Polyene systems

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

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. 

Adjacent polyenes are hydrocarbons with double bonds between each pair of atoms in the polyene system they are called cumulenes2. Two types of cumulenes exist those with an even number of adjacent double bonds and those with an odd number. The former can exhibit chirality, the latter geometric isomerism. Allenes (propadienes) are the simplest members of the even-numbered type of cumulenes. 

In this work we present a description of the quantification of structural effects on reactivities and properties of polyene systems. 

Two types of chemical reactivity of data sets can be distinguished those in which the diene or polyene system acts simply as a skeletal group, and those in which it acts in whole or in part as a reaction site. 

Electrocyclic processes are also observed with ionic systems, both cationic and anionic. The decisive factor in the stereochemistry is the number of electrons in the delocalized polyenic system. Thus the mode of ring opening or ring closure depends on the n electrons in the open polyene. 

Transition metal catalysts can alter the normal thermal reactions of a polyene system in several ways. 

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

The electronic spectroscopy of polyene systems has been so extensively investigated and is so thoroughly covered in numerous books and reviews1,3,11,21 38 that only a brief general summary need be presented here. 

Direct ,Z-photoisomerization is sufficiently ubiquitous in the photochemistry of all polyene systems that it will be considered first in its own separate section, although the... 

Dienes and polyenes can undergo a variety of intermolecular cyclization reactions, the exact nature of which is dependent on the number of double bonds, the relative positions of these bonds with respect to each other, the preferred conformation of the diene or polyene system and the reaction partner. 

Several effects can influence the electronic structure of Cjq upon metal complex formation. One is the removal of one double bond from the remaining 29 fullerene double bonds. As in any polyene system, this decreased conjugation is expected to raise the energy of the LUMO and therefore decreases the electron affinity of the system. Conversely, the d-orbital backbonding transfers electron density from the metal into n orbitals of the remaining double bonds, which also decreases the electron affinity. 

By introducing fluorine atoms to the polyenic system of retinal, the geometry, electronic properties, hydrophobicity, and absorption properties of the molecule will be modified. Thus, fluoro derivatives of retinal are useful tools to understand the interactions between retinal and opsin, especially on the level of charge and hydrophobic effects at the protein site. Moreover, fluorine atoms are probes in NMR and allow studies on model molecules of visual pigments Consequently, syntheses of mono-, di-, and trifluoro derivatives of retinal have been the subject of many investigations. 

There is continuing interest in the coordination compounds of cyclic polyenes, such as 1//-azepines, as it has become apparent that complexation with tricarbonyliron can lead to marked changes in the reactivity of the polyene system towards electrophilic substitution. In fact, 1 -methoxycarbonyl- 1H-azepinetricarbonyliron at 0°C with acetic anhydride and... 

Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19-40b) as their light-harvesting pigments. These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or... 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

It is in this area that qualitative MO procedures have great success because there are general characteristics of the 77 molecular orbitals of mono-cyclic, conjugated polyene systems that predict differences in the properties of cyclobutadiene, benzene, cyclooctatraene, and other similar compounds that are not obvious from the simple VB method. 

Pd-catalysed decarboxylation-elimination offers a useful method for regioselective generation of conjugated dienes. The polyene system of vitamin A derivative 381 is prepared from the /f-acetoxycarboxylic acid 379 by decarboxypalldation, as shown by 380 [172],... 

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