Diels-Alder reaction in polymer

Figure 21.4 RAFT agents capable of undergoing RAFT-hetero-Diels—Alder reactions in polymer—polymer conjugations. Note that the C=S double bond in sulfonyldithiofor-...

Stenzenberger H. D. Thermosetting Polyimides from Bismaleimides via Diels-Alder Reaction in Polyimides Other High-Temp. Polym., Proc. Eur. Tech. Symp., 2nd" 1991 215, Eds. Abadie M. J. M. and Sillion B., Pb. Elsevier Amsterdam... 

Cycloadditions. Both hetero-Diels-Alder reactions of polymer-bound aldimines with Danishefsky s diene to afford 2-aryl-2,3-dihydro-4-pyridones and 1,3-dipolar cycloadditions of nitrones with a, 3-unsaturated amides are catalyzed by YbfOTflj. Remarkable switch in diastereoselectivity by solvent is observed in the latter reactions. 

A variety of solid Lewis and Br0nsted acids has been shown to catalyze Diels-Alder reactions. In several instances the results obtained with heterogeneous catalysts were better than those with homogeneous Lewis acid catalysts. Most of the reported reactions of interest in the synthesis of fine chemicals were catalyzed by (modified) zeolites, clays, alumina, or silica. Catalysts with interesting properties were obtained when support materials such as zeolites, alumina, or silica were treated with Lewis acids. These catalysts were moderately selective in diastereo-selective Diels-Alder reactions with chiral dienophiles and induced enantioselec-tivity (up to 31 % e. e.) in the reaction of cyclopentadiene with methacrolein after treatment with chiral derivatives. Excellent enantioselectivity in this reaction (up to 95 % e. e.) was observed with a polymer-supported chiral oxazaborolidinone. Because of their facile recovery and recycling, we expect that solid-acid catalysts will find increasing use in Diels-Alder reactions in the future. 

The progress of the Diels-Alder reaction was assessed by contact angle measurements performed at room temperature (Fig. 19.7). Again, the reaction was studied systematically at six different temperatures. We observed that the Diels-Alder reaction could be described as a pseudo-second-order reaction (Fig. 19.8). Similarly to the Diels-Alder reaction on monolayers, the third-order rate constants koA. calculated from the least-squares fits shown in Fig. 19.8 for the Diels-Alder reaction in the polymer thin film, obey the Arrhenius equation (Fig. 19.9). The activation energy 3 = 48.1 3.7 kj moh and the activation entropy AS = -538.2 16.4 J mol at 298 K (Table 19.2) are determined at the polymer surface in the same way as for the monolayers. 

Wei, H.L., Yang, Z., Zheng, L.M., Shen, Y.M., 2009. Thermosensitive hydrogels synthesized by fast Diels-Alder reaction in water. Polymer 50, 2836—2840. 

The enantioselective Diels-Alder reaction is another main motif in chiral Lewis acid catalysis. In 1996, Itsuno and coworkers reported an asymmetric Diels-Alder reaction using polymer-supported catalysts under flow conditions. Immobilized chiral oxazoboloridune (34) was prepared from a copolymer of N-sulfonylvabne and borane having styrene moiety, affording the Diels-Alder adduct in an enantioselective manner (up to 71% yield) [126], The authors used a gravity-fed-type column for the flow reaction. Ti-TADDOL-functionalized monolithic resins (35) were developed by Altava and Luis for the asymmetric Diels-Alder reaction (Scheme 7.30). 

Lehn and coworkers reported a series of reactive diene and pentafulvenes involving reversible Diels-Alder reactions at ambient temperature. The results provide a foundation for the implementation of the reversible Diels-Alder reaction in constitutional dynamic chemistry (Scheme 7.26) [27]. With variation of the functionalized substituted fulvenes 108 and cyanoolefin esters 109, the methodology may be applied for the preparation of dynamic combinatorial libraries and provides an entry to reversible dynamic polymers and the discovery of biologically active substances. 

CH = CH — CH = CH — are said to have conjugated double bonds and react somewhat differently from the other diolefins. For instance, bromine or hydrogen is often added so that a product of the type -CHBr-CH=CH-CHBr- is formed. Also, these hydrocarbons participate in the Diels-Alder reaction see diene reactions). They show a tendency to form rubber-like polymers. Hydrocarbons not falling into these two classes are said to have isolated double... 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

Aqueous ring-opening metathesis polymerization (ROMP) was first described in 1989 (90) and it has been appHed to maleic anhydride (91). Furan [110-00-9] reacts in a Diels-Alder reaction with maleic anhydride to give exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3—dicarboxylate anhydride [6118-51 -0] (24). The condensed product is treated with a soluble mthenium(Ill) [7440-18-8] catalyst in water to give upon acidification the polymer (25). Several apphcations for this new copolymer have been suggested (91). 

Commercial manufacture of this polymer was first announced by CdF Chimie in 1975 following about 10 years of study on the Diels-Alder reactions of alkenes with cyclopentadiene. In 1976 a plant with a capacity of 5000 tonnes per annum was put on stream and product marketed as Norsorex. Manufacturing under licence is now being extended to American Cyanamid whilst CdF Chimie has also signed a marketing agreement with Nippon Zeon. 

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

Polymer-supported BINOLs thus prepared were treated with Zr(Ot-Bu)4 to form polymer-supported zirconium 20. In the presence of 20 mol% of various zirconium 20, the model aza Diels-Alder reactions of imine Id with Danishefsky s diene (7a) were performed results from selected examples are shown in Table 5.8. Whereas the 4-t-butylphenyl group resulted in lower enantiomeric excess (ee), higher ee were obtained when 3,5-xylyl, 4-biphenyl, 4-fluorophenyl, and 3-tri-... 

Bis-o-quinodimethanes have also been used to functionalize [60]-fullerene by Diels Alder reaction. An example is the preparation of main-chain polymers with incorporated [60]-fullerene units [48] illustrated in Scheme 2.20. Cycloaddition of bis-diene 50 generated in situ from bis-sulfone 49 with [60]-fullerene leads to an oligomer mixture 51. Another type of functionalization is based on the... 

Supported Lewis acids are an interesting class of catalysts because of their operational simplicity, filterability and reusability. The polymer-bound iron Lewis-acid 53 (Figure 3.8) has been found [52] to be active in the cycloadditions of a, S-unsaturated aldehydes with several dienes. It has been prepared from (ri -vinylcyclopentadienyl)dicarbonylmethyliron which was copolymerized with divinylbenzene and then treated with trimethylsilyltriflate followed by THF. Some results of the Diels-Alder reactions of acrolein and crotonaldehyde with isoprene (2) and 2,3-dimethylbutadiene (4) are summarized in Equation 3.13. 

Another interesting scavenger is polymer-supported anthracene, developed by Porco for the scavenging of dienophiles [109]. An example of its application to the synthesis of a complex 5,8-dihydro-(l,2,4)triazolo[l,2-a]pyridazine-l,3-diones via hetero-Diels-Alder reaction followed by removal of the excess of triazole-3,5-dione under microwave irradiation is depicted in Scheme 24. For this particular example, moving from thermal heating (toluene, 100 °C) to a microwave-assisted protocol (DCE, 150 °C) reduced scavenging time from 3 h to just 15 min. 

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