Functional transformation via the activation of unreactive bonds is classified into two types of synthetic modes. One is a method to activate by the assist of intramolecular chelation from the directing group in which the activation of unreactive bonds is performed by oxidative addition of lower-valent metal species. The other method is to cleave relatively high acidic C-H bonds with higher-valent metal species. In this method, it is often necessary to use reoxidants for reproducing higher-valent metal species. Even in reactions using higher-valent metal species, the assist of intramolecular chelation from the directing group is effective for performing the activation of unreactive bonds regioselectively.
Metal species utilized for functional transformation via the activation of unreactive bonds are palladium, ruthenium, rhodium, iridium and so on, with most of them being noble metals. Recently, relatively inexpensive metals such as nickel, copper and iron have also been used in these transformations. Much research has been performed and a number of useful transformations such as arylations, alkenylations, alkynylations, carbonylations and heteroatom functionalizations have been reported. Many of them activate C-H bonds on unsaturated carbons of aromatic rings or alkenes to transform into other functional groups. However, currently, cleavage of C-H bonds on sp3 carbon which are generally harder to activate are being studied and successful transformations have been developed. There are various ways to activate unreactive bonds. The detail of these chemical properties and reactivities are explained as follows.
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