| As significant materials and intermediates in the synthesis of fine chemicals, aromatic ketones have a wide range applications in many areas such as pharmaceuticals, agrochemicals, dyes and fragrances. Friedel-Crafts acylation is one of the classical methods for their synthesis. Whereas, this traditional method is against development goals of green chemistry because it usually uses plenty of Lewis acids such as AlCl3, FeCl3, which give large volumes of waste. In addition, another representative method for the synthesis of aryl ketones is the oxidation of the corresponding secondary alcohols by chromium reagents such as chromium trioxide, and this method is high cost since Cr(VI) of reaction liquid has great toxicity which is very difficult to deal with. In the past decade, the formation of C-C bonds via the C-H bond cleavage of arenes substrate bearing directing groups catalyzed by transition-metal is one of the simple and effective methods for the synthesis of aromatic ketones. It has advantages over other methods since it avoids the prefunctionalization of substrates. To date, palladium salts catalyzed synthesis of aromatic ketones through directed ortho C-H bond activation of arenes has been widely investigated. However, supported palladium nanoparticles (PdNPs) catalyzed this reaction has been rarely reported and the research field still remains significant challenges. In this study, we prepared Pd/y-Al2O3 and examined the catalytic performance of the catalyst under different conditions in the C-C coupling reaction of 2-arylpyridines with aromatic aldehydes and alkylbenzenes, respectively.1. Supported palladium nanoparticles catalysts (Pd/y-Al2O3) were prepared by impregnation-reduction method using ?-Al2O3 as support. The effect of different preparation conditions on the characteristics of catalysts were investigated by X-ray diffraction, N2 adsorption and desorption, transmission electron microscopy, atomic absorption spectroscopy, and so on. The results show that the addition of L-lysine during the preparation process protects PdNPs against agglomeration. The catalyst (mean PdNP diameters of 3.00 nn) had a good performance in the C-C coupling reaction of 2-arylpyridines with aromatic aldehydes and alkylbenzenes. It is shown that particle size of PdNPs is the key factor to affect the catalytic performance of catalyst for C-C coupling reaction of 2-arylpyridines with aromatic aldehydes and alkylbenzenes.2. The catalyst was used for the C-C coupling reaction of 2-arylpyridines with aromatic aldehydes to synthesize aromatic ketones.2-Phenylpyridine acylation with benzaldehyde was used as the model reaction. The catalyst had a good performance with the conversion of 2-phenylpyridine was 88% and the selectivity of phenyl(2-(pyridin-2-yl) phenyl)methanone was 99% in the presence of TBPB refluxing in cumene at 130? for 24 h. The effects of palladium loading, supports, solvent, oxidant, reaction temperature and time for this reaction were studied. The recyclability of catalyst was relatively good and the conversion of 2-phenylpyridine decreased of 44 percentage points, the selectivity of desired product held above 92% even after recycling five times. Based on relative literatures and our results, a tentative mechanism was proposed. A broad scope of aldehydes, including aromatic aldehydes bearing both electron-withdrawing and electron-donating groups, could react with 2-phenylpyridines to synthesize the corresponding ketones in moderate to good yields.3. The catalyst was used for the C-C coupling reaction of 2-arylpyridines with alkylbenzenes to synthesize aromatic ketones.2-Phenylpyridine acylation with toluene was used as the model reaction. The catalyst had a good performance with the conversion of 2-phenylpyridine was 84% and the selectivity of phenyl(2-(pyridin-2-yl) phenyl)methanone was 97% in the presence of TBPB refluxing in toluene at 110? for 24 h. After reused for five times, the catalyst also have a good activity and the conversion of 2-phenylpyridine only decreased of 13 percentage points and the selectivity of desired product held above 99%. What's more, the effects of palladium loading, oxidant, reaction temperature and time for this reaction were also examined and a possible mechanism was proposed. A broad scope of alkylbenzenes, including alkylbenzenes bearing both electron-withdrawing and electron-donating groups, could react with 2-phenylpyridines to synthesize the corresponding ketones in moderate to good yields. |
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