Mechanisms hydroformylation, 36

The essential features of the alkene hydroformylation mechanism proposed by Heck and Breslow [61] remain intact, after many investigations using a variety of techniques. The cycle shown in Scheme 3.3 is that for the unmodified, cobalt... 

Scheme 3.3 Alkene hydroformylation mechanism for an unmodified cobalt catalyst.

The first basic step in a simplified version of the hydroformylation mechanism is alkene coordination to the cobalt center ... 

The basic steps in the hydroformylation mechanism do not change in the presence of ligand-modified cobalt, but the kinetics of the reaction is affected.30 First-order dependence of the rate on hydrogen partial pressure and an inverse first-order dependence on CO partial pressure were observed in the unmodified system.36 At high CO pressures the rate is first-order in both alkene and cobalt concentrations. The last, product-forming, step—the cleavage of the acylcobalt... 

The metal hydride mechanism was first described for the cobalt-carbonyl-catalyzed ester formation by analogy with hydroformylation.152 It was later adapted to carboxylation processes catalyzed by palladium136 153 154 and platinum complexes.137 As in the hydroformylation mechanism, the olefin inserts itself into the... 

It was shown15) by asymmetric hydroformylation of linear butenes that asymmetric induction in hydroformylation occurs substantially before or during metal alkyl intermediate formation, which, according to the accepted hydroformylation mechanism 2I-57,58), is the second step in the catalytic process (see also Sect. 5.1). 

Finally, the above research has resulted in a better understanding of the hydroformylation mechanism, particularly of the first substantially irreversible step of the reaction and, consequently, has allowed us to formulate a possible model for the regulation of the isomeric composition of the hydroformylation products which, on the basis of our experiments, seems to be mainly connected with steric factors. 

Modified from reference mthenium-catalyzed hydroformylation mechanism invoiving seif-assembiing ligands and informed by ESI(+)-MS analysis. 

Rhodium catalyzed hydroformylation mechanism P. W. N. M. van Leeuwen, C. Claver, Rhodium catalyzed hydroformylation, Kluwer Academic Publishers, Dordrecht, 2002. 

Unlike the hydrogenation catalysts, most iridium catalysts studied for hydroformylation chemistry are not particularly active and are usually much less active than their rhodium counterparts see Carbonylation Processes by Homogeneous Catalysis). However, this lower activity was useful in utihzing iridium complexes to study separate steps in the hydroformylation mechanism. Using iridium complexes, several steps important in the hydroformylation cycle such as alkyl migration to carbon monoxide were studied. Another carbonylation reaction in which iridum catalysis appears to be conunercially viable is in the carbonylation of methanol. ... 

Simplified version of the hydroformylation mechanism. Note that other ligands on the Rh are not shown for ease in illustrating what happens with reactants and products. 

These few examples may show that the hydroformylation mechanism is still under investigation, even after more than 50 years of successful operation in industrial plants. 

The hydroformylation mechanism for phosphine-modified rhodium catalysts follows with minor modifications the Heck-Breslow cycle. HRh(CO)(TPP)3 [11] is believed to be the precursor of the active hydroformylation species. First synthesized by Vaska in 1963 [98] and structurally characterized in the same year [99], Wilkinson introduced this phosphine-stabilized rhodium catalyst to hydroformylation five years later [100]. As one of life s ironies, Vaska even compared HRh(CO)(TPP)3 in detail with HCo(CO)4 as an example of structurally related hy-drido complexes [98]. Unfortunately he did not draw the conclusion that the rhodium complex should be used in the oxo reaction. According to Wilkinson, two possible pathways are imaginable the associative and the dissociative mechanisms. Preceding the catalytic cycle are several equilibria which generate the key intermediate HRh(CO)2(TPP)2 (Scheme 4 L = ligand). 

Figure 9. Dissociative hydroformylation mechanism with the model substrate ethylene (M = Rh, Ir L = CO, PH, cf. [7]).

The alkyl mechanism is the preferred pathway of isomerization, if the catalytic species contains a metal-bonded hydride as illustrated in Scheme 2. It is reminis-cent of the hydroformylation mechanism (see Section 2.1.1). 

Scheme 5. The catalytic cycle proposed for the hydroformylation mechanism with rac-1 as catalyst precursor. The two intramolecular hydride transfer processes reflect the cooperativity of the two rhodium centers.

The proposed bimetallic hydroformylation mechanism for 15r is shown in Fig. 13 and is based on our DFT calculations. In many ways the mechanistic steps parallel those for the dicationic catalyst, and bimetallic cooperativity once... 

Recent results of model reactions with isolated hydrido-, alkyl-, and acylcobalt carbonyls, and their kinetic and spectroscopic investigations have provided a more detailed picture of the classic Heck and Breslow mechanism. This may serve toward a better understanding of the hydroformylation mechanism with other metal catalysts. 

The first investigations in the framework of hydroformylation by Otto Roelen and successors were based on ligand-free systems. Of course, such catalysts are not constituted by the naked metal. They host, besides H and CO, diolefins, carboxylates, or halogenides, which are replaced either in the beginning or throughout the hydroformylation mechanism by solvent molecules, substrates, or reagents. Such unmodified catalysts are still in use and give excellent results in numerous applications. 

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