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求翻译下面一段化学合成内容 请帮忙 英文 或麻烦翻译下第1--3页
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Initially o-phenylenediamine and benzaldehyde were used in a 1:1 molar ratio in water for the titled transformation, but it failed to produce the desired 1,2-disubstituted benzimidazole 3a as a major product, instead a mixture of products was isolated (Scheme 1). Increase in the reaction time, temperature, and variation in the molar proportion of the reactants did not make any influence on the ratio of the product distribution. At this juncture and based on the literature report46,47 on SHFP (sodium hexafluorophosphate) we undertook the above study by using 1 mmol of anhydrous SHFP in the above reaction mixture of o-phenylenediamine and benzaldehyde (1:2 mmol). This revised protocol after a very simple work-up gave only a single isolated product in 76% yield. However, considering the recent emphasis on the development of greener protocols we further worked on the above transformation to replace with a suitable greener catalyst the use of SHFP, which has been reported to produce even more hazardous waste due to its decomposition to phosphorus oxides and toxic hydrofluoridric acid.47 Working in this direction and looking into the recent literature of resin bound greener catalyst we used PF6 ion bound Amberlite 900 (Cl) as a potential catalyst for the above transformation. Polymeric resin bound hexafluorophosphate ion (PHP) was prepared48 (Scheme 2) by washing Amberlite 900 resin (Cl) packed in a column with 10% aqueous sodium hexafluorophosphate solution repeatedly until the washing gave a negative response for the chloride ion (Scheme 6). Finally the solid was washed several times with water and then dried under vacuum. The binding of hexafluorophosphate ion on the surface of Amberlite 900 resin was confirmed by IR spectroscopy49 in the following way. IR spectrum of PHP was taken in KBr discs (Fig. 1). PF6 belongs to Oh point group, where six kinds of vibration modes are included in the molecule, whose basic frequency number is 15. From the characteristics of Oh point group, it can be calculated that for PF6 two energy levels belonging to F1u have infrared activity which results from the flexing vibration and bending vibration of F–P bonds, respectively. Therefore, two strong absorption peaks of PF6 should appear at 820–860 and 550–565 cm1. In the IR spectra of PHP, strong absorption peak appeared at 557 and 832 cm1, respectively (Fig. 1). Appearance of the above said peaks did confirm the binding of PF6 ion on polymeric resin (Amberlite 900). This resin bound PF6 ion (PHP) was then applied in place of SHFP as the catalyst and it also showed excellent chemoselectivity by giving 3a as the major product in water at room temperature (Scheme 3). In a bid to increase the yield of the desired product, we performed the same reaction in other green solvents, such as ethanol, methanol, and mixtures of both water–methanol and water-ethanol in varying proportions at room temperature as well as in 50 C (Table 1). Of the entire solvent systems studied, methanol gave similar results as water, but ethanol performed poorly (entry 3). A further study with mixed solvent systems indicated that a 1:1 mixture of water and methanol gave an excellent yield of 3a within 1 h at ambient temperature (Table 1, entry 13); increase in reaction temperature had no effect on the isolated yield of the reaction (entry 14). |
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2015-05-15 09:51:21, 4.02 M
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