硝基的还原是一种常用的合成伯胺的方法，特别是芳香伯胺。
有机化学合成中通过芳香族化合物的硝化反应获得硝基化合物，再还原硝基可得芳香伯胺。这是每一个有机合成人员都应该掌握的基础方法。
硝化反应—芳香族化合物引入硝基的机理和五种方法，反应经验分享
而还原硝基的方法多种多样，一般而言，最干净和简便的还原方法就是通过Pd/C或Raney Ni（雷尼镍）加氢。
但是当分子内存在对加氢敏感的官能团，如：卤素（Cl, Br ，I），双键，三键时，我们应该采用其他方法，比如酸性条件下的铁粉的还原，SnCl2，保险粉和乙醇，等方法避免原料变坏，产生副产物。
反应的方法：
1、硝基加氢还原为胺
利用Pd/C或Raney Ni催化加氢还原硝基是合成胺的最干净和简便的还原方法。
缺点就是条件比较强，卤素和双键，炔键可能也会被还原。
Amixture of N,N-dimethyI-5-nitropyridin-2-amine (3.5 g, 21 mmol) and10percent palladium on carbon (540 mg) in 25 mLMeOH and 25 mLEtOAc wasshaken under 45 psi hydrogen pressure at RT for 1.5 h. The mixture was filteredthrough Celite andthe filtrate concentrated to a red oil (2.8 g, 100percent).
To a solution of the starting material (3 g, 16 mmol) in ethanol (100 mL) was added 10 percent Pd/C (1 g) and the resulting mixture shaken under a hydrogen atmosphere (4 bar) for 3 days. The catalyst was removed by filtration and the solvent evaporated. To the resulting oil was added a saturated solution of ethanolic hydrochloric acid (5 mL) and the solvent was evaporated. Addition of ether gave a white solid which was collected and washed with ether to provide 3-amino-1,1,1-trifluoro-2-pentanol hydrochloride (1.3 g) as a white solid;
Hydrogenation of the material (7 g, 20.2 mmols) in tetrahydrofuran (300 mL) and methanol (150 mL) in the presence of Raney nickel (1.5 g) for 1 hour yielded crude product. The mixture was hydrogenated for 5 hours at atmospheric pressure. The catalyst was removed by filtration and the filtrate was evaporated to give the crude product, then it was purified by silica gel chromatography.
2、还原性金属氯化物还原硝基
还原性金属氯化物还原硝基制备胺还原性的金属氯化物（ 如二氯化锡，三氯化钛等）可以将硝基还原为胺基，此类方法一般不还原双键，也不会脱卤。
Thesolid product (2.0 g, 9.25 mmol) wasadded to a mixed solution of ethanol (20 ml) and conc. hydrochloric acid (5ml), SnCl2 (5.26 g, 27.75 mmol) wasadded to the mixture, and the resulting solution was stirred at 50° C. for 3hours. And then the product was filtered, washed, and dried to prepare6-phenoxypyridine-3-amine (1.5 g, 87percent).
A stirred solution of stannous chloride dihydrate (38 g, 0.183 mol) in concentrated hydrochloric acid (100 ml, 1.2 mol) was treated portionwise with the starting material (10 g). Ethanol (10 mL.) was added to clarify the solution and the mixture is stirred at 60 °C for 3 hours. The solution was evaporated to near dryness and three 100 mL. portions of ethanol are added sequentially and evaporated from the residue. The residual syrup was stirred with 800 mL. of ether and the resulting solid is collected by filtration, washed with ether and dried at reduced pressure. This solid (@ tin complex) is dissolved in 150 mL of water and the tin is precipitated as the sulfide by bubbling hydrogen sulfide gas into the solution. The mixture is filtered and the filter cake washed with water.
The filtrate and washings are combined and evaporated at reduced pressure. The residue is repeatedly evaporated with 100 mL portions of ethanol. The final residual gum is triturated with 30 mL. of ethanol to give as a crystalline solid product dihydrochloride, which is collected by filtration, washed with ethanol and dried at reduced pressure; m.p. 155 °C.
3、 铁酸还原硝基制备胺
常用的条件：铁粉，氯化铵在甲醇和水混合溶液中回流。
此类反应的后处理，一般情况下都是趁热过滤，防止冷却后，产品吸附到剩余的铁粉和氧化铁上。
另外由于铁粉反应后氧化物的颜色较深，不好纯化，有时会用锌粉代替铁粉进行反应，条件类似，常用 AcOH/H2O体系，锌粉可以用磁力搅拌，其次AcOH/H2O体系溶解性很好，醋酸在这里即作为溶剂，又可作为氢源。当然如果溶解度实在不好，可以加点乙醇或者四氢呋喃。
反应完毕，直接萃取浓缩，调pH，再萃取浓缩，结晶即可。
To asolution of N-(2-nitrobenzyl)-5-(pyridin-4-yl)thiophen-2-amine (535 mg, 1.7 mmol) andNH4Cl (95 mg, 1.8 mmol) in20 mL of70% aqueous EtOH wasadded iron dust (482 mg, 8.6 mmol) andthe reaction mixture was heated at 78 oC.After 1 h the reaction mixture was filtered through a pad of Celite andthe pad was washed with hot EtOH. Thefiltrate was concentrated in vacuo andthe residue was azeotropedtwice with benzene to give the crudeN-(2-aminobenzyl)-5-(pyridin-4-yl)thiophen-2-amine.
反应机理：
硝基化合物还原成胺, 通常是通过亚硝基化合物、羟胺、偶氮化合物等中间过程最后还原为胺。
铁粉还原反应是通过电子的转移而实现的。铁是电子给体， 被还原物的某个原子首先在铁粉的表面得到电子生成负离子 自由基，后者再从质子给体（如水）得到质子而生成产物。以芳香族硝基化合物被铁粉还原成芳伯胺的反应为例。
4：不饱和硝基氢化锂铝（LAH）或NaBH4——Lewis酸还原
一般情况下硝基化合物不用氢化锂铝(LAH)还原，因为氢化锂铝(LAH)无法将硝基还原彻底，从而得到混合物， 但对于不饱和的共轭硝基化合物其可通过氢化锂铝(LAH)还原或NaBH4-Lewis酸的方法进行还原得到饱和的胺。
To a slurry of LiAIH4 (15.0 g, 0.395 mole) and anhydrous ether (500 mL) in a 5 liter,3-necked flask fitted with a condenser, mechanical stirrer and dropping funnel was added thestarting material (20.0 g, 0.084 mol) dissolved in 2 liters of ether. The addition was madeover a period of about 4 hours while refluxing the ether slurry. When the addition was completed, refluxing was continued for an additional 1 to 2 hours. After the addition of diatomaceous earth (20 g) and then water (70 mL) slowly, dropwise, with cooling in an ice bath, the supernatant ether was decanted, the salts were washed with fresh ether several times followed by decantation and finally filtration. The solvent was removed by distillation and more thoroughly on a rotary evaporator. Cooling in an ice bath gave 15.91 g (90% yield) of the desired product as a slightly yellow solid, m.p., 45°C. High vacuum distillation gave an analytical sample.
Boron trifluoride etherate (17.7 g) was added under nitrogen with ice-cooling to sodium borohydride (3.94 g, 0.104 mol) suspended in tetrahydrofuran (150 mL). After stirring at room temperature for fifteen minutes, the starting material (5.78 g, 0.208 mol), dissolved in tetrahydrofuran (80 mL) was added dropwise. The mixture was refluxed overnight, quenched by the addition of ice-water, acidified with 5M hydrochloric acid and refluxed for a further two hours. The aqueous layer was washed with ether and the amine liberated by the addition of ammonia solution which was extracted into dichloromethane. The extracts were washed with water, dried and evaporated to give pure product as an oil.
5：硝基保险粉还原硝基
连二亚硫酸钠，也称为保险粉，是一种无机物，化学式为Na2S2O4，为白色结晶性粉末。
连二亚硫酸钠是一种能够还原醛、酮、不饱和酮、醌类、双不饱和酸、偶氮类、硝基、亚硝基化合物、亚胺、肟、吡啶盐、吡嗪、乙烯砜以及进行分子内 Marschalk 环化反应的多功能试剂，也可用于二溴代化合物的脱卤化和 Claisen 重排。
Na2S2O4可以还原硝基的机理如下：
1. Na2S2O4在水中溶解时，会形成亚硫酸根离子（S2O4^2-）和钠离子（Na+）。
2. 当Na2S2O4溶液中加入硝基化合物时，硝基会与亚硫酸根离子发生反应，生成亚硝酸根离子（NO2^-）和硫酸根离子（SO4^2-）。
3. 亚硝酸根离子进一步与Na2S2O4发生反应，生成氨基化合物和硫酸根离子。
4. 最终生成的产物是氨基化合物和硫酸根离子。
To a suspension of 3-(p-nitrobenzyl)-7-benzyladenine bromaide (2.5 mmol) and sodium dithionite (1.74 g, 10 mmol) in EtOH (100 mL) is added 0.2 N aq. NaH (100 mL, 20 mmol). The mixture is stirred at 40°C for 24 h. After cooling to room temperature, the solution is concentrated under reduced pressure to a volume of 70 mL. The precipitate obtained is filtered with suction, washed water (2X25 mL) and then dried in vacuo at 100°C in 65% yield. Analytical samples of 7-benzyladenine are obtained by recrystallization in water/EtOH.
6：水合肼和Raney Ni还原硝基
水合肼和Raney Ni还原硝基的方法，也比较常用，但由于水合肼有一定的毒性而受到限制。
To a refluxing mixture of 19.2 g (0.058 mol) of the above material and 2 teaspoonsful of raney nickel in 200 mL ethanol is added over a 15-minute period a solution of 9.6 g of 99% hydrazine hydrate in 30 mL ethanol. The reaction is refluxed 1 hour, cooled, and evaporated to a residue which is dissolved in ether. The ether solution is treated with decolorizing carbon and evaporated to give 3-amino-4-benzyloxyphenyl 2-pyridyl ketone as an oil, 14.2 g (yield 86%).
7：TiCl3还原硝基
三氯化钛可以通过一系列还原反应将硝基还原为氨基。
1. 先把三氯化钛与稀氢氧化钠溶液反应，生成氯化钛和氢氧化钠：
TiCl3 + 3NaOH -> Ti(OH)3 + 3NaCl
2. 继续加热氯化钛和氢氧化钠溶液，进行水解反应，生成氧化钛和氨水：
Ti(OH)3 + NaOH -> TiO2 + 2H2O
TiO2 + 4NH4OH -> TiO2 * 4H2O + 4NH3
通过上述反应，三氯化钛中的硝基被还原为氨基。
The material (2.9 g, 7.4 mmols) was dissolved in acetone (60 mL) and the solution stirred at 5°C in an ice bath. 30 percent titanium trichloride solution (40 mL) in hydrochloric acid was added dropwise over 30 minutes and the reaction mixture stirred at 5°C for a further 45 minutes before warming to room temperature overnight. The reaction mixture was poured into water and extracted with diethyl ether. The combined ethereal extracts were washed with water, dried (MgSO,), filtered and the filtrate evaporated under reduced pressure to give a pale yellow solid (1.7 g). This was triturated with a 1:1 petroleum/ether mixture and filtered to yield the desired product (1.2 g) as a cream solid.
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