Читать книгу Computational Methods in Organometallic Catalysis - Yu Lan - Страница 14

Creating new material is always entrusted with the important responsibility for the development of human civilization [1–3]. In particular, synthetic chemistry becomes a powerful tool for chemists, as it exhibits great value for the selective construction of new compounds [4–8]. Various useful molecules could be prepared by the strategies of synthetic chemistry, which provides material foundation, technological support, and drive force for science [9–20]. Synthetic chemistry is also the motivating force for the progress of material science, pharmaceutical science, energy engineering, agriculture, and electronics industry [21–41]. In this area, organic synthesis reveals broad interests from a series of research fields, which could target supply to multifarious functional molecules.

The synthetic organic chemistry usually focuses on “carbon” to widen related research, which could afford various strategies for the building of molecular framework, functional group transformations, and controlling stereochemistry in more sophisticated molecules [9, 22, 42–50]. Therefore, selective formation of new covalent bond between carbon atom and some other atom involving nitrogen, oxygen, sulfur, halogen, boron, and phosphorus becomes one of the most important aims for synthetic organic chemistry. In particular, nucleophiles and electrophiles are important for the construction of new covalent bonds.

A nucleophile, which is a molecule with formal lone‐pair electrophiles, can donate two electrons to its reaction partner for the formation of new covalent bond. Alternatively, an electrophile, which is a molecule with formal unoccupied orbitals, can accept two electrons from its partner for the formation of new covalent bond. Thereinto, coupling reactions could be categorized as redox‐neutral cross‐coupling with an electrophile and a nucleophile, oxidative coupling with two nucleophiles, and reductive coupling with two electrophiles (Scheme 1.1).

Scheme 1.1 Cross‐coupling reactions with nucleophiles and electrophiles.

In organic chemistry, the nucleophile is an electron‐rich molecule that contains a lone pair of electrons or a polarized bond, the heterolysis of which also could yield a lone pair of electrons (Scheme 1.2). According to this concept, organometallic compounds, alcohols, halides, amines, and phosphines with a lone pair of electrons are nucleophiles. Some nonpolar π bonds, including olefins and acetylenes, which could donate the π‐bonding electrons, are often considered to be nucleophiles. Moreover, the C—H bonds of hydrocarbons can be considered to be nucleophiles because the electronegativity of carbon is higher than that of hydrogen, which could deliver a proton to form a formal carbon anion. Correspondingly, the electrophile is an electron‐deficient molecule that contains unoccupied orbitals or low‐energy antibonding molecular orbital, which could accept the electrons from nucleophiles. In this chemistry, cationic carbons, which usually come from the heterolysis of carbon—halogen bonds, are electrophile. Polar π bonds, including carbonyl compounds and imines, also could be considered to be electrophile, which involve a low‐energy π antibond. Interestingly, Fisher‐type singlet carbene has an electron pair filling one sp² hybrid orbital and an unoccupied p orbital, which could be considered to be either nucleophile or electrophile in coupling reactions.

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.