Diels-Alder reactions poly

An alternative synthesis of a thermally stable cyclopentadienyl functionalized polymer involved ring bromination of poly(oxy-2,6-diphenyl-l,4-phenylene), followed by lithiation with butyl lithium to produce an aryllithium polymer. Arylation of 2-norbornen-7-one with the metalated polymer yielded the corresponding 2-norbornen-7-ol derivative. Conversion of the 7-ol to 7-chloro followed by treatment with butyl lithium generated the benzyl anion which undergoes a retro Diels-Alder reaction with the evolution of ethylene to produce the desired aryl cyclopentadiene polymer, 6. 

Linders et al. [73] studied the Diels-Alder reaction of 6-demethoxy-/J-dihydrothe-baine with an excess of methyl vinyl ketone (used both as reactant and solvent), which gives a mixture of two isomeric adducts. When the reaction was performed using classical heating extensive polymerization occurred, whereas much less poly-... 

The electrochemical oxidation of phenols produces quinones that can be used as dienophiles for the Diels-Alder reaction. A typical example is shown in Scheme 14, where a lithium perchlorate/nitromethane system and an electrode coated with a PTFE [poly-(tetrafluoroethylene)] fiber, to create a hydrophobic reaction layer. 

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

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. 

Side chain furan functionalization has been accomplished by acetalization (Section 1.11.4.4.3) of poly(vinyl alcohol) (180) with furan-3-carbaldehyde (181 Scheme 87) (72MI11106). The adduct (182) was utilized as the diene reactant in a Diels-Alder reaction to bind vinyl-functional siloxane oligomers to the polymer. 

The kinetics of the Diels-Alder reaction between cyclopentadiene and N-n-butyl-maleimide in mixtures of water with MeOH, MeCN, and poly(ethylene glycol) have been investigated. The results were analysed using the Abraham-Kamlett-Taft model. 

Scheme6 NOBIN derivatives bearing dendritic poly(aryl ether) wedges. These systems have been studied in the titanium-catalyzed hetero-Diels-Alder reaction of Danishefsky s diene with aldehydes...

The above-mentioned examples belong to the captivating field of covalent reversible polymers. This field includes examples like reversible polymerization of cyclic oligomers from poly (ethylene terephthalate) [62], thermo-reversible polymers based on Diels-Alder and retro-Diels-Alder reaction (see, for example [63-69]), reversible photo-polymerization in the solid state of molecules... 

Inoue K, Yamashiro M, Iji M (2009) Recyclable shape-memory polymer poly(lactic acid) crosslinked by a thermoreversible Diels-Alder reaction. J Appl Polym Sci 112 876-885... 

If the reaction mixture is contained in an internal poly(tetrafluoroethylene) cell transmitting IR radiation (see Sec. 6.7.2), quantitative analysis via high-pressure spectroscopy (Figures 6.7-16 to 6.7-18)) yields precise values of the rate coefficients of these Diels-Alder reactions. As an example, the pressure dependence of the rate coefficient k of the cycloaddition of 2 (with R = CF3) and 3 in CH2CI2 solution at 45 °C (Buback et al., 1989) is shown in Fig. 6.7-19. 

Figure 6.19 shows an example of the application of this technique to select a catalytic site an RNA sequence that catalyzes Diels-Alder reactions. A library containing random sequences of RNA was first prepared using a uridine derivative with a pyridyl moiety instead of the usual uridine. Each oligo-RNA chain in the library was connected to a diene part via a flexible poly(ethylene glycol) (PEG) chain. 

The tetrakis(methylidene)-7-oxabicyclo[2.2.1]heptane (527) offers an interesting route to linear poly-annulated ring systems, e.g. 4Klemethoxydaunomycinone (641), via a sequence of two Diels-Alder reactions (Scheme 124). Monoadduct (528) could be isolated and subjected to the [4 + 2] cycloaddition of benzyne. An alternative synthesis of anthracyclinone analogs starts with the Diels-Alder addition of 3-methoxybenzyne to a derivative of the bis-cisoid tetraene (527). ... 

Rare earth metal triflates are recognized as a very efficient Lewis acid catalysts of several reactions including the aldol reaction, the Michael reaction, allylation, the Diels-Alder reaction, the Friedel-Crafts reaction, and glycosylation [110]. A polymer-sup-ported scandium catalyst has been developed and used for quinoline library synthesis (Sch. 8) [111], because lanthanide triflates were known to be effective in the synthesis of quinolines from A-arylimines [112,113]. This catalyst (103) was readily prepared from poly(acrylonitrile) 100 by chemical modification. A variety of combinations of aldehydes, amines, and olefins are possible in this reaction. Use of the polymer-supported catalyst has several advantages in quinoline library construction. 

As was shown earlier with the Suzuki coupling reactions, organopalladium intermediates can show good stability to water and other protic sources. This stability has been exploited in the synthesis of polyacetylene under air- and moisture-stable conditions. It was found that simple palladium(II) salts (PdCl2, Pd(CH3C02)2, etc.) can be used to initiate the 1,2-insertion polymerization of strained cyclic alkenes in water (eq. (8)) [31]. Once formed, poly-8 can be converted to polyacetylene through a retro-Diels-Alder reaction. 

Recently, Mullen et al. have reported a convergent approach134 to poly(phenylene) dendrimers (Scheme 5) similar to the divergent [4 + 2] cycloaddition route they had previously developed.135 The process was initiated by a Diels—Alder reaction between tetra-substituted cyclopentadienone 61 and dialkynyl mono-... 

Diblock copolymers of the two substrates were also prepared. Both the above-mentioned copolymers undergo retro Diels-Alder reactions to form poly (ketovinylene) the former material gave complete conversion only at a rather high temperature (280°C). 

Reusability is a characteristic of the sensitizers prepared by stirring Ceo-fullerene with aminomethylated poly(styrene/vinylbenzene). They have been used to promote the standard O2 oxidation processes such as ene and Diels-Alder reactions (Scheme 7), and catalysts suitable for photoxidations in water have been prepared from them by reaction with poly(allylamine). The same reactions have been carried out using a novel solvent-free procedure which involves loading a porphyrin into solvent-swollen polystyrene beads and carrying out the photo-oxidation in neat liquid substrate. The formation of the allylic hydroperoxide (89) from p-pinene, with complete conversion and in 84% yield, is particularly noteworthy, as the standard liquid-phase reaction can be problematic. It is suggested that the possibility of using this approach under solar conditions is further evidence of the sustainable, green chemistry potential of synthetic photochemistry. 

Esters and other derivatives of alkenyl- and alkynyl-phosphonic acids, and of the correspondingly unsaturated phosphinic acids, undergo a wide range of addition and cycloaddition reactions which will be considered more fully in Chapter 6. These reactions include applications of the Diels-Alder reaction to yield carbocyclic phosphonic acids based on mono- and poly-cyclic ring systems, some of which are precursors to other aromatic phosphonic acids. A typical example of this procedure is the addition of dimethyl (bromoethynyl)phosphonate to 2,3-dimethylbutadiene elimination of HBr from the 1 1 cycloadduct, using Et3N, affords dimethyl (2,3-dimethylphenyl)phosphonate. The addition of carbenes to alkenylphosphonic derivatives yields those of cyclopropylphos-phonic acids. ... 

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