Hexa-2,4-dienes

The dienes used are such that the double bonds in the polymer are either on a side chain or as part of a ring in the main chain. Hence should the double bond become broken the main chain will remain substantially intact. Dienes commonly used include dicyclopentadiene, ethylidene-norbomene and hexa-1,4-diene Table 11.17). 

The Diels-Alder reaction with triple bond dienophiles gives access to cyclo-hexa-1,4-diene derivatives. Further reaction of a reactive intermediate thus produced or a subsequent oxidation step can then lead to a six-membered ring aromatic target molecule. 

With a substitution pattern as found in l-vinyl-2-methylcyclopropane 7, a retro-ene reaction (see Ene reaction) may take place to yield hexa-1,4-diene 8 ... 

The isoprene units in the copolymer impart the ability to crosslink the product. Polystyrene is far too rigid to be used as an elastomer but styrene copolymers with 1,3-butadiene (SBR rubber) are quite flexible and rubbery. Polyethylene is a crystalline plastic while ethylene-propylene copolymers and terpolymers of ethylene, propylene and diene (e.g., dicyclopentadiene, hexa-1,4-diene, 2-ethylidenenorborn-5-ene) are elastomers (EPR and EPDM rubbers). Nitrile or NBR rubber is a copolymer of acrylonitrile and 1,3-butadiene. Vinylidene fluoride-chlorotrifluoroethylene and olefin-acrylic ester copolymers and 1,3-butadiene-styrene-vinyl pyridine terpolymer are examples of specialty elastomers. 

Ethylene-propylene (30-60 mole per cent) copolymers produce substances which are rubbery in nature. They are prepared by using Ziegler catalysts based on vanadium oxychloride/aluminium trihexyl by solution process at 40°C using chlorobenzene or pentane as a solvent. These can be vulcanised with peroxides. Ethylene-propylene-hexa 1, 4-diene terpolymers are rubbers which can be vulcanised with sulphur. 

The relative fluorination rates for nonequivalent C = C bonds in substituted perfluorocyclo-hexa-1,4-dienes depend on the electronic properties of the substituents and indicate the electrophilic nature of vanadium(V) fluoride."4... 

Z groups C-terminal benzyl esters are removed simultaneously. To avoid the higher temperatures at which some protecting groups are not sufficiently stable, the use of cyclo-hexa-1,4-diene was proposed as it is a more efficient hydrogen donor that can be used at room temperature. By this procedure Z groups, benzyl esters, and tyrosine benzyl ether are removed within 2 hours with 10% Pd/C as the catalyst. 

The synthesis of hexa-1,4-diene has been achieved by the nickel-catalyzed homogeneous addition of ethylene to butadiene. The nickel is introduced in the form of the complex Ni[P(OEt)3]4. The reaction is carried out in acid media, and the active catalyst is the cationic complex NiH[P(OEt)3]3 which is a 16-electron molecule. In Fig. 7 the sequence of reactions that leads to the catalytic formation of isomeric hexadienes is... 

Fig. 7. Catalytic synthesis of hexa-1,4-diene from ethylene and butadiene.

The preparation of mono- and diadducts 5 and 6, respectively, of dibromocarbene with cyclo-hexa-1,4-diene is described. The synthesis of the diadduct encounters difficulties in its isolation, due to the formation of an emulsion and polymeric products. Such problems occur occasionally if an alkene is allowed to react with bromoform under phase-transfer catalysis conditions. 

The isomerization of c -l-methyl-2-vinylcyclopropane (1) to (Z)-hexa-1,4-diene (3) at 180 C in the gas phase, first described in 1964, is the prototype for all homo [1,5] hydrogen migration reactions of substituted cyclopropanes. 

Skinner,G. A., Viney,M., Wallis,S.R. Cis-tram isomerism in Ziegler-catalysed terpolymerization of hexa-1, 4-diene with ethylene and propylene. Polymer (London) 13,242-244 (1972). 

A second example of this is provided by the pyrolysis of cis-l-methyl-2-vinylcyclopropane, which decomposes at about 120 °C lower than its trans isomer to give cis-hexa-1,4-diene (17, 18, 58) ... 

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