Cyclic ethers 1.3- dioxolane

The transformation of oxiranes into other oxygen-containing heterocyclic systems (e.g., cyclic ethers, dioxolanes, orthoesters, lactones, and cyclic carbonates), via a variety of heterolytic, homolytic, enzymatic, and single-electron transfer processes, has been reviewed <2004RJ01227>. 

With certain cyclic ethers (dioxolane, oxiranes, etc.), the use of particular conditions results in a limitation of the back-biting reactions and a certain control of the polymerization. For instance, in polymerizations carried out in the presence of an alcohol and with a very low instantaneous monomer concentration, obtained by a slow addition of the monomer solution, almost the totality of electrophilic entities carried by the initiator reacts with the monomer to give protonic species ( activated monomer) the latter then react with the nucleophilic sites that are the hydroxy groups of alcohols. Propagation occurs through nucleophilic attacks of hydroxyls onto activated monomer molecules ... 

Various cyclic ethers are reported to be superior solvents for secondary lithium metal batteries. 1,3-Dioxolane [94, 95] and... 

Leaist, D.G., MacEwan, K., Stefan, A., and Zamari, M. Binary mutual diffusion coefficients of aqueous cyclic ether at 25 °C. Tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, 1,3-dioxane, tetrahydropyran, and trioxane, J. Chem. Eng. Data, 45(5) 815-818, 2000. 

Under certain conditions, irreversible chain-breaking reactions are absent and cationic ROPs of cyclic ethers proceed as living polymerizations. These conditions are found for polymerizations initiated with acylium and l,3-dioxolan-2-ylium salts containing very stable counterions such as AsFg, PFg, and SbClg or with very strong acids (fluorosulfonic and... 

Polyoxymethylene, also referred to as acetal resin or POM, is obtained either by anionic polymerization of formaldehyde or cationic ring-opening copolymerization of trioxane with a small amount of a cyclic ether or acetal (e.g., ethylene oxide or 1,3-dioxolane) [Cherdron et al., 1988 Dolce and Grates, 1985 Yamasaki et al., 2001]. The properties and uses of POM have been discussed in Sec. 5-6d. 

If a cyclic ether such as dioxolane or dioxane is substituted for the acyclic ether the result is copolymerization of the cyclic ether (25) (Fig. 6). 

Certain cyclic ethers which will not homopolymerize will copolymerize with THF (25, 52). These cyclic ethers are stable five and six-membered ring compounds such as 2-methyltetrahydrofuran (2-MeTHF), and 1,4-dioxane (DOX). It is probable that 4-phenyl-l,3-dioxane (PhDOX), tetrahydropyran (THP), and 4-methyl-l,3-dioxolane (MDOL) which do not homopolymerize but which have been reported to copolymerize with BCMO (107, 108) would also copolymerize with THF. In the copolymerizations a correlation was again found between the basicity of the attacking ether and its reactivity with the cyclic oxonium ion. The... 

A number of pyrans, including 3-hydroxy-tetrahydropyran (both axial conformer, 29 and equatorial conformer, 30), 2-methoxy-tetrahydropyran 33, 3-methyl-tetrahydropyran 32, and several 4-substituted tetrahydropyrans, along with 2-methyl-l,3-dioxolane and the rigid cyclic ethers 7-oxabicyclo[2.2.1]heptane and 1,8-cineole, were studied extensively by NMR. These empirical results, in conjunction with the literature data for a variety of acyclic and cyclic ethers, were used to examine the reliability of O-substituent chemical shift models in these systems. The empirical data correlate well with predictions made from the model and it is concluded that ethereal oxygen substituent chemical shifts are due to both steric and electrostatic terms <1998J(P2)1751>. 

This section deals only with solvents whose reduction products are insoluble in the presence of lithium ions. The list includes open chain ethers such as diethyl ether, dimethoxy ethane, and other polyethers of the glyme family cyclic ethers such as THF, 2Me-THF, and 1,4-dioxane cyclic ketals such as 1,3-dioxolane and 1,3-dioxane, esters such as y-butyrolactone and methyl formate and alkyl carbonates such as PC, EC, DMC, and ethylmethyl carbonate. This list excludes the esters, ethyl and methyl acetates, and diethyl carbonate, whose reduction products are soluble in them (in spite of the presence of Li ions). Solutions of solvents such as acetonitrile and dimethyl formamide are also not included in this section for the same reasons. Figure 6 presents typical steady state voltammo-grams obtained with gold, platinum, and silver electrodes in Li salt solutions in which solvent reduction products are formed and precipitate at potentials above that of lithium metal deposition. These voltammograms are typical of the above-mentioned solvent groups and are characterized by the following features ... 

Fuchigami and his co workers found that the fluorination of cyclic ethers like tet-rahydrofuran, 1,4-dioxane, and 1,3-dioxolane was achieved by anodic oxidation of a mixture of a large amount of liquid cyclic ethers and a small amount of Et4NF 4HF (only 1.5-1.7 equiv. of F to the ether) at a high current density (150 mA... 

All the approaches described have been used to prepare functional polymers by cationic ring-opening polymerization. From this point of view, groups of monomers that have been investigated most are cyclic ethers (tetrahydrofuran), cyclic acetals (1,3-dioxolane), cyclic imines (N-f-butylaziridine), and oxazolines, i.e., these monomers for which the living conditions can be approached. 

In conclusion, in the kinetics of dioxolane polymerizations with many catalysts, the initiation mechanism is complex and inefficient. The degree of efficiency seems to be related both to the cation and to the anion. Again as in the case of cyclic ethers and cyclic sulphides, an independent measurement of the number of active sites seems essential for precise kinetics. The most probable fep for the polymerization seems to be of the order of 10—501 mole sec . With careful choice of polymerization conditions a kinetically reversible polymerization occurs, but the molecular weight of the polymer produced is not related to the initiator concentration, probably as a result of a transfer reaction. 

Polyformaldehyde can also be prepared by polymerization of trioxane, the cyclic trimer of formaldehyde. Trioxane polymerizes by ring opening polymerization and cationic initiators are the only effective initiators. Formaldehyde is always present when trioxane is polymerized because the growing polyoxymethylene chains by depropagation may lose one monomer unit, which is formaldehyde not trioxane. In spite of the fact that formaldehyde plays an (as yet incompletely understood) role in trioxane polymerization, which is a cyclic ether polymerization like dioxolane or tetrahydrofurane [5], trioxane will not be discussed in this review. 

Unlike anionic initiators or anionically growing alkoxide chains which can only grow (or terminate), cationic initiators (Lewis, Bronsted acids or preformed initiators) or the cationically growing chain may cause acetal-interchange reactions. These reactions are also called transacetalization and cause rearrangement in the molecular weight distribution in homopolymers. The rates of transacetalization are relatively slow compared to that of polymerization except at high temperatures. In the presence of cyclic ethers or cyclic formals, for example, dioxolane, polyformaldehyde can incorporate randomly the co-monomer polyoxyethylene units into the polymer under transacetalization conditions. 

As with polystyrene sulfonic resins, Nafion-based acid catalysts are highly efficient for hydration and dehydration processes and, in general, for condensation reactions that occur with the formation of water or similar secondary products. Formation of ethers has been studied for various alcohols [109-111]. Dehydration of 1,4- and 1,5-diols at 135 °C affords the corresponding cyclic ethers such as 20 in excellent yields (Scheme 10.7), while 1,3-diols experience different transformations depending on their structure [112]. The dehydration of 1,2-diols mainly proceeds via the pinacol rearrangement. Further condensation of the initially formed carbonyl compound and unreacted diol affords 1,3-dioxolanes [113]. The catalyst could be efficiently reused following a reactivation protocol. Formation of aryl ethers is also possible, and the synthesis of dibenzofurans 21 (X = O) from 2,2 -dihydroxybiphenyls has been reported (Scheme 10.7) [114]. The related reaction... 

Processes that do not terminate were described for the polymerization of five- and seven-membered cyclic ethers THF (4, 91) and oxepane (93). Under proper working conditions both cyclic acetals, namely 1,3-dioxolane and 1,3-dioxepane (94. 95). as well as bicyclic acetals or 1,6-anhydro-l,3,4-tri-0-benzyl-a-D-glucopyranose (96) can polymerize under living conditions. 

Thermal stability of polyacetal is achieved by incorporating into the backbone —CH2—CH2— units, i.e., disrupting the sequence of —CH2—O— units susceptible to unzipping. This is done, by copolymerization of TXN with other cyclic ethers or acetals, preferably with 1,3-dioxolane or ethylene oxide (<5%mol)42,53). 

The cationic pohmierizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from greater nucleophilicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is true of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane may exist in equilibrium with macroalkoxy carbon cations and tertiary oxonium ions. By comparison, the active propagating species in polymerizations of cyclic ethers, like tetrahydrofuran, are only terdaiy oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

Like THF, cyclic acetals (e.g., 1,3-dioxolane and 1,3,5-trioxane) are polymerizable only with cationic initiators. The ring-opening polymerization of 1,3,5-trioxane (cyclic trimer of formaldehyde) leads to polyoxymethylenes (see Example 3.24), which have the same chain structure as polyformaldehyde (see Example 3.22). They are thermally unstable unless the semiacetal hydroxy end groups have been protected in a suitable way (see Example 5.7). Like the cyclic ethers, the polymerization of 1,3,5-trioxane proceeds via the addition of an initiator cation to a ring oxygen atom, with the formation of an oxonium ion which is transformed to... 

The Celanese route for the production of polyacetal yields a more stable copolymer product via the reaction of trioxane, a cyclic trimer of formaldehyde, and a cyclic ether, such as ethylene oxide or 1,3 dioxolane. The structures of these monomers are shown in Figure 3.25. The polymer structure is represented in Figure 3.26. 

Scheme 4.2. There is partial loss of methanol from the dimethylacetals (DMA) under the heat conditions of the GC injection port resulting in the corresponding alk-1-enyl methyl ethers (AMEs).The latter occur in the cis and trans configuration. DMA can be converted to stable cyclic acetals (dioxolane derivatives) by reaction with 1,3-propanediol in the presence of p-toluenesulfonic acid (/>-TSA) (85,127). The cyclic acetals are stable under the GC injection conditions see bottom GC chromatogram in Fig.

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