Cyclopentadiene, dimerization

Table 3 lists all polyenes whose radical cations have been investigated by one or other of the above-described techniques and some of the structures listed are shown below the table. Note that some nonconjugated dienes do not retain their structure upon ionization [e.g. semibullvalene 104 (equation 61) or the cyclopentadiene dimers 126 and 294 (equation 62)] but break a bond to form a bisallylic radical cation, a rather common tendency of radical cations that have this possibility. 

Hetero Diels-Alder reactions are very useful for constructing heterocyclic compounds, and many important chiral molecules have thus been synthesized. Although the retro Diels-Alder reaction does not itself involve the asymmetric formation of chiral centers, this reaction can still be used as an important tool in organic synthesis, especially in the synthesis of some thermodynamically less stable compounds. The temporarily formed Diels-Alder adduct can be considered as a protected active olefin moiety. Cyclopentadiene dimer was initially used, but it proved difficult to carry out the pyrrolytic process. Pentamethyl cyclopentadiene was then used, and it was found that a retro Diels-Alder reaction could easily be carried out under mild conditions. 

Cyclopentadiene dimerizes rapidly at room temperature and should be used immediately (Note 5) or stored at Dry Ice temperatures. As obtained above, the product has a refractive index of about 1.433 at 25° and is quite satisfactory as a starting material for the following preparation (Note 6). The yield, which is determined by weighing the receiving flask plus product, depends upon the quality of dicyclopentadiene employed (Note 7). 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

Interestingly, when using copper(I)triflate, the cyclopentadiene dimer 14 reacts in an intermolecular way, leading to the cydobutane 15 (reaction 5) [22], When the same substrate is transformed in the presence of the triplet sensitizer acetone, an intramolecular [2 + 2] cycloaddition takes place and the cage hydrocarbon compound 16 is formed. Obviously, the formation of a copper complex intermediate involving both alkene double bonds of the substrate is unfavorable in this case. 

Simple dimerization reactions will favour the dimer at low temperatures and the monomer at high temperatures. Two monomer molecules have more entropy than one molecule of the dimer. An example is the dimerization of cyclopentadienp. On standing, cyclopentadiene dimerizes and if monomeric material is needed the dimer must be heated and the monomer used immediately. If you lazily leave the monomer overnight and plan to do your reaction tomorrow, you will return in the morning to find dimer. 

Cyclopentadiene featured in Chapter 35 as an important diene in the Diels-Alder reaction. If you try to buy cyclopen tadi-ene you will find that the catalogues list only dicyclopentadiene or cyclopentadiene dimer . The dimerization of cyclopentadiene is reversible the monomer dimerizes by a Diels-Alder reaction at room temperature to give the dimer and the reaction is reversed on heating. So the dimer is a good source of the monomer. 

Monomeric 1,3-cyclopentadiene is obtained by the thermal cracking of cyclopentadiene dimer which is heated at about 150° in a nitrogen atmosphere in a distillation apparatus with a water-jacketed condenser. The fraction which boils below 44° is collected at 0° and is then degassed and stored in vacuo at — 78°. ... 

The boiling points of 1,3-cyclopentadiene and cyclopentadiene dimer (3a, 4, 7, 7a-tetra-hydro-4,7-methanoindene), are 40.0° and 170°, respectively. 

The procedure used for the formation of 1.3-cyclopentadiene from cyclopentadiene dimer is given in Synthesis 30. 

Roth, Schilling and coworkers [47-54] have investigated the nuclear spin polarization behavior of cation-radicals of numerous strained hydrocarbon systems produced by PET to strong electron acceptors such as chloranil, anthraquinone and cyanoaromaties. These systems include cyclopentadiene dimers, methyl-enebicyclo[2.2.0]hexenes [48], bicyclo[1.1.0]butanes [49], hexamethyl (Dewar benzene) [50], norbomadiene [53], quadricyclene [53], and 1,2-diphenylcyclopropanes [54]. 

With its concerted mechanism implying little charge distribution change along the pathway, the Diels-Alder reaction has been understood to have little rate dependence on solvent choice. For example, the relative rate of cyclopentadiene dimerization increases only by a factor of 3 when carried out in ethanol. The relative rate for the Diels-Alder reaction of isoprene with maleic anhydride (Table 7.1) varies by only a factor of 13 with solvents whose dielectric constants vary by almost a factor of 10, but the rate acceleration is not a simple function of the solvent polarity. Furthermore, the dimerizations of cyclopentadiene and 1,3-butadiene proceed at essentially identical rates in the gas and solution phases. ... 

SYNS BICYCLOPENTADIENE BISCYCLOPEKTA-DIENE 1,3-CYCLOPENTADIENE, DIMER DICYKLOPENTADIEN (CZECH) DIMER CYKLOPENTADIENU (CZECH) 3a,4,7,7a-TETRA HYDRO-4,7-METHANOINDENE... 

CYCLOPENTADIENE, DIMER see DGWOOO pi-CYCLOPENTADIENYL COMPOUND with NICKEL see NDA500... 

Dipolar cycloaddition of 3//-1.2.4-triazole-3,5(4// )-diones to strained bicyclic alkenes generally leads to the formation of rearranged 1,2,4-triazolidines (Section7.2.10.1.), although bicyclo[2.2.2]-octene does not react and cyclopentadiene dimer gives exclusively the cne reaction product24. 

In contrast to those unreactive dienes that can t achieve an s-cis conformation, other dienes are fixed only in the correct s-cis geometry and are therefore highly reactive in the Diels-Alder cycloaddition reaction. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cyclopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self Diels-Alder reaction. 

A reactive diene like 1,3-cyclopentadiene readily undergoes a Diels-Alder reaction with itself that is, 1 -cyclopentadiene dimerizes because one molecule acts as the diene and another acts as the dienophile. 

The cyclopentadiene dimer, when treated with potassium permanganate and triethylbenzylammonium chloride as a phase-transfer agent, furnishes an 83% yield of exo,cis-diol resulting from the hydroxylation of the double bond in the cyclohexene ring [900] (equation 82). 

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