Bis dienophiles

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] ... 

The bifunctionality of the bis-diene and bis-dienophile monomers is apparent from the condensation product, structure [XXI], which still contains a diene and a dienophile in the same molecule. This polymer is crystalline, indicating a high degree of stereoregularity in the condensed rings. It decomposes to a graphitic material before melting. 

The sunuhaneous double Diels-Alder addition of l,l-bis(3,5-dimethylfur-2-yl)ethane (8) with a biS dienophile such as diethyl ( , )-4-oxohepta-2,5-diene-1,7-dioate was proposed as new, asymmetric synthesis of long-chain polypropionate fragments and analogues <96TL4149>. 

The synthesis of the macrocycles 43 (Scheme 9) is an example of repetitive, highly stereoselective Diels-Alder reaction between bis-dienes 41 and bis-dienophiles 42, containing all oxo or methano bridges syn to one another. The consecutive inter- and intramolecular Diels-Alder reactions only succeed at high pressure. Obviously, both reactions are accelerated by pressure. The macrocycles are of interest in supramolecular chemistry (host-guest chemistry) because of their well-defined cavities with different sizes depending on the arene spacer-units. 

If the oxo (or methano) bridges are not exclusively syn to one another in either the bis-dienophiles or bis-dienes, then the pressure-induced repetitive Diels-Alder reactions (proceeding again highly stereoselectively) produce rigid ribbon-type oligomers on a nanometer scale (Scheme 10 entry 1). Bis-diene 45 reacts less stereoselectively than bis-diene 44 and forms with bis-dienophiles such as 46 the ribbon-type oligomers 47... 

The diepoxide products obtained as described here are excellent bis-dienophiles hence, they are useful starting materials for further synthesis. They have... 

An example of the second type of modification is the application of the Diels-Alder cycloaddition reaction to polders and copol ers containing pendant or backbone furan moieties. The use of bis-dienophiles such as propiolic acid and its esters or bis-maleimides provides a means of crosslinking based on multiple bridging by the double interchain lycloadditions. The thermal reversibility of these reactions allows the return to the original linear structure (thermoplastic material) by simply heating the gel. 

The final copolymer was obtained in 90% yield it had a molecular weight of 10,800 and a polydispersity index of 2.1. In this case Diels-Alder copolymerization dominates over the cyclobutane homopolymerization of a bi(dienophile). This means that the Diels-Alder addition of the dienophile to the diene is substantially faster than the competing addition of the dienophile cation radical to the neutral dienophile. 

Scheme 44. Diels-Alder reaction with bis-dienes and bis-dienophiles [ 142b]...

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]...

Structure can be obtained by repetitive Diels-Alder reactions of the difuranocyclo-octane as the iris-diene and dimethyl acetylenedicarboxylate as the bis-dienophile. The cage compound is formed in an undesirable side-reaction [70]. The extension of the spacer-unit of a bis-diene can be achieved by the pressure-induced Diels-Alder reaction of the bis-diene with allenylchloromethylsulfone followed by basic HCl and SO2 elimination [71]. 

Another example is condensation of bis dienophiles with dienes [202] ... 

The following exanples of DA cycUzations (Schemes 6.2-6.4) illustrate schematically the formation of the corresponding adducts using either a mono- or a bis-dienophile and emphasize the reversibOity of these reactions which is promoted by an increase in temperature. 

A tris(4-bromophenyl)ammonium hexachloroantimonate catalyst has been utilized to promote a cation radical mechanism in the Diels-Alder cycloaddition polymerization of a bis(diene) with an ionizable bis(dienophile) (Scheme 2). The polymers were obtained with molecular weights up to ca 10 000 and a polydispersity index of ca 2. The electron-transfer reactions of phenols and its derivatives are also important to the polymer industry for the stabilization of polymers, fats and oils. Pulse radiolysis of naphthols and hydroxybiphenyls in n-butyl chloride at room temperature forms two species-phenol-type radical cations and phenoxyl-type radicals. Two different electron-transfer channels are proposed. The naphthol and hydroxybiphenyl radical cations show increased stability compared with phenol radical cations, presumably due to extensive delocalization over the whole aromatic system. 

Polymerization of a bis(diene) with a bis(dienophile) has yielded high molecular weight polymer. The reaction can be carried out in solvents such as j-tetrachloroethane and dimethylformamide, or in bulk. Optimum reaction temperatures vary with the particular case. No catalysts are required. An example of such a polymerization is shown in Eq. (III-3) (<5). 

Bis(dienes) containing ferrocene groups have been polymerized with related bis(dienophiles). Polymer [16] was obtained from l,l -bis(dimethyl-4-iso-prenylsilyl)ferrocene and l,l -bis(dimethylvinylsiIyl)ferrocene (//). 

Method 2. Polymerization of Bis(dienophiles) with Pseudobis(dienes)... 

The polymerization of bis(dienophiles) with cyclopentadienones, a-pyrones, and thiophene dioxides has yielded some very high molecular weight... 

Stoddart and cowotkers have extensively exploited the principle of repetitive arene and bis-arene cycloaddition to furans for the design and synthesis of novel macropolycyclic stractures [22,23]. For example, bis-dienophile 21 obtained from ortho-xylene 20 with butyllithium followed by trapping of the resulting bis-arene equivalent 22 with 2 equiv of furan represents a suitable building block for the construction of the macropolycylic belt-like molecule 23. Diels-Alder reaction of 21 with an appropriate bis-diene under alternate thermal and high-pressure conditions afforded macrocycle 23 in good yield, together with an acyclic isomer (Scheme 13.8) [22,23]. 

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