Solvent effects cyclic ether reactions

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Ordinary ketones are generally much more difficult to cleave than trihalo ketones or p-diketones, because the carbanion intermediates in these cases are more stable than simple carbanions. However, nonenolizable ketones can be cleaved by treatment with a 10 3 mixture of t-BuOK—H2O in an aprotic solvent such as ether, dimethyl sulfoxide, 1,2-dimethoxyethane (glyme), and so on, or with sohd t-BuOK in the absence of a solvent. When the reaction is applied to monosubstituted diaryl ketones, that aryl group preferentially cleaves that comes off as the more stable carbanion, except that aryl groups substituted in the ortho position are more readily cleaved than otherwise because of the steric effect (relief of stain). In certain cases, cyclic ketones can be cleaved by base treatment, even if they are enolizable. " OS VI, 625. See also OS VH, 297. 

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. 

Section II, A,3. The harmful effect of cyclic ethers as solvents is emphasised by a recent study of their reaction with Me3SiMn(CO)5 (cf. Section III,B,3) (437) ... 

The indan-based a-amino acid derivatives can be synthesized by PTC. Kotha and Brahmachary [11] indicated that solid-liquid PTC is an attractive method that offered an effective way of preparing optically active products by chiral PTC. They found that ethyl isocyanoacetate can be easily bisalkylated in the presence of K2CO3 as the base and tetrabutylammonium hydrogen sulfate as the catalyst. The advantage of isolating water from the reaction medium is to avoid the formation of unwanted hydroxy compounds in the nucleophilic substitution reaction. If liquid-liquid PTC is applied in the system with the strong base NaOH and dichloromethane as the organic solvent, the formation of dihydroxy or cyclic ether can be observed. 

So how do we find G AC or GBC Well, first we must remove the more powerful specific style of catalysis by working at constant pH because SAC or SBC depends on pH alone. If we find that the rate of the reaction changes with the concentration of a weak base at constant pH, we have GBC. The formation of three- and five-membered cyclic ethers shows the contrast between GBC and SBC. The formation of epoxides is straightforward SBC with a simple linear dependence on pH between pH 8 and 12, and no acceleration at constant pH by carbonate (CO ") ions. There is an inverse solvent isotope effect and an aryl substituent at the electrophilic carbon atom gives the small positive p value expected for Sn2 with an anion. 

Both diethyl ether and the cyclic ether tetrahydrofuran, shown below, are common solvents for organic reactions. Diethyl ether was formerly used as an anesthetic (known simply as ether in that context), but it had significant side effects. 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifiuoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction may be carried out in an inert solvent such as hexane at about 60°C. The copolymer is obtained as a slurry and is collected, washed, and dried. 

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