Migratory insertions into metal-heteroatom bonds

In spite of very common mentioning of insertion process in a variety of modern studies and in spite of widespread application of addition reactions in modern organic chemistry, molecular picture of this fascinating transformation remains unexplored. The aim of this chapter is to point on understanding at molecular level of insertion (migratory insertion) of unsaturated organic molecules into metal-heteroatom bond. 

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. 

The versatility of CO as a synthon also stems from its ability to undergo insertion reactions into a variety of metal-heteroatom bonds. The migratory insertion of CO into transition metal-hydride bonds, while thermodynamically unfavorable, generates metal-formyl complexes M-C(0)H (Equation (19)), a few examples of which have been isolated independently. This reaction is assumed to be a key step in both the homogeneous and heterogeneous catalytic hydrogenation (i.e., reduction) of CO, including the Fischer-Tropsch synthesis of hydrocarbons and... 

A few final comments should be made on the insertions of substrates containing C-C multiple bonds into the bonds between a transition metal and an electronegative heteroatom. First, insertions of olefins into related thiolate and phosphide complexes are as rare as insertions into alkoxo and amido complexes. Reactions of acrylonitrile into the metal-phosphorus bonds of palladium- and platinum-phosphido complexes to give products from formal insertions have been observed, and one example is showm in Equation 9.90. However, these reactions are more likely to occur by direct attack of the phosphorus on the electrophilic carbon of acrylonitrile than by migratory insertion. Second, the insertions of alkynes into metal-oxygen or metal-nitrogen covalent bonds are rare, even though the C-C ir-bond in an alkyne is weaker than the ir-bond in an alkene. 

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