CO insertion mechanism

Another aspect of stereochemistry of the CO insertion which has received attention concerns the actual process of formation of the acyl moiety from the coordinated CO and R. Three possible pathways may be envisaged. First, the alkyl moves from the metal onto an adjacent CO. This is known as the alkyl migration mechanism. Second, a coordinated CO moves to insert into the M—R bond—a CO insertion mechanism. Third, both CO and R move in a cooperative manner. These three pathways are represented schematically in Eq. (46). 

The presented CO insertion mechanism (Scheme 11.2) partly follows the one proposed by Pichler and coworkers,31 32 but differs in several important points. In... 

The mechanisms proposed over the last 50 years for the Fischer-Tropsch synthesis, principally on the basis of studies using heterogeneous catalyst systems, may be divided into three main classes (a) metal-carbide mechanisms (b) hydroxyl carbene, =CH(OH), condensation mechanisms and (c) CO insertion mechanisms. 

The seemingly plausible Scheme shown in 4 is inconsistent with the results of the 13C0 labeling study as are most schemes which do not involve CO insertion for the chain propagation. We believe that ethylene arises from the same sequence of steps as the other hydrocarbon products. The role of the second metal center in the reduction cannot be described. We believe that the iron-iron bond is cleaved early in the reaction since the reduction in the presence of PBu3 produced the unsubstituted species, LiCpFe(C0)2. While there is too little information currently available to assess the importance of Scheme 3, our results on reduction in this iron system are not consistent with the normal CO insertion mechanism or with carbene oligomerization. We suggest Scheme 3 until further research can be accomplished. 

The question of the mechanism of Fischer-Tropsch reaction is of considerable controversy. Three principal routes for product formation have been proposed the carbide mechanism, the hydroxymethylene mechanism, and the CO insertion mechanism. Numerous modifications were also introduced in attempts to account for some details in the complex chemistry of the process.205 207 208 211 229-233... 

The 18-electron complexes CH3Mn(CO)5 and CpFe(CO)2Me have proved particularly amenable to studies of the CO insertion mechanism. With I3CO and I4CO as labeled reagents, it has been shown that... 

We then focus on the Pichler-Schulz CO insertion mechanism (39). This reaction has been much less investigated than the carbide mechanism. We recognize that in homogeneous catalysis, alkene hydroformyla-tion has been investigated extensively it appears that hydroformylation is much more difficult on metallic surfaces than in the presence of mononuclear cationic metal complexes (40). 

The CO insertion mechanism is of interest because its sequence allows CO dissociation and initiation of the reaction to become rate limiting. The chain-growth intermediate that is incorporated into the growing chain is CO itself and not a "Ci" species that first has to be generated. 

We conclude from the available computational results that it is unlikely that chain growth proceeds via the CO insertion mechanism. However, the growth of alkyl chains can be terminated by CO insertion, and alcohols, aldehydes, or even carboxylic acids can be produced as a result. 

The CO insertion mechanisms of the Pichlcr-SchuU type have been largely influenced by principles of homogeneous catalysis and are related to the mechanism proposed for the hydroformylation reaction (sec Mlydroformylation this volume. 

The significance of these results was, however, questioned by Ilcnrici-Olive and Olive, who claimed that all of these findings were compatible with a CO insertion mechanism as well [126]. Titus, further studies are ncccssar> in order to obtain an unified picture of the Pischer-Tropsch mechanism. 

The first convincing example of CO insertion into a metal-alkyl bond dates back to 1957, when Coffield demonstrated the reversible reaction of MnCH3(CO)5 with CO to give Mn(CH3CO)(CO)5 [9]. This CO insertion mechanism, which has been extensively studied by kinehc and/or spectroscopic means since the 1960s, has been regarded as a prototype of other CO insertion systems [10-12]. The important conclusions derived through these studies are summarized below. 

In Section 24.7, we stated that the distribution of the products in Figure 24.15 is consistent with the migration of the Me group, and not with a mechanism that involves movement of the inserted CO. Confirm that this is true by determining the distribution of products for the CO insertion mechanism and comparing it with that for the Me migration mechanism. 

The three mechanisms that consider the chemical identity of the monomeric building block and the nature of hydrocarbon chain growth include the CO-insertion mechanism, the hydroxycarbene mechanism, and the carbene mechanism. Each mechanism is proposed in the literature as the probable Fischer-Tropsch reaction paths [10-12] ... 

Although these experimental results aU support the double carbonylation mechanism through the bis-acylpalladium intermediates, a report has been made claiming the consecutive CO insertion mechanism. The work seems to need further validation. 

Fig. 11 Schematic diagram of the CO insertion mechanism. Reprinted from ref. 16 with permission from Elsevier.

Chain initiation. As illustrated by both the carbide mechanism and the CO insertion mechanism, the FTS reaction is initiated through CO activation. The divergence for two mechanisms is that the carbide mechanism requires CO to firstly dissociate, while the CO insertion mechanism proposes CO to be hydrogenated at the initial step. Some recent density functional studies based on periodic catalyst models have evaluated the possibility of these different activation pathways by calculating the activation energy barriers of the involved steps under different reaction conditions (temperatures, pressures and coverages). Here, we will discuss these results in both direct and H-assisted routes. 

CO insertion. The CO insertion mechanism, in which CO inserts into R-CH2 directly, was originally proposed by Pichler and Schulz. Hu and Saeys have calculated the activation barrier for the CH2-I-CO coupling step. The barrier for insertion is about 1.87 eV, being too high to... 

Fig. 14 Possible propagation reaction paths for the CO insertion mechanism. The reaction starts by C-C coupling (" + CO "), followed by hydrogenation (" + H ") and C-O scission steps ("O "). The full arrows indicate the dominant reaction path and the dotted arrows the minor reaction paths. R represents hydrogen or an alkyl group. Reprinted from ref. 39 with permission from Elsevier.

---