Conjugation, extended

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

There are two types of bleachiag oxidative and reductive. Oxidative bleaching cleaves carbon—carbon double bonds thus destroyiag chromophores. Reductive bleaching reduces carbon—carbon double bonds to single bonds. The loss of extended conjugation ia the chromophore leads to loss of color. 

The analogous structures 309, 310, and 311 have been suggested for the pyrophthalones. Early work favored structure 309 (see reference 385 and references therein). Later, ultraviolet spectral data showed the presence of an extended conjugated system, and comparison with the spectra of the A -methyl compounds suggested structure 310. ... 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

It would be expected that the stabilization of the adsorbed species by an extended conjugated system should increase with the number of aromatic rings in the adsorbed azahydrocarbon. However, data suitable for comparison are available only for phenanthridine, benzo-[/]quinoline, and benzo[h] quinoline. The large difference in the yields of biaryl obtained from the last two bases could be caused by steric interaction of the 7,8-benz-ring with the catalyst, which would lower the concentration of the adsorbed species relative to that with benzo[/]quinoline. The failure of phenanthridine to yield any biaryl is also noteworthy since some 5,6-dihydrophenanthridine was formed. This suggests that adsorption on the catalyst via the nitrogen atom is possible, but that steric inhibition to the combination of the activated species is involved. The same effect could be responsible for the exclusive formation of 5,5 -disubstituted 2,2 -dipyridines from 3-substi-tuted pyridines, as well as for the low yields of 3,3, 5,5 -tetramethyl-2,2 -bipyridines obtained from 3,5-lutidine and of 3,3 -dimethyl-2,2 -... 

Azo-coupled products are widely used as dyes for textiles because their extended conjugated tt electron system causes them to absorb in the visible region of the electromagnetic spectrum (Section 14.9). / -(Dimethylamino)azobenzene, for instance, is a bright yellow compound that was at one time used as a coloring agent in margarine. 

In view of the limited capacity of the sulfur atom in the sulfoxide and sulfone functional groups to transmit conjugative effects due to the insulating effect of the LUMO sulfur d-orbitals45,46,56, the application of the UV technique even in the case of the cyclic vinyl sulfones (e.g. thiete dioxides 6b) cannot be expected to find extensive use. UV spectra of substituted thiete dioxides in which an extended conjugated system (e.g. 194) exists in the molecule, did provide useful information for structure elucidation231. However, the extent... 

Polymers for these conductive systems may be synthesised by a variety of means including Ziegler-Natta polymerisation or nucleophilic displacement reactions. The molecules tend to be rigid because of the need for them to possess extended conjugation. This lack of free rotation about carbon-carbon bonds within the molecule imposes a high energy barrier to solvation, thus making these molecules difficult to dissolve. This lack of solubility in turn... 

The line structures of three biologically important molecules that contain extended conjugated systems. The conjugated systems are highlighted with color screens. 

There have been many applications of the Wittig reaction in multistep syntheses. The reaction can be used to prepare extended conjugated systems, such as crocetin dimethyl ester, which has seven conjugated double bonds. In this case, two cycles of Wittig reactions using stabilized ylides provided the seven double bonds. Note the use of a conjugated stabilized ylide in the second step.250... 

The use of the term leuco dye is a common paradox. Leuco color formers are materials that undergo controlled chemical or physical changes resulting in a shift from a colorless state to an intense color. The preparation of leuco color formers takes advantage of the very nature of colored materials themselves. The existence of extended conjugated -system in dyes is responsible for the absorption in the visible region. The chemistry of such rc-system is noted for facile reactivity, particularly to reactions such as reduction, oxidation, and hydrolysis (not hydrolytic cleavage). When n-... 

---