【硝化反应】烯烃的硝化反应
烯烃硝化后通常得到共轭的硝基烯烃，而它是有机合成中非常有用的中间体。硝基烯烃一般通过烯烃的硝化或者是2-硝基醇的Henry脱氢制备得到。使用硝酸对烯烃进行硝化通常得到适中的收率，然而这种方法因为缺乏选择性且烯烃容易分解，因而并不适用于实验室中的有机合成。甾体坎利酮在硝酸和醋酸酐作用下，选择性的硝化在4-位上，收率为52%1。
Absolute nitric acid (0.1 mL, 2.4 mmol) is added to acetic anhydride (0.3mL, 3.2 mmol) with cooling in ice. To this mixture, the solution of compound 1(100 mg, 0.3 mmol) in methylene chloride (1.25 mL) is added while cooling in ice. After 1 h at room temperature, chloroform (10 mL) is added, and the mixture is washed with water, aqueous KHCO3 solution (10%, just until the beginning of brown coloration) and water again, dried with Na2SO4, and evaporated. Preparative TLC of the residue on four TLC sheets (10 cm×20 cm) with butylacetate/toluene (40:60 v/v, 3×). Elution of the main band(Rf 0.57) in front of compound 1 (Rf 0.30) with acetone yields 59 mg of compound 2 (52%).1
基于与氮氧化合物的硝化反应为共轭硝基烯烃的制备提供了一个便捷的方法。3-硝基丙烯酸甲酯，是一种非常重要的有机合成试剂，它可以通过在碘存在下，丙烯酸甲酯与四氧化二氮反应，之后经过醋酸钠作用得到2。
In a dry, 5-L flask is placed methyl acrylate (298.4 g, 3.47 mol) and anhydrous diethyl ether (2500 mL). The solution is cooled to 0 oC and iodine(250 g, 0.98 mol) is added. The mixture is stirred for 15 min and then dinitrogen tetroxide (76.6 mL, 1.24 mol) is introduced rapidly. The reaction solution is stirred at 0 oC for 30 h. The solution is cooled to -4 oC and washed with saturated sodium sulfate and NaCl, and dried over Na2SO4 at -4 oC overnight. The solution is filtered and concentrated until it contains approximately equal volumes of product and ether. The solution is then cooled to -78 oC for 30 min and filtered to give the product, and the filtrate is eluted through chromatographic column to give another batch of product. The combined product is dried at 0 oC under a vacuum to give 396-401 g(78%-79%) of methyl 2-iodo-3-nitropropionate (containing a small amount ofmethyl (E)-3-nitroacrylate).
To an hydrousdiethyl ether (2800 mL) was added anhydrous sodium acetate (95 g, 1.16 mol) followed by methyl 2-iodo-3-nitropropionate (150 g, 0.579 mol). The reaction is stirred vigorously at 0 oC for 40 h. The flask is then removed from the ice bath and may be kept, until needed, in a freezer for 5-7 days. The ethereal solution is then decanted and filtered to remove the sodium acetate. The solution is washed by aq. NaHCO3 and brine, dried over Na2SO4 in a freezer at -4 oC. After concentration, the residue is recrystallized by 50% diethyl ether/petroleum ether to give 67-69 g of methyl(E)-3-nitroacrylate (88-91% from dried methyl 2-iodo-3-nitropropionate) as yellow plates.2
烯烃通常可以用硝基氧化物、亚硝酸钠、亚硝酸钾、亚硝酸银和C(NO2)4等硝化基进行硝化。下面的表格Table介绍了一些烯烃的硝化反应条件和收率。
最直接和常用的制备硝基链烯烃的方法就是通过一氧化氮 (NO) 和链烯烃反应。芳香族链烯烃、烯丙基化合物、丙烯酸衍生物的直接硝化在室温下就能很平稳的进行，并具有高区域选择性和良好的收率3。
In an autoclave under an argon atmosphere the styrene (2mmol) was dissolved in 1,2-dichloroethane (20 mL). Autoclave was filled with NO gas under pressure 2 bar, and the mixture was stirred at room temperature for 3 hours. The solvent was removed under vacuum, and the crude product was purified by column chromatography (eluents: hexane/ethyl acetate) to give nitro compound with 90% yield (E/Z 94:6).3
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