Crown ethers (crown compounds) are macro- heterocyclic compounds containing in their cycles more than 11 atoms, of which at least four are heteroatoms that are linked together by ethylene bridges.
As a rule, the heteroatom is an oxygen atom. If one or more oxygen atoms are replaced by nitrogen or sulfur atoms, then the corresponding compounds are called azacrown or thiacrown ethers, respectively. If crown ethers are condensed with benzene or cyclohexane rings, then they are benzene crown or cyclohexane crown ethers. Crown ethers containing P, Si, As atoms in the cycle, as well as amide, ester, and some other functional groups, were obtained.
Content
Nomenclature
In the trivial names of crown ethers, the total number of atoms in the cycle and the number of heteroatoms are indicated by the numbers that put before and after the word “crown”. According to IUPAC, crown ethers are presented as derivatives of cyclic hydrocarbons:
- formula I - 12-crown-4; (1,4,7,10-tetraoxacyclododecane)
- formula II - 1,10-diaza-18-crown-6; (4,7,13,16-tetraoxa-1,10-diazacyclooctadecane)
- formula III - 1,7-dithia-15-crown-5
- formula IV - dibenzo-18-crown-6
- formula V - cyclohexano-15-crown-5
Properties
Crown esters are viscous liquids or crystalline substances that are highly soluble in most organic solvents and poorly soluble in water. Their chemical properties depend on the nature of the heteroatoms and functional groups in the cycle.
Crown ethers form stable lipophilic complexes with metal cations , especially alkaline and alkaline earth. In this case, the metal cation is included in the intramolecular cavity of the crown ether and is retained due to the ion-dipole interaction with heteroatoms. The most stable are those complexes in which the size of the metal ion is close to the size of the cavity of the crown ether molecule. Complexes, unlike the inorganic salts that form them, are often soluble in organic solvents, which makes it possible to create a homogeneous environment for many chemical reactions (for example, “purple benzene” is used as a solution of potassium permanganate in benzene in the presence of crown ethers). Due to the high stability of the complex ion, the existence of such unusual alkali metal compounds as alkalides and electrodes is possible .
Getting
Crown ethers are obtained by condensation of dihaloalkanes or d-esters of p- toluenesulfonic acid with polyethylene glycols in tetrahydrofuran , 1,4- dioxane , dimethoxyethane , dimethyl sulfoxide , tert- butanol in the presence of bases (hydrides, hydroxides, carbonates); intramolecular cyclization of monotosylates of polyethylene glycols in dioxane, diglyme or tetrahydrofuran in the presence of alkali metal hydroxides, as well as cyclooligomerization of ethylene oxide in the presence of BF 3 and alkali and alkaline earth metal borofluorides.
Azacrown esters are obtained by acylation of di- or polyamines with partially protected amino groups by dicarboxylic acid chlorides, followed by reduction of the macrocyclic diamides formed; by alkylation of dithosyl diamines with dihalogen derivatives or glycol ditosylates in the presence of alkali metal hydrides or hydroxides.
Thiacrown ethers are prepared from thiaanalogs of polyethylene glycols in the same way as conventional crown ethers or by alkylation of dithiols with dihalides or ditosylates in the presence of bases.
Application
Crown ethers are used for the concentration, separation, purification and regeneration of metals, including rare earths; for the separation of nuclides, enantiomers; as medications, antidotes, pesticides; to create ion-selective sensors and membranes; as catalysts in reactions involving anions. The use of some crown ethers to enhance the permeability of drugs through the mucous membranes of the eyes has been shown [1] .
Tetrazacrown ether is cyclene, in which all oxygen atoms are replaced by nitrogen [2] , is used in magnetic resonance imaging as a contrast medium .
Notes
- ↑ PWJ Morrison, NN Porfiryeva, S. Chahal, IA Salakhov, C. Lacourt, II Semina, RI Moustafine, VV Khutoryanskiy. Crown Ethers: Novel Permeability Enhancers for Ocular Drug Delivery? // Molecular Pharmaceutics. - 2017-08-21. - ISSN 1543-8384 . - DOI : 10.1021 / acs.molpharmaceut.7b00556 .
- ↑ http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV8P0152 Organic Syntheses, Coll. Vol. 8, p. 152 (1993); Vol. 68, p. 227 (1990).
Literature
- Duff's reaction - Copper // Chemical Encyclopedia in 5 volumes. - M .: Big Russian Encyclopedia, 1990. - T. 2. - 671 p.
- Neyland, O. Ya. Organic Chemistry. - M .: Higher. school, 1990. - 751 p.