Olefin hydrogenation and isomerization

Fig. 14. Proposed catalytic cycle of olefin hydrogenation and isomerization catalyzed on [HRu3(CO), (OSi=)] and [HOs3(CO)i,(OSi=)] species.

The rates of olefin hydrogenation and isomerization by Group VIII metal-phosphine complexes are increased by the presence of hydroperoxides and/or oxygen. A similar rate enhancement is observed in the hydroformylation of alkenes catalysed by [RhCl(CO)(PPh3)2]. The addition of small amounts of cyclohexenyl hydroperoxide is considered to effect the unusual transformation of [RhCl(CO)(PPh3)2] to cw-[RhCl(CO)2(PPh3)], which appears to be a very active alkene hydroformylation and isomerization catalyst. Asymmetric induction in hydroformylation reactions has been achieved. ... 

Kokes and Dent by combining IR spectroscopy with hydrogen isotope techniques, and by applying kinetic and stereochemical considerations were able to determine the intermediate surface species in the hydrogenation and isomerization of simple olefins over zinc oxide. 

Methanation and olefin hydrogenation or isomerization (secondary and side reactions)... 

Olefin hydrogenation catalyzed by Fe(CO)s normally requires somewhat severe conditions, typically 150°C and 10 atm H2 (/, p. 64). With near-ultraviolet irradiation the carbonyl becomes effective at ambient conditions for hydrogenation (and isomerization) of olefins (448, 449). Photoinduced labilization of carbonyls is thought to give tricarbonyl species as the active catalysts, e.g.,... 

Hydrogenation and Isomerization Reactions of Olefinic Alcohols Catalyzed in... 

The overall reaction of olefins, hydrogen, and carbon monoxide can be complex since not only are both linear and branched aldehydes (and hence alcohols) formed, but the same catalysts promote olefin isomerization. The products formed may be summarized as follows ... 

Olefin Reductions—Alkylation capacity would increase with any reduction in olefin content because it would provide a way to use the C4 and some C5 olefins in gasoline. New isomerization capacity would be needed if substantial quantities of C5 and C6 olefins had to be removed. The C5/C6 olefins would have to be hydrogenated and isomerized to make up part of the octane loss. 

Scheme 1. Hydrogenation and isomerization of olefin via common intermediates.

CDTECH, Inc. Ether Refinery C4 and C5 streams Produces high iso-olefin/ether products by hydrogenation and isomerization 5 1992... 

This mechanism, which appears to be well-established in certain systems (iO), has close analogies with suspected mechanisms for heterogeneous hydrogenation and isomerization of olefins (6). The second suggested mechanism involves hydrogen abstraction from the olefin with the reversible formation of a 7r-allylic species. 

The ready hydrogenation and isomerization of methyl oleate and palmitoleate with Fe(CO)s confirm the results of Ogata and Misono (18) with monounsaturated aliphatic compounds. In the isomerization of monoolefins Manuel (15) suggested the occurrence of equilibria involving either 7r-olefin HFe(CO)3 and a-alkyl Fe(CO)3 complexes, or TT-olefin Fe(CO)3 and 7r-allyl HFe(CO)3 complexes. The formation of olefin-iron tetracarbonyl complexes has been reported (19). The reaction of butadiene and Fe2(CO)9 has been observed to lead to the formation of butadiene-Fe(CO)4 and butadiene-[Fe(CO)4]2 complexes in which one or both double bonds are pi-bonded to the iron (16). A mechanism involving both monoene-Fe(CO)4 (I) and allyl-HFe(CO)3 complexes (II) is postulated for the isomerization of methyl oleate (Scheme II) and for its homogeneous hydrogenation. 

The causes of the selectivity of hydrogenation catalysts, similarly to the mechanism of hydrogenation and isomerization reactions of olefins are complicated problems, which so far have resisted any satisfactory solution 46). 

The low rates of isomerization are not unexpected in view of results given in the last section. Evidence that the mechanisms of olefin exchange and isomerization are closely related is provided by a brief study of the isomerization of the butenes in the presence of deuterium (31). The rates of entry of deuterium into the reactant olefin were small, and in the case of cis-2-butene its deuterium number increased linearly with the fraction which had isomerized. Olefin exchange was, as expected, particularly slow in the case of irans-2-butene. Hydrogen exchange was also slight, especially with 1-butene. 

It has been revealed that the ruthenium carbene complex 6 can promote not only the olefin metathesis but also hydrogenation and isomerization. Louie et al. reported that the one-pot tandem ring-closing... 

The proposed kinetic schemes of primary olefin- and paraffin formation and of olefin secondary hydrogenation and isomerization are presented in Fig.6 together with comparative conclusions for iron and cobalt as the catalysts. 

With cobalt, the situation is different In addition to reducing the average product-molecule size, the methanation reaction and the secondary olefin reactions of hydrogenation and isomerization are enhanced by increasing the reaction temperature, indicating the release of their suppression [16, 19] (and the dynamic nature of the regime). 

Together with hydrogenation and isomerization, epoxidation completes the trio of commercially significant applications of enantioselective homogeneously catalyzed reactions. Stereospecific olefin epoxidation is distinctive in that it creates two chiral centers simultaneously. The enantioselective epoxidation method developed by Sharpless and co-workers is an important asymmetric transformation known today. This method involves the epoxidation of allylic alcohols with tert.-butyl hydroperoxide and titanimn isopropoxide in the presence of optically active pure tartrate esters (Eq. 3-14). 

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