Olefin hydrogenation pathways

At higher Co site densities, the probability of chain termination to olefins is lower and decreases somewhat faster with hydrocarbon chain size (Fig. 13b). Thus, the heavier product obtained on materials with high site densities reflects a lower probability of chain termination to olefins, an effect that arises from the enhanced readsorption of such olefins within transport-limited pellets with high site density. In contrast, chain termination to paraffins is not influenced by Co site density (Fig. 13c). Site density does not affect primary chain growth and termination chemistry or inhibit secondary olefin hydrogenation pathways, effects that could otherwise account for the higher Cs+ selectivity observed on higher site density pellets. 

Example 8.9. Olefin hydrogenation with Wilkinson s catalyst. Wilkinson s catalyst is a dihydrido-chloro-phosphino complex of rhodium, H2RhClPh3, where Ph is an organic phosphine such as triphenyl phosphine [48-52]. The dominant mechanism of olefin hydrogenation with this catalyst, established chiefly by Halpem [53-55] in detailed studies that included measurements of equilibria in the absence of reactants and of reaction rates of isolated participants, backed by independent NMR studies [56] and ab initio molecular orbital calculations [57], is shown as 8.69 on the facing page (without minor parallel pathways and side reactions). 

Olefin hydrogenation reactions control C5+ selectivity by intercepting reactive a-olefins. Readsorption and chain initiation steps reintroduce olefins into chain growth pathways and reverse the most frequent chain termination... 

Thus, a preliminary analysis of olefin production pathways can be performed based on the methane-to-ethylene ratio and on temperature dependence of the (C3 = )-to-(C2 =) ratio. A more detailed elaboration can be reached from experiments with varied oxygen concentration and from the detailed analysis of the product distribution (including hydrogen formation). However, ethylene formation itself is strong evidence for the contribution of the radical route in product formation. The analysis of experimental data about product distribution during propane oxidation (Kondratenko et al, 2005) demonstrates that over rare-earth oxide catalysts radical route is prevailing in olefin formation. On the other hand, over supported Y-containing catalysts, propylene... 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

Photochemical olefin hydrogenations catalysed by Fe(C0)g follows a radical pathway, according to high-pressure IR and UV studies. Irradiation with visible light of the system triethanolamine/Ru(bipy)3CIj/DMF induces the reduction of COj to formate. A similar system with a Ru(IH) complex as electron acceptor cleaves acetylene photolytically ( > 400 nib) to methane (eqn.6). ... 

A plausible hypothesis for olefin hydrogenation appears to be a non oxidative hydrogenolysis of the metal alkyl bond (formed by alkene insertion into a preformed M—H bond) via a four-center transition state. Such a pathway is well documented in main group chemistry, the alkyls of Li and A1 being particularly relevant examples. 

Phenoxaphosphino-modified xantphos-type ligands in the rhodium-catalyzed hydroaminomethylation of internal olefins were found to give linear amines. Hydro-aminomethylation and each of its individual steps were monitored by high-pressure infrared spectroscopy. The results suggest that hydroaminomethylation takes place by a sequential isomerization/hydroformylation/amination/hydrogenation pathway [170]. 

Scheme 3 Plausible pathway for hydrogenation of olefin catalyzed by the Fe-H complex...

CeDs solution (Scheme 7). Both t -arene complexes were also determined by the X-ray diffraction and showed no reaction to hydrogen and olefins. Therefore, it was considered that the formation of the t -arene complexes was a deactivation pathway in the catalytic hydrogenation. 

In the presence of a suitably disposed /i-hydrogen—as in alkyl-substituted thiirane oxides such as 16c—an alternative, more facile pathway for thermal fragmentation is available . In such cases the thiirene oxides are thermally rearranged to the allylic sulfenic acid, 37, similarly to the thermolysis of larger cyclic and acyclic sulfoxides (see equation 9). In sharp contrast to this type of thiirane oxide, mono- and cis-disubstituted ones have no available hydrogen for abstraction and afford on thermolysis only olefins and sulfur monoxide . However, rapid thermolysis of thiirane oxides of type 16c at high temperatures (200-340 °C), rather than at room temperature or lower, afforded mixtures of cis- and trans-olefins with the concomitant extrusion of sulfur monoxide . The rationale proposed for all these observations is that thiirane oxides may thermally... 

In discussing the reaction pathways, we believe that the general evidence leads to the conclusion that hydrogenolysis proceeds via adsorbed hydrocarbon species formed by the loss of more than one hydrogen atom from from the parent molecule, and that in these adsorbed species more than one carbon atom is, in some way, involved in bonding to the catalyst surface. In the case of ethane, this adsorption criterion is met via a 1-2 mode or a v-olefin mode. Mechanistically it is difficult to see how the latter could be involved in C—C bond rupture in ethane. With molecules larger than ethane, other reaction paths are possible One is via adsorption into the 1-3 mode, and another involves adsorption as a ir-allylic species. 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

RuCl(PPh3 )3(alkyl) (90). Because of the fact that the orthometallated complex reacts with H2 to re-form HRuCl(PPh3)2, catalytic hydrogenation of olefins can result via such pathways, although product formation via reaction (11) is kinetically preferred (88). 

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