Dioxepanes

With aldehydes or their derivatives, butanediol forms acetals, either 7-membered rings (1,3-dioxepanes) or linear polyacetals the rings and chains are easily intraconverted (126,127). 

By utilizing a combination of RAFT and cationic ROP, the synthesis of [poly(methyl methacrylate)][poly(l,3-dioxepane)][polystyrene] miktoarm star terpolymers was achieved [182], The approach involved the synthesis of PS functionalized with a dithiobenzoate group by RAFT polymerization and subsequent reaction with hydroxyethylene cinnamate (Scheme 98). The newly created hydroxyl group was then used for the cationic ring opening polymerization of 1,3-dioxepane (DOP). The remaining dithiobenzoate group was used for the RAFT polymerization of methyl methacrylate. 

The base-catalysed ring contraction of 1,3-dioxepanes offers an attractive route to 4-formyl tetrahydropyrans (Scheme 14) , whilst fused exo-cyclic dienes 27 result from the radical cyclisation of alkenyl iodides 26 (Scheme 15) <00OL2011>. Intramolecular radical addition to vinylogous sulfonates is highly stereoselective, leading to the ci s-2,6-disubstituted tetrahydropyran (Scheme 16) . 

There are relatively few entries in the non-fused dioxepin area, and most of these focus on reactions of these systems. For example the triflic acid-initiated polymerisation of 1,3-dioxepane in the presence of acetic acid and hexanedicarboxylic acid has been studied and mechanistic aspects discussed <00JPS(A)1232>. Biodegradable microspheres for the controlled delivery of drugs have been made from copolymers and homopolymer blends of L-lactide and l,5-dioxepan-2-one <00PP1628>. Ring contraction of 5-methylene-l,3-dioxepanes (eg. Ill) on reaction with trimethylsilyl trifluoromethanesulfonate in the presence of base afforded the exo tetrahydropyrans, in good yields <00TL2171>. 

P2t Z = 2 Dx = 1.281 R = 0.06 for 1,338 intensities. The molecule contains a seven-membered, 1,3-dioxepane ring which has a conformation close to the twist-chair, with a two-fold axis through the mid-... 

For the polymerisation of 1,3-dioxolan and 1,3-dioxepan by HC104 Plesch and Westermann [20] were able to show by conductivity measurement that Xxr = [HClO4]0 and hence obtained values of kp These values are probably composite, and the contributions from k v and k"p are not known, but is has been explained in Section 3 that they are unlikely to differ by more than one power of ten. 

Most of the kp values which seem reasonably reliable are shown in Tables 1 and 2. At first sight the kp values fall into two groups For olefin polymerisations they lie between ca. 103 and 109 M 1 s 1 and for the O- and 5-heterocyclics they lie in the range 10 3 to 10 (with the exception of 1,3-dioxepan). 

Studies of 1,3-dioxepan by Plesch and Westermann [8] showed that its polymers, produced by perchloric acid catalysis, also have a completely regular structure similar to that of the polydioxolans, and that depolymerisation produces no product other than the monomer. 

Subsequent experiments by Jones and Plesch [11] showed that polymers of (I) and of 1,3-dioxepan made with various triethyloxonium salts as catalysts, were also almost devoid of end-groups and therefore the great majority of the molecules must also be cyclic. 

The ring expansion mechanism is of course only a special case of the well-known mechanism by which dioxolan reacts with non-cyclic formals e.g., (I) and CH2-(OMe)2 give (MeOCH2OCH2-)2 in this way. It also accounts in a simple manner for the cleanness of the monomer-polymer equilibrium and for the high yields of cyclic dimer (without any detectable linear fragments) which are obtainable from 1,3-dioxane and 1,3-dioxepan [8]. 

The object of the work described was to discriminate between the two principal rival theories concerning the polymerisation of 1,3-dioxacycloalkanes by anhydrous perchloric acid, the Mainz theory and the Keele theory . By means of Jaaks s method for determining tertiary oxonium ions we found that in polymerisations under the driest conditions the concentration of these is negligibly small. This was done with 1,3-dioxolane (1), 4-methyl-l,3-dioxolane (4), and 1,3-dioxepane (5), and the findings are supported by determinations of the content of hydroxy groups of polymers prepared and killed under different conditions. 

Materials 1,3-Dioxolane (1) and 1,3-dioxepane (5) were prepared and purified conventionally. Compound 1 contained no impurities detectable by GLC, but 5 contained a trace of tetrahydrofuran (THF) which could not be removed even by distillation on a Fenske column with a reflux ratio of 50 1 4-methyl-l,3-dioxolane (4) was prepared by Astle s method [10]. All monomers were dried preliminarily by storage over LiAlH4 in reservoirs attached to a conventional high-vacuum line fitted only with all-metal valves, and then stored with liquid Na-K alloy until used. Methylene dichloride was purified conventionally, distilled on a Normatron 1.5 m column, dried i.vac. over LiAlH4 on a conventional high-vacuum line, and then stored for 24 h over a fresh sodium film immediately before use, in a reservoir attached to the vacuum line. 

Figure lc Simultaneous plots of temperature and electrical conductivity against time for the polymerisation of 0.6 M 1,3-dioxepane (5) wjth g x lO-4 M HC104 at -40 °C... 

Figure 2b The dependence of kx on [HC104] for the polymerisation of 1,3-dioxepane (5) at - 40 °C. (Calorimeter experiments, PHW)...

Table 2 The content of fert-oxonium ions in polymers of 4-methyl-1,3-dioxolane and 1,3-dioxepane (5) ...

It is not difficult to show by conductance measurements that when the polymerisation of dioxolan becomes of first order, roughly at the first half-life, all the available acid has reacted with monomer or oligomer to produce active centres [10]. Thus on the Mainz theory one would expect to find a number of OH groups of the same order of magnitude as the number of perchloric acid molecules introduced this is evidently not so, as long as the water concentration in the system is significantly less than that of the acid [10]. For 1,3-dioxepan the protonation is very much faster than for DXL and is complete long before the first half-life of the polymerisation. 

The method of Jaacks and his co-workers was used by Firat and Plesch to study the nature of the cations present when DXT, 4-methyl-DXL, and 1,3-dioxepan (DXP) were polymerised by anhydrous perchloric acid in methylene dichloride. They showed that when these polymerisations are done under the most rigorously dry conditions, the concentration of tert.-oxonium ions in the polymerising mixtures is at most close to the limit of detection and much less than the original concentration of perchloric acid and the concentration of ions in the polymerising solution. 

Hydride Ion Transfer between Triphenylmethyl Cation and Tetrahydrofuran, 1,3-Dioxolan and 1,3-Dioxepan, Kabir-ud-Din and P.H. Plesch, Journal of the Chemical Society, Perkin II, 1978, 937-938. 

This indicates the possibility of making addition polymers biodegradable by the introduction of ester linkages in to the backbone. Since the free radical ring-opening polymerization of cyclic ketene acetals, such as 2-methylene-1,3-dioxepane (1, Scheme I), made possible the introduction of ester groups into the backbone of addition polymers, this appeared to be an attractive method for the synthesis of biodegradable addition polymers. 

J. F. Stoddart and W. A. Szarek, Conformational studies on 1,3-dioxepans. Part III. 2,5-0-methylene-D-mannitol and some related compounds, J. Chem. Soc. B (1971) 437 442. 

Some of the more important monomers whose ring opening polymerisations have been induced by stable cation salts include, 1,4-epoxides, notably tetra-hydrofuran (20,112,113), 1,2-epoxides (114), 1,3-episulphides (thietans) (33,53), 1,2-episulphides (thiiranes) (53), azetidines (115,116), aziridines (117), the cyclic formals, 1,3-dioxolan (23,54, 118-120), and 1,3-dioxepan (118,119), trioxane (121,122) and more recently lactones (123). Aldehydes (124) may also be included since these molecules can be regarded as the smallest possible oxygen hetero-... 

Probably the first reference to the polymerisation of cyclic acetals (formals) was by Hill and Carothers (143). Since then a number of groups of research workers have shown interest in these monomers, particularly in 1,3-dioxolan 12 (54, 144-147) and to a lesser extent 1,3-dioxepane 13 (148-150). 1,3-Dioxan 14 does not appear to polymerise but merely forms crystalline dimer and trimer. 

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