Cyclopentadienones, Diels-Alder addition

When benzyne is generated in the presence of potential dienes, additions at the highly strained triple bond occur. Among the types of compounds that give Diels-Alder addition products are furans, cyclopentadienones, and anthracene. 

Convergent synthesis (Fig. 4.18) also uses cyclopentadienone for construction of the dendrons. However, in this case two molecules of the diene are added via Diels-Alder reaction to 4,4 -diethynylbenzil. The resulting second-generation den-dron is reacted with dibenzylacetone in a twofold Knoevenagel reaction. The newly functionalised cyclopentadienone dendron obtained again undergoes (fourfold) Diels-Alder addition to the tetraethynylated tetraphenylmethane core unit [34a]. 

The main side reactions are cyclotrimerization (of the alkyne), co-cyclotrimer-ization (alkyne/alkyne/alkene), and formation of cyclopentadienones 5 and 6. In addition, spirofuranones such as 7 and 8 can form, with the latter being the Diels-Alder product of 7 with ethylene. It is generally observed that high ethylene and CO pressures disfavor the formation of side products. Further details on the side-product formation were given in [4d]. 

Kinetic data on addition reactions, and their implications as to the reaction mechanism, have been reviewed in the recent past this is particularly true for the Diels-Alder reaction - - and its reverse reaction , and for 1,3-cyclo-additions . As well, kinetics and mechanism have been reviewed for particular cases, like Diels-Alder reactions of cyanogen and nitroso derivatives as dienophiles, and of cyclopentadienones as dienes. Other discussions of mechanism, of the Diels-Alder reaction and of 1,2-cycloadditions - , are also worth noting. Another particular case is that of 1,3-cycloadditions of nitrile oxides ", ... 

Various polymerization route with unique starting monomer [64] have been developed, e.g., topochemical poly(diacetylene) synthesis with pyrrole-, thiophene-, -Si-groups dienophilic addition of maleic imides, Diels-Alder polymers via cyclopentadienone derivatives. 

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