Basicity of cyclic ethers

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

Yamashita et al. have shown that the basicity of cyclic ethers, represented by pKb, plays an important role [195]. The plot of —log r, vs. pKb of monomer M2 is often linear. Nucleophilic attack of a cyclic ether on the cationic end is thus one of the driving forces of copolymerization. The slope... 

The basicities of cyclic ethers have been investigated widely, as they provide information not only on the nature of the chemical reactions but on theoretical questions as well. Earlier monographs may be consulted for analyses and conclusions relating to the relevant results. ... 

Relative basicities of cyclic ethers have been determined by several methods [146,152,153]. By one method Wirth and Slick [152] obtained relative basicities by infrared from a study of the equilibrium constant at 27°C for the distribution of BF3 between two ethers in benzene solution. In another case, Yamashita et al. [146], following a method similar to that of Gordy and Stanford [154], measured the infrared absorption spectra of cyclic ethers in the presence of 0.1 mole r methanol-d. They then calculated the basicity of the monomers from the shift value of O—D stretching band. Nuclear magnetic resonance measurements of the equilibrium constant between BF3. Et O and the cyclic ether were also used [146]. 

Yamashita et al. [146] point out some features of the basicity of cyclic ethers. The basicity is affected by chemical structure, ring size, and substituents. The ring size affects basicity in the order 4 > 5 > 6 > 3. In 5-membered rings the basicity order is ether > lactone > formal. Methyl substitution increases the basicity and chloromethyl substitution decreases basicity. The relative basicities of the monomers are broadly in agreement with their reactivity in copolymerization experiments [58, 122]. 

Darensbourg DJ, Chung WC (2013) Relative basicities of cyclic ethers and esters. Chemistry of importance to ring-operting co- and teipolymerization reactions. Polyhedron 58 139-143... 

Basicity of cyclic ethers decreases in the order as shown in Scheme 2. 

The reaction processes shown in Scheme 8 not only accomplish the construction of an oxepane system but also furnish a valuable keto function. The realization that this function could, in an appropriate setting, be used to achieve the annulation of the second oxepane ring led to the development of a new strategy for the synthesis of cyclic ethers the reductive cyclization of hydroxy ketones (see Schemes 9 and 10).23 The development of this strategy was inspired by the elegant work of Olah 24 the scenario depicted in Scheme 9 captures its key features. It was anticipated that activation of the Lewis-basic keto function in 43 with a Lewis acid, perhaps trimethylsilyl triflate, would induce nucleophilic attack by the proximal hydroxyl group to give an intermediate of the type 44. 

It has long been accepted that cyclic ethers with more than four-membered rings are, in contrast to epoxides, only cationically polymerizable due to the high basicities of their ether oxygen atoms. The cationic polymerization involves 0-... 

It is generally agreed that propagation in the cationic polymerization of cyclic ethers occurs after nucleophilic attack by the monomer oxygen atom (equation 3). Therefore, many authors attempt to explain their copolymerization data by noting that the more basic monomer has the higher reactivity with the active chain end. The order of basicity which has been established (36, 38) is ... 

Tanaka [198] made an attempt to determine the contribution of ring strain and basicity to the reactivity of cyclic ethers in copolymerization by means of Gibbs polymerization energy. He derived a relation between the relative reactivities of an m-membered ring with i substituents to that of an n-membered cyclic ether with j substituents. He included a linear combination of their basicity differences, A(pKb)m and of Gibbs energy, A(AG)m., in his relation (a, b, and c are constants)... 

Detailed studies were performed for a series of cyclic ethers and K values measured for (C2Hs)30 BF4 and THF (K = 36 at 35 C in CH2CI2), THP (K = 23.4) and OXP (K = 26) >. Cyclic ethers are thus not only more basic but also more nucteophilic (reaction (38)) than linear ethers. 

These findings represent new and significant conclusions about the relative reactivity of cyclic ethers. Prior to the work of Saegusa et al. [54], most work has suggested that a monomer s reactivity as reflected in its fep is related to basicity and/or possibly ring strain. More will be said about these results at the end of this review after data for all the cyclic ethers and sulphides have been presented. 

The factors most commonly mentioned as important in studies of copolymerization of these cyclic compounds are relative basicity and ring strain. It seems fairly well established that relative basicity plays an important role in controlling the relative reactivity of cyclic ethers. This lends further support to the belief that propagation proceeds by nucleophilic attack of monomer oxygen in the cationic copolymerizations. Basicity seems to be less important than other factors such as ring strain in determining relative reactivity of cyclic sulphides. 

Tanaka [155] has attempted to make a quantitative estimate of the contribution of ring strain and basicity to reactivity of cyclic ethers in cationic copolymerization. Free energy of polymerization was used as a measure of ring strain. The relationship he derived related the logarithm of relative reactivity, l/r , of m-membered ring ethers with i substituents to n-membered ring compounds with j substituents to a linear combination of the differences in basicity, A(pXb)m,i- ,/ and in free energy, viz. 

Thus a plot of [M2 ] /O against [Mj ] allows determination of the value of Ki from the intercept and r2 from the slope. The resulting kinetic parameters are summarized in Table 13. The comparison with the basicity values, here given in terms of pi b > ain shows that in a copolymerization of cyclic ethers, reactivity correlates well with basicity. 

In copolymerizations of cyclic ethers, too, basicity of the monomers seems to correlate well with the reactivity ratios determined from copolymer composition. Recently, Saegusa et al. [54] have pointed out that the apparent values of the monomer reactivity ratios in cyclic ether copolymerizations may be more influenced by the exchange... 

Heterocyclics containing P-atoms are usually strong nucleophiles. For example the basicity of 2-methoxy-ODP (pKa = 3.1) is much higher than that of cyclic ethers or sulfides. Therefore, a wide range of initiators, i.e., carbenium or oxonium salts, Lewis and protonic acids, and relatively nonreactive alkyl halides and organoalu-minum compounds have been used. Usually, reactions were carried out in a N2 atmosphere, although no special precautions (e.g. vacuum) were used to avoid contamination with water. 

Aoki et al. 21) proposed the following formula relating l/rt to basicity and ring strain in the copolymerization of cyclic ethers with BCMO ... 

Under the same conditions, the reactivity of three-membered cyclic ethers in anionic copolymerization with cychc anhydrides is higher than that of four-membered ethers Higher membered cyclic ethers can polymerize or copolymerize with anhydrides only by a cationic mechanism whereby not only alternating copolymer but also a great number of polyether sequences are formed. This difference in reactivity is evidently associated with the basicity of cychc ethers, three-membered ethers having the lowest basicity The lower basicity causes a lower reactivity of the epoxide (cychc ether) in competitive reactions or in copolymerization with other cychc monomers compared with the expected reactivity which follows from the strain in the ring. The strain energy, taken as the difference between the experimental and calculated heats of formation was found to be 54.4kJ/mol for ethylene oxide... 

When lactones copolymerize with cyclic ethers, such as j -ptopiolactone with tetrahydrofiiran, in the early steps of the reaction the cyclic ethers polymerize almost exclusively. This is due to the greater basicity of the ethers. When the concentration of the cyclic ethers depletes to the equilibrium value, their consumption decreases markedly. Polymerizations of the lactams com-mence. The products are block copolymer. 

In our previous review paper, presented at the Rouen Symposium on Cationic Polymerization, we stressed some of the major differences between the cationic po -lymerization of cyclic ethers and cyclic acetals 11 ]. These differences are mainly caused by the much larger basicity (nucleophilicity) of cyclic ethers, than that of cyclic acetals moreover, cyclic ethers are more basic (nucleophilic) than their polymers, whilst polyacetals seem to be more basic than their corresponding monomers. 

Tanaka Y (1967) Contribution of ring strain and basicity to reactivity of cyclic ethers in cationic copolymerizatiorL J Maraomol Sci A Chem 1(6) 1059-1068... 

Cyclic ethers constitute an important class of heterocyclic monomers that polymerize by ionic mechanisms. Studies of the mechanism, kinetics, and thermodynamics of cyclic ether polymerization were essential in establishing basic principles of ionic ring-opening polymerization (ROP). There are several book chapters and reviews summarizing this field and although some of them date back to early 1980s, the main conclusions are still t id and provide a basis for more recent developments. " ... 

In this chapter, the basic features of CROP of cyclic ethers will be briefly outlined on the basis of published earlier chapters and reviews, mainly the first edition of Comprehensive Polymer Science, and then more recent developments will be discussed based on the original literature. 

Knowledge of the order of basicities of cyclic and linear ethers is important for understanding certain phenomena in cyclic ether polymerization. As indicated earlier, chain transfer to polymer is a general feature of the cationic polymerization of cyclic ethers because the nucleophilic site of the monomer molecule (oxygen atom) is transferred to the polymer unit. To what extent chain transfer to polymer competes with propagation depends on the relative nucleophilicity of monomer and polymer unit. Thus, for five-membered THF, the polymer unit is a weaker base than the monomer. This makes the polymer less reactive than the monomer in nucleophilic substitution type reactions. Consequently, for this monomer, chain transfer to polymer is slow as compared to propagation. In contrast, in the polymerization of three-membered EO, the polymer unit is more basic than monomer. Therefore, reactions involving the polymer chain are important in this system. Practical consequences will be discussed in the subsequent sections devoted to polymerization of different classes of cyclic ethers. 

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