Carbonyl migratory insertion reactions

The main steps in the catalytic MeOH carbonylation cyde which were proposed for the Co catalysed process [2] have served, with some modification perhaps in the carbonylation of MeOAc to AC2O, to the present day and are familiar as a classic example of a metal catalysed reaction. These steps are shown in Eigure 5.1. They are of course, (i) the oxidative addition of Mel to a metal center to form a metal methyl species, (ii) the migratory insertion reaction which generates a metal acyl from the metal methyl and coordinated CO and (iii) reductive elimination or other evolution of the metal acyl spedes to products. Broadly, as will be discussed in more detail later, the other ligands in the metal environment are CO and iodide. To balance the overall chemistry a molecule of CO must also enter the cycle. 

Compared with Rh systems, where the two principal species are well resolved, it can be seen that as well as more Ir species the bands also overlap, making quantification more difficult. Qualitatively some conclusions can be drawn from the spectra. Forster identified for example that in the presence of I", a potent catalyst poison, much of the Ir could still be present as [IrMe(CO)2l3] . Similarly, as [H2O] is increased the carbonylation rate falls. This is consistent with increased [T] since equilibrium [HI] increases with [H2O] as described above, inhibiting the migratory insertion reaction of [IrMe(CO)2l3] . 

Bianchini has reported that the migratory insertion reactions of [Pd(R)(CO)(P-P)]+ complexes (R = Me, Et) are reversible and follow first-order kinetics irrespective of the chelating diphosphine (P-P = dppp, dppe, meso-dppb, rac-dppb, meso-bdpp, rac-bdpp) [5e, f]. The free energies of activation for these reactions were calculated from the half-life times (tj 2) obtained by P( H HP NMR spectroscopy as all the rates of conversion of the methyl carbonyl complexes were independent of the CO pressure. Therefore, the AG values associated with the migratory insertion of the methyl carbonyl complexes could be straightforwardly calculated from the values using the equation AG = RT(ln k -ln kT/h) with = In First-order... 

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 migratory insertion see Migratory Insertion) reactions of manganese carbonyls have been studied for many... 

Protolysis also is a useful way of preparing Z —CO— adducts with the early transition metal centers. Typically, the early transition metal alkyls have carbanionic character and readily react with the acidic metal carbonyl hydrides. In the reaction of Cp2ZrMe2 with CpMo(CO)3H, methane is evolved and Cp2ZrMe2(OC)Mo(CO)2Cp forms (60,61). A low-frequency Z —CO— stretching frequency is observed at 1545 cm - and the formulation is confirmed by an X-ray structure determination. When this complex is placed under a CO atmosphere, a migratory insertion reaction occurs (Scheme 1) to produce a species having both f/2-acetyl and Z —CO— ligands. 

An example of the promotion of CO migratory insertion by a BrOnsted acid is found for the enhanced rate of CO uptake by Mn(CO)s(CH3) in the presence of dichloroacetic acid. Judging from the kinetic data, CO uptake is enhanced by both the monomer and the dimer of the acid (200). It was postulated that the basis for this enhancement in the migratory insertion reaction is hydrogen bonding with a carbonyl oxygen (24). In this connection,... 

The compound (dppe)Pt(OMe)2, which is formed in the reaction between NaOMe (2 equiv.) and (dppe)PtCl2, reacts with CO to give (dppe)Pt(C02 Me)2 Mechanistic studies reveal that the reaction rate is first order in (dppe)Pt(OMe)2 and first order in CO concentrations and, furthermore, when the reaction is carried out in CD3OD less than 10% of CD3O is incorporated into the Pt—COjMe ligand. This provides the first example of a carbonyl migratory insertion into an M—OR bond, rather than the more common attack by OR on a carbonyl ligand in the formation of metallacarboxylates. 

The transformation of an alkylmetal carbonyl complex into a metal-acyl complex is one of the most common types of migratory insertion reactions (Equation 9.3). Examples of CO insertion into metal-alkyl complexes are known for all of the transition elements. This reaction class has been the subject of review articles. These reactions occur by a family of diverse, delicately balanced reaction pathways the dominant mechanism depends on the reaction conditions, especially the solvent. Although these pathways are now imderstood in considerable detail, the precise identities of the intermediates in some of these reaction pathways are unknown. 

Mechanistic studies show that the first reaction proceeds via a migratory insertion reaction. In contrast the second was shown to proceed via initial displacement of aryloxide by CO followed by nucleophilic attack at Ihe coordinated carbonyl ligand. In the case of the bis(aryloxycarbonyl), subsequent thermolysis under CO leads to formation of a metal dicarbonyl, CO2, ArCOaAr and deoxygenation of one of the initial aryloxide ligands. A competing reaction is elimination of phenol and formation of a metallolactone. 

Migratory insertion reactions allow the generation of different kinds of Ni-G bonds. GO insertion is a classic example that has received much attention due to its involvement in catalytic and stoichiometric G-G bond formation processes.Although the carbonylation of cr-organonickel complexes is often followed by reductive elimination processes, many stable Ni-acyl complexes have been isolated. Recent examples of such reactions are shown in Equations (68) and (69). " Their formation is usually reversible, as demonstrated by the equilibrium shown in Equation (70), which indicates that the insertion of GO into Ni-aryl bonds is thermodynamically favored over the insertion into Ni-alkyl bonds. Acyl complexes containing Bp or Tp ligands have been prepared by carbonylation of the corresponding alkyl or aryl precursors. The ready carbonylation of the Tp derivative 137 (Equation (71))... 

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