Conjugated polyenic 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. 

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. 

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 ... 

Molar absorptivities in the range of 5000 to 30,000 are typical for the tt — tt transitions of conjugated polyene systems. Such large molar absorptivities are helpful, since spectra may be obtained with very small amounts of sample. On the other hand, samples and solvents for UV spectroscopy must be extremely pure. A minute impurity with a large molar absorptivity can easily obscure the spectrum of the desired compound. 

The conjugated polyene systems taking part in pericyclic reactions are linear, so all the tt molecular orbitals of the starting materials have a different energy. The distribution of energy levels is important because it affects the electronic configuration of the starting materials (see Example 6.19) and consequently the course of the reaction. 

In retro-carotenoids, all the single or double bonds of the conjugated polyene system have shifted by one position. The carbon atoms defining the new conjugated system are indicated in the nomenclatnre as a prefix prior to the term -retro-. For example, 4, 5 -didehydro-4,5 -retro-p,p-carotene-3,3 -diol (eschscholtzxanthin) (Fignre 3.10b). 

In previous total synthesis of other natural compounds, we have been interested in the stereoselective construction of conjugated polyenic systems. These olefinic sequences can be obtained by palladium-catalyzed cross-... 

The conjugated polyene system typical of carotenoids gives rise to only very weak bands in IR spectroscopy. However, this technique has proved to be of value for detecting certain special structural features such as acetylenic, allenic, hydroxy and unreactive keto groups, e.g. like those in fucoxanthin and capsanthin (for a comprehensive discussion of carotenoid chemistry see Vetter et aL, 1971). The structures of allenic and acetylenic carotenoids may be determined with the help of IR spectroscopy, because of the unusual peak at 1928 cm" indicative of allenic groups, and the weak band at 2170 cm, characteristic for acetylenic carotenoids (Vetter et al., 1971). 

Molecular Orbitals of Alkenes and Conjugated Polyene Systems Molecular Orbitals of Conjugated Ions or Radicals... 

In an electrocyclic reaction, a cyclic system (ring closure) is formed through the formation of a a-bond from an open-chain conjugated polyene system at the cost of a multiple bond and vice versa (ring opening). These reactions are unimolecular in nature as the rate of reactions depends upon the... 

MOLECULAR ORBITALS OF ALKENES AND CONJUGATED POLYENE SYSTEMS... 

In order to understand and explain the results of the various pericychc reactions on the basis of different theoretical models, a basic understanding of the molecular orbitals of the molecules, particularly those of alkenes and conjugated polyene systems and their symmetry properties, is required. 

There are two independent symmetry elements, viz., mirror plane, m, and twofold axis, C2, that are used to characterize various molecular orbitals of alkenes or conjugated polyene systems. 

As shown in Figure 1.13, irradiation of an alkene or conjugated polyene system promotes an electron from its ground state HOMO to the ground state LUMO, which then becomes the highest occupied molecular orbital in the excited state, for example, W3 of butadiene becomes HOMO upon excitation of an electron from W2 to W3 on irradiation. 

While the Hiickel method has now been supplanted by more complete treatments for theoretical analysis of organic reactions, the pictures of the tt orbitals of both linear and cyclic conjugated polyene systems are correct as to symmetry and the relative energy of the orbitals. In many reactions where the 77 system is the... 

Although a number of iron carbonyl complexes have been prepared with cyclic conjugated polyene systems such as cyclooctatetraene, linear conjugated polyene systems have been studied only up to 1,3,5-hexatriene,... 

The prefix retro stated together with two positions of a retinoid structure indicates a shift in the conjugated polyene system. The first position gives the carbon atom from which a proton has been formally transferred to the carbon atom at the second position, e.g., 14-hydroxy-4,14-retroretinoic acid methyl ester (30) and 4,14-retroretinyl acetate (31). 

It is assumed that the hydroperoxyl radical ROO generated by autoxidation of Hpids is not reduced to hydroperoxide as in the case of phenohc antioxidants, but is captured by the conjugated polyene system, represented by the formula 5-28, with the formation of relatively stable fS-carotene radicals, these being stabilised by resonance ... 

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