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Scheme 1.2 Some selected examples of nucleophiles.

      Superficially, at least, the reaction between nucleophile and electrophile could construct a covalent bond undoubtedly. However, the familiar nucleophiles and electrophiles, used in cross‐coupling reactions, are usually inactive, which could not react with each other rapidly. Moreover, when more active nucleophiles and electrophiles are used in coupling reactions, it would become out of control, which would not selectively afford target products. In effect, introducing transition metal catalysis can perfectly solve this problem. The appropriate transition metal can be employed to selectively activate the nucleophiles and electrophiles and stabilize some others, which led to a specially appointed cross‐coupling reaction.

      High‐valence transition metal can obtain electrons from nucleophile, which led to the transformation of nucleophile into electrophile. The newly generated electrophile can couple with other nucleophiles to form covalent bond, which is named oxidative coupling reaction [51–53]. Meanwhile, the reduced transition metal can be oxidized by exogenous oxidant for regeneration. Correspondingly, low‐valence transition metal can donate electrons to electrophile leading to the transformation of electrophile into nucleophile, which can react with another electrophile to form covalent bond. Accordingly, it is named reductive coupling reaction. The oxidized transition metal also can be reduced by exogenous reductant.

      The d orbital of some transition metals could be filled by unpaired electrons, which led to a unique catalytic activity in radical‐involved reactions. The homolytic cleavage of transition metal–carbon (or some other atoms) bond is an efficient way for the generation of a radical species, which can promote further transformations. On the other hand, free radical can react with some transition metal leading to the stabilization of radical, which can cause further radical transformations [54–57]. Moreover, nucleophiles and electrophiles, activated by transition metals, also can react with radical to form new covalent bonds.

      Although there is no electron barrier due to the appropriate symmetry of frontier molecular orbitals, a great deal of uncatalyzed pericyclic reactions would occur under harsh reaction conditions, which could be often attributed to the low‐energy level of highest occupied molecular orbitals (HOMOs) and high‐energy level of lowest unoccupied molecular orbitals (LUMOs) in reacting partners. Transition metals can play as a Lewis acid, which could significantly reduce the LUMO of coordinated organic moiety. Therefore, it has been widely adopted to catalyze pericyclic reactions, which leads to moderate reaction conditions and adjustable selectivity [58–62]. Moreover, the node of d orbital can change the symmetry of a conjugative compound, which involves a transition metal. Therefore, transition metal itself also could participate in a pericyclic reaction to reveal unique catalytic activity.

As an overview of organometallic chemistry, the core is the formation of a metal–carbon bond and its further transformation. Different from organocatalysis, organometallic catalysis process usually goes through multiple steps as well as complicated catalytic cycles, which originated from the complex bonding pattern of metallic catalyst and the variation of valence state for the central metal element. Consequently, improving the reaction efficiency and yield for organometallic catalysis encountered more difficulty than conventional organocatalysis. Moreover, the design of catalysis and ligand for transition metal‐catalyzed reaction is still facing both opportunities and challenges. To solve the above‐mentioned issues, the understanding of reaction mechanism is imperative, which could give more information for the detailed reaction process, and help to improve the reaction efficiency and yield.

      1.1.2 A Brief History of Organometallic Chemistry

As an interdiscipline of organic and inorganic chemistry, organometallic chemistry has a history of almost 200 years since the first complex K[(C₂H₄)PtCl₃]·H₂O was reported by Zeise when he heated ethanol solution of PtCl₄/KCl [62]. The history of organometallic chemistry can be roughly divided into four stages. The chemists majorly focused on main group organometallic compounds in the nineteenth century. Later in the first half of the twentieth century, chemists paid more attention to understanding the structures of organometallic compounds involving transition metals. Then in the latter half of the twentieth century, various transition metal‐catalyzed reactions had been widely reported. Since this century, chemists have been keen on using transition metal catalysis to selectively construct more complex organic compounds. The outlined history of organometallic chemistry could be concluded in Scheme 1.3 [63].