【英文名称】18-Crown-6
【分子式】C12H24O6
【分子量】264.32
【 CAS 登录号】17455-13-9
【缩写和别名】18C6, 1,4,7,10,13,16-Hexaoxacyclooctadecane
【物理性质】白色结晶，mp 39-39.5 °C, bp 116°C/ 26.6Pa , 可溶于水。
【制备和商品】国内外试剂公司均有销售。通常采用威廉姆逊合成法制备，即用醇盐与卤代烷反应（式1)。也可由环氧乙烷在模板法的条件下低聚制备（式2)。
【注意事项】具有较大的毒性，对大鼠的口服致死量为 300 mg /kg 。对眼睛和皮肤有刺激性，应避免直接接触。
What are the unique properties and applications of 18-Crown-6?
18-Crown-6 has the ability to form stable complexes with various metal salts, ammonium salts, and organic cations. For example, the cavity of the 18-Crown-6 molecule can accommodate a potassium ion with a diameter of 266 pm (式3).
By utilizing this property, various salts can be dissolved in organic solvents. The resulting complexes can be soluble in non-polar organic solvents due to the outwardly arranged alkyl groups, while the uncoordinated anions remain in the solvent in their naked form, exhibiting high reactivity. 18-Crown-6 is widely used in organic synthesis, optical resolution, heavy metal chelation, separation, analysis, and pharmaceutical and biochemical applications.
18-Crown-6 can be used as a phase transfer catalyst, enabling reactions that are difficult or do not occur under traditional conditions to proceed smoothly. These reactions have the advantages of fast reaction rates, simple conditions, convenient operation, and high yields. For example, the condensation reaction of benzaldehyde in an aqueous solution has a very low yield. However, by adding 7% of 18-Crown-6 to the water solution, the yield of benzaldehyde can reach 78% (式4).
In the addition reaction between amine derivatives and imines catalyzed by alkali, the addition of 18-Crown-6 in a low-polarity solvent at low temperature can effectively promote the dissolution and ionization of the catalyst potassium tert-butoxide, thus catalyzing the reaction efficiently (式5).
18-Crown-6 exhibits typical ether properties, and its α-hydrogen can be removed to generate radicals in the presence of oxidants. For example, in the presence of tert-butyl peroxide, 18-Crown-6 undergoes dimerization reaction (式6).
The synthesis of many nitrogen-containing derivatives of 18-Crown-6 has also been reported. They have similar effects to 18-Crown-6 in alkali metal chelation and phase transfer catalysis (式7 and 式8).
1. Pedersen, C. J. Science 1988,241，536.
2. Pedersen, C. J. J. Am. Chem. Soc. 1967,89, 7017.
3. Gokel, G W.; Cram, D. J.; Liotta, C. L.; Harris, H. P.; Cook, F. L. Org. Synth. 1977,57,30.
4. Chen, Y.; Seki, K.; Yamashita, Y.; Kobayashi, S. J. Am. Chem. Soc. 2010,132, 3244.
5. Beaujean, M.; Mignani, S.; Merenyi, R.; Janousek, Z.; Viehe, H. G：Tetrahedron 1984, 40, 4395.
6. Alison M. S.; Jose A. V. J. Org. Chem. 2007, 72,3735.
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