Hydrogenation mechanisms hydrogen-transfer processes

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Nevertheless, the mechanism of the Shvo s catalyst has been one of the most controversial regarding the nature of the hydrogen-transfer process (84). The analysis of this reaction mechanism served as an example of comparison of both the inner- and outer-sphere reaction pathways for hydrogenation of polar, C=0 (85-87) and C=N (88—95) and unpolar bonds (95). In the next subsections are presented the mechanistic studies we carried out for the hydrogenation of ketones, imines, alkenes, and alkynes (29,87,95). 

This observation can be used to derive a possible reaction mechanism for the hydrogen transfer process. We assume that the first step is the dehydrogenation of the alcoholic group, followed by isomerization of the unsaturated ketone to the conjugated one ... 

Intra- and intermolecular hydrogen transfer processes are important in a wide variety of chemical processes, ranging from free radical reactions (which make up the foundation of radiation chemistry) and tautomeriza-tion in the ground and excited states (a fundamental photochemical process) to bulk and surface diffusion (critical for heterogeneous catalytic processes). The exchange reaction H2 + H has always been the preeminent model for testing basic concepts of chemical dynamics theory because it is amenable to carrying out exact three-dimensional fully quantum mechanical calculations. This reaction is now studied in low-temperature solids as well. 

GTPCH, FTPS, and SR are required and sufficient to carry out the proper stereospecific reaction to BH4 (45). With the crystallographic structures (Figure 13), including the characteristics of the active centers of all three enzymes, the essential information for the interpretation of the reaction mechanism is available. Moreover, NMR studies on the reaction mechanisms of all three enzymes revealed the details of the hydrogen transfer process and the stereochemical course of the reactions." ... 

A wide variety of oxidants has been used to produce a-oxy-alkyl radicals from alcohols and ethers hydrogen peroxide, hydroperoxides, perborate, per-oxydicarbonate, peroxydisulphate. This last gives good results the mechanism of the oxidation is controversial. Electron transfer from oxygen [Eq. (19)] has been proposed on the basis of spin-trapping experiments 5 ), followed by a hydrogen-transfer process [Eq. (20)]... 

Both quantum mechanical and force field-based methods have been employed in the attempt to model hydrogen transfer reactions. In free-radical chemistry, by far the most significant hydrogen transfer process involves the reaction between a carbon-centered radical and a chain-carrying reagent such as tributyltin hydride... 

The mechanism of thermal degradation of polyetheleneterephthalate (PET) has been studied over many years by off-line analysis of the pyrolysis products " and by Py-GC/MS studies. The results suggested a jS-CH hydrogen transfer process leading to the formation of oligomers with olefin and carboxylic end-groups (Eq. 5.4). 

As Fig. 5 shows, the reaction on the doublet PES follows similar mechanisms with the quartet state. The overall reaction is endothermic by 7.3 kcal moP Note that the spin transition from the quartet to doublet state is probably involved in the hydrogen transfer process from f5 to (6. The spin transition from f6 to the more stable f6 will significantly lower the barrier height for the second C-H bond activation. 

Curran, G.P., R.T. Struck and E. Gorin. Mechanism of the Hydrogen Transfer Process to Coal and Coal Extract. lEC Proc. 

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