Migratory reductive elimination process

The migratory reductive elimination process is consistent with the kinetic data reported for C-C reductive elimination from c/5-PdMe(C6H4-p-Y)(PEt2Ph)2 complexes (Scheme 9.5) [16], The rate constants exhibit a good Hammett correlation with Ojt values of Y, which are synthetic parameters introduced by Yukawa and Tsuno for isolating resonance effects. On the other hand, no correlation was observed with o or Op values. Positive sign of the p value (-1-3.2) indicates a major contribution of tt-electrophilicity of aryl ligand in the C-C bond formation. 

Ruthenium complex-catalyzed addition of C-H bonds of aromatic esters to olefins involves C-C reductive elimination as the product forming step [74], for which a migratory reductive elimination process involving a zwitter ionic intermediate 21 has been proposed (Scheme 9.33) [75]. 

A proposed mechanism for this reaction is shown in (Scheme 24). Insertion of the less hindered olefin moiety of a diene into the [Y]-H bond forms G-I. Car-bocyclization of G-I gives G-II, and the subsequent cr-bond metathesis with a hydrosilane yields the product and regenerates Cp 2YH THF. The observed high catalyst activity of the Cp 2YMe and Cp 2LuMe complexes relative to Ni(0) and Rh(I) complexes for this carbocyclization is ascribed to the Lewis acidity of the metal center and the presence of an open coordination site. These features favor both /3-migratory insertion and a-bond metathesis over oxidative addition and reductive elimination processes that are preferred in the Ni(0) and Rh(I) catalyst systems. 

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))... 

The current mechanistic understanding of these reductive cyclization processes is largely conjecture. Stepwise oxidative addition, migratory insertion, and reductive elimination (see Scheme 26) is a widely proposed mechanism. However, other mechanisms - such as initial cyclometallation - are to afford a rhodacyclopentadiene followed by either oxidative addition to a rhodium(v) intermediate or (perhaps more likely) bond metathesis with an additional molecule of silane (Scheme 28). 

The basic steps are well known after oxidative addition of HCN, we find coordination of ethene, migratory insertion of ethene into the nickel hydride bond, and reductive elimination of ethyl cyanide (propanenitrile). More detailed studies by Du Pont s McKinney and Roe [3] have shown that the productive cycle involves the reductive elimination by the process shown in Figure 11.2. 

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. 

Hydroformylation (the oxo process) involves the addition of H2 and CO to an olefin to form aldehydes (eq. 2.8), which have a number of important industrial applications. Extensive mechanistic studies have shown that this reaction involves migratory insertion of a bound alkyl group (formed by insertion of an olefin into a metal hydride) into a bound CO, followed by reductive elimination of the aldehyde. The rate-limiting step for the hydroformylation in liquids is either the reaction of olefin and HCo(CO)4 or the reaction of the acyl complex with H2 to liberate the product aldehyde. The high miscibility of CO in sc C02 is therefore not necessarily a major factor in determining the rate of the hydroformylation. Typically, for a-olefins, linear aldehydes are preferred to branched products, and considerable effort has gone into controlling the selectivity of this reaction. 

The previous sections have described C-P bond formation by classical organome-tallic processes, such as migratory insertion and reductive elimination. However, there is evidence in some other systems that the metal catalyst activates the organophosphorus substrate for direct nucleophilic attack on an electrophile [43]. 

Even without mechanistic information, one can begin to rationalize and, perhaps more importantly, predict various catalytic organopalladium reactions in consultation with Table 3 and Scheme 3. For example, the following four reactions shown in Scheme 5 are representative of the four most important types of Pd-catalyzed C—C bond formation processes discussed in detail in Parts III-VI. It is useful to note that only four patterns in Table 3, that is, (i) carbopalladation, (ii) reductive elimination, (iii) migratory insertion, and (iv) nucleophilic (or electrophilic) attack on ligands, can achieve C—C bond formation. This summary can also be appropriately modified for the formation of other types of bonds, such as C—H, C— M, C— X, and X— X bonds, where M is a metal and X is a heteroatom. 

A synthesis of bicyclo[5.3.0]decatrienes through a Rh(I)-catalysed cycloisomerization of 3-acyloxy-4-ene-l,9-diynes has been reported " to proceed by [l,2]-acyloxy migration, 6n electrocyclization, migratory insertion, and reductive elimination. The overall process viewed as a intramolecular 5 -I- 2-cycloaddition with concomitant [l,2]-acyloxy migration (Scheme 146). 

Much of the organometallic reactivity of late-metal-amido complexes is presented in later chapters of this text. In general, these complexes are reactive toward many classic organometallic processes, such as reductive elimination, migratory insertion, and 3-hydrogen... 

Early-transition-metal-alkoxo complexes are also stable toward other typical organome-tallic processes, such as reductive elimination. Hydridometal-alkoxo, alkylmetal-alkoxo, and arylmetal-alkoxo complexes of the early metals are known, but none of them undergoes reductive elimination to form a carbon-oxygen bond. Early-metal-alkoxo complexes also do not tend to undergo migratory insertion processes. In fact, this lack of insertion was exploited by Jordan to illustrate coordination of alkenes to d metal centers. " ... 

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