In organic chemistry, an aza-crown ether is an aza analogue of a crown ether (cyclic polyether). 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), 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. The 18-membered ring in (CH<sub>2</sub>CH<sub>2</sub>NH)<sub>6</sub> can be synthesized by combining two triamine components. 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 linkers: Aza crowns often feature trimethylene ((CH<sub>2</sub>)<sub>3</sub>) as well as ethylene ((CH<sub>2</sub>)<sub>2</sub>) linkages. One example is cyclam (1,4,8,11-tetraazacyclotetradecane).
Tertiary amines: In many aza-crown ethers some or all of the amines are tertiary. One example is the tri(tertiary amine) (CH<sub>2</sub>CH<sub>2</sub>NCH<sub>3</sub>)<sub>3</sub>, known as trimethyltriazacyclononane. Cryptands, three-dimensional aza crowns, feature tertiary amines.
Mixed ether-amine ligands: Another large class of macrocyclic ligands feature both ether and amines. 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)]<sub>2</sub>.
Lariat crowns: The presence of the amine allows the formation of Lariat crown ethers, which feature sidearms that augment complexation of cation.
References