Commonly used catalysts —- Palladium on Barium Sulfate
【English name】Palladium on Barium Sulfate
CA Registry Number][7440-05-3
[Abbreviation and alias] Rosenmund’s Catalyst
Structural formula] Pd/BaSO4
Preparation and Commodity] Barium sulfate loaded Pd catalyst or activated carbon loaded Pd catalyst are available at major reagent companies, or can be made in-house.
Precautions] Palladium/barium sulfate catalysts are best stored in airtight containers.
The mixed reagent formed by loading palladium onto barium sulfate or activated carbon can be used to catalyze the hydrogenation of an acyl chloride, converting the acyl chloride to the corresponding aldehyde compound.
The palladium-catalyzed hydrogenation of an acyl chloride to an aldehyde is also known as the “Rosenmund reaction” (Eq. 1) [1]. The initial methods for this reaction were accomplished by passing hydrogen gas through a xylene or toluene solution of the acyl chloride. Although this method can be applied to most of the chloride compounds, further reduction reactions such as the conversion of aldehydes to alcohols and further side reactions such as the formation of esters, ethers, and hydrocarbon compounds can severely affect the yield of the Rosenmund reaction.
Subsequent studies have revealed a more feasible method of operation, i.e., the hydrogenation of naphthalenyl chloride in xylene or toluene solvent using palladium/barium sulfate as catalysts and quinoline-sulfur as moderator, and the reflux reaction after the passage of hydrogen was able to realize the hydrogenation of naphthalenyl chloride in high yields (Eq. 2) [2].
In order to further overcome the high temperature used in the traditional Rosenmund reaction and the danger of passing hydrogen, palladium/barium sulfate was replaced by palladium/carbon, and anhydrous sodium acetate was added as the hydrochloric acid absorber, which was able to realize the Rosenmund reaction of the closed system under mild conditions (Eq. 3)[3], thus laying a good foundation for its industrial application.
The addition of hydrochloric acid absorbents such as N,N-dimethylacetamide, sodium acetate and ethyldiisopropylamine to the reaction system can effectively promote the reaction, and realize the hydrogenation reaction of sterically forbidden acyl chloride substrates under mild conditions, e.g., by using the traditional Rosenmund reaction, the hydrogenation of 1-tert-butyryl cyclohexanoyl chloride mainly yields tertiary-butylcyclohexane products, while the addition of the hydrochloric acid absorbent, ethyl diisopropylamine, can achieve a high level of hydrochloric acid absorption. isopropylamine, the corresponding aldehyde compound (Eq. 4) was obtained in high yield [4].
Recent studies have shown that both traditional and new methods are very successful in realizing the conversion of chlorides to aldehydes, e.g., the conversion of unsaturated chlorides to unsaturated aldehydes (Eq. 5) can be effectively realized by the traditional method [5].
Related reactions
Rosenmund reduction
The reaction in which an aldehyde is obtained by hydrogenation reduction of an acyl chloride catalyzed by BaSO4, quinoline-S or thiourea-passivated Pd catalysts. If the passivation is not carried out, the resulting aldehyde continues to be reduced to an alcohol, so the possible by-products are alcohols, esters and alkanes. Common solvents used for the reaction are toluene, xylene, etc.
Commonly used catalysts —-Lindlar Catalysts
Commonly used catalysts —-Palladium Carbon (Pd/C)
English name] Palladium on Carbon
CAS No.] 64741-65-7
[Physical Properties] Black powder or small balls containing 0.5% ~ 30% Pd. Insoluble in all organic solvents and acidic solutions.
[Preparation and Commodity] Domestic and foreign reagent companies have sales.
[Precautions] It is safe in air-containing airtight containers, but keep away from solvents and compounds containing sulfur and phosphorus. When used in organic solvents, it must be protected by nitrogen, and the residue must not be dried during filtration. If filtration aids must be used and recovery of the catalyst is desired, cellulosic materials should be used.
Pd/C is capable of catalyzing the hydrogenation of olefins, alkynes, ketones, nitriles, imines, azides, nitro compounds, benzene rings, and heterocyclic aromatic compounds. It can also be used for the hydrogenolysis of cyclopropanes, phenyl derivatives, epoxides, hydrazines and halides. Also used for dehydrogenation of aromatic compounds and deacylation of aldehydes.
Carbon-Nitrogen Bonds
Pd/C catalyzes the hydrogenation of nitriles to primary amines in acidic or ammonium solutions. In acidic solution, Pd/C can also hydrogenate nitrile to aldehydes or alcohols and alkylate oximes to secondary amines. Using α-methylaniline as a chiral auxiliary, highly diastereoisomerically selective reductive amination can be realized. Nitrile reduction to amines
Aniline can be hydrogenated to obtain less alkylated amines. The carbon-nitrogen bond is able to be broken under both transfer hydrogenation and conventional hydrogenation. Allylamines can also be de-allylated under Pd/C catalyzed reactions. 1-Aziridines can be hydrogenated to give ring-opening amines, and the reactive carbon-nitrogen bond can be selectively cleaved preferentially.
Carbon Halide Bonds
Aromatic halides (Cl, I, Br) can undergo Pd/C-catalyzed hydrolytic dehalogenation in the presence of a base. In the absence of a base, the dehalogenation is very slow or even incomplete.
Nitrogen-Nitrogen Bonds
Under Pd/C catalyzed hydrogenation, azides or diazides can be reduced to amines. These groups can also be regarded as potential amines, which undergo hydrogenation and then react with intramolecular and amine-sensitive groups to give different amines.
The azofuranose ring reduces the azido group to an amine under Pd/C hydrogenation conditions, and therefore undergoes ring expansion to form piperidine derivatives. Similarly, pyranosyl azides can undergo a similar reaction to give aza-heptameric rings.
Dehydrogenation
Pd/C is also an effective dehydrogenating reagent at high temperatures for carbocyclic and heterocyclic aromatic compounds.
Enones can be converted to phenols.
Recently, a new composite material, carbon nanofibers, has been discovered. Since carbon nanofibers have a larger specific surface area, they are more suitable as carriers for metal catalysts. The application of carbon nanofiber-loaded palladium catalysts under liquid-phase conditions can lead to the dehydrogenation of tetrahydronaphthalene to produce naphthalene.
Hiroa [Sakurai, H. J. Org. Chem. 2002, 67, 2721] et al. achieved very good results in the Pd/C-catalyzed Suzuki coupling reaction of iodophenol with arylboronic acid at room temperature using water as solvent. This method is of great significance for large scale production because of its claimed clean production.
D. S. Ennis [D. S. Ennis, Org. Process Res. Dev. 1999, 3,248] et al. used Pd /C as catalyst for Suzuki-coupling reaction in large scale for the production of antidepressant drug, SB-245570.
