【硝化反应】肟氧化为硝基化合物

将羰基转化成硝基 (retro Nef Reaction) 是制备硝基化合物的一个重要的合成方法。这种转化通常是通过用强氧化剂（如CF₃CO₃H）把肟氧化成硝基来实现¹。

无水CF₃CO₃H很难处理，因此有文献报道了该反应的优化方法²。即用三氟乙酸酐与过氧化脲的络合物(UHP) 在乙腈中0 °C反应得到过氧三氟乙酸的溶液，将醛肟氧化得到相应的硝基化合物，并且收率较高。该方法对酮肟不适用，只能得到相应的酮。

To a stirred suspension of UHP (60 mmol, 5.8 g) in CH₃CN (30 mL), TFAA (50 mmol, 10.5 g) dissolved in CH₃CN (50 mL) at 0 ^oC. After stirring at this temperature for 4 h, most of the solvent was removed under reduced pressure and the residue was dissolved in NaHCO₃sol satd. and extracted with ether. The ethereal solution was washed with 5% Na₂SO₃(starch-iodide test negative) and dried over MgSO₄. The crude material, after evaporation of the solvent, was purified by flash chromatography to give nitro compound with 75% yield.²

文献中还报道了很多新的方法将肟氧化成硝基化合物。Olah报道了用过氧硼酸钠在乙酸溶液把肟氧化成硝基化合物的方法³。

Bicyclo[2.2.1]norbornanone-2-oxime (1.0 g, 8 mmol) was dissolved in 50 mL of glacial acetic acid in dry three-necked round bottomed flask. The flask was immersed in an oil bath at a temperature of 55 ^oC. In portions, solid sodium perborate tetrahydrate (6.20 g, 40 mmol) was added with vigorous stirring over a period of 30 minutes. After the addition, the reaction mixture was stirred at the same temperature for 4 h. After cooling, the reaction mixture was treated with 4×25 mL ice cold saturated solution of sodium bicarbonate, followed by extraction with ether (2×50 mL). The organic layer was washed several times with cold water, dried over anhydrous MgSO₄ and evaporated. The crude product was passed through a silica gel column eluted with chloroform-pentane (1:4). The obtained product was further purified by vacuum distillation to give 2-nitrobicyclo[2.2.1]-norbornane (0.74 g) as a yellow liquid in 65% yield.³

用Mo (IV)的过氧化合物可以将肟氧化成硝基化合物，这个方法的好处是既可以氧化醛肟也可以氧化酮肟,⁴可以很好的弥补UHP方法的不足。

In a typical experiment 5 mL of CH₃CN containing 0.5 mmol of oxodiperoxo complex was added to a stirred CH₃CN solution containing 1 mmol of the oxime at 40 ^oC. The progress of the reaction was monitored by following the disappearance of the peroxide(iodometric titre). The oxidation products were isolated by column chromatography (silica-gel, pentane-chloroform 4:1).

m-CPBA作为一个常用的氧化剂，也被用于某些肟转化成硝基化合物的反应⁵。

To a solution of m-chloroperbenzoic acid (80%, 6.45 g,0.03 mol) in a minimum volume of dichloromethane (~90 mL) is added diisopropyl 1-hydroxyimino-alkane phosphonate (0.03 mol) at room temperature. The reaction mixture is stirred and the progress of reaction is monitored by TLC (silicagel, chloroform-ethyl acetate, iodine). After about 72 h the oximinophosphonates have disappeared. The mixture is extracted with saturated aqueous sodium hydrogen carbonate containing sodium sulphite (2×50 mL), water(2×50 mL), brine (50 mL), and dried with sodium sulphate. The solvent is removed and the residue is distilled under reduced pressure to give pure diisopropyl 1-nitroalkane phosphonate with 65% yield.⁵

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