illustrated in Fig. 6.11(b). 

Using the steady-state approximation it can easily be shown that the rate constant of the interconversion between A and D is given by 

The primary and secondary kinetic isotope effects of a given reaction step ij can be written in the form 

For the case of degenerate HH-transfers, the following isotopic reaction rate constants have been derived [18a-c, 26] 

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The presence of a significant kinetic isotope effect for the losses of OH from the nitroazoles (ThAd = 4.6) supports the suggestion that the fragmentations are adequately described by a stepwise hydrogen transfer, followed by cleavage of the OH moiety or, alternatively, rearrangement to structures which can eliminate OH and H2O98. 

Fig. 13 Combined PES for concerted and stepwise hydrogen transfer calculated by Handgraaf and coworkers...

Figure 2. Stepwise hydrogen atom transfer from Tetralin...

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

One of the earliest studies of the reaction of C2H4 with D2, in which a full mass spectrometric analysis of the products was performed, used a nickel wire as catalyst [115,116]. Some typical results are shown in Fig. 11. These results showed that ethylene exchange was rapid and the deutero-ethylenes are probably formed in a stepwise process in which only one deuterium atom is introduced during each residence of the ethylene molecule on the surface, that is there is a high probability of ethylene desorption from the surface. From Fig. 11(a) it can also be seen that the major initial products are ethane-d0 and ethane-d,. This is consistent with a mechanism in which hydrogen transfer occurs by the reaction... 

Hydrogen transfer is stereospecifically cis and occurs in a stepwise manner,2 passing through a metal alkyl complex (species H in Fig. 2), the so-called half-hydrogenated state. 

Although very little is known about the mechanism of such hydrogenations, Abley and McQuillin assumed that hydrogen transfer occurred in a stepwise fashion. It was claimed that the configuration of the methyl 3-phenylbutanoate... 

This work has led to more studies on the reaction mechanism of reduction by hydrosulfite, Chung [17] proposed that the mechanism proceeds by stepwise electron transfer from the reducing agent to the carbonyl group to form a ketyl radical intermediate that can then abstract a hydrogen atom from the medium, dimerize to pina-col, or undergo further reduction (Figure 13.2). 

Isomerization of terminal olefins by HCo(CO)4, or more likely by the coordinatively unsaturated HCo(CO)3 in equilibrium with it, proceeds rapidly at room temperature. The isomerization is catalytic but the HCo(CO)3 4 is consumed irreversibly by simultaneous hydroformylation, which removes 2 mol of the hydridocarbonyl for each mole of reacted olefin. The competition between these two reactions (as well as the bimolecular decomposition of the hydrocarbonyl to and Co2(CO)g) depends on the conditions of the experiment. The results obtained with 4-methyl-1-pentene under one atmosphere of N2 are shown in Fig. 1 and the catalytic cycle, which rationalizes the stepwise isomerization, is shown in Fig. 2. In Fig. 2 HM represents either HCo(CO)4 or HCo(CO)3. In experiments on the isomerization of PhCD2CH=CH2 with HCo(CO)4 in the presence of unlabeled p-allyltoluene both PhCI>=CHCH3 and labeled 4-propenyltoluene were found in the products indicating hydrogen transfer between olefins via complexed HM. The... 

Figure 6.12 Free energy correlation (shown schematically) for the H and D zero-point vibrations for a degenerate stepwise double hydrogen transfer reaction according to Eq. (6.31), where secondary kinetic isotope effects and isotopic fractionation between the initial and the intermediate state were neglected. Adapted from Ref [18c],...

The derivation of expressions for the multiple kinetic isotope effects of the triple hydrogen transfer case is analogous to the HH-transfer but more tedious. Therefore, the reader is referrred to refs. [25] and [26]. The main results are included in Table 6.2. As in the case of the HH-transfer, the kinetic isotope effects derived for the stepwise transfers are valid in the presence of turmeling and are independent of the tunneling model used. By contrast, the kinetic isotope effects of the single barrier reaction are affected by tunneling. 

Intramolecular Single and Stepwise Double Hydrogen Transfer in H-bonds of Medium Strength... 

H/D isotope effects during compression assisted concerted HH-transfer in two cooperatively coupled hydrogen bonds, (c) Geometric H/D isotope effects during compression assisted stepwise HH-transfer in two anticooperatively coupled hydrogen bonds. 

Aminyl radicals have also been detected indirectly during the reaction of hydroxyl radicals (HO ) or their conjugated base ( 0 ) with the free amino group of amino acids (Reactions (3.9) and (3.10)) [40-43], and identified by time-resolved EPR experiments [44]. Similar reactions may be expected for peptides. While Reactions (3.9) and (3.10) show a net hydrogen transfer, they likely proceed via a stepwise electron-transfer and proton-transfer (Reaction (3.11)), a reaction commonly referred to as proton-coupled electron transfer (PCET). Proton transfer from the ami-nium radical cation to the base (OH ) will likely occur within the solvent cage. 

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