Rhenium Catalysts

Rhenium is a silvery white heavy metal that belongs to the sixth cycle transition metal in the periodic table. It is a very rare and dispersed element with content of only 7-10% in the earth's crust, mainly in molybdenite. It is mainly used as a catalyst for the petroleum industry. In addition, it has high electron emission performance and high melting point and can be widely used in radio, television, vacuum technology, high-temperature coating materials for rockets or missiles, instruments and high-temperature components for spacecraft.

Applications:

• Friedel-Crafts Reaction: There are two main types of Friedel-Crafts reactions: alkylation and acylation. This reaction is the most direct synthesis of carbon-carbon bonds, but the traditional Friedel-Crafts reaction often requires equivalent or even excess Lewis acid or Bronsted acid to catalyze, but a small amount of carbonyl rhenium compound can play a good catalytic effect. Since the rhenium carbonyl compound has Lewis acidity, the reaction of the aromatic substrate with the paraformaldehyde can be catalyzed by using pentacarbonyl rhenium chloride as a catalyst. The reaction product is an N-arylmethane derivative, and ReBr(CO)₅ exhibits good catalytic activity in the reaction. In addition, rhenium pentacarbonyl bromide ReBr(CO)₅ can also catalyze the electrophilic substitution reaction of an acid chloride with a benzene ring.

Figure 1. Electrophilic substitution reaction catalyzed by carbonyl rhenium.

• Nucleophilic addition reaction: The construction of carbon-carbon bonds by nucleophilic addition reaction is also a very effective and common method in the synthesis. The rhenium carbonyl compound can also catalyze the allylation of an aldehyde, the diacetylation of an aldehyde, etc., and exhibits the characteristics of its Lewis acid in such a reaction. Some researchers have studied a kind of chiral rhenium complex to catalyze the asymmetric addition reaction of diphenylzinc to aldehyde, and its ee value is as high as 98%. Compared to similar ferrocene derivatives, rhenium complexes exhibit better corresponding selectivity.

Figure 2. Asymmetric addition reaction of diphenylzinc and aldehyde

• Addition reaction to carbon-carbon double and triple bonds: For non-polar unsaturated bonds, it is often difficult to achieve a nucleophilic addition reaction. However, by using the soft Lewis acidity of the transition metal, it can be coordinated and complexed with a carbon-carbon double bond or a triple bond to activate the substrate to make the reaction easier. Some researchers have used rhenium catalysts to achieve the addition reaction of ethyl acetoacetate and its analogs to unactivated terminal alkynes.

Figure 3. Rhenium catalyst catalyzed addition reaction.

• Cyclization reaction: Researchers have studied the tandem cyclization of cetylene glycol silyl ether catalyzed by carbonyl rhenium chloride. Then, by using a substrate containing an enol silyl ether and an alkyne structure in the molecule, a substituted phenol product was successfully obtained under the action of ReBr(CO)₅.

Figure 4. Rhenium catalyst catalyzed cyclization reaction.

• Boride reaction: Under photochemical conditions, only the catalytic amount of C₅Me₅Re(CO)₃ can activate the C(sp³)-H bond of the terminal alkane, and then react with the pinacol borate to finally achieve the boronation of the alkane. Moreover, the reaction can be carried out at room temperature.

References

1. Hua R M, He J Y Sun H B.(2007). “ReCl(CO)₅-catalyzed Reactions of Aromatic Compounds with 1,3,5-Trioxane or Aqueous Formaldehyde Affording Diarylmethanes”. Chinese Journal of Chemistry. 25(1):132-135．
2. Kuninobu Y Matsuki T, Takai K. (2009). “Rhenium-catalyzed regioselective alkylation of phenols”. Journal of the American Chemical Society.131(29):9914-9915.