Schiemann反应
由芳胺制备四氟硼酸的芳香重氮盐(ArN2+BF4-)热分解得到芳香氟化物的反应。此反应也被称为Balz-Schiemann反应。
反应机理
反应实例
光催化的Schiemann反应。
参考文献
1. Balz, G.; Schiemann, G. Ber. 1927, 60, 1186-1190. Günther Schiemann was born in Breslau, Germany in 1899. In 1925, he received his doctorate at Breslau, where he became an assistant professor. In 1950, he became the Chair of Technical Chemistry at Istanbul, where he extensively studied aromatic fluorine compounds.【Günther Schiemann生于德国的弗罗茨瓦夫，1925年获得弗罗兹瓦夫大学的博士学位，并出任副教授。1950年出任伊斯坦布尔大学的化工学院主席，在那里他着重研究氟化学。】
2. Roe, A. Org. React. 1949, 5, 193-228. (Review).
3. Sharts, C. M. J. Chem. Educ. 1968, 45, 185-192. (Review).
4. Montgomery, J. A.; Hewson, K. J. Org. Chem. 1969, 34, 1396-1399.
5. Laali, K. K.; Gettwert, V. J. J. Fluorine Chem. 2001, 107, 31-34.
6. Dolensky, B.; Takeuchi, Y.; Cohen, L. A.; Kirk, K. L. J. Fluorine Chem. 2001, 107, 147-152.
7. Gronheid, R.; Lodder, G.; Okuyama, T. J. Org. Chem. 2002, 67, 693-720.
8. Heredia-Moya, J.; Kirk, K. L. J. Fluorine Chem. 2007, 128, 674-678.
9. Gribble, G. W. Balz-Schiemann reaction. In Name Reactions for Functional Group Transformations; Li, J. J., Ed.; Wiley: Hoboken, NJ, 2007, pp 552-563. (Review).
10. Pomerantz, M.; Turkman, N. Synthesis 2008, 2333-2336.
编译自：Name Reactions （A Collection of Detailed Reaction Mechanisms）, Jie Jack Li, Schiemann reaction，page 537-538.
此反应的往期介绍
四氟硼酸的芳香重氮盐(ArN2+BF4-)热分解得到芳香氟化物的反应称为Balz-Schiemann反应。一般的重氮盐不稳定，但四氟硼酸盐的重氮盐很稳定，而且制备产率很高。四氟硼酸盐的芳香杂环重氮盐也可发生此反应。重氮盐可以在四氟硼酸存在下对芳胺进行重氮化得到。当用六氟磷酸盐(PF6-)或六氟锑酸盐(SbF6-)而不是四氟硼酸盐作为平衡离子时，芳香氟化的产率可以提高。此反应有一个缺点是，当大量的六氟硼酸的重氮盐热分解时，有爆炸的危险。然而，不需要加热和光分解也可以发生，在吡啶氟化氢溶液中分解时，大量的底物也可以温和的进行。当制备有极性取代基(OH, OMe, CF3, etc.)的芳香氟代物时，此方法非常有用。
反应机理：反应机理中包含正离子中间体，此正离子是被BF4-进攻而不是氟离子。此反应中四氟硼酸盐的重氮盐热分解和光分解分解速率相同，这正证明了芳基正离子的存在。分解遵循一级反应速率理论，因此是SN1反应。
反应实例：
参考文献：
编译自Strategic Applications of Named Reactions in OrganicSynthesis, László Kürti and Barbara Czakó, Balz-Schiemann reaction, page34-35.
芳香胺类化合物通过重氮盐热解而得到相应的氟化物，是合成氟化合物的一个重要方法，我们称为Balz－Schiemann反应。一般情况下将重氮盐转变为不溶于水的硼氟酸盐ArN2BF4，或直接在硼氟酸存在下重氮化，再加热分解重氮盐，便得到氟化物。
对一些热稳定性差的化合物，也可以通过光解或超声波分解得到相应的氟化物。
用六氟磷酸代替氟硼酸，得到的重氮盐ArN2PF6溶解性更小，因此提高了反应产率。
另外，其他重氮盐如ArN2SbF6，ArN2AsF6，ArN2SiF6也有报道。
这个方法的缺点是有时候硼氟酸盐ArN2BF4不太稳定，容易分解，其制备和分离比较困难，反应的重复性较差。
反应示例13c：
A. o-Bromobenzenediazonium hexafluorophosphate. A solution of 95 mL of 12N hydrochloric acid in 650 ml. of water is added with stirring to 60 g. of o-bromoaniline (0.35 mole; in a 2-L. three-necked flask equipped with stirrer and thermometer. Solution is effected by heating the mixture on a steam bath. A solution of 29 g. (0.42 mole) of sodium nitrite in 75 ml. of water is added with stirring while the mixture is maintained at −5° to −10° by means of a bath of ice and salt or of dry ice and acetone. At the end of the addition there is an excess of nitrous acid, which can be detected with starch iodide paper. Seventy-four milliliters (134 g., 0.60 mole) of 65% hexafluorophosphoric acid is added in one portion, with vigorous stirring, to the cold solution of the diazonium salt. Cooling and slow stirring are continued for an additional 30 minutes, and the precipitated diazonium hexafluorophosphate is then collected on a Büchner funnel. The diazonium salt is washed on the funnel with 300 ml. of cold water and with a solution of 80 ml. of methanol in 320 ml. of ether. The salt is partly dried by drawing air through the funnel for 2 hours. It is then transferred to a pile of several filter papers, powdered with a spatula, and dried at about 25°/1 mm. for at least 12 hours. The dried o-bromobenzenediazonium hexafluorophosphate is cream-colored; weight 108–111 g. (94–97%); m.p. 151–156° (dec.).
B. 1-Bromo-2-fluorobenzene. Caution! This step should be carried out in a hood because the PF5 evolved on thermal decomposition of the diazonium salt is poisonous. The apparatus consists of a 1-L, three-necked, round-bottomed flask equipped with a thermometer, a condenser, a magnetic stirrer (optional), and a 250-mL Erlenmeyer flask that is attached by means of a short rubber Gooch connecting tube. The dry powdered hexafluorophosphate salt is placed in the Erlenmeyer flask, and 300 mL of heavy mineral oil is placed in the round-bottomed flask. The mineral oil is heated to 165–170° by means of an oil bath or electric heating mantle and maintained at this temperature while the salt is added rapidly in portions over a period of 30 minutes. The flask is cooled rapidly to room temperature, the side flask is removed, and 400 mL of 10% aqueous sodium carbonate is added slowly through the condenser. The mixture is steam-distilled until no more oil is visible in the distillate.
The oil, which is heavier than water, is separated, and the aqueous layer is extracted with three 50-mL portions of methylene chloride. The oil and extracts are combined, dried over anhydrous sodium sulfate, and distilled from a Claisen flask with an indented neck. Colorless 1-bromo-2-fluorobenzene is collected at 58–59°/17 mm. or 156–157°/760 mm.; weight 45–47 g. (73–75% based on o-bromoaniline); nD25 1.5320–1.5325.
这个反应的另一个改进方法就是直接用HF/NaNO2进行重氮化，或在一些碱如吡啶，2－羟基吡啶，哌啶等存在下重氮化，然后热解生成相应的氟化合物14，苯胺类化合物和氨基取代的杂环化合物，如吡啶，嘧啶等都能发生这种反应。
反应示例14：
In a 100-mL PFA made reactor equipped with a reflux condenser, 4-aminopyridine (470 mg, 5mmol) was added to HF/Pyridine (70% HF, 10 mL) at 0oC. The mixture was allowed to stand at room temperature until 4-aminopyridine was completely dissolved. After cooling down to –78oC, NaNO2 (380 mg, 5.5 mmol) was added to a solution of 4-aminopyridine in HF, the stirred solution was allowed to stand at 0oC for 30 min and then stand at 60oC for 15-30 min. By quenching the resultant solution with ice water, followed by the neutralization with cold saturated NaHCO3, the resulting mixture was extracted with CH2C12, the combined organic phase was dried over MgSO4, and evaporated under reduced pressure to give the crude product, which was distilled to give the desired product (460 mg, 95%).
反应示例：
To the alanine (1.74 g, 20 mmol) in pyridinium polyhydrogen fluoride (50 mL) in polyolefin bottle with good stirring at 0oC was added NaNO2 (2.1 g, 30 mmol) in three prortions over aperiod of 30 min. Stirring was continued at room temperature for 5 h, then quenched with 100 mL of ice water and extracted with ether. The organic layer was washed with brine and dried over Na2SO4. The solvent was removed under reduced pressure, the residue was recrystallized to give the desired product (1.39 g, 76%)