Cyclic conjugated polyenes

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. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

General reactivity trends for alkenes were established for hydrozirconahon by way of qualitative studies terminal alkene > internal alkene > exocyclic alkene > cyclic alkene trisubshtuted alkene. The rate of hydrozirconahon decreases with increasing substitution on the alkene. This property was used for selechve monohydrozir-conation of conjugated and non-conjugated polyene derivahves (Scheme 8-8) [84-86]. 

Carotenoids are natural compounds characterized by a conjugated polyenic chain connecting two terminal groups, as represented in Scheme 7. Terminal groups can be of different natures they often contain a cyclic double bond and allow one to classify carotenoids. 

It is possible in principle to deduce from the position of the first absorption band the twisting angle between the double bonds in cyclic conjugated polyenes, e.g. on the basis of a simple LCBO model50. This has been attempted for cycloocta-l,3,5-triene 184257 and cyclooctatetraene 239260. For the former it was deduced that the --system becomes... 

Campbell, A. D., and S. N. Slater Cyclic Conjugated Polyenes The attemp-... 

One further example of selection rules for reactions is provided by the intramolecular conversion of an open-chain, conjugated polyene to a cyclic olefin with one less pair of n electrons. The simplest example is the butadiene-cyclobutene interconversion ... 

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. 

There is a way around the An+ 2 rule that is not very important for substances analogous to benzene, but. is quite important for cycloaddition reactions. Let us see how this works for a cyclic conjugated polyene. 

Strong evidence in confirmation of the above explanation of the chemical shifts of aromatic hydrogens is provided by a study of the cyclic conjugated polyene [18]annulene, which has hydrogens both inside and outside the ring ... 

Previously in Chapter 3 we introduced the Huckel molecular orbital theory and applied it to the n system of a number of conjugated polyene chains. In this section we will apply this approximation to cyclic conjugated polyenes, taking advantage of the symmetry properties of these systems in the process. 

Another example of a cyclic conjugated polyene system is naphthalene. A coordinate system for this molecule, along with the labeling of the 7t atomic orbitals, is shown in Fig. 7.1.13. The lOx 10 secular determinant has the form ... 

In the previous section, we discussed the construction of the a molecular orbitals in AH systems. In this section, we confine our treatment to the jz molecular orbitals in cyclic conjugated polyenes. In most molecules, there are a bonds as well as n bonds, and these systems can be treated by the methods introduced in these two sections. 

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