Olefin hydrogenation intermediates

To test this theory, a mixture of n-hexane and Relabeled 1-hexene was reacted in hydrogen over the catalyst at various space velocities. The specific activity of each of the products (the n-hexenes were lumped together) are shown in Figure 2. The important observation is made at zero conversion. When extrapolated to Infinite space velocity, the benzene has approximately the same specific activity as the hexene, thus clearly indicating that essentially all the benzene is formed in a reaction sequence that involves equilibrium with gaseous n-hexenes. It may then be concluded that olefins are intermediates in the aromatiza-tion process. 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Kinetic analyses and deuterium-labeling experiments have demonstrated that, remarkably, the reductive elimination of TEA and the formation of intermediate C is the rate-determining step in the (de)hydrogenation cycle. Accordingly, hydrogenation of the acceptor appears to be slower than dehydrogenation of the alkane substrate. This contrasts with the fact that catalytic olefin hydrogenation is well-established in transition-metal-mediated chemistry [10]. 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

P-31 NMR was a powerful tool in studies correlating the structure of tertiary-phosphine-rhodium chloride complexes with their behavior as olefin hydrogenation catalysts. Triphenylphosphine-rhodium complex hydrogenation catalyst species (1) were studied by Tolman et al. at du Pont and Company (2). They found that tris(triphenylphosphine)rhodium(I) chloride (A) dissociates to tri-phenylphosphine and a highly reactive intermediate (B). The latter is dimerized to tetrakis(triphenylphosphine)dirhodium(I) dichloride (C). 

Although hydrogenation of carbocyclic aromatics usually leads to the fully saturated derivative, there is abundant evidence that many reductions proceed through intermediate olefins. Hydrogenation of cre-sols was assumed to proceed through formation of all kinds of 1,2-dihydro-cresols produced in equal amounts (38) and hydrogenation of xylenes to afford equal quantities of all tetrahydroxylenes as intermediates (36). 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

Rhodium-olefin complexes have been identified as intermediate species in rhodium-catalyzed olefin-to-olefin addition reactions (5a, 150a, 151) and olefin hydrogenation reactions (450). Although the ethylene-Rh(I) complexes are not in themselves catalysts for dimerization of ethylene, both [(C2H4)2RhCl]2 and (C2H4)2Rh( Cac) react with... 

Diimide occurs only as an unstable intermediate in the hydrolysis of azodi-carboxylic acid, in the alkaline cleavage of benzene sulfonyl hydrazide, hydroxyl-amine-O-sulfonic acid and chloramine, in the oxidation of hydrazine, and in several other organic decomposition reactions . At room temperature it readily undergoes decomposition, disproportionation, and in the presence of symmetrical multiple bonds (like the ones in olefins) hydrogen transfer reactions ... 

Mills et al. IS) proposed formal reaction schemes in which conversion proceeds through olefinic reaction intermediates, such as the one illustrated by Fig. 10 for the Ce-hydrocarbons. They introduced the concept of separate catalytic functions, in terms of an isomerization activity—operative on the olefinic intermediate—associated with the acidic oxide base and a hydrogenation activity associated with platinum. Ciapetta and Hunter... 

Ethers are also attacked by metal atoms e.g., tetrahydrofuran (THF) oxidatively adds to Mo atoms to yield a reactive intermediate that may serve as an olefin hydrogenation catalyst... 

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