Insertion into Metal-Carbon Bond

Among elementary processes involved in transition metal-catalyzed organic synthesis, 1,1-insertion and a-elimination constitute two fundamental processes quite important in transforming organic substrates. The 1,1-insertion can be expressed as a general form as given below (Eq. 7.1). 

Various unsaturated compounds can be inserted into the metal alkyl, aryl, and alkenyl complexes to give new organometallic complexes having various functional groups. The insertions of carbon monoxide (CO) and isocyanide (CNR) into transition metal-carbon a-bond are particularly important processes, since a carbon unit can be increased in the process and the acyl type complexes formed by the insertion processes can be subjected to further transformations to synthesize useful organic compounds. For example, the CO inserhon constitutes a fundamental step in industrially important processes such as hydroformylation of olefins, acetic acid synthesis from methanol and CO, Fischer-Tropsch process, amidocarbonylation, olefin and CO copolymerizahon processes as well as in a variety of laboratory syntheses of carbonyl containing compounds. 

Insertion processes are reversible in certain cases, subject to thermodynamic factors in Eq. 7.1. Particularly important among the deinsertion processes is the decarbonylation by which a compound with one less carbon unit is produced. De-carbonylation of acyl halides and aldehydes are utilized for removing a carbonyl 

Current Methods in Inorganic Chemistry, Volume 3 Editors H. Kurosawa and A. Yamamoto 2003 Elsevier Science B.V. All rights reserved 

Unsaturated compounds such as CO can be formally inserted into a metal-heteroatom bond such as hydroxide, alkoxide, amide, and sulhde. However, it presents a difficult problem to determine whether the overall insertion process proceeds through a migratory insertion process or by an alternative process. For example, the anionic ligand such as OH and alkoxides may dissociate first to provide a vacant coordinahon site for the CO ligand that is subsequently attacked by the anion to give the same product as that obtained by the migratory insertion of the anionic ligand on the coordinated CO. 

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Comparative data of the mechanism of polymerization on active centers containing non-transition metals. Olefin insertion into metal-carbon bond is also a key step not only for catalytic polymerization but also for olefin oligomerization with organoaluminium compounds. The latter process includes the same steps as the catalytic polymerization in the presence of transition metal compounds... 

Insertion into metal carbon bonds for alkylidene [reaction (j)] and alkylidyne [reaction (k)] complexes that yield metallacycles will also be considered. 

At present there appear to be no well-defined examples of O2 insertion into metal-carbon bonds. There are, however, reactions that are difficult to explain by a free radical mechanism ... 

Carbon dioxide may insert into metal-carbon bonds to yield mono- or bidentate carboxylate complexes, as in reaction (a), depending on the availability of coordination sites on the metaF. ... 

Synthesis via Sulfur Insertion into Metal-Carbon Bonds... 

Nitriles can also undergo [2 + 2] cycloadditions with metal unsaturated bonds, e.g., at metallocenes with Zr=E (with E = 0 or s)180,181 or Ti=CH2182-187 moieties, giving unstable 4-membered metallacycles which are intermediates for other products. Other formal cyclo-additions involving nitrile insertions into metal carbon bonds are also known. Examples of cycloadditions of nitriles with C N bond formation to produce metallacycles were presented above ((11), Table 1 (7) and (8), Table 3). 

Y. Kayaki and A. Yamamoto, 1,1 -Insertions into Metal-Carbon Bonds, In Fundamentals of Molecular Catalysis, H. Kurosawa and A. Yamamoto, Eds., Elsevier Amsterdam, 2003, pp. 390-395. 

This has two detrimental effects on polymerization. First, chelation strengthens the metal-olefin interaction, thereby raising the barrier for the insertion step. Second, it forces insertion through the endo face, in sharp contrast to the known propensity for norbornene to insert into metal-carbon bonds through the less hindered exo face [3 a, 5]. Consistent with this hypothesis has been our observation of the preferential uptake of the exo isomer in the polymerization of functional norbornene derivatives by Pd(PRj)(Me). For example. Fig. 9.3 shows the uptake profile versus time for the polymerization of 5-norbornene-2-carboxyhc acid ethyl ester starting with a monomer isomer ratio of 22% exo to 78% endo. Indeed, under certain conditions a polymer can be obtained from the exo isomer but not the endo isomer [10]. 

Insertion and Carbonyl and alkene groups may insert into metal-carbon bonds the reverse process gives eliminition elimination of a ligand. Together with oxidative addition and reductive elimination steps, these reactions form the basis for many catalytic applications. 

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