A substance that contains palladium and has a catalytic function is referred to as a palladium catalyst. Palladium is a silver-white transition metal with good ductility and plasticity. It is chemically inert and stable in air and humid environments at room temperature. It is heated to 800 ℃ and a palladium oxide film is formed on the surface of palladium. Palladium nanoparticles and many palladium compounds have very good catalytic activity and are therefore mainly used for the production of catalysts in chemistry.
Palladium catalysts are widely used in the field of organic synthesis due to their advantages of good catalytic activity, simple preparation and good stereoselectivity.
- Suzuki coupling reaction: Carbon-carbon coupling reactions play an important role in organic synthesis. The Suzuki cross-coupling reaction is one of the most effective and flexible methods for the selective construction of carbon-carbon bonds. The homogeneous catalyst of palladium has the advantages of good dispersibility, high catalytic activity and good selectivity, and is widely used to catalyze various types of coupling reactions. In addition, heterogeneous palladium catalysts prepared by loading palladium compounds onto various supports can also be used to catalyze a variety of coupling reactions. For example, a Pd/Al2O3-CELL catalyst obtained by supporting palladium acetate on a composite carrier of cellulose and alumina can catalyze a Suzuki coupling reaction of bromoacetophenone and phenylboronic acid.
Figure 1. Palladium catalyst catalyzed Suzuki coupling reaction
- Carbonylation: Asymmetric carbonylation is a reaction in which a carbonyl group is introduced into an organic compound. Palladium catalysts have very important applications in asymmetric carbonylation reactions. Asymmetric carbonylation reactions catalyzed by palladium catalysts include asymmetric hydroesterification, asymmetric hydrocarboxylation, and lactonization. For example, a complex formed by complexing PdCl2 with a bisphosphine ligand can catalyze the asymmetric monohydroesterification of styrene. The catalytic system formed by complexation of Pd(OAc)2 with bis(1-adamantyl)-n-butylphosphine can catalyze the asymmetric hydrogen carboxylation of ethylene aromatic hydrocarbons to form α-arylpropionic acid series compounds.
Figure 2. Palladium catalyst catalyzed carbonylation
- Nucleophilic substitution reaction: The transition metal catalyzed allyl substitution reaction is a better method for constructing C-C, C-N, C-O, C-F, C-S, C-H bonds and C-P bonds. Due to the presence of the empty d orbitals, metallic palladium readily complexes with some ligands to form highly efficient heterogeneous catalysts for catalyzing asymmetric allyl substitution reactions. For example, a palladium catalyst formed by complexation of palladium with a phosphite can catalyze the nucleophilic substitution reaction of allyl acetate with a 2-substituted malonate. A palladium catalyst formed by complexing palladium with a novel chiral oxazoline type ligand catalyzes the nucleophilic substitution reaction of allyl ester with propylene glycol or malonate. Palladium catalysts have high-efficiency regional chemoselectivity for most substrates when used in the catalysis of nucleophilic substitution reactions.
Figure 3. Palladium catalyst catalyzed nucleophilic substitution reaction
- Diels-Alder reaction: The Diels-Alder reaction is one of the most important methods for constructing a six-membered ring, and it is also an important method for forming new carbon-carbon bonds. The coordination compound formed by palladium (II) and the ligand has a central ion valence electron arrangement of d8, and its empty orbit can accept electrons in a d2sp hybrid manner. Therefore, the palladium complex is a good Lewis acid. It can be used to catalyze asymmetric Diels-Alder reactions. In recent years, the development of palladium chiral Lewis acid catalysts and their applications in asymmetric Diels-Alder reactions have made great progress. Many of the catalysts have high catalytic efficiency, low dosage, and ease of preparation, and have many unique advantages in the synthesis of chemically selective and regioselective six-membered and multi-substituted benzenes. For example, the catalyst [Pd(PPh3)4] can catalyze the Diels-Alder reaction of tetracyanoethylene with 1-bromostyrene and obtain the product in high yield.
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