Olefin isomerization, catalysis

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

Practically all the heavy transition metals can be made to eatalyze olefin isomerization, presumably through transient formation of metal hydrides. A stable platinum hydride has been shown to react with ethylene to form a cT-CjHjPt complex which can eliminate ethylene to regenerate the hydride. The commercially successful processes for the conversion of ethylene to acetaldehyde and ethylene to vinyl acetate via PdClj catalysis have stimulated enormous interest in the mechanism of these reactions, their application to other conversions, and their extension to other catalytic systems. The various stages in the conversion of ethylene are quite well-understood and an important step in the reaction involves hydride migration. The exact role of Pd in the migration has not yet been elucidated. It seems almost certain that the phenomenal interest in the whole area of transition metal isomerization in the last several years will be more than matched by the wealth of work that is certain to pour out of research laboratories in the next few years. 

The use of base catalysis for various reactions, such as olefin isomerization, cyclohexadiene dehydrogenation, aromatic alkylation, and olefin polymerization, has been demonstrated. In some cases, compounds are formed which are difficult to prepare by other means, and some highly selective reactions have been found. 

One of the landmark achievements in the area of enantioselective catalysis has been the development of a large-scale commercial application of the Rh(I)/BINAP-catalyzed asymmetric isomerization of allylic amines to enamines. Unfortunately, methods for the isomerization of other families of olefins have not yet reached a comparable level of sophistication. However, since the early 1990s promising catalyst systems have been described for enantioselective isomerizations of allylic alcohols and aUylic ethers. In view of the utility of catalytic asymmetric olefin isomerization reactions, I have no doubt that the coming years will witness additional exciting progress in the development of highly effective catalysts for these and related substrates. 

Hydrides of Ni(I) and Ni(II) are known (37). A Ni(II) hydride appears to be an intermediate in the catalysis of olefin isomerization by phosphine complexes of nickel (61). Dilworth (62) has pointed out that stable hydride species are not obtained in model complexes with sulfur ligands. However, they may be possible within the confines of a protein chelate. 

Recently, it has been discovered that catalysis by rhodium compounds is more effective than by the older cobalt catalyst when tris(triphenylphosphine)rhodium chloride is treated with carbon monoxide, the catalyst bis(triphenylphosphine)rhodium carbonyl chloride is formed. This catalyst is very effective under very mild conditions (49-51). It is believed that the tr-ir rearrangement is also important with this catalyst and operates in a manner analogous to that in the cobalt-catalyzed process, since stablization of the cr complex has been shown to lead to olefin isomerization and lower linear selectivity (52). 

The behavior of stilbene radical cations in the semiconductor catalysis is in keeping with the result of photoisomerization of other olefins like 6-methylstyrene sensitized by electron acceptors like chloranil in polar solvents (48). The semiconductor photocata-lyzed isomerization of strained cyclobutanes to strained dienes (isomerization of quadricyclene to norbomadiene and similar reactions of complex cage compounds (49)) is related to the olefin isomerization discussed above. 

Olefin isomerization, with Claisen rearrangement, 1, 365 Olefin metathesis with alkyllead, 9, 415 in aqueous media, 1, 834 ESI—MS studies, 1, 812 in high-throughput catalyst discovery, 1, 365 in ionic liquids, 1, 869 for polymerization characteristics, 1, 149 Grubbs catalysts, 1, 151 Schrock catalysis, 1, 150... 

As in the hydroformylation of olefins, isomerization (of excess epoxide) occurs, producing ketones (23). Since the catalysis by dicobalt octacarbonyl is promoted by methanol, which is known to cause disproportionation,... 

The first example of fully aqueous metal catalysis of olefin isomerization was reported by Grubbs et al. in 1994 [2]. These authors adopted [Ru(H20)6](tos)2 (tos = p-toluenesulfonate) [8] as a catalyst, which is highly active for the ring-opening polymerization of strained cyclic olefin. Both allylic alcohol and allylic ethers undergo isomerization in the presence of [Ru(H20)6](tos)2. 

Olefin isomerization is common in petrochemical refining processes (heterogeneous catalysis) and, of course, follows the thermodynamic driving forces fran -olefins are more stable than their cis isomers, and internal olefins more stable than terminal olefins (eq. (1)). 

As far as the information goes, those metals which form ethylene complexes will also form complexes with other olefins, and vice versa this is paralleled in catalysis by the observation made in the first paragraph of this section, that the extent of olefin isomerization and exchange is always characteristic of the metal and substantially independent of the molecular weight of the olefin. 

In comparison to intense activities in the field of the solid superacids, not much work has been done on solid superbase catalysis[l]. Here we report first the preparation of a solid superbase which exhibits remarkably strong basicity, and then its synthetic application to the olefin isomerizations[2] and side-chain alkylations of alkylbenzenes with olefins[3]. The reactions proceed smoothly under mild reaction conditions to give the products quantitatively. 

The first successful achievements using asymmetric homogeneous transition metal catalysis were obtained in the asymmetric hydrogenation of alkenes24 25, This method has been successfully used in many synthetic applications (Section D.2.5.1.)26-29. In addition, chirally modified versions of the transition metal catalyzed hydrosilylation of olefins and carbonyl compounds (Sections D.2.3.1. and 2.5.1.) and olefin isomerization (Section D.2.6.2.) have been developed. Transition metal catalyzed asymmetric epoxidation constitutes one of the most powerful examples of this type (Section D.4.5.2.). 

In the last years a great interest was paid to the catalytic properties of iron-containing zeolites that show interesting activities in different industrial reactions. The Fe-BEA zeolite is reported to be a good catalyst in the vapour phase alkylation processes [1], the Fe-TON zeolite shows very high activity and selectivity in the olefin isomerization [2, 3]. Finally, new applications of zeolitic catalysts in the partial oxidation catalysis, such as the Solatia Inc. processes for benzene hydroxylation to phenol using Fe-MFI, open a novel route for the use of zeolites in oxidation processes [4, 5]. On the other hand, the catalytic properties of the metal-modified MOR type zeolite in the isomerization process are well known. 

Compounds Ni(olefin)2 have significant application in catalysis. Olefin-carbonyl complexes of the type [Ni(CO)j, (olefin) ] are not known. Their existence as unstable intermediate compounds in many reactions results from catalytic properties of tetracarbonylnickel in olefin oligomerization processes, oxo synthesis, olefin isomerization, etc. 

Coordinatively Unsaturated Cobalt Carbonyls Relevant to Hydro-formylation. The negative effect of carbon monoxide partial pressure on the rate of hydroformylation was the first indication of the participation of coordinatively unsaturated cobalt carbonyls in the catalysis of aldehyde formation and of the accompanying olefin isomerization. The retarding effect of carbon monoxide has also been observed in cobalt-catalyzed olefin and aldehyde hydrogenation and in various other reactions of cobalt carbonyls as well. It was assumed that in these reactions in fast reversible carbon monoxide dissociation highly reactive coordinatively unsaturated complexes are formed in very low concentrations, undetectable by conventional analytical methods. By using sophisticated new methods, in some cases the detection and characterization of these elusive species has become possible. 

In zeolite catalysis, carbenium- or carbonium-ion intermediates are energetically located at the top of the reaction energy barriers. In contrast, in superacid solutions, these protonated intermediates are ground-state reactants. The zeolite carbonium- and carbenium-ion transition state concepts are illustrated for C-C activation and olefin isomerization reactions below. ... 

In 2011, a general and highly enantioselective olefin isomerization via bio-mimetic proton-transfer catalysis with a novel cinchona alkaloid derived catalyst QDla was reported (Scheme 3.10). ... 

Trombetta, M., Busca, G., Rossini, S., Piccoli, V., andComaro, U. FT-IR studies on light olefin skeletal isomerization catalysis. Part I the interaction of C4 olefins with pure y-alumina. J. Catal 1997,168, 334-348. 

The same effect has been observed in the case of an encapsulated organogold catalyst of cycloi-somerization of enyne 441 by Scheme 5.23, and it has been attributed [22] to the hydrophobic environment within the coordination capsule 575, preventing carbenium-ionic intermediates of the catalytic process from their side reactions with water. The same caging catalyst has been used in [23] to perform the combined enzymatic and transition metal catalysis of tandem reactions by Scheme 5.24. The authors of this work also developed a tandem olefin isomerization-reduction reaction by Scheme 5.25 with the encapsulated organoruthenium cation as a catalyst on its... 

One of the present challenges, of particular relevance to commercial hydrosilation, is side reactions that lead to olefin isomerization and the formation of byproducts derived from the olefin. The cost associated with these nuisance reactions is huge, both in terms of raw material waste as well as waste-disposal expenses, not to mention production capacity loss and sometimes detrimental effects on the catalysis itself. All of these in turn drive up the price of the products affected, in an industry (chemical) that has become highly competitive. 

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