Olefin isomerization catalytic cycle

Kinetics. Since the hydrogenation of 1-hexene is accompanied by isomerization to the internal olefin, the catalytic cycle involves an alkyl intermediate which must be formed by inserting the coordinated 1-alkene. Reaction 7 proposes a mechanism for the hydrogenation ... 

The formation of isomeric aldehydes is caused by cobalt organic intermediates, which are formed by the reaction of the olefin with the cobalt carbonyl catalyst. These cobalt organic compounds isomerize rapidly into a mixture of isomer position cobalt organic compounds. The primary cobalt organic compound, carrying a terminal fixed metal atom, is thermodynamically more stable than the isomeric internal secondary cobalt organic compounds. Due to the less steric hindrance of the terminal isomers their further reaction in the catalytic cycle is favored. Therefore in the hydroformylation of an olefin the unbranched aldehyde is the main reaction product, independent of the position of the double bond in the olefinic educt ( contrathermodynamic olefin isomerization) [49]. 

The catalytic cycles that have been documented, namely alkyne eyelotrimerization and olefin isomerization, demonstrate that addition and elimination from dimetal centers can occur readily in the presence of metal-metal bonds and alkoxide 1igands. 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

The [Os3(CO)io( t-H)( t-OSi)]surface catalyzes the isomerization and hydrogenation of olefins. When the hydrogenation of ethylene is carried out at 90 °C the trinuclear framework of the initial cluster remains intact in all the proposed elementary steps of the catalytic cycle [133]. However, at higher reaction temperatures the stability of the [Os3(CO)io( t-H)( t-OSi)]sujface depends on the nature of the reactant molecule. It is moderately active in the isomerization of 1-butene at 115 °C but decomposes under reaction conditions to form surface oxidized osmium species that have a higher activity [134]. 

If a catalytic cycle composed of several elementary processes is promoted on an isolated single site, we could make distinctions about the function of the active sites. For example, some metal complexes which are active for the isomerization reaction of olefins via alkyl intermediates are not effective catalysts for the hydrogenation reaction, and such differences in catalytic ability of the metal complexes is explained by the numbers of coordinatively unsaturated sites which are available for the reactions as described schematically in Scheme 7. 

It has been our goal to design a catalytic system theoretically. To the end of this goal, we have so far analyzed the organometallic reactions by using the ab initio MO calculations. Recently, we have completed the theoretical study of the catalytic cycle of hydrogenation by the Wilkinson catalyst (2), of which mechanism has been proposed by Halpern (3). This catalytic cycle shown in Scheme 1 consists of oxidative addition of H, coordination of olefin, olefin insertion, isomerization, and reductive elimina-... 

There are two possibilities in the reactions of la. The first is that la isomerizes to 1 due to the steric repulsion and that the same reaction path l- 2- 3- 3b is followed. The second possibility is that H2 oxidative addition to la takes place to give directly 3a and thus in the subsequent catalytic cycles intermediates always have cis phosphines. Olefin coordination to 3a is prohibited because of the steric repulsion between the bulky olefin and two bulky phosphines cis to the olefin. Thus 3a->3b isomerization has to take place before the catalytic cycle proceeds. 

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

Scheme 14. Proposed catalytic cycle for the isomerization of olefins catalyzed by H2Os3(CO)i0, according to (348). [From G. Siiss-Fink and F. Neumann, in The Chemistry of the Metal-Carbon Bond (F. R. Hartley, ed.), Vol. 5, p. 303. Wiley, New York, 1989. Reprinted by permission of John Wiley Sons, Ltd.]...

Isomerization of terminal olefins by HCo(CO)4, or more likely by the coordinatively unsaturated HCo(CO)3 in equilibrium with it, proceeds rapidly at room temperature. The isomerization is catalytic but the HCo(CO)3 4 is consumed irreversibly by simultaneous hydroformylation, which removes 2 mol of the hydridocarbonyl for each mole of reacted olefin. The competition between these two reactions (as well as the bimolecular decomposition of the hydrocarbonyl to and Co2(CO)g) depends on the conditions of the experiment. The results obtained with 4-methyl-1-pentene under one atmosphere of N2 are shown in Fig. 1 and the catalytic cycle, which rationalizes the stepwise isomerization, is shown in Fig. 2. In Fig. 2 HM represents either HCo(CO)4 or HCo(CO)3. In experiments on the isomerization of PhCD2CH=CH2 with HCo(CO)4 in the presence of unlabeled p-allyltoluene both PhCI>=CHCH3 and labeled 4-propenyltoluene were found in the products indicating hydrogen transfer between olefins via complexed HM. The... 

Koga and Morokuma also investigated the alternative catalytic cycle proposed based on molecular modeling and NMR experiments performed by Brown [14-18], which suggested that isomerization takes place at an early stage after oxidative addition of molecular hydrogen, but before olefin coordination [19]. The cyclohexene substrate studied in this work coordinates more easily to cis-bisphosphine intermediates in comparison to the frans-bisphosphines studied previously (Scheme 2). 

Besides primary reactions that constitute the above-described chain growth cycle, readsorption of olefins, and in particular ethylene, can result in the formation of alkyl species, some of which depolymerize to form CH2 species, and thus, should be accounted for at results interpretation. The list of other secondary reactions of 1-olefins includes their isomerization, cracking, and hydrogenolysis. However, the contribution of olefins into the catalytic cycle is strongly determined by reaction conditions. 

---