Polymer synthesis, Diels-Alder reaction

Scheme 45. Synthesis of ladder polymers by Diels-Alder reactions route A reaction of bis-dienes with bis-dienophiles [143] route B aromatic compounds employing a diene and a dienophile moiety [143 c]...

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] ... 

A polymer-supported silyl triflate and subsequent functionalization synthesis and solid-phase Diels-Alder reactions of silyloxydienes [25]... 

Smith JG J., Ottenbrite R. M. Synthesis of Polyimides Utilizing the Diels-Alder Reaction Contemp. Top. Polym. Sci. 1992 7 83-93... 

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. 

Abstract An overview on the microwave-enhanced synthesis and decoration of the 2(lH)-pyrazinone system is presented. Scaffold decoration using microwave-enhanced transition-metal-catalyzed reactions for generating structural diversity, as well as the conversion of the 2(lH)-pyrazinone skeleton applying Diels-Alder reactions to generate novel heterocyclic moieties are discussed. The transfer of the solution phase to polymer-supported chemistry (SPOS) is also described in detail. 

Saha AK, Hossain MM (1993) Synthesis of a polymer-bound iron Lewis acid and its utilization in Diels-Alder reactions. Tetrahedron Lett 34 3833-3836 Olson AS, Seitz WJ, Hossain MM (1991) Transition metal catalysis of the Diels-Alder reaction. Tetrahedron Lett 32 5299-5302... 

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. 

In the field of polymer chemistry the regio- and stereoselectivity of the Diels-Alder reaction is used for the concerted synthesis of structurally homogeneous double-stranded ladder polymers [39], which are useful materials with nonlinear optical properties and high electrical conductivity. It has turned out that the repeated Diels-Alder method is superior to an alternative two-step process, in which first an open chain precursor is formed followed by polymer ring closure as structural defects can occur [40]. 

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

Oikawa M, Ikoma M, Sasaki M (2005) Parallel synthesis of tandem Ugi/Diels-Alder reaction products on a soluble polymer support directed toward split-pool realization of a small molecule library. Tetrahedron Lett 46 415-418... 

The Diels Alder reaction, involving the [4 + 2]-cycloaddition of an unsaturated group (di-enophile) to a 1,3-diene, has been studied for the synthesis of ladder polymers, such as the reaction of 2-vinyl-l,3-butadiene with benzoquinone [Bailey, 1972] (Eq. 2-247). Related polymerizations are those utilizing the [2 + 2]-cycloaddition reaction [Dilling, 1983]. While [4 + 2]-cycloaddition reactions are thermally induced, [2 + 2]-cycloaddition reactions are... 

There are many examples of AB monomers in the polymer literature [104]. In particular there are examples of the Diels-Alder reaction being used in successful AB type approaches to polymer syntheses. There are however several practical problems associated with this type of approach. If the diene or dienophile is highly reactive, the synthesis and purification of the monomer can be very difficult. Furthermore, these reactive monomers can be difficult to store without some partial advancement of the molecular weight. If on the other hand the diene or dienophile is not very reactive under moderate conditions, then low molecular weight polymers are obtained. If more stringent conditions for polymerization are employed, the retro Diels-Alder reaction and other side reactions can become more important and therefore lead to the formation of a low molecular weight polymer. 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

Following the first reports in the literature of catalytic imprinted beads, a number of authors also reported applications of this polymer format to several imprinting systems. Busi et al. [64] reported the preparation of catalytic active beads for the Diels-Alder reaction using a TSA as a template. Jakubiak and co-workers developed imprinted beads for the oxidation of phenols based on a Cu(II) complex as catalytic centre [65]. Say and collaborators described the synthesis of microbeads also based on a Cu(II) complex with esterase activity towards paraoxon (60), a potent nerve agent [66]. The imprinted beads enhanced the rate of reaction over the non-imprinted polymer by a factor of 40, as resulted from the ratio of the corresponding kciil. 

C. MacMillan, /. Am. Chem. Soc. 2000, 122, 4243-4244. For the synthesis of polymer-bound derivatives of the MacMillan catalyst and applications in Diels-Alder reactions, see M. Benaglia, G. Celentano,... 

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. 

---