Cyclic tetramers

Crown 4 and 18 crown 6 are a cyclic tetramer and hexamer respectively of repeat mg —OCH2CH2— units they are polyethers based on ethylene glycol (HOCH2CH2OH) as the parent alcohol... 

Pyrroles react with the conjugate acids of aldehydes and ketones to give carbinols (e.g. 67) which cannot normally be isolated but which undergo proton-catalyzed loss of water to give reactive electrophiles (e.g. 68). Subsequent reaction may lead to polymeric products, but in the case of reaction of pyrrole and acetone a cyclic tetramer (69) is formed in high yield. 

The elastomers consist of very high moleculcU weight (-0.5 X 10 ) linecu gums cross-linked after fabrication. In order to achieve such polymers it is necessary that very pure difunctional monomers be employed since the presence of monofunctional material will limit the molecular weight while trifunctional material will lead to cross-linking. Where dimethylsilicone rubbers are being prepared, the cyclic tetramer, octamethylcyclotetrasiloxane, which may be obtained free from mono- and trifunctional impurities, is often used. This tetramer occurs to the extent of about 25% during the hydrolysis of dichlorosilanes into polymers. 

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

In addition to the monophosphorus phosphoryl and thiophosphoryl compounds discussed above, several poly-phosphoryl and -thiophosphoryl halides have been characterized. Pyrophos-phoryl fluoride, 0=PP2—O—P(=0)p2 (mp —0.1°, bp 72° extrap) and the white crystalline cyclic tetramer [0=P(F)—0)4 were... 

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)...

The yields of the CPOs tend to be inversely proportional to the size of the ring. Although the connection maimers are different, square-shaped cyclic tetramer 14 was isolated in 7% yield [20], whereas the smaller square 15 was obtained in 22% yield [19] (Fig. 7). 

Phosnic 390 from two different lots, IJ-3 and IL-22, was obtained and used for pol3rmerization without further purification. According to gas and liquid chromatographic analyses, the Phosnic 390 sanqjles consist of 91% cyclic trimer and 9% cyclic tetramer (i.e., octa-chlorocyclotetraphosphazene). 

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxane have been determined. In this case (unlike the cyclotetrasiloxane in Fig. 26), the cis isomer does contain an intramolecular hydrogen bond, and intermolecular hydrogen bonds link the molecules into cyclic pairs 57. The trans isomer cannot form intramolecular hydrogen bonds, but forms cyclic tetramers which are further hydrogen-bonded to form infinite sheets 58 (280). 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

This may suggest that either propagation is very slow due to the low reactivity of the derived species or that the equilibrium constant K=kp/kpolymer chain. This is apparently in agreement with the high yield (up to 40%) of cyclic tetramer the yield of the polymer in the classical... 

Kipping experiment of the coupling of diphenyldichlorosilane had to be very low because in addition to cyclic tetramer, large quantities of cyclic pentamer and hexamer were formed. Only very recently was existance of polysilanes with two aryl substituents proved by Miller who prepared... 

For example, the cyclic tetramer is the 8-membered ring compound 2. On the basis of equation 1, the adjacent NH and SiH groups provide the functionality which permits the molecular weight of the [CH3SiHNH]n cyclics to be increased. Thus, [CH3SiHNH]n cyclics will be linked together via four-membered rings as shown in 3. 

The synthetic process for oligomeric species is to react the aminopropyl-functional disiloxane with the cyclic tetramer as illustrated in Scheme 4. 

The number average molecular weight at equilibrium is governed by the initial molar ratio of the cyclic tetramer to the disiloxane. In general, the DSX incorporates more readily using the quaternary ammonium catalyst. 

The vapor of CH3OH also contains some cyclic tetramer, (CH3OH)4,... 

Other organometallic compounds of aluminum include the alkyl hydrides, R2A1H. Molecular association of these compounds leads to cyclic tetramers. When the dimeric and trimeric compounds are dissolved in a basic aprotic solvent, the aggregates separate as a result of formation of bonds between A1 and the unshared pair of electrons on the solvent molecule. Toward Lewis bases such as trimeth-ylamine, aluminum alkyls are strong Lewis acids (as are aluminum halides). 

Although a cyclic tetramer is also known, it is not an important compound. 

In another category of template synthesis, ethylene oxide is cyclized in the presence of BF3 and alkali, alkaline earth, or transition ions as their fluoroborate, fluorophosphate or fluoroantimonate salts - see [4.4], This procedure yields cyclic tetramers, pentamers and/or hexamers with the particular product(s) obtained being quite dependent on the metal present. Thus Ca(BF4)2 gives a 50% yield of tetramer, Cu(BF4)2 and Zn(BF4)2 give the pentamer in 90% yield, whereas Rb(BF4) and Cs(BF4) yield exclusively hexamer (Dale Daasvatn, 1976). 

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