Rhodium Catalysts

A substance that contains rhodium and has a catalytic function is referred to as a rhodium catalyst. Rhodium is a silvery, hard metal with high reflectivity. It is chemically stable and does not form oxides. Even when heated to melt, rhodium doesn’t react with oxygen. Rhodium is a platinum group metal and has the characteristics of low crustal content, high price and high catalytic activity. Rhodium and its alloys, rhodium-containing compounds, rhodium complexes and the like all have catalytic functions and can be used as catalysts.

Applications:

Although the rhodium catalyst is expensive, it is widely used in organic synthesis research and industrial production due to its good catalytic activity and good stereoselectivity.

• Cycloaddition reaction: The application of transition metal catalysts to cycloaddition is a hot spot in metal organic chemistry research. Many cyclization reactions that are difficult to occur or have very low yields are improved by the action of transition metal catalysts. The cycloaddition reaction catalyzed by rhodium (II) catalyst has attracted wide attention due to its advantages of high enantioselectivity, short reaction time and high conversion rate. Rhodium acetate is used in the cyclopropanation reaction to synthesize the simplest three-membered ring. Many important cyclopropane derivatives can be obtained by the addition reaction of metal carbene with unsaturated double bonds. For example, rhodium acetate as a catalyst can effectively catalyze the intermolecular cycloaddition reaction of 2-azofluorenone and substituted olefins, and a series of substituted cyclopropane derivatives are obtained in high yield. Rhodium acetate can also catalyze the cycloaddition reaction of diazobenzylphosphonate with aromatic imine to obtain the product aziridine-2-phosphonate in a highly selective and stereospecific manner.

Figure 1. Rhodium catalyst catalyzed cycloaddition reaction

• Asymmetric hydrogenation: Asymmetric catalytic hydrogenation of carbon-carbon double bonds by using a complex of a chiral ligand and a metal as a catalyst, and synthesizing a chiral compound having enantioselectivity by stereo control and chiral induction are research hotspots in the organic field. The complex of chiral ligand and rhodium has excellent catalytic performance and can catalyze many different types of asymmetric hydrogenation reactions, including asymmetric hydrogenation of enamides, asymmetric catalytic hydrogenation of enol esters, symmetrical hydrogenation of non-acrylic acid and the like. Among them, the asymmetric hydrogenation of the enamide by the complex of the chiral ligand and the transition metal rhodium is one of the most effective methods for synthesizing the optically active α-amino acid compound, and has been successfully applied to the synthesis of industrial pharmaceutical intermediates. For example, a complex formed by DuPHOS with rhodium can catalyze the asymmetric hydrogenation of an a-aryl olefinamide with high selectivity and high yield to give the product.

Figure 2. Rhodium catalyst catalyzed asymmetric hydrogenation

• Polymerization: The transition metal complex catalyzes the substitution of alkynes to form helical polymers, which has great application value. The rhodium compound is potentially highly reactive toward alkynes and is capable of inducing acetylene to form stereoactive polymers. Therefore, rhodium catalysts play a very important role in the catalytic substitution of acetylene chiral polymerization. Among them, the rhodium compound containing a diene ligand exhibits high catalytic efficiency in the catalytic polymerization reaction, and is the most commonly used rhodium catalyst in the polymerization of the alkynes. For example, the rhodium olefin complex [Rh(nbd)Cl]₂ can catalyze the asymmetric polymerization of achiral substituted phenylacetylene monomers.

Figure 3. Rhodium catalyst catalyzed polymerization

Hydroformylation: The olefin hydroformylation reaction is one of the most important reactions in the industry. The rhodium catalyst is used as a catalyst for the hydroformylation reaction of olefins, and has the advantages of mild reaction conditions and low side reactions. At low pressures, rhodium catalyzed hydroformylation of olefins is more efficient, energy-efficient and environmentally friendly. The rhodium catalysts used at low pressure mainly include Rh(acac)CO(PPh₃)₃ and HRhCO(PPh₃)₃, among which Rh(acac)CO(PPh₃)₃ has been more widely used.

References

1. Syu, Jin-Fong. (2019). “Asymmetric Synthesis of β-Aryl β-Imido Sulfones Using Rhodium Catalysts with Chiral Diene Ligands: Synthesis of Apremilast.” Organic Letters 21(12), 4614-4618.
2. Sakhabutdinova, G. N. (2018), “Rhodium(II)-Catalyzed Reaction of Salicylaldehyde and Its Derivatives with Diazocarbonyl Compounds.” Russian Journal of Organic Chemistry 54(12), 1772-1776.
3. Swyka, Robert A. (2019), “Rhodium-Catalyzed Aldehyde Arylation via Formate-Mediated Transfer Hydrogenation: Beyond Metallic Reductants in Grignard/Nozaki-Hiyama-Kishi-Type Addition.” Journal of the American Chemical Society 141(5), 1828-1832.