Polyenes, conjugated

2 Conjugated Polyenes Table 3.4 contains examples of conjugated alkenes with the major site of reaction indicated. This table also included some examples of functionalized polyenes. 

For conjugated trienes, the central double bond is not reactive while a traras-alkene is more reactive than a terminal one, a mixture usually results with the major product arising from reaction of the trans-alkene. If the end of the triene unit is a cis-alkene rather than a trans-alkene, then only reaction at the terminal alkene is observed. 

For the oxidation of conjugated dienes with an aryl substituent at one of the systems and both alkenes being trans, the major product arises from reaction of the alkene not attached to the aryl unit, unless that moiety is naphthyl [259]. 

Molecules with alternating single and double bonds ( conjugated polyenes ) often exhibit unusual physical and chemical properties. Chemists have postulated the involvement of zwitterionic resonance structures to account for these properties. 

Consider 1,3-pentadiene and 1,4-pentadiene. Which, if either, would benefit from the type of resonance described above Draw appropriate resonance contributors for this isomer. Indicate the likely importance of different zwitterionic structures which you might draw. Compare the energies of 1,3-pentadiene and 1,4-pentadiene. Which one is more stable  

Compare carbon-carbon bond distances in each diene. Which bond distances are unusually long or short Use propene as a model of a molecule with normal CC single and double bonds. Can resonance account for all of the unusual bond distances Is this the only explanation  

Compare electrostatic potential maps for the two dienes. Does the more stable isomer show greater delocalization of negative charge  

Obtain the energies, carbon-carbon bond distances and electrostatic potential maps for 3-ethyUdene-l,3-pentadiene and 1,3,5-heptatriene. Which data are consistent with the usual resonance argument and which are not  

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This method of diene formation with definite E and Z structures has wide synthetic applications [518], particularly for the syntheses of natural products with conjugated polyene structures. Bombykol and its isomers (650 and 651) have been prepared by this method[5l9]. The synthesis of chlorothricolide is... 

All of the evidence indicates that cyclooctatetraene is not aromatic and is better considered as a conjugated polyene than as an aromatic hydrocarbon... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Among planar monocyclic fully conjugated polyenes only those possessing (4n + 2) TT electrons where n is a whole number will have special stability that be aromatic... 

The An + 2) tt electron standard follows from the pattern of orbital energies m monocyclic completely conjugated polyenes The tt energy levels were shown for ben zene earlier m Figure 114 and are repeated m Figure Figure 11 13a and 11 13c... 

Poor correlations are obtained for cross-conjugated polyene systems such as... 

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

Although the Hiickel method has now been supplanted by more complete treatments for theoretical analysis of organic reactions, the pictures of the n orbitals of both linear and cyclic conjugated polyene systems that it provides are correct as to symmetry and the relative energy of the orbitals. In many reactions where the n system is the primary site of reactivity, these orbitals correctly describe the behavior of the systems. For that reason, the reader should develop a familiarity with the qualitative description of the n orbitals of typical linear polyenes and conjugated cyclic hydrocarbons. These orbitals will be the basis for further discussion in Chapters 9 and 11. 

In general conclusion, the HMO and SCF methods both appear able to make reasonably accurate predictions about the stabilization in conjugated moleeules. The stabilization is general for benzenoid compounds but quite restricted in nonbenzenoid systems. Because the HMO method of estimating stabiUty is based on the ideas of HMO theory, its general success vindicates the ability of this very simplified MO approach to provide insight into the structural nature of the aimulenes and other conjugated polyenes. More sophisticated MO methods, of course, are now accessible and should be applied for more detailed analysis of the structures of these molecules. 

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