Cyclopentadiene dimer formation

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. 

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-... 

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. 

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

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

Diazomethane generateti in the presence of palladium compounds has been shown to undergo addition to the double bond of the norbornene fragment of the cyclopentadiene dimer, e.g. formation of 14 and This was attributed to the formation of a 7t-alkene complex of... 

A,ii, Cyclopentadienylirondicarbonyl (Fp) Compounds. A common organoiron reagent is cyclo-pentadienylbis(carbonyliron) (Fp, 711). " This derivative can be converted to an alkyl derivative (713, where R = alkyl) by reaction with an alkyl halide, or to the protio derivative (713, R = H) by reaction with an acid. The anionic reagent 712 is prepared by reaction of iron pentacarbonyl with the dimer of cyclopentadiene (710 - sec. 115.B) at 200°C to give the dimeric species 711. Treatment of 711 with base leads to loss of cyclopentadiene and formation of 712. A byproduct of this process is ferrocene (71 4) formed by extrusion of carbon monoxide from 711.595... 

Diels-Alder reaction of 4,7-dihydro-1,3-dioxepins with cyclopentadiene dimers leads to bi-cyclic 1,3-dioxepanes, for example formation of (93) and (94) (Scheme 15) <80GEP(0)2918168, 83GEP(O)3204627>. 

ANSWER The formula of CioHi2 is a clue. Clearly, two C5H6 molecules have joined to make the C10H12 product. The reaction is, in fact, successful. The problem is that cyclopentadiene dimerizes in a Diels-Alder reaction very quickly.The molecule you isolate is the Diels-Alder dimer. Note formation (msdnly) of the endo adduct. Anytime you see a 1,3-diene, especially a cyclic one, think Diels-Alder. 

Product formation was postulated to arise from the spontaneous rearrangement of the initially formed hetero Diels-Alder adducts 127. The similar reaction when performed with an alkyl glyoxylate in the absence of water (neat) or in organic solvent (toluene) inevitably leads to cyclopentadiene dimerization instead of the expected cycloaddition. 

The formation of the transition state is accompanied by a decrease in volume because two species are unified to form one associate. The study of cyclopentadiene dimerization under a pressure allowed the estimation of the activation volume =... 

If pure monomer is to be used ia a reactioa, it must be used iaimediately or stored at < — 20° C to preveat dimerization to any appreciable extent. Chemical inhibition does not prevent dimerization low temperature is preferred. If the monomer has to be stored for more than a few hours, it must be protected against oxygen to prevent peroxidation and polymer formation. Cyclopentadiene monomer reacts spontaneously with oxygen of the air to form brown, gummy peroxide-containing products. 

Sulphines may react as dienophiles with 1,3-dienes with the formation of cyclic sulphoxides. Unstable 2,2-dichloro-5,6-dihydro-2ff-thiin-l-oxide 191 was formed in an exothermic reaction between 173aandcyclopentadieneat — 40 (equation 101). The simplest, parent sulphine, CH2 = S = O, prepared in situ by treatment of a-trimethylsilylmethanesulphinyl chloride with cesium fluoride, reacts with cyclopentadiene to give bicyclic, unsaturated sulphoxide 192 as a mixture of two diastereoisomers in a 9 1 ratio (equation 102). On the other hand, a,j8-unsaturated sulphine 193 (generated by thermolysis of 2-benzylidene-l-thiotetralone dimer S-oxide) in boiling toluene behaves as a 1,3-diene and was trapped by norborene forming sulphoxide 194 in 78% yield ° (equation 103). 

Cyclic dienes which are locked in the cisoid conformation, e.g. (82), are found to react very much faster than acyclic dienes in which the required conformation has to be attained by rotation about the single bond (the transoid conformation is normally the more stable of the two). Thus cyclopentadiene (82) is sufficiently reactive to add to itself to form a tricyclic dimer, whose formation—like most Diels-Alder reactions—is reversible. 

In contrast, exposure of 14-VE (diene)MCp Cl complexes (M = Zr, Hf) to CO (1 atm) results in the formation of cyclopentadienes70. The mechanism proposed for this transformation was elucidated with a carbon labeled CO ( CO) as requiring an initial coordination of CO to generate a (diene)MCp (CO)Cl complex 153 (Scheme 37). For the hafnium complex, the intermediate 153 (M = Hf) was observed by infrared spectroscopy. Insertion of CO into the a2, jt diene generates a metallacyclohexenone, which undergoes reductive elimination to generate the dimeric metallaoxirane species 154. -Hydride elimination from 154 (M = Zr, Hf) followed by 1,2-elimination produces substituted cyclopentadienes and the polymeric metal-oxide 155. Treatment of (diene)TiCp Cl with CO leads to isolation of the metallaoxirane complex 154 (M = Ti). 

The trimerization of cyclopentadiene (6) is catalyzed by a homogeneous bifunctional palladium-acid catalyst system.7 The reaction gives trimers 7 and 8 as a 1 1 mixture in 70% yield with bis(acetylacetonato)palladium(II) [Pd(acac)2] or with bis(benzylideneacetone)-palladium(O) as the palladium component of the catalyst. As the phosphorus component, phosphanes like trimethyl-, triethyl-, or triphenylphosphane, and triisopropylphosphite or tris(2-methylphcnyl)phosphite, are suitable. A third component, an organic acid with 3 < pK < 5, is necessary in at least equimolar amounts, in the reaction with cyclopentadiene (6), as catalytic amounts are insufficient. Acids that can be used are acetic acid, chloroacetic acid, benzoic acid, and 2,2-dimethylpropanoic acid. Stronger acids, e.g. trichloroacetic acid, result in the formation of poly(cyclopentadiene). The new catalyst system is able to almost completely suppress the competing Diels-Alder reaction, thus preventing the formation of dimeric cyclopentadiene, even at reaction temperatures between 100 and 130°C. 

Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with NzO at room temperature, but isolated cuprous ions are entirely inactive for this reaction 60). More interesting selectivity may be found in the reaction of olefins with methylene complexes the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in /i-CH2Co2(CO)2(Cp)2 61), /<-CH2Fe2(CO)8 (62), and /<-CH2-/i-ClTi(Cp)2Al(Me)2 (65) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). 

In ////-prepared l-adamantyl-2-diphenyl-phosphaalkene dimerizes spontaneously and forms an 1,2-diadamantyl-lA3,2A3-diphosphetane <2006IC5225>. Repetitive cycloaddition reactions between trimethylsilylphosphaalkyne 44f and cyclopentadiene, in a molar ratio of 3 2, lead to formation of the polycyclic triphospha compound 91 <1999EJ0363>. [WICOATHF] triggers [2+2] cycloaddition reactions of Mes -arsaalkyne 92 by rr-complexation... 

The linear coupling reaction of acetylene and acrylonitrile afforded 2,4,6-hep-tatrienenitrile by dimerization of acetylene and insertion of one molecule of acrylonitrile [88] (Eq. 66). The reaction involves the formation of a ruthena-cyclopentadiene complex, which also catalyzed the reaction. 

The efficient formation of cyclopentadienylsodium is of paramount importance for the entire reaction sequence. Variations in the yield of methyl 2,4-cyclopentadiene-l-acetate have been traced to the degree of efficiency in producing a fine sodium sand which is used to produce cyclo-pentadienylsodium. In the alkylation reaction of cyclopentadienylsodium with methyl bromoacetate, the entire process nust be carried out in an Inert dry atmosphere at -78°C. At higher temperatures, the desired product can undergo undesired dimerization and double bond migration side reactions. Methyl 2,4-cyclopentadiene-l-acetate, once formed, is used Immediately. 

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