Dimers cyclic

Auto-association of A-4-thiazoline-2-thione and 4-alkyl derivatives has been deduced from infrared spectra of diluted solutions in carbon tetrachloride (58. 77). Results are interpretated (77) in terms of an equilibrium between monomer and cyclic dimer. The association constants are strongly dependent on the electronic and steric effects of the alkyl substituents in the 4- and 5-positions, respectively. This behavior is well shown if one compares the results for the unsubstituted compound (K - 1200 M" ,). 4-methyl-A-4-thiazoline-2-thione K = 2200 M ). and 5-methyl-4-r-butyl-A-4-thiazoline-2-thione K=120 M ) (58). 

Experimental values are collected in the McClellan book (B-63MI40400) and in a review on dipole moments and structure of azoles (71KGS867). Some selected values are reported in Table 3. The old controversy about the dipole moment of pyrazole in solution has been settled by studying its permittivity over a large range of concentrations (75BSF1675). These measurements show that pyrazole forms non-polar cyclic dimers (39) when concentration increases and, in consequence, the permittivity value decreases. 

The spectra of pyrazolidinones have often been described (63PMH(2)161, 67BSF3502, 70CHE1568). In the l-aryl-3-pyrazolidones (79), both the r (CO) and r (NH) bands changed considerably between the crystal (cyclic dimer) and the solution spectra. Typical values for j (CO) range from 1700 to 1730 cm" in solution. For the l-aryl-5-pyrazolidones (80),... 

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

This polymer first appeared commercially in 1965 (Parylene N Union Carbide). It is prepared by a sequence of reactions initiated by the pyrolysis of p-xylene at 950°C in the presence of steam to give the cyclic dimer. This, when pyrolysed at 550°C, yields monomeric p-xylylene. When the vapour of the monomer condenses on a cool surface it polymerises and the polymer may be stripped off as a free film. This is claimed to have a service life of 10 years at 220°C, and the main interest in it is as a dielectric film. A monochloro-substituted polymer (Parylene C) is also available. With both Parylene materials the polymers have molecular weights of the order of 500 000. 

Pyrolysis studies have often been applied to chlorotnfluoroethylene and tetrafluoroethylene Pyrolysis of chlorotnfluoroethylene at 560-590 °C yields 70-83% of a mixture containing both linear and cyclic dimers of chlorotri... 

N6H2 Probably a cyclic dimer of N3H N7H9 hydrazinium azide monohydrazinate, N2H5N3. N2H4... 

After these findings, evidence was presented that pyrazoles which crystalline as cyclic dimers 9a/9b and tetramers 11a are subject to double and quadruple proton transfers respectively (92JA9657 95JOC1965). An im-... 

Reaction of the cyclometallated complexes 244-246 with pyrazole and excess sodium hydride affords cyclic dimers 247 (990M3991), where C N denotes the corresponding cyclometallated ligand in accordance with structures 244-246. The [Pt2(thienylpyridine)2(/j.-pz)](C104)3] is known as well. 

Under the phase-transfer catalysis conditions, 2-bromo-8-methylquino-line (67) was coupled with 2-pyridylboronic ester 68 to furnish 2-(2-pyridyl)-8-methylquinoline (69) in 56% yield (91JOC6787). At this point, it is opportune to mention that the simple 2-pyridylborane, in contrast to 3- and (4-pyridyl)boranes, is considered an unsuitable Suzuki coupling partner because it forms an unusually stable cyclic dimer resembling a dihydroanthracene. In this case, the obstacle was circumvented by using 2-pyridylboronic ester in place of 2-pyridylborane (Scheme 9). 

The thermal [2 + 2] cycloaddition is limited to certain activated alkenes. For instance tetrafluoroethylene, tetrachloroethylene, allenes e.g. 17, ketenes and ena-mines can form cyclic dimers or react with other alkenes ... 

Metathesis of a cycloalkene initially yields a cyclic dimer, i.e. the size of the ring is doubled ... 

This interesting solvent effect was explained by Baro et al. on the basis of the formation of cyclic dimers (13.7) as suggested earlier by Hunter (1937) and by Le Fevre and Vine (1937). It was later confirmed in an X-ray investigation by Omel chenko and Kondrashev (1967). The X-ray analysis of 1,3-diphenyltriazene copper(i) (Brown... 

The concentration of the lactam in the final product is determined by (3.11). Cyclic dimers can also form, and these also take part in the polymerization12 the reactions are acid catalyzed. The kinetics of this ring-opening polymerization with the three reactions in (3.10)—(3.12) is complex. The reaction rate constants and equilibrium constants have been described by several authors,5 6,8,12 28 and more pragmatic approaches for describing the reaction kinetics have also been given.28,31,33... 

CV. See Cyclic voltammogram (CV) Cyanate resins, phenolic, 418 Cyclic dimers, 153... 

The diacetylated ethane-1-hydrox-1,1-diphosphonic acid is prepared by dissolving the diphosphonic acid in acetic acid and adding acetic anhydride [114]. The sodium salt can be directly converted to the free acid form by passing it through an hydrogen cation exchanger. The bicyclic dimer is prepared by basic hydrolysis of diacetylated cyclic dimer, as shown in Eq. (70) ... 

Attempted polymerization of ds-7,9 dioxabicyclo[4.3.0]nonane 43 with phosphorus pentafluoride as initiator at temperatures ranging from -25 to 0 °C provided only a cyclic dimer 44 in high yield35. Under similar conditions trans-7,9-dioxabicyclo-[4.3.0]nonane 45 polymerized almost instantly to polymer 46 with number average molecular weight of several thousands, along with a small amount of a cyclic dimer 47. 

The effect of the temperature on the polymerization of 53 in methylene chloride is presented in Table 3. The upper half of the data in the table shows the temperature effect on the products in the initial stage of the reaction, and the lower half is that for the middle to final stages of the reaction. Obviously there is a drastic change in the reaction products between -20 and -30 ° Below —30 °C, the cyclic dimer is the predominant or even sole product after the reaction of 48 hours, while above —20 °C, the low molecular weight polymer is exclusively formed. The cyclic oligomers once formed in the initial stage of the reaction are converted to the polymer in the later stage of the reaction above —20 °C. 

Time dependence of the reaction products can be seen more clearly in the time-yield curves of oligomerization in methylene chloride at —40° (Fig. 4). The yield of mixture of the cyclic tetramer and hexamer (mostly the latter), passed through a maximum value of about 40% and then decreased to nearly zero after 48 hours. On the other hand, the yield of the cyclic dimer increased rather sigmoidly with reaction time. 

Temp., °C Time, hr Cyclic dimer Yield 5, % Cyclic tetramer Polymer + cyclic hexamer Mnc... 

Fig. 4. Time-yield curves of the oligomerization of 6,8-dh>xabicyclo[3.2.1 Joctan-7-one. Monomer, 4 g CH2Cl2 4 ml BF3Et20, l mol% to monomer temp., -40 °C , cyclic dimer o, cyclic tetramer plus cyclic hexamer45)...

Table 4 summarizes the most suitable reaction conditions for the selective preparation of cyclic dimer, tetramer, and hexamer from 53. It must be pointed out... 

The cyclic dimer, tetramer, and hexamer can be crystallized in acetonitrile, and also in chloroform (the former two oligomers). X-ray analysis of the crystals of the cyclic dimer47 disclosed that it consisted of a pair of different enantiomers of 53 and that all of the four substituents attached to the two tetrahydropyran rings occupied the axial position as illustrated in Fig. 54s). Such a conformation is in... 

Fig. 5. Structure of the cyclic dimer of 6,8-dioxabicydo-[3.2.1 octan-7-one4S ...

An alternative explanation for the exclusive formation of the even-numbered cyclic oligomers would be that they are produced by the polymerization of the cyclic dimer. However, the cyclic dimer isolated from the reaction mixture was found not to polymerize under the reaction conditions used for the oligomerization. Therefore, if this concept were correct, another cyclic dimer with a different configuration and higher reactivity must have been formed during the reaction. Its presence, however, has not yet been verified by experiment. 

The cyclic dimeric borazine derivatives 2 and 3 can be prepared by reaction of 2,4-dichloro-l,3,5,6-tetramethylborazine with aromatic 1,3-dihydroxy and l-amino-3-hydroxy compounds (Fig. 2). The products are obtained in yields... 

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