1,3-Cyclopentadiene, cycloaddition with polymerization

Catalytic activity of these polymer-supported Ti and A1 species was assayed for the Diels-Alder reaction between cyclopentadiene and methacrolein [6]. As polymerization of cyclopentadiene competes with the cycloaddition process, all reactions were carried out using an excess (1.5 times) of the diene. Catalysts to dienophile ratios of ca. 0.05-0.2 were used for the different experiments. 

Cross-linked polymers bearing IV-sulfonyl amino acids as chiral ligands were converted to polymer bound oxazaborolidine catalysts by treatment with borane or bromoborane. In the cycloaddition of cyclopentadiene with methacrolein, these catalysts afforded the same enantioselectivities as their non-polymeric counterparts238. 

As was mentioned, cycloaddition of unactivated hydrocarbons, namely, that of cyclopentadiene, has practical significance. 5-Vinyl-2-norbomene is produced by the cycloaddition of cyclopentadiene and 1,3-butadiene546,547 [Eq. (6.96)] under conditions where side reactions (polymerization, formation of tetrahydroindene) are minimal. The product is then isomerized to 5-ethylidene-2-norbomene, which is a widely used comonomer in the manufacture of an EPDM (ethylene-propylene-diene monomer) copolymer (see Section 13.2.6). The reaction of cyclopentadiene (or dicyclopentadiene, its precursor) with ethylene leads to norbomene548,549 [Eq. (6.97)] 550... 

Various types of chirally modified Lewis acids have been developed for asymmetric Diels-Alder cycloadditions. Some of these, including Ti-TADDOLates, have been attached to crosslinked polymers [11]. A recent example of this approach involved polymeric monoliths 103 containing TADDOL subunits (Scheme 3.29). The treatment of 103 with 71X4 afforded Ti-TADDOLates, which were used for the asymmetric Diels-Alder reachon of cyclopentadiene 104 and 105. The major product obtained in this reachon was the mdo adduct with 43% ee [58]. The supported Ti-catalysts showed an exhaordinary long-term stabihty, being achve for at least one year. 

The first example of supported titanium-catalysed DA reaction was reported by Luis et al. in 1992 (Scheme 7.49). In this work, the authors described the preparation of several polymeric alcohols derived from Merrifield resin and their efficiency as ligand in titanium-catalysed cycloaddition between methacrolein and cyclopentadiene with yields between 83-99% and good exo selectivity. With the aim to evaluate the enantioselectivity of the reaction, catalyst 80 was then prepared almost quantitatively by double esterification of tartaric acid with chloromethylated polystyrene and a 1 1 mixture of titanium tetrachloride/titanium tetraisopropoxide. " Despite the good conversion and selectivity toward the exo-cycloadduct, only 3% enantiomeric excess was recorded, which was attributed to the very high reactivity of the system. Recycling of the catalyst was successfully performed by filtration from the reaction media and washing with dichloromethane, and catalyst 80 was reused seven times without significant loss of catalytic activity. 

Dienes, such as cyclopentadiene, are often used to trap the more reactive ketenes, which otherwise would undergo oligomerization or polymerization reactions. In the reactions of monoketenes with cyclopentadiene endo cycloaddition products are usually obtained, with the exception of t-butylketene, where the exo adduct is predominantly formed . In the reaction of MeC(X)=C=0 (X = Cl, Br) with cyclopentadiene, a strong solvent effect on the exo/endo ratio is observed . The more polar solvents favor the exo isomer, indicating the presence of a polar intermediate. 

U+2-Cycloaddit ion React ions ° The idea that the Diels-Alder addition possibly could be usefxil as a polymer-forming reaction was carried over from Illinois where one of Marvel s students had attempted to make a bisbutadiene in which the diene units were connected by a phenylene unit. Some work with cyclopentadiene which we had started convinced me that polymerization via a Diels-Alder reaction might be possible with a biscyclopentadiene monomer. Although only low molecular weight polymers were obtained, this marked the beginning of research on the synthesis of aromatic polymers by cycloaddition reactions. 

Another example relates to the plasma polymerization of maleic anhydride on silicon substrates [55]. After functionalization of the anhydride groups of the polymer with dienophile groups such as allyl amine groups, bicyclo[2.2.1]hept-2-ene groups are attached to the surface via the Diels-Alder cycloaddition of a diene. The cycloaddition of cyclopentadiene or [(trimethylsilyl)methyl]cydopentadiene to the cyclic imide-functionalized plasma polymer is shown in Scheme 1.17. In this case, the extent to which maleic anhydride groups are converted is controllable, such that the number density of functionalized groups can be tailored to comply with the adhesion between different types of solid surface. 

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