Crown ethers 18-membered ring formation

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Alkali metal picrates have been used to measure formation constants for crown ethers in solution, but the selectivity of benzo crown ethers for metal picrates, relative to the analogous chlorides, nitrates, perchlorates, and thiocyanates, may vary significantly. Apparently, it—it interactions between the picrate ions and the aromatic ring(s) on the crown are responsible for the difference. The importance of the picrate effect rises as the number of benzo groups in the crown ether is increased, and it varies with their location in the macrocycle. The dependence of the picrate NMR chemical shift on the metal cation and/or macrocycle identity has been used to study picrate-crown ether 7r-stacking in large crown ether (18, 21, and 24-membered) complexes. 

Selective monotosylation of symmetric diols was also reported. i This technique was further applied to the s)mthesis of cyclic ethers (eq 22). This process, performed in the presence of at least 3 equiv of Ag20, was particularly favorable when steri-cally undemanding five- or six-membered rings could be formed. However, the technique was also used for the formation of the three-membered oxirane ring and of a substituted 5-0-crown ether. 

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