Other Conjugated Systems

The (Horner-)Wadsworth-Emmons reaction generally is superior to the Wittig reaction, and has found application in many cases for the synthesis of a ,/3-unsaturated esters, a ,/3-unsaturated ketones and other conjugated systems. Yields are often better then with the original Wittig procedure. However the Wadsworth-Emmons method is not suitable for the preparation of alkenes with simple, non-stabilizing alkyl substituents. 

Other conjugated systems, including trienes, enynes, diynes, and so on, have been studied much less but behave similarly. 1,4 Addition to enynes is an important way of making allenes ... 

The saturated hydrocarbons are very susceptible to electron beam damage, both in the monolayer and multilayer forms. While aromatic hydrocarbons and other conjugated systems exhibit minimal or no beam damage effects during the time necessary to carry out the LEED experiments, the ordered structures of paraffins disappear after 5 sec of electron beam exposure as a result of desorption or partial dissociation of the organic adsorbates. 

In principle, pyrylium salts can be transformed to pyrans by the addiiion of an appropriate reagent to the 2-, 4-, or 6-position of the heterocycle. Although most transformations seem to be nucleophilic additions (because of the electrophilic character of the pyrylium substrates) some reactions, such as hydrogenation or one-electron reductions, are of a radical nature. Many nucleophilic additions to pyrylium salts proceeding via unstable pyran intermediates seem to be of general synthetic interest, especially in the fields of aromatic and other conjugated systems, l97 200 but are of little interest in pyran chemistry. 

The dye is positively solvatochromic, the absorption maximum in a polar solvent such as dimethyl sulfoxide being 620 nm, that is, 12 rnn higher than in a nonpolar solvent such as carbon tetrachloride [5],Characteristic of indigo is the unusually deep shade compared with other conjugated systems of similar size [6], This is explicable in terms of the special arrangement of the atoms in the basic... 

One can apply this method to other conjugated systems as well. In this manner, the resonance energy is the difference between the variational energies of the full state and the reference VB structure. As such, the resonance energy itself is variational. Tests of these variational resonance energies show that they reproduce experimentally determined values, for example, for benzene and cyclobutadiene (5). 

Organozinc reagents have been coupled with halides in the synthesis of dienes and other conjugated systems to form new carbon-carbon bonds such reactions have been reviewed477-479. In particular, the reaction of alkynyl zinc reagents with vinyl halides has been used in natural product synthesis, usually catalyzed by palladium complex catalysts. 

Q Explain how to construct the molecular orbitals of butadiene and other conjugated systems. 

Visualizing benzene as a resonance hybrid of two Kekule structures cannot fully explain the unusual stability of the aromatic ring. As we have seen with other conjugated systems, molecular orbital theory provides the key to understanding aromaticity and predicting which compounds will have the stability of an aromatic system. 

Aromatics and other conjugated systems can also be reduced by sodium in liquid ammonia with a trace of alcohol for a proton source. 

As the title of this article suggests, the material covered will deal with the electronic structure of conductive conjugated polymers in relation to their physicochemical properties. This subject is so broad that the theme is confined herein to polyacetylene and other conjugated systems having aromatic rings. The research in this area, however, is rapidly developing and, on reflection, yielding fruitful results that have wide applications and ramifications in quantum chemistry. 

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