Dienes and polyenes

Bryson and Pye have reported the synthesis of the 1,4-diene (107) and its strategic use in a stitching and riveting hydroboration-carbonylation procedure in an intriguing steroid synthesis.  

Thermal [2,3] sigmatropic rearrangement of the sulphonium ylide (114) gives a 54% yield of a mixture of four isomeric products, the major (70%) of which is the allenic thioether (115), a species which contains the entire artemisyl carbon skeleton hydrolysis with mercuric chloride then leads to artemisia ketone in 80% yield. The second route involves methallylation of the methyl thioacetal monoxide (116), the unstable intermediate (117) on distillation being transformed into the trienic sulphide (118). The latter species on hydrolysis gives artemisia ketone in 57% yield. 

Derguini-Boumechal, R. Lome, and G. Linstrumelle, Tetrahedron Letters, 1977, 1181. 

Reagents i, PhSH-02, then BuLi-THF to (119b) ii, ==- Br iii, dil. H2SO4 iv, Li-EtNH2 V, PBr3-py vi, (119b) 

Reagents i, Me3SiCH2MgCl-NiCl2(catalytic) ii, 2LiPr2-CuI iii, —/ Br iv, 2MeONa v, LiN(SiMe3)2-Me2NCH2CH2NMc2 / 

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Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

Before we start with a systematic discussion of the syntheses of difunctional molecules, we have to point out a formal difficulty. A carbonmultiple bond is, of course, considered as one functional group. With these groups, however, it is not clear, which of the two carbon atoms has to be named as the functional one. A 1,3-diene, for example, could be considered as a 1,2-, 1,3-, or 1,4-difunctional compound. An a, -unsaturated ketone has a 1.2- as well as a 1,3-difunctional structure. We adhere to useful, although arbitrary conventions. Dienes and polyenes are separated out as a special case. a, -Unsaturated alcohols, ketones, etc. are considered as 1,3-difunctional. We call a carbon compound 1,2-difunctional only, if two neighbouring carbon atoms bear hetero atoms. 

Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly-... 

Cyclic dienes and polyenes, monocyclic as well as bicyclic, can be metathesized in the same way as cyclic monoenes. As expected, cyclobutene 27), 1,5-cyclooctadiene, and 1,5,9-cyclododecatriene 28) yield the same polyalkenamer, in this case polybutenamer (1,4-polybutadiene), since these reactants consist of the same base units, i.e.—(CH2)2CH==CH— ... 

It will be convenient to consider under one heading all those unsaturated systems which do not fall under any of the sections previously discussed. There are four categories which are included in this catch-all heading. They are pyridones, quinones, multiple substitution at nonequivalent sites, allenes and cumulenes, and conjugated dienes and polyenes. 

A. Andreetta, F. Conti, and G. F. Ferrari Selective homogeneous hydrogenation of dienes and polyenes to monoenes, pp. 204-295. 

The Chemistry of Dienes and Polyenes. Volume 1 Edited by Zvi Rappoport Copyright 1997 John Wiley Sons, Ltd. 

Contribution of quantum chemistry to the study of dienes and polyenes... 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

The aim of this chapter is to present the most recent theoretical contributions to the study of structure, properties and reactivity of dienes and polyenes. Earlier stages in these areas are covered in the above-mentioned reports1-5. 

The dienes and polyenes are compounds which intervene in a large number of organic reactions, as will be seen in different chapters of this book. Several excellent reviews have been devoted to theoretical studies about their reactivity, with special emphasis on the mechanism of pericyclic reactions3-5. As was mentioned in the introduction, this section will only treat, as an example, the Diels-Alder reaction, since it has been the most studied one by theoreticians. Our goal is not to cover all aspects, but instead to show the high potential and usefulness of theoretical methods in order to interpret and rationalize the experimental results. In the rest of the chapter we will concentrate on the last ab initio calculations. 

All along this chapter, we have covered some of the most significant and recent contributions of Quantum Chemistry to the study of dienes and polyenes. 

The enormous development of powerful computers and the implementation of new theoretical methods continuously extends the field in which theory can provide results with chemical accuracy. This fact allows us to foresee that in the near future the structure and properties of dienes and polyenes will be more thoroughly understood. 

The structural chemistry of dienes and polyenes is extremely diverse and intricate since about 12% of all determined structures of organic compounds contain two or more double... 

II. DIENES AND POLYENES A. Linear and Branched Dienes and Polyenes... 

Sterically strained conjugated acyclic dienes and polyenes. 

Rotational isomerism normally complicates the study of gaseous nonconjugated dienes and polyenes because many conformers appear simultaneously, and hence only few structures of free molecules in this category have been studied. 

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