Migratory Insertion and Elimination

The migration of one ligand to another within the coordination sphere of the metal is distinct from the mechanism of reactions that form products containing the same connectivity (but with different stereochemistry) by intermolecular attack of a nucleophile (Equation 9.2) or electrophile onto the coordinated ligand M-Y. The reactions occurring by nucleophilic or electrophilic attack onto M-Y are discussed in Chapters 11 and 12. 

Migratory insertions are one step of many different types of catalytic processes, several of which are conducted on large industrial scales and are presented in later chapters of this text. For example, the mechanism of carbonylation processes, such as hydroformylation, includes ttie insertion of CO into a metal-carbon bond. Likewise, catalytic hydrogenation occurs by insertion of an olefin into a metal-hydride bond, and olefin polymerizations and couplings of olefins with haloarenes occur by insertions of olefins into metal-carbon bonds. The reverse of these reactions, p-hydride, p-alkyl, and p-aryl eliminations, are principal pathways for the decomposition of metal-alkyl complexes. 

Changes In Geometry and Electron Count During Migratory Insertion and Elimination 

Migratory insertion does not lead to a change in formal oxidation state, unless the inserting ligand Y is an alkylidene, alkylidyne, or isoelectronic ligand bound by a metal-ligand multiple bond. 

The groups undergoing the migratory insertion process must be coordinated cis to each other within the coordination sphere of the metal. 

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The reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

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). 

A key feature of the mechanism of Wilkinson s catalyst is that catalysis begins with reaction of the solvated catalyst, RhCl(PPh3)2S (S=solvent), and H2 to form a solvated dihydride Rh(H)2Cl(PPh3)2S [1], In a subsequent step the alkene binds to the catalyst and then is transformed into product via migratory insertion and reductive elimination steps. Schrock and Osborn investigated solvated cationic complexes [M(PR3)2S2]+ (M=Rh, Ir and S= solvent) that are closely related to Wilkinson s catalyst. Similarly to Wilkinson s catalyst, the mechanistic sequence proposed by Schrock and Osborn features initial reaction of the catalyst with H2 followed by reaction of the dihydride with alkene for the case of monophosphine-ligated rhodium and iridium catalysts [12-17]. Such mechanisms commonly are characterized... 

Insertion and -elimination. A catalytic cycle that involves only one type of elementary reaction must be a very facile process. Isomerisation is such a process since only migratory insertion and its counterpart P-elimination are required. Hence the metal complex can be optimised to do exactly this reaction as fast as possible. The actual situation is slightly more complex due to the necessity of vacant sites, which have to be created for alkene complexation and for P-elimination. 

Despite the fact that the CMM system (see Section 6.2.2, Equation 6.4) showed modest achvities, it nevertheless demonstrated the feasibility of catalytic OHA governed by classic organometaUic reactivity, namely oxidative addition, migratory insertion and reduchve elimination. The authors were able to propose a catalytic cycle as outlined in Scheme 6.1 based on the following experimental observations ... 

While the main carbonylation cycles are now understood in considerable detail for these apparently simple catalytic systems, there will undoubtedly be considerably more work done on these and related sytems to understand the factors influencing the principal steps of oxidative addition, migratory insertion and reductive elimination and, in particular, further work to understand the unwanted reactions that lead to by-products. 

The coplanar migratory insertion and the subsequent reductive elimination occurring with retention of configuration at the M—C bond ensure an overall syn addition... 

As discussed earlier, the generally accepted mechanism for the Heck reaction involves the steps of oxidative addition, coordination of the alkene, migratory insertion, and P-hydride elimination [2,3], With the intramolecular Heck reaction emerging as an important synthetic reaction over the past decade, the individual steps of this mechanism have come under closer scrutiny, and attention is beginning to be directed at determining the identity of the enantioselective step [41],... 

The glyoxylative-decarbonylative coupling rationalizes as follows (Scheme 27). After the oxidative addition of indole-3-glyoxylyl chloride 38, adduct 39 undergoes a migratory de-insertion and elimination of carbon monoxide furnishing the acyl-Pd... 

RBr - RCHO. The reagent reacts with primary bromides in the presence of triphenylphosphine (25°) to give the corresponding aldehyde in high yield (75-85%, isolated) after protonation with acetic acid. The reaction is considered to involve oxidative addition, migratory insertion, and reductive elimination. 

Insertions and -eliminations are also the microscopic reverse of each other. In an insertion, an A=B tt bond inserts into an M—X bond (M—X -I- A=B M—A—B—X). The M—X and A=B tt bonds are broken, and M— A and B—X bonds are formed. Insertion is usually preceded by coordination of the A=B tt bond to the metal, so it is sometimes called migratory insertion. In an insertion, an M—X bond is replaced with an M—A bond, so there is no change in oxidation state, d electron count, or total electron count. However, a new a bond is formed at the expense of a tt bond. The nature of the reaction requires that the new C—and C—H bonds form to the same face of the A=B tt bond, so that syn addition occurs. The reaction of a borane (R2BH) with an alkene to give an alkylborane is a typical insertion reaction that you have probably seen before. 

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. 

In conventional Mizoroki-Heck chemistry, catalytic turnover is ensured by a generally accepted sequence of fundamental steps (cf Section 7.2.1). Within this catalytic process, both migratory insertion-and importantly 3-hydride elimination-are diastereospecific, the latter usually necessitating a synperiplanar arrangement of a Cp—H bond and the Pd" bond. The absence of conformationally accessible... 

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). 

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