금속 나노촉매를 이용한 짝지움 반응에 관한 연구
- 금속 나노촉매를 이용한 짝지움 반응에 관한 연구
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- Cross-coupling reaction catalyzed by transition metals is one of the valuable methods in synthetic chemistry. Milder and more efficient reaction conditions to construct carbon-carbon bonds have been developed. However, common cross-coupling reactions using organometallic reagents and other surrogates involve the preparation of functionalized reagents and generation of the undesired byproduct. Consequently, green and sustainable chemistry which minimize the formation of byproduct are of significant concern. Particularly, one of the alternative pathways is that the reaction involves activation of aryl C-H bond. This reaction is attractive in the view of environmental and economical synthesis because they can reduce the number of synthetic steps, avoid the generation of waste, and limit the formation of by-product. The formation of C-C bond directly from a simple C-H bond is the most promising but a challenging goal in catalysis. Therefore, the design of new catalysts for more efficient and greener reactions is important research topics in organic synthesis.
The transition metal nanoparticle catalysts embedded in aluminum oxyhydroxide, [M/AlO(OH)], were synthesized from readily available reagents. The metal nanoparticle catalysts were prepared from metal precursors and support precursors in alcoholic solvent through a sol-gel process. Metal alkoxides were transformed into fibrous structures through hydrolysis and condensation. Various metal nanoparticles entrapped in several types of supports, such as AlO(OH), TiO_(2), and SiO_(2), were synthesized and compared to their catalytic activities in the various reactions including aerobic oxidation of alcohols, hydrogenation of alkenes and arenes, and cyclization. Catalysts showed their distinct catalytic activities depending on the metal used. The catalysts embedded in aluminum oxyhydroxide showed higher activity and selectivity than the catalysts embedded in TiO_(2) and SiO_(2). Moreover, the synthesis of 2-aminobenzothiazoles via an oxidative C-H activation/intramolecular C-S bond formation was carried out in the presence of our catalytic systems. The catalyst 2 showed highest activity and selectivity and use of other catalysts led to the formation of N-phenylbenzamide as a major product.
Dehydrogenation of alcohols was carried out using ruthenium nanoparticles entrapped in aluminum oxyhydroxide, Ru/AlO(OH) (1)
the catalyst showed high activity and selectivity. Furthermore, the catalyst 1 was applied to the one-pot oxidation-Wittig reaction to afford a,b-unsaturated esters. This method does not require additives except oxygen, which promotes the catalytic dehydrogenation of alcohols. The desired a,b-unsaturated esters were selectively formed in the cases of benzylic alcohols as well as aliphatic alcohols, whereas the yields were somewhat low in the cases of aliphatic alcohols. The catalyst 1 was recycled five times with a gradual decrease in catalytic activity.
A direct arylation was investigated with various metal nanoparticle catalysts. Among the catalysts, the copper nanoparticle catalyst, Cu/AlO(OH) (3), showed high activity in the meta-selective direct arylation of anilide derivatives with arylating reagents under mild conditions. According to the XPS analysis, the surface of copper nanoparticle catalyst is covered with Cu(I) species as well as Cu(II) species. In comparsion with commercially available Cu(I) and Cu(II) reagents, we observed that the use of Cu(I) reagents is more effective than that of Cu(II) ones for the direct arylation. In addition, the catalyst 3 was applied to the synthesis of 3-amino-4’-chloro-4-ethylbiphenyl, a common key intermediate of potent herbicides, through the reduced number of steps. The recovered catalyst 3 showed almost the same activity for three runs.
Oxidative olefination of anilides was examined with palladium catalysts under mild conditions. Commercially available Pd catalyst, Pd(OAc)_(2), showed high activities in the ortho-arylation and ortho-olefination. Interestingly, Pd on activated carbon was also effective in the oxidative olefination of anilides with olefins in the presence of an acid. Moreover, this method was applied for iterative metal-catalyzed C-H bond functionalizations to provide more complex products. Unfortunately, other heterogeneous palladium catalysts, including commercially available catalysts and our catalysts, were inactive under the reaction conditions.
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