Cyclic compound, formation

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

Chapter 12 - Spatial-energy criterion of structure stabilization was obtained. The computation results for a hundred binary systems correspond to the experimental data. The basic regularity of organic cyclic compound formation is given and its application for carbon nanostructures is shown. 

These products are consumed consecutively, probably to form benzene and polycyclic compounds. Toluene may also react consecutively to benzene. The ratio of toluene or xylenes to benzene was about twice that obtained in the thermal reaction of ethylene, respectively, at temperatures from 703° to 854°C and at conversions up to 40 mole %. The ratio of styrene to benzene was about one-third as large as that obtained in the thermal reaction of ethylene. Addition of butadiene in the thermal reaction of propylene increased the selectivity of cyclic compound formation, although the increase was smaller than in the case of ethylene. These facts support the mechanism for the formation of monocyclic aromatic compounds proposed by Wheeler and Wood (24) this is discussed in detail later. 

Next, in order to learn more about the rates of dehydrogenation of cyclohexenes resulting from Diels-Alder reactions between butadiene and olefins, VCH, HCH and MCH were earlier subjected to thermal reactions at 530- 665 C ( ). The main reactions in these cases were reverse Diels-Alder reactions and dehydrogenations. Dehydrogenations which are related to the productions of cyclohexa-diene and benzene homologues were 1 10 in selectivity as compared to that of the reverse Diels-Alder reaction. An interesting observation related to cyclic compound formation is that, in the case of MCH pyrolysis, cyclohexadiene and cyclopentene are formed at almost the same rates as butadiene and propylene. So that, in this case, about 60% of MCH is employed in the formation of cyclic compounds. 

Based on the kinetic data obtained above, a tentative calculation was earlier performed to determine whether or not the rate of cyclic compound formation in actual pyrolysis reactions can be accounted for through the Diels-Alder reactions between butadiene and olefins. The actual rate of cyclic compound formation was much greater than the rate calculated from the concentrations of butadiene and olefins in the actual pyrolysis conditions. 

Use of dihydrides introduces the possibility of formation of cyclic or polymeric organotin compounds by appropriate choice of bifunctional reaction partners. The first example of cyclic compound formation was reported by Henry and Noltes 58), who carried out the reaction shown in Eq. 20, with M being silicon or germanium. The same reaction involving diphenyldivinyltin yielded... 

The kinetics of this type of polymerization are the same as for simple condensation for this reason, the use of the term polycondensation is perhaps more appropriate. Unless kinetic evidence suggests otherwise, polymerizations involving the formation of chain polymers from cyclic compounds, following ring scission, are classed as condensation polymerizations. Some important con-... 

Various fluorinated cyclic compounds containing -N=N- bonding can be decomposed by the elimination of nitrogen The photolysis of phenylfluorodiazi-rine results in the formation of the intermediate phenylfluorocarbene, which can react instantaneously with olefins [7<5] (equation 45)... 

A variety of condensation processes can lead to cyclic hydroxamic acids. These involve either the condensation of two molecules or the intramolecular cyclization of a single compound. In some cases, a primary hydroxamic acid function is already present and formation of a cyclic compound can arise by suitable reaction on nitrogen. These processes will be dealt with first. 

During the cracking process, fragmentation of complex polynuclear cyclic compounds may occur, leading to formation of simple cycloparaffins. These compounds can he a source of Ce, C7, and Cg aromatics through isomerization and hydrogen transfer reactions. 

In the construction of carbocycles, five-membered ring formation has been used for preparing fused cyclic compounds, such as functionalized diquinanes. ° The reaction of 36 with (TMSlsSiH furnished the expected product 37 in 80% yield and in a or.fi ratio of 82 18, as the result of a kinetic controlled reaction (Reaction 43). 

The strategy of cleaving cyclic compounds can bo taken much further. All four chiral centres in (20) are fixed by its formation from cage compound (19). 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

From the reactions presented in this section one can conclude that cyclic acetal formation via addition to a carbene intermediate is a general reaction for type I cleavage of cyclobutanones, tricyclic compounds, and certain bridged bicyclics as minor products. No acetal has been isolated from photolyses of cyclopentanones or cyclohexanones except for the special case of an a-sila ketone previously discussed. 

Cycloaddition reactions, which increase molecular complexity by formation of a cyclic compound and, simultaneously, two C-C or C-X bonds [1], are among the most widely used reactions in organic synthesis. The reactions are also regio- and stereoselective. For these reasons, such processes are usually the key step in the multistep synthesis of natural products. 

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

Mechanistic studies of the nickel-catalyzed cyclization of butadiene have been carried out. The formation of various cyclic compounds catalyzed by nickel complexes is explained via the intermediacy of ir-allylic nickel complexes 11 and 12. 

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