Cyclic ethers solvent effects

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

In addition to exploiting solvent effects on reactivity, there are two other valuable approaches to enhancing reactivity in nucleophilic substitutions. These are use of crown ethers as catalysts and the use of phase-transfer conditions. The crown ethers are a family of cyclic polyethers, three examples of which are shown below ... 

In early work no such NMR chemical shift changes relative to those of the parent components were observed for polypseudorotaxanes with aliphatic backbones and aliphatic crown ethers as the cyclic species [108, 109]. Model studies were performed with 18-crown-6 (18C6), which is so small that it cannot be threaded. The recovery of intact 18C6 under conditions identical with those for the syntheses of the polyrotaxanes ruled out the possibility of side reactions. The effective removal of the small crown ether by precipitation into a solvent which was poor for backbone but good for the cyclic demonstrated the effectiveness of the purification procedure. In addition, reaching a constant min value after multiple precipitations and the absence of the peak for free crown ether in GPC traces indicated that the larger crown ethers detected by NMR in the purified polymeric products were indeed threaded rather than simply mixed. 

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. 

Solvent effects in the peroxycarboxylic acid epoxidations are significant. The epoxidation rates in ether or ethyl acetate are approximately one-tenth of those in benzene or chloroform. The much slower epoxidation with peroxycarboxylic acids with intermolecular hydrogen bonding is indicative of the development of a cyclic transition state. 

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) ... 

Although lithium cuprates bearing two alkynic ligands notoriously resist transfer of this group, when admixed with an equal amount of the lithium alkynide (i.e. 3RC CLi Cul) exclusive 1,2-addition occurs with cyclic enones. While prior efforts to effect this chemistry in strictly ethereal solvents (diox-ane) have been unsuccessful, use of HMPA (- 20%) as cosolvent now leads to efficient couplings in these cases. The initial products may be isolated as such, or oxidatively worked up to provide, in the case of (73), the rearranged material (74 Scheme 11). 

The homologation of ketones by the addition of diazoalkanes complements the Tiffeneau-Demjanov rearrangement. Epoxide formation is a side reaction which can be minimized if polar aprotic solvents are avoided (Scheme 7). Rearrangement i.e. homologation) is maximized in ether solvents or by Lewis acid catalysis. The reaction is most effective in the ring expansion of cyclic ketones. 

Diazirine 63 was photolyzed in various solvents and within CyDs. The solution results are summarized in Table 8. The conventional solvents were used to gauge whatever effects the CyD hosts had on carbene 64. Hydrocarbon solvents, like pentane ( -C5Hi2) and cyclohexane (c-C6H12), were used to mimic the inner cavities of CyDs, which are also nonpolar, hydrophobic environments. Tetrahydrofuran (THF) was employed because the cyclic ether resembles the D-Glcp monomer units of the CyDs. Moreover, since CyDs also possess many hydroxyl (O-H) groups, it seemed appropriate to perform control experiments in alcoholic solutions of diazirine 63. Finally, chloroform (CHC13) was used to assess the spin-state of carbene 64. 

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. 

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