Conjugated dienes conformations

The two most stable conformations of conjugated dienes are the s cis and s trans The s trans conformation is normally more stable than the s cis Both conformations are planar which allows the p orbitals to overlap to give an extended tt system... 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

Another reaction unique to conjugated dienes is the Diels-Alder cycloaddition. Conjugated dienes react with electron-poor aikenes (dienophiles) in a single step through a cyclic transition slate to yield a cyclohexene product. The reaction is stereospecific, meaning that only a single product stereoisomer is formed, and can occur only if the diene is able to adopt an s-cis conformation. 

Most of the reported X-ray studies have been carried out to establish structures or conformations of different compounds. The X-ray of fully conjugated diene compound 20 has been reported showing supramolecular assemblies resulting in a 7i-stacking interaction between two adjacent molecules <2004TL7363>. The other structures reported in Figure 2 have been determined and parameters described in the corresponding papers. 

Compared to the application of ordinary conjugated dienes, the use of vinylallenes as diene components is advantageous from the viewpoint of both reactivity and stereoselectivity. The equilibrium between the s-trans and s-cis conformers is more on the side of the s-cis isomer for vinylallenes than it is for 1,3-dienes. Consequently, vinylallenes exhibit a higher reactivity. 

Reaction of linear conjugated dienes with la at —10 °C in hydrocarbon solvent in the presence of McsSiCl/AlCls affords stereospecific tra i-l-silyl-3-vinyl-cyclopentanes, indicating a [3 + 2] cycloaddition of the allyl group of la with a carbon arbon double bond of the diene [Eq. (9)]. In the [3 + 2] annulation reaction, of greater significance is the tram conformation of the trimethylsilyl group and vinyl groups. 

Cyclic conjugated dienes that are s-cis conformation, e.g. cyclopentadiene and 1,3-cyclohexadiene, are highly reactive in Diels-Alder reactions. In fact, cyclopentadiene is reactive both as a diene and as a dienophile, and forms dicyclopentadiene at room temperature. When dicyclopentadiene is heated to 170 °C, a reverse Diels-Alder reaction takes place and reforms the cyclopentadiene. 

The third important reaction of singlet oxygen is 1,4-cycloaddition to conjugated dienes, resulting in the formation of endoperoxides.360-364,371 -373 Acyclic dienes give monocyclic endoperoxides that exist in rapidly equilibrating half-chair conformations 410,411... 

Certain chromium complexes also exhibit similar selectivities in the hydrogenation of conjugated dienes.139 140 Coordination of the diene in the cisoid conformation and 1,4 addition can account for the selectivity. 

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