What Is Transfer Hydrogenation?

Catalytic transfer hydrogenation refers to a reaction in which hydrogen is transferred from a donor of hydrogen to a reaction substrate of an organic compound under the action of a catalyst.

If using one

Pinaverium bromide is a selective gastrointestinal calcium antagonist used to treat irritable bowel syndrome with good results and mild adverse reactions. Pinaverium bromide is prepared from nobitol as a starting material through multiple steps such as catalytic hydrogenation. The reaction for the catalytic hydrogenation of nobyl alcohol to dihydronobyl alcohol uses Raney nickel as the catalyst and ethanol as the solvent , Under high temperature and pressure. Through experiments, a simple, safe and easy-to-reach hydrogenation method was found. That is, Pd-C was used as the catalyst, cyclohexene was used as the hydrogen donor, toluene was used as the solvent, and the reaction was carried out at reflux for 48 h. The yield was high (86%) Dihydronorbitol was obtained. The route is as follows.

Preparation of dihydronorbitol: 17.0 mL (0.1 mol) of nobitol, 10 mL (0.1 mol) of cyclohexene, 40 mL of toluene, 0.1 g of 5% Pd-C were added to a 250 mL eggplant-shaped flask, and the temperature was raised to reflux for 48 h , Cooled to room temperature, filtered, the filtrate was evaporated under reduced pressure to remove the solvent, and separated on a silica gel column (V (petroleum ether): V (dichloromethane) = 3: 1 as the eluent), and the eluent was evaporated to give a pale yellow color. 14.5 g of oil, 86% yield. ^[2]

In 1944 Thomas proposed a catalytic transfer hydrogenation (CTH) reaction. The CTH reaction refers to a reduction reaction in which a hydrogen acceptor is used for catalytic transfer hydrogenation under the condition that a hydrogen donor is used as a hydrogen source, and the reaction does not require external hydrogen. Common hydrogen donors include hydrocarbons, alcohols, formic acid and its salts, hydrazine hydrate, and triethylsilane. Compared with ordinary hydrogenation reaction, CTH has the advantages of mild reaction conditions, good safety and easy operation. Isopropanol as a hydrogen source can catalyze the reduction of carbon-containing unsaturated bonds in organic materials, the reduction of nitrogen- and sulfur-containing unsaturated bonds, and the reduction of unsaturated bonds in inorganic salts. It has good selectivity, mild conditions, and can change organic substances. It can be used as a solvent or a catalyst, and is an ideal hydrogen donor. In recent years, relevant experimental research has developed rapidly, and some experimental results have been applied industrially. ^[3]

1 Reduction of carbon unsaturated bonds in organic matter

The reduction of carbon-carbon unsaturated bonds in the CTH reaction mainly includes the reduction of carbon-carbon double bonds, and the reduction of carbonyl and aromatic groups.

1.1 Reduction of carbon-carbon double bonds

The earliest catalytic transfer hydrogenation reactions appeared in the petrochemical industry, such as the dehydroaromatization of cyclohexane, and the reduction of olefins to isoparaffins by hydrogenation. In the petroleum refining industry, catalytic transfer hydrogenation is one of the characteristic reactions of catalytic cracking. The extent of catalytic transfer hydrogenation can change the distribution of petroleum cracked products within a wide range. Controlling the catalytic transfer hydrogenation reaction conditions can control the olefin content in petroleum products, which has attracted the attention of researchers. Isopropanol is used as a hydrogen donor. In the presence of a catalyst, such as Pd / C as a catalyst, it is oxidized to acetone, and the carbon-carbon double bond is reduced. Under the action of some excellent catalysts, such as divalent Ru complexes, etc., especially the new non-metal active center catalysts developed by Adolfsson, when the nitro, carbonyl, sulfonic acid group and other groups exist, the carbon in aliphatic compounds Carbon double bonds can also be accurately reduced. Mikhalenko et al. Used the high selectivity of isopropanol reduction to catalyze the reduction of cyclopentadiene to cyclopentene. In the research of using carbon isopropanol to reduce carbon-carbon double bonds, the faster development is the use of Ni (0) nanoparticles (NiNPs) as a catalyst for the CTH reaction. Compared with ordinary catalysts, this catalyst has the most prominent advantages. The conditions are milder and the conversion rate is higher. As the number of rings contained in olefins increases, the conversion of hydrogen transfer will decrease rapidly. ^[3]

1.2 Reduction of carbonyl

In organic compounds, the carbon-oxygen unsaturated bond refers to the aldehyde group of the aldehyde and the ketone carbonyl group, and the reduction of the carbon-oxygen unsaturated bond occupies a large proportion in the hydrogen transfer. The reduction of such unsaturated groups mainly uses alcohols such as isopropanol as a hydrogen source. Under the action of a catalyst, the carbonyl group is reduced to -OH or deeply reduced to the corresponding hydrocarbon, but the products of such reduction reactions are generally not Stereo selectivity. Relevant discussions show that under the action of catalysts such as Ru (mostly used), Ir and other metal complexes, latent chiral ketones can be reduced to chiral alcohols by CTH reaction. At present, there are many such studies. Isopropanol as a hydrogen source can perform limited reduction of aldehydes and ketones to prepare corresponding alcohols and aldehydes. Luo Ge et al. Used homogeneous hydrogen transfer of acrolein and isopropanol to produce allyl alcohol and studied the deactivation and reuse of the reaction catalyst. Isopropanol can be homogeneously reacted with acrolein to produce allyl alcohol. Reduction of ketones with isopropanol can produce corresponding alcohols. It is worth noting that in this type of reaction, aluminum isopropanol, which can be formed by the reaction of isopropanol, aluminum and mercury chloride, is used as a catalyst to reduce ketones and aldehydes to alcohols, while aluminum isopropoxide- Isopropanol is oxidized to acetone by itself, which has the advantages of mild reaction conditions and high yield. Some researchers have described in detail the synthesis of aluminum isopropoxide, its unique physical and chemical properties, and its role in catalytic reactions (catalytic conversion to ester aldehydes) (isopropanol was later found to have catalytic activity). Although this reaction is homogeneous Instead, it can recover aluminum isopropoxide by adding a high-boiling-point solvent and using reduced-pressure distillation after the reaction, and it is easy to industrialize production, so it has unparalleled advantages compared with other catalysts. Danielle et al. Used Wilkinson catalyst [RhCl (PPh ₃ ) ₃ ] to study the reduction of acetophenone, benzophenone and n-hexene with isopropanol, which has broad application prospects in pharmaceutical, daily chemical and energy. In addition to Ru-containing catalysts used to reduce carbonyl groups, the aforementioned NiNPs can also reduce carbonyl groups. Compared with other forms of Ni-containing catalysts, NiNPs can achieve higher conversion rates, and the products can be separated by decantation after the reaction. . ^[3]

2 Reduction of sulfur and nitrogen unsaturated bonds

A large part of the research on the use of isopropanol as a hydrogen source for catalytic transfer hydrogenation is directed to the catalytic hydrogen transfer reduction of sulfur-containing and nitrogen-containing unsaturated groups. In particular, it has important application prospects in petrochemicals, medicine, pesticides and environmental protection, but extensive and in-depth research is needed.

2.1 Reduction of sulfur-containing unsaturated bonds

Some sulfur-containing compounds, such as thiophenol and its derivatives, are important raw materials for medicine, pesticides and optics, and are widely used. The general preparation method of thiophenols is the substitution reaction of halogenated aromatics with H ₂ S at high temperature (500 600 ), alkyl substituted benzene as the starting material, and alkyl substituted benzene is prepared by sulfonation reaction, acid chloride reaction, etc. Instead of thiophenol, the traditional method has the disadvantages of environmental pollution, waste of resources, and equipment corrosion. Isopropyl alcohol is used as a hydrogen donor to reduce benzenesulfonic acid, benzenesulfonyl chloride, etc. to prepare thiophenol precursors. Save resources, easy to operate and so on. At present, there are few studies on the application of isopropanol for direct reduction of thiophenols.

2.2 Reduction of nitrogen-containing unsaturated bonds

2.2.1 Reduction of nitrogen-containing unsaturated bonds such as azido, azo, and imino, carbon-nitrogen double bonds

In the above reaction, the hydrogen transfer reduction of the carbon-nitrogen double bond of the imine substance usually uses isopropanol as a hydrogen source. Kayaki et al. Explored the reaction mechanism of reducing carbon-nitrogen double bonds with isopropanol as a hydrogen source, and used atomic tracing to find that the hydrogen at the alpha position of the hydroxyl group was mostly transferred to the carbon atom connected to nitrogen in the imine, and the hydroxyl Nitrogen is usually transferred to the imine.

2.2.2 Nitro reduction

Catalytic hydrogen transfer is an environmentally friendly nitro reduction method. Heterogeneous catalytic hydrogen transfer is performed using isopropanol as a hydrogen donor, and the catalyst is easy to recover and recycle. Nitro can be reduced to amino by catalyzing hydrogen transfer. Shi Qixun and other nickel-iron composite oxides obtained from Ni-Fe composites as precursors and isopropanol as hydrogen donors reduced arylnitro compounds. ^[3]