Subject conjugated polyenes

If one considers only hydrocarbons, and more especially the so-called alternant hydrocarbons, i.e. first of all the conjugated polyenes and the aromatic hydrocarbons of the benzene series, the greater part of their physical properties, ionization potentials, lower electronic transitions etc., can be interpreted qualitatively and often quantitatively in terms of the electronic structure of the n system alone. As the number of n electrons is small with respect to the total number of electrons of the molecule, a considerable simplification of the quantum-mechanical problem is obtained. However, it must be noted immediately that the assumptions of a complete a—n separation and of a rigid a frame are not sufficient to eliminate the a electrons completely from the theory because the n electrons of an unsaturated molecule are not attracted by bare nuclei, but are subject to an effective potential containing Coulomb and exchange contributions from the a electrons. 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

Numerous dienes and polyenes have been subjected to hydroxymercuration-demercuration.311 With nonconjugated dienes, the products can usually be predicted by applying what one has learned from the corresponding simple alkenes. Isolated double bonds are more reactive than conjugated double bonds and frequently one of the double bonds is sufficiently more reactive than the others that monohydroxy-lated products can be obtained. Improvements in selectivity have been reported by using mercury(II) tri-fluoroacetate339 or by adding sodium lauryl sulfate.340... 

Polyene antibiotics, such as amphotericin B (II) which contains seven conjugated double bonds (heptaene moiety), are subject to attack by peroxyl radicals, leading to aggregation and loss of activity. ... 

Color Development. The absorption spectrum in the visible region of PVC subjected to heat degradation shows several maxima that are related to the degree of conjugation of the polyenes formed by dehydrochlorination. Bengough and Varma (29) have studied the... 

There are many synthetic examples that use radical cyclization as a key step, and the radical precursor is not limited to iodides or bromides. In Pattenden s synthesis of pentalenene, conjugated selenyl ester 156 was treated with Bu3SnH and AIBN to give a 45% yield of tricyclic ketone 159. Loss of PhSe generated the acyl radical 157, which exists in equilibrium with the ketene radical 156. Radical cyclization via the latter intermediate leads to 159. Cyclization via aryl radicals is also possible. In Schultz s synthesis of hexahydro-phenanthren-2-one derivatives, " aryl bromide 160 was cyclized to 161 in 78% yield under standard conditions. Radical cascade reactions have become quite popular for the synthesis of polycyclic ring systems. In these reaction, polyenes are subjected to radical cyclization, generating tricyclic or even tetracyclic ring systems. 5 Chiral auxiliaries have been used effectively in radical cyclization reactions. ... 

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