Alkyl migration 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 inversion of configuration at the iron atom in Eq. (13) is in accord with the proposed alkyl migration mechanism for the decarbonylation reaction22,32,12K... 

The third experiment differentiates conclusively between Mechanisms 2 and 3. The CO migration of Mechanism 2, with CO cis to the acyl ligand, requires migration of CO from the acyl ligand to the vacant site. As a result, 25% of the product should have no CO label and 75% should have the labeled CO cis to the alkyl, as shown in Figure 14-10. On the other hand, alkyl migration (Mechanism 3) should yield 25% with no label, 50% with the label cis to the alkyl, and 25% with the label trans to the alkyl. Because this is the ratio of cis to trans found in the experiment, the evidence supports Mechanism 3, which is the accepted pathway for this reaction. 

When octahedral alkylpentacarbonyl derivatives of manganese(I) undergo carbon monoxide insertion assisted by a nucleophile L different from carbon monoxide (L = tertiary phosphines, amines, CO, etc.), the cis product is initially formed this observation, however, is not sufficient to permit one to distinguish between the alkyl migration mechanism [reaction (a)] and the insertion of a precoordinated carbonyl group into the manganese-carbon bond [reaction (e)]. In both cases mutually cis positions are involved. 

Carbonylation of the isomeric cationic solvated complexes of platinum(II) C and D [the corresponding palladium(II) species react at a much faster rate] proceeds via an alkyl migration mechanism in a study based on NMR detection of the intermediates. This conclusion is in agreement with the available data in the literature kinetic data should, however, complement the information concerning the palladium(II) and plati-num(II) systems. 

As mentioned in 11.3.2.1.1, product analysis and mechanistic studies on these compounds and on the corresponding ruthenium analogues led to the conclusion that these processes involve exclusively an alkyl migration mechanism. 

Because of the importance of the CO insertion into a transihon metal-carbon a-bond in relation to the transition metal-catalyzed carbonylation, the mechanism of the CO insertion has attracted considerable attenhon [1,2]. Through fundamental studies on model complexes, the route of CO insertion has been established for most transition metal complexes to be the one through alkyl migration mechanism [3-8]. While CO insertion can occur with various metal-heteroatom o-bonds, only insertion into metal alkyls (hydrocarbyls) will be discussed in this chapter. 

Although the CO insertion proceeds by the alkyl migration mechanism, especially for the group 10 transition metal alkyls, the acyl complex may undergo the subsequent isomerization, depending on the nature of the ligand situated at the site trans to the acyl ligand. The presence of the rapid equilibrium makes the clear-cut determination of the mechanism a difficult matter. 

Extended CNDO calculations support the alkyl migration mechanism for the well-known process in ... 

XIV) and L [Eq. (50)] may be best rationalized on the basis of the intermediacy of square-pyramidal (XVII), which arises from (XIV) via a concerted approach of Et and CO(l), accompanied by the opening of the Cl(l)—Ir—L bond angle. It is entirely possible that this mechanism of cooperative migration may be as general, if not more so, than that of alkyl migration. 

Key steps of the polymerization mechanism are the coordination of the monomer to the coordinatively unsaturated cationic metal center followed by alkyl migration (insertion) into the metal-carbon bond to form the polymer chain and recreating the vacant coordination site, which allows the subsequent coordination of the next monomer ... 

The validity of this mechanism has been demonstrated for a number of CO insertion reactions. Thus when chemists use the term CO insertion, they usually mean alkyl migration. Several things to keep in mind when considering a reaction of this type are (I) it involves ligands which are cis to one another. (2) in the course of the reaction a vacant coordination becomes available, and (3) the reverse reaction cannot occur unless a ligand is first eliminated. 

Metal-alkyl bonds can be carbonylated and metal-acyl bonds can be decarbonylated32). The main mechanism suggested for the carbonylation reaction is alkyl migration from the metal to a terminal CO group with entrance of a ligand L, and vice versa for decarbonylation32). 

The plumbylene species described, PbMe2, disproportionated into metallic lead and Me4Pb. This is analogous to the proposed mechanism involving 1,2-alkyl migrations from... 

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