The magnesium catalyst refers to an elemental magnesium or magnesium compound which has a catalytic function. Magnesium is a silver-white alkaline earth metal which is malleable. Metallic magnesium is chemically active. In the air, a very thin oxide film is formed on the surface of the magnesium, making it difficult for the air to continue to react with the magnesium. Powder or ribbon magnesium emits intense white light when burned in air. Magnesium reacts with alcohol and water to form hydrogen. Magnesium and halogenated hydrocarbons react more violently under anhydrous conditions to form Grignard reagents. Many magnesium halide compounds have catalytic properties that catalyze some organic synthesis reactions. In addition, some magnesium salts and magnesium complexes also have catalytic functions.
Magnesium catalysts are rich in variety and have good catalytic effects, so they can catalyze many types of reactions in the field of organic synthesis.
- Cyclization reaction: Pyrrole derivatives are an important class of compounds with significant biological activity. The Paal-Knorr cyclization reaction is one of the most important and simple methods in the synthesis of pyrrole derivatives. Lewis acid magnesium catalysts have been widely used in the formation of C-C bonds and in organic chemical reactions of functional group conversion. Magnesium diiodide can catalyze the Paal-Knorr cyclization reaction of 1,4-diketone with various primary amines, thereby synthesizing various chiral pyrrole derivatives in high yield and high stereoselectivity. In addition, the complex formed by magnesium metal and some chiral ligands also has a catalytic function to catalyze some cyclization reactions. For example, an oxazoline-phenolic hydroxyl group is used as a chiral ligand to form a metal magnesium catalyst in situ. This catalyst can achieve an intermolecular asymmetric dearomatization reaction between β-naphthol and aziridine compounds.
Figure 1. Magnesium catalyst catalyzes cyclization reaction
- Asymmetric Diels-Alder reaction: The Diels-Alder reaction can synthesize compounds with good biological activity, and thus has always played a very important role in the synthesis of six-membered carbocyclic rings and heterocyclic rings and in the field of stereochemistry. Lewis acid has catalytic activity for the Diels-Alder reaction. The asymmetric Diels-Alder reaction of chiral 2-oxazolinone as a dienophile can be catalyzed by magnesium perchlorate as Lewis acid catalyst. With magnesium perchlorate as a catalyst, the reaction product can be obtained in stereochemical selectivity and in high yield.
Figure 2. Magnesium catalyst catalyzes the asymmetric Diels-Alder reaction
- Hantzsch reaction: The hexahydroquinoline has a skeleton of a 1,4-dihydropyridine ring and thus has similar activities such as calcium ion regulating pharmacoction. Hexahydroquinolinlogical compounds have strong physiological and pharmacological activities, such as antibacterial, anti-inflammatory and analgesic, antitumor and antiviral activities and the like. Mg(ClO4)2 is a strong Lewis acid that exhibits very good catalytic performance in the Hantzsch reaction. As a magnesium catalyst, MgSO4 can catalyze the Hantzsch reaction of aromatic aldehyde, cyclohexanedione and ethyl acetoacetate under mild conditions to obtain a hexahydroquinoline compound.
Figure 3. Magnesium catalyst catalyzes the Hantzsch reaction
- Hydroxylation reaction: Hydroxylation of phenol by hydrogen peroxide is one of the best methods for preparing benzenediol. Magnesium ferrite has a good hydroxylation activity in the hydroxylation reaction of phenol, and thus can be used as a catalyst for such a hydroxylation reaction. The magnesium ferrate (MgFe2O4) catalyst catalyzes the hydroxylation of phenol by a free radical mechanism. Hydrogen peroxide first reacts with a catalyst to form hydroxyl radicals. Under the action of the catalyst, the hydroxyl radical continues to react with phenol to form diphenyldiol. The use of magnesium ferrite as a catalyst has the advantages of mild reaction conditions, simple experimental operation and high product yield.
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