Hydrogen homolytic activation

The products of reductive cyclization incorporate two non-exchangeable hydrogen atoms. Homolytic and heterolytic hydrogen activation pathways may now be discriminated on the basis of hydrogen-deuterium crossover experiments. Reductive cyclization of the indicated nitrogen-tethered enyne under a mixed atmosphere... 

The formation of hydrides by activation of molecular hydrogen implies the transformation of H2 into MH ", MH or MH. The hydrogen activation can be either homolytic or heterolytic, and can occur at single or multiple metal centres. Examples of homolytic activation are given in Equations (1) and (2). 

Both modes of hydrogen activation, heterolytic and homolytic, may be achieved either directly or through a two-step mechanism involving the intermediate formation of ri -H2 complexes. 

The mechanism of oxygen transfer from the peroxide (29) to the substrate still remains a matter of controversy. Current opinions favor the formation of a high-valent FeO (or Fe =0 or Fe —0-) (30) active species, which acts as a homolytic hydrogen abstractor from the substrate. An alternative mechanism considers the Fe -peroxide complex as the actual hydroxylating reagent, by analogy with the reactivity of vanadium(V)-peroxo and -alkylperoxo complexes and that of chromium(Vl)-peroxo complexes (see below). ... 

In reaction with the active heme complex, compound 1, it is often assumed that the first stage is homolytic hydrogen abstraction, resulting in radical formation. This is certainly likely to occur with aliphatic hydroxylation. Ideally a calculation of the transition state should be carried out, but this is difficult in practice. Relative radical stabilities have therefore been used as an approximation. In a typical calculation one generates all possible radicals for a substrate, optimizes them, and determines their relative stabilities. They are then docked into the 3D protein structure using constraints between the heme and the sites predicted from the radical calculations. Early descriptions of this approach made use of homology models, but the same techniques can obviously be used with crystal structures. 

Hydrogen is one of the key components in HDS as well as of all the other reactions implicated in hydrotreating, and therefore the way in which hydrogen reacts with the catalysts and the nature of the sites at which such activation takes place are of prime concern. Two main types of hydrogen activation processes have been considered [14, 15, 18] one involves a simple homolytic splitting on surface (S-S) " units to produce two -SH groups (Eq. 1.3), which some authors think is the major pathway for hydrogen activation ... 

Fe —0-) (30) active species, which acts as a homolytic hydrogen abstractor from the substrate. 

Obviously, there is a great deal of analogy between the mechanisms of hydrogen activation and hydrogenation in aqueous and nonaqueous systems. The most common made of activation of dihydrogen is its homolytic splitting by oxidative addition [Eq. (1)]. 

The unsupported Mo(W)S2 catalysts exhibit hexagonal coordination. It is reasonable to assume that the same coordination is retained in the supported catalysts. Under hydroprocessing conditions, the corner and edge sulfur ions in Mo(W)S2 can be readily removed. This results in the formation of the coor-dinatively unsaturated sites (CUS) and/or sulfur ion vacancies that have the Lewis-acid character. Double and even multiple vacancies can be formed. Because of the Lewis-acid character, CUS can adsorb molecules with the unpaired electrons e.g., N-bases) present in the feed. They are also the sites for hydrogen activation. In this case, H2 may be homolytically and heterolytically split to yield the Mo-H and S-H moieties, respectively." It is this active hydrogen that is subsequently transferred to the reactant molecules adsorbed on or near CUS. Part of the active hydrogen can be spilt over on the support and to a certain... 

The activation of H2 by Ga+ is proposed to proceed differently since no protons are observed experimentally when H2 or alkane adsorbs. Homolytic hydrogen dissociation... 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

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