Olefins hydroformylation catalytic cycle

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]. 

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). 

Two possible mechanisms are suggested for the hydroalkoxycarbonylation of alkenes. One is similar to that of hydroformylation in which a catalytic cycle starts with a hydridometal complex (Scheme 14, path Here, olefin... 

Figure 1 outlines the key intermediates of a catalytic cycle where the rate-determining step is the formation of an n-alkyl derivative of the trans-bisphosphine via a coordinated olefin complex. This presumed catalytic cycle appears to satisfy proposals by Cavalieri d Oro et al. (9), C. V. Pittman et al. (10), and J. Hjortkjaer (II). Although no single mechanism of hydroformylation was established, the cycle is shown here to illustrate the key nature of the equilibrium between the trisphosphine (D) and the trans-bisphosphine (E). 

The widely accepted mechanism for olefin hydroformylation using a HRh(PR3)2(CO) catalyst system was proposed over 30 years ago by Wilkinson et al [104]. The catalytic cycle comprises many of the fundamental reac-... 

The catalytic cycle for the cobalt-based hydroformylation is shown in Fig. 5.7. Most cobalt salts under the reaction conditions of hydroformylation are converted into an equilibrium mixture of Co2(CO)8 and HCo(CO)4. The latter undergoes CO dissociation to give 5.20, a catalytically active 16-electron intermediate. Propylene coordination followed by olefin insertion into the metal-hydrogen bond in a Markovnikov or anti-Markovnikov fashion gives the branched or the linear metal alkyl complex 5.24 or 5.22, respectively. These... 

One major advantage offered by the dppf ligand in Rh-catalyzed olefin hydroformylation is exemplified in its higher linear aldehyde selectivity when present in a dppf Rh ratio of 1.5 or higher [37,242]. This result leads to the proposed key intermediate of a Rh dimer with both chelating and bridging phosphine in the catalytic cycle. It also confirms the significance of the tris (phosphine) moieties at the point when the aldehyde selectivity is determined, i.e., the step in which the hydride is inserted into the M-olefin bond. This involvement of a dinuclear or tris (phosphine) intermediate appears to differ from the intermediate RhH(CO)(PR 3)z (olefin) (which is converted into the square planar Rh(R)(CO)(PR 3)2 by hydride insertion) commonly accepted for hydroformylation catalyzed by monophosphine complexes. P NMR studies also established the existence of the equilibrium in which the disphosphine can be... 

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... 

Another catalytic cycle studied by Matsubara, Morokuma, and coworkers [77] is the hydroformylation of olefin by an Rh(I) complex. Hydroformylation of olefin by the rhodium complex [78-80] is one of the most well known homogeneous catalytic reactions. Despite extensive studies made for this industrially worthwhile reaction [81, 82], the mechanism is still a point of issue. The active catalyst is considered to be RhH(CO)(PPh3)2, 47, as presented in Fig. 25. The most probable reaction cycle undergoes CO addition and phosphine dissociation to generate an active intermediate 41. The intramolecular ethylene insertion, CO insertion, H2 oxidative addition, and aldehyde reductive elimination are followed as shown with the surrounding dashed line. Authors have optimized the structures of nearly all the relevant transition states as well as the intermediates to determine the full potential-... 

In this review we summarized the results of the latest ab initio studies of the elementary reaction such as oxidative addition, metathesis, and olefin insertion into metal-ligand bonds, as well as the multistep full catalytic cycles such as metal-catalyzed hydroboration, hydroformylation, and sila-staimation. In general, it has been demonstrated that quantum chemical calculations can provide very useful information concerning the reaction mechanism that is difficult to obtain from, and often complementary to, experiments. Such information includes the structures and energies of unstable intermediates and transition states, as well as prediction of effects of changing ligands and metals on the reaction rate and mechanism. 

The catalytic processes so far described contained catalytic cycles comprised of a few elementary processes and elucidation of the mechanisms was relatively straightforward. By combination of multiple elementary processes the scope of the catalysis can be further expanded. We have already dealt with hydroformylation of olefins, which uses three types of substrates and the catalytic process consists of at least three elementary processes. For obtaining the aldehydes in good selectivity, the catalysis must proceed by combination of proper sequence of elementary processes. In the following examples, we shall deal with other types of catalytic processes composed of multiple steps of elementary processes. 

The mechanism of hydroformylation by Pt/Sn systems has been investigated with the help of model complexes (Scheme 42). Only platinum SnCla complexes react with H2 to give EtCHO and close the cycle. 4-Pentenal is cyclized to cyclopentanone by cationic rhodium catalyst such as [Rh(dppe)2] in nitromethane or dichloromethane at 20 °C. The initiating step of the process is the oxidative addition of aldehyde-CH to the Rh(I) centre, a reversal of the final step in an olefin hydroformylation sequence. The mechanism was probed by deuteration studies direct evidence for the catalytic intermediates by NMR was unobtainable. The intermediates are involved in the reversible formation of side products, although selectivity to cyclopentanone can be as high as 98%. The essential features of the reaction are outlined in Scheme 43. ... 

The complexes are arranged in order of increasing catalytic activity in hydroformylation (substrate conversion after 6-h reaction) as follows (REOP(OPPh))2Rh(acac) < (REPPh2)2Rh(acac) < (REOPPh2)2Rh(acac) < ((REO)2PPh)2Rh(acac). It was shown that the catalysts, which remain in the aqueous phase after the reaction, can be reused (for five catalytic cycles in 6 h, a constant conversion of the initial olefin is attained). 

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