CO insertion/methyl migration

A. Heterolysis of the Metal-Carbon a-Bond Homolysis of the Metal-Carbon a-Bond Oxidation of Lm iM +1-R Followed by Homolysis P-Hydride Shift Reactions P-Elimination Reactions P-Elimination of Carboxylates CO Insertion/Methyl Migration... 

The Monsanto acetic acid process produces acetic acid from methanol and CO gas under fairly mild conditions (I80 C, 30-40 atm). The process utilizes a square planar Rh(l) catalyst. As shown in Figure 19.33, the first step in the catalytic mechanism is the OA of methyl iodide to form an 18-electron compound. In the second step, CO insertion (alkyl migration) occurs, resulting in a 16-electron species. Carbon monoxide adds to the vacant coordination site to r enerate a saturated compound, which then undergoes RE of CH3COI to regenerate the catalyst. The CH3COI product is further processed by reaction with water to make acetic acid and HI. The latter... 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

Unsymmetrical vicinal diols can be prepared from a three-component reaction of aldehydes, CO, and aminotroponiminate-ligated titanium dialkyl complexes. Solutions of Me2TiL,2 (L = N -dimethylaminolroponiminalc) react rapidly with CO at room temperature. Double methyl migration to CO produces an 2-acclonc complex which inserts the aldehyde to afford a titana-dioxolane and releases the unsymmetrical diol upon hydrolysis [65]. 

Insertion and migration refer to the process in which an unsaturated molecule inserts to a metal-anion bond. The two ways of describing this elementary step have been depicted in Figure 2.2 and 2.3 In the platinum complex shown an acetyl fragment is formed from a co-ordinated CO and a methyl group, both attached to platinum. Clearly, the two reacting groups should occupy positions cis to one another, otherwise the reaction cannot occur [1]. 

A detailed study of the CO insertion, or methyl migration, observing formation and decomposition of the transients, was performed so far only for one Cu(I) model system (93). It was reported that methyl radicals form transient complexes containing metal carbon -bonds with carbonmonoxide (n = 1, 3, 4) complexes of Cu(I). These complexes decompose yielding Cu(II) and acetaldehyde as final products via an copper acetyl intermediate formed by insertion of /migration of CH3 as described in Scheme 4. 

The product of this reaction appears to have formed by insertion of a CO group into an Mn—CHj bond. The reverse of this reaction is called dccarbonylation but may also be called dcinsertion or. more broadly, elimination. Infrared studies with °CO have revealed that the reaction actually proceeds by migration of Ihe methyl ligand rather than by CO insertion. 

Evidence for CO insertions into CF3-metal bonds, a reaction which is presumably the reverse of the methyl migration step, has been sought in several instances (25), but, as yet, no CF3-metal derivative generated in this manner has been isolated. However, upon UV irradiation of CF3Mn(CO)5 held at 17 K, vibrational spectra were obtained that were consistent with the formation of a trifluoroacetyl manganese species. The product was not separated (26). 

Elementary steps involving insertion or migration reactions are of prime importance for catalysis employing alkenes and carbon monoxide. Two examples taking place on a platinum centre have been depicted in Figs. 4.18 and 4.19. In these reactions an acetyl fragment is formed on the platinum centre from a coordinated CO and a methyl group. The important mechanistic difference... 

Oxidative addition of methyl iodide to the coordinalively unsaturated cobalt (I) species (1) gives the methyl complex (2) which undergoes CO insertion, probably via methyl migration. Elimination of iodine from the acetyl complex (3) and oxidative addition of hydrogen gives (5). Reductive elimination of the primary product acetaldehyde leads to the unsaturaied complex (6) which oxidatively adds iodine. The catalytic cycle is closed by the elimination of hydrogen iodide from (7), which is consumed by reaction with methanol to give methyl iodide. 

A special case of substitution reaction is the migratory insertion (see Migratory Insertion) reaction of alkyl or aryl metal carbonyls (see equation 58), by which an alkyl or aryl metal carbonyl is converted into an acyl or aroyl metal carbonyl by the action of a Lewis base. This reaction has been studied extensively, and in the case of Mn(Me)(CO)5 is found to proceed through a coordinatively unsaturated tetracarbonyl resulting from methyl migration on to one of the terminal CO groups in a cis position, (see equation 59). ... 

The CO of Ihe acetyl ligand has a choice of four cis positions into which it may shift, displacing the CO that is already there. One of these sites is occupied by CO. Thus we would predict that 25% of the product would have no CO and the other 75% would have a CO ligand cis to the methyl group. Experimentally it is found that 25% of the product is devoid of tagged CO. 25% of the product has CO Irans to CH. and 50% of the product has CO cis to CH,. Therefore CO insertion must be eliminated as a mechanistic possibility, a methyl migration mechanism, however. i.s consistent with these experimental results. 

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