Conjugated dienes interesting examples

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

The cyclization step of Equation 28-8 is a photochemical counterpart of the electrocyclic reactions discussed in Section 21-10D. Many similar photochemical reactions of conjugated dienes and trienes are known, and they are of great interest because, like their thermal relatives, they often are stereospecific but tend to exhibit stereochemistry opposite to what is observed for formally similar thermal reactions. For example,... 

An interesting example of high chemoselectivity in the coordination polymerisation of conjugated dienes is the polymerisation of 1,3-cyclohexadiene in the presence of Ziegler-Natta [186] and re-allylnickel [187] catalysts, which leads to polymers consisting mainly of 1,4 monomeric units. 

An interesting example of Pd-catalyzed oxidation of a 1,3-conjugated diene is shown in Scheme 20. This reaction type produces stero-defined 1,4-functionalized dienes, which are useful synthetic building blocks and reaction intermediates. Benzoquinone oxidation of an ) -allyl-Pd intermediate is a key step in releasing the organic product. ... 

Other complexes of great interest are the / -conjugated diene metallocenes. They are conveniently prepared by the one-pot reaction of the photochemically induced metallocene with the diene at low temperature. Alternative routes include the treatment of the metallocene dichloride with a conjugated diene dianion equivalent or the coupling of alkenyl ligands in the coordination sphere of the metal. The resulting systems show unusual behavior. For example, buta-1,3-diene directly leads to the s-trans complex (8), which can be thermally equilibrated into the s-cis isomer (9). This form exhibits a substantial metallacyclopentene character and is best described as a d metal (cr, 7r)-type complex (10), as shown by X-ray crystal analysis. On the other hand, the s-trans isomer should be formally regarded as a real d metal -alkene 7T-complex. 

Although acyclic dienes in general gave a lower yield than the cyclic ones, the 1,4-stereocontrol obtained for internal conjugated dienes is of interest in synthesis. For example, E,E) and ( , Z)-hexa-2,4-dienes were stereoselectively transformed to the d,l and meso-1,4-diacetate, respectively vide infra). 

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