Oxyethylene unit crown ethers

Interestingly the highest selectivity toward Li cations among paracrowno-phanes is obtained by 34c (s. Scheme 1) with five ethereal oxygen atoms, whose structure differs from the so-called lithiophilic crown compounds with four ethereal oxygen atoms such as 12-crown-4 (37), 14-crown-4, and their derivatives (38) (Structure 20). These results clearly suggest that the combination of number of oxyethylene units and the shape of the ether cavity leads to this suitable Li" selective ionophore. 

Furthermore, the hydrolysis of butyl acetate and methyl pivalate in benzene in the presence of KOH at 25 °C as well as the reaction of potassium phenolate with benzyl chloride in boiling acetonitrile are accelerated by addition of polyoxyethylene [183]. The catalytic effect of POE is augmented by an increase in the number of oxyethylene units, i.e. 1 <6< 12. PEO is also an interfacial catalyst of the reaction of phenol and 2,4,6-trimethylphenol with methyl iodide in water-chloroform and dichloromethane. The kinetic study of the reaction between benzyl chloride and potassium acetate in the presence of PEO of variable molecular weight in toluene and butanol has been performed with IR spectroscopy [184]. The dissolution of a reagent of poor solubility is apparently a rate-limiting step of the reaction in a solution of low polarity (toluene). The presence of PEO impurities in toluene has been detected. Moreover, effect of PEO and crown ethers as phase transfer catalysts has been compared. In a low-polarity solvent, oligoethylene oxides are more effective catalysts, while in a polar solvent (butanol) the effectiveness of PEO and crown ethers as phase transfer catalysts is similar. 

Crown ethers are heterocycles consisting of oxyethylene units, and their size depends on the number of these repeating units. They are known to capture cations into their cavities selectively. Specific cation recognitions of the crown ethers led to the development of various applications such as metal cation sensors and phase transfer cata-lysts.2 Since the hydrophilicities of the crown ethers increase through complexation with cations, introducing crown ether units into thermosensitive polymers should control their solubilities and LCST behaviors in the presence of cations as stimuli. 

There are fewer reports of linear, acyclic, ion-binding polymers. It has been reported that poly(oxyethylene) improves the solubility of alkali metals in ethers such as tetrahydrofuran, dime thoxy ethane, and diglyme, stabilizes fluorenyl alkali metal compounds, accelerates Williamson reactions and accelerates several other nucleophilic reactions.All of these effects were attributed to the ability of poly(oxyethylene) to complex with cations in solution. Yanagida and coworkers studied the alkali metal cation complexation of poly(oxyethylene), using a picrate salt extraction technique similar to the one used by Pedersen and Frensdorff. Polymers with more than 23 oxyethylene units were effective iono-phores for potassium, with degrees of extraction (percent extracted) comparable to crown ethers. The extractability per oxyethylene unit was nearly constant, and the complex stability increased linearly with increasing numbers of repeating oxyethylene units. Seven oxyethylenes were the minimum number of repeat units necessary to bind potassium ion effectively in the aqueous phase. The less efficient extraction of short-chain poly(oxyethylene) is apparently caused by its hydrophilic character. 

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