Dicyclopentadiene Dienophiles

The starting material is called dicyclopentadiene . Cyclopentadiene itself is not stable it dimerizes to dicyclopentadiene slowly at room temperature by a Diels-Alder reaction. It does this even though it is not an electron-deficient dienophile, demonstrating the enormous reactivity of cyclopentadiene as a diene in the Diels-Alder reaction. 

Cyclic conjugated dienes that are s-cis conformation, e.g. cyclopentadiene and 1,3-cyclohexadiene, are highly reactive in Diels-Alder reactions. In fact, cyclopentadiene is reactive both as a diene and as a dienophile, and forms dicyclopentadiene at room temperature. When dicyclopentadiene is heated to 170 °C, a reverse Diels-Alder reaction takes place and reforms the cyclopentadiene. 

The dicyclopentadiene that is formed in the dimerization of cyclopentadiene has the ring of the dienophile in an endo orientation to the cyclopentadiene ring that acts as the diene. Usually, substituents on the dienophile are found to be endo in the adduct if the substituents contain pi bonds. Another example is provided by the reaction of cyclopentadiene and maleic anhydride illustrated in the following equation ... 

Retrocycloaddition Many cycloaddition reactions require moderate heating to overcome the activation energy, but if it is heated too much the equilibrium will favour cycloreversion or retrocycloaddition. For example, cyclopentadiene slowly undergoes cycloaddition with itself one molecule of cyclopentadiene acts as a [4ir]-electrons diene and the other as a [2tt]-electrons dienophile. The product is an endo-tricyclo[5.2.1.0]deca-3,8-diene (8.4), often called dicyclopentadiene. The product 8.4 gives back cyclopentadiene on heating at 150° C for an hour. 

When heated, dicyclopentadiene undergoes a retro Diels-Alder reaction, and two molecules of cyclopentadiene are re-formed. If cyclopentadiene is immediately treated with a different dienophile, it reacts to form a new Diels-Alder adduct with this dienophile. 

Another example of the retention of volatile DA reagents is that of cyclopentadiene in a tandem retro-DA-DE prime reaction [15, 38]. This reaction type is the thermal decomposition of a DA adduct (A) and the generation of a diene (generally the initial diene) which is trapped in situ by a dienophile leading to a new adduct (B) [39]. Cyclopentadiene (22) (b.p. 42 °C) is generated by thermolysis of its dimer at approximately 160 °C [40]. An equimolar mixture of commercial crude dicyclopentadiene (21) and dimethyl maleate was irradiated in accordance with the GS-MW process, in an open reactor, under 60 W incident power, for 4 min... 

The customary laboratory source of 1,3-cyclopentadiene is a compound called dicyclopentadiene (C10H12). Dicyclopentadiene is the Diels-Alder cycloaddition product of two molecules of 1,3-cyclopentadiene. One molecule acts as diene, the other as a dienophile. Heating dicyclopentadiene causes it to undergo a retro Diels-Alder reaction to give 1,3-cyclopentadiene. 

Figure 4.51. A cartoon representation of the Diels-Alder reaction between two cyclopenta-diene equivalents one of which acts as a diene and the other as a dienophile. Dicyclopentadiene (3a, 4,7,7a-tetrahydro-4,7-methano-lH-indene, tricyclo[5.2.1.0 ]deca-3,8-diene) is commercially available and is thermally decomposed ( cracked ) at its boiling point (170°C) to produce cyclopentadiene (bp 46°C). The latter slowly dimerizes at room temperature. Dicyclopentadiene exists almost completely as the endo isomer, which, it is argued, forms as result of maximum overlap of n orbitals during the reaction (the Alder Endo rule). The other potential isomer (the exo isomer) is not observed.

Another important example of application of FMO method on Diels-Alder reaction is dimerization of cyclopentadiene to yield dicyclopentadiene. In this case endo isomer is formed rather than exo although exo isomer is thermodynamically more stable. Reason behind this is that in case of endo isoiper favourable secondary interactions are there between diene and dienophile ... 

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