Aza-crown ether explained
In organic chemistry, an aza-crown ether is an aza analogue of a crown ether (cyclic polyether).[1] [2] That is, it has a nitrogen atom (amine linkage, or) in place of each oxygen atom (ether linkage,) around the ring. While the parent crown ethers have the formulae, the parent aza-crown ethers have the formulae, where n = 3, 4, 5, 6. Well-studied aza crowns include triazacyclononane (n = 3), cyclen (n = 4),[3] and hexaaza-18-crown-6 (n = 6).
Synthesis
The synthesis of aza crown ethers are subject to the challenges associated with the preparation of macrocycles.[4] The 18-membered ring in (CH2CH2NH)6 can be synthesized by combining two triamine components.[5] By reaction with tosyl chloride, diethylene triamine is converted to a derivative with two secondary sulfonamides. This compound serves as a building block for macrocyclizations.
Variants
Many kinds of aza crown ethers exist.
- Variable length linkersAza crowns often feature trimethylene ((CH2)3) as well as ethylene ((CH2)2) linkages. One example is cyclam (1,4,8,11-tetraazacyclotetradecane).
- Tertiary aminesIn many aza-crown ethers some or all of the amines are tertiary. One example is the tri(tertiary amine) (CH2CH2NCH3)3, known as trimethyltriazacyclononane. Cryptands, three-dimensional aza crowns, feature tertiary amines.
- Mixed ether-amine ligandsAnother large class of macrocyclic ligands feature both ether and amines..[6] One example is the diaza-18-crown-6, [(CH<sub>2</sub>CH<sub>2</sub>O)<sub>2</sub>(CH<sub>2</sub>CH<sub>2</sub>NH)]2.[7]
- Lariate crownsThe presence of the amine allows the formation of Lariat crown ethers, which feature sidearms that augment complexation of cation.[8]
References
- 10.1016/S0010-8545(98)00243-4. Proton coordination by polyamine compounds in aqueous solution. 1999. Bencini. Andrea. Bianchi. Antonio. Garcia-España. Enrique. Micheloni. Mauro. Ramirez. José Antonio. Coordination Chemistry Reviews. 188. 97–156.
- 10.1039/b106367g. Metal complexes of azacrown ethers in molecular recognition and catalysis. 2002. Reichenbach-Klinke. Roland. König. Burkhard. Journal of the Chemical Society, Dalton Transactions. 2. 121–130.
- Org. Synth. . 1,4,7,10-Tetraazacyclododecane . David P.. Reed . Gary R. . Weisman . 78 . 73 . 2002 . 10.15227/orgsyn.078.0073.
- 10.1021/cr00094a008. Synthesis of aza-crown ethers. 1989. Krakowiak. Krzysztof E.. Bradshaw. Jerald S.. Zamecka-Krakowiak. Daria J.. Chemical Reviews. 89. 4. 929–972.
- Org. Synth. . 1,4,7,10,13,16-Hexaazacyclooctadecane . T. J.. Atkins. J. E.. Richman. W. F.. Oettle . 58 . 86 . 1978 . 10.15227/orgsyn.058.0086.
- 10.1021/ja00732a007. Stability constants of cyclic polyether complexes with univalent cations. 1971. Frensdorff. Hans K.. Journal of the American Chemical Society. 93. 3. 600–606.
- Organic Syntheses. Vincent J.. Gatto. Steven R.. Miller. George W.. Gokel . 4,13-Diaza-18-Crown-6 . 68 . 227. 1990 . 10.15227/orgsyn.068.0227.
- G. W.. Gokel. L. J. . Barbour. R. . Ferdani . J.. Hu . Lariat Ether Receptor Systems Show Experimental Evidence for Alkali Metal Cation Interactions. Acc. Chem. Res. . 2002 . 35. 10. 878–886 . 10.1021/ar000093p. 12379140.