Insertion involving carbonyls migratory

The carbomethoxy cycle starts with the attack of a methoxy group at a coordinated carbonyl group or a migratory insertion of CO in a palladium methoxy bond. Any type of methoxy species will have a low concentration in the acidic medium of the reaction. In Figure 12.20 many details of these reactions, discussed above in section 12.2, have been omitted and only a shorthand notation is presented. Subsequently insertion of ethene takes place. It is known from stoichiometric experiments that both reactions are relatively slow. In the final step a formal protonation takes place, which as we saw before, may actually involve enolate species. 

The mechanism of palladium-catalyzed carbonylation of organic halides is generally assumed to involve oxidative additon of R-X to a Pd(0) species which is formed from the precursors on the action of CO + OH . Migratory insertion of R onto a coordinated CO followed by reaction with a nucleophile generates the product and gives back the catalytically active palladium(O) species (Scheme 5.4 A). 

Negishi, E. L, de Meijere, A., Eds. Palladium-Catalyzed Carbonylation and Other Related Reactions Involving Migratory Insertion. In Handbook of Organopalladium Chemistry for Organic Synthesis-, Wiley-Interscience New York, 2002 Vol. 2, pp 2309-2714. 

There are a few reports of silyl-carbonyl complexes that decompose by processes that may involve a migratory insertion. These reactions lead to siloxide derivatives by ill-defined processes that involve inter- or intramolecular silyl migration6,7 1014 and most commonly result in cleavage of the carbon monoxide C-O bond (e.g. equation 93)247. 

A dynamic density functional study of methane photo-carbonylation by the Rh(PH3)2Cl catalyst has been reported [73]. It involves C-H bond activation to produce Rh(PH3)3Cl(H)(CH3)(CO) followed by migratory insertion of CO into the Rh-CH3 bond to generate Rh(PH3)2Cl(H)(CH3CO) and, finally, the elimination of acetaldehyde to form Rh(PH3)2Cl(CO) H)(CO)(CH3CO)... 

The preparation of metal carbamoyl [M-C(0)NR2] and silaacyl [M-C(0)SiR3] derivatives by the migratory insertion of CO in the appropriate metal amide (M-NR2) or silyl (M-SiR3) complexes is also precedented. " Although CO reacts with metal-acyl bonds to give a-ketoacyl compounds, this and related reactions that appear to be double carbonylations but do not actually involve two consecutive CO migratory insertions have also been teported. ... 

In order to avoid or minimize redundancy, the 19 General Patterns are classified into six categories, as summarized in Table 1, and all patterns in each category are discussed as a unit. Many processes of Pd complexes involve a pair of patterns that are miCToscopic reversals of each other, such as complexation and decomplexation (or dissociation) in ligand substitution. Migration of Pd via a series of hydropalladation-dehydropalladation and reversible carbonylation via migratory insertion-deinsertion are additional representative examples. 

This reaction profile, also called carbonylation, governs the reactivity of Pd-carbonyl complexes. Anionic M[Pd(CO)l3], for instance, catalyzes the reductive carbonylation of esters.f On the other hand, Pd(CO)(PPh3)3 was reported to catalyze the carboxymethy-lation of organic halides and the cyclocarbonylation of cinnamyl halides.f " However, the Pd-CO complexes are most often generated in situ from preformed alkyl -palladium complexes and CO under stoichiometric or catalytic conditions, for example, in the copolymerization of alkenes and CO. Decarbonylation reactions also involve the intermediacy of Pd-CO complexes. In this case, migratory deinsertion (Sect, n.3.1), that is, the microscopic reversal of the migratory insertion, takes place. 

The third route involves abstraction of CO from an organic compound. This can happen for aldehydes, alcohols, and even CO2. In the example shown in Eq. 4.7, the reaction requires three steps the second step is the reverse of migratory insertion. The success of the reaction in any given instance relies on the thermodynamic stability of the final metal carbonyl product, which is greater for a low-valent metal. Note that the first step in the case of an aldehyde is oxidative addition of the aldehyde C—H bond. It is much more difficult for the metal to break into a C—C bond. This means that ketones, R2CO, are usually resistant to this reaction. 

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