Polyenes bonds

Treatment of acetylated or tetrahydropyranylated -hydroxysulfones 273 with t-BuOK resulted in the formation of acetylenic or polyenic bonds -. ... 

Dewar s definition of RE is based on the possibility of approximating the X electron energy of polyenes by summing up all polyene bond enetipes,... 

TABLE 1. Polyene bond lengths and angles. Carbon atoms are numbered sequentially starting with the outer carbon of the double bond closest to the chain end. 

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

Before we start with a systematic discussion of the syntheses of difunctional molecules, we have to point out a formal difficulty. A carbonmultiple bond is, of course, considered as one functional group. With these groups, however, it is not clear, which of the two carbon atoms has to be named as the functional one. A 1,3-diene, for example, could be considered as a 1,2-, 1,3-, or 1,4-difunctional compound. An a, -unsaturated ketone has a 1.2- as well as a 1,3-difunctional structure. We adhere to useful, although arbitrary conventions. Dienes and polyenes are separated out as a special case. a, -Unsaturated alcohols, ketones, etc. are considered as 1,3-difunctional. We call a carbon compound 1,2-difunctional only, if two neighbouring carbon atoms bear hetero atoms. 

The charge distribution may be symmetrical when the system possesses similar nitrogen atoms and identical or unsymmetrical nuclei approaching more or less a polyene-like bond state when the nuclei are of different... 

As noted earlier planar annulenes with 4n tt electrons are antiaromatic A mem ber of this group [16]annulene has been prepared It is nonplanar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

The perturbation of the PMD symmetry is accompanied by a decrease in the charge alternation and by the appearance of bond alternation from one end group to another. The bond alternation ampHtude has been revealed to be proportional to the asymmetry degree, which can be calculated as the difference of topological indexes = 4>gj — 4>gg. The effect is maximum if Tgj > 45° and Tgg < 45°. If A4>j2 = 90°, the ideal polyene state is... 

Studies show polymethine chain lengthening in highly asymmetrical dyes to be accompanied by strong quadratic increases in deviations (3,7,9,10,30,31). In contrast to polymethines, the deviations of the related asymmetrical polyenes are negative as the break of the symmetry leads to a decrease in bond alternation (32). 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

Natamycin. Natamycin or pimaricin (2) is a polyene with only four conjugated double bonds. This tetraene is obtained from Streptomjces natalensis. Like the other polyenes, pimaricin induces release from cells with membranes containing 20—40 mol % ergosterol. This is also the ergosterol concentration in the membrane of Saccharomjces cerevisiae. 

Since highly basic heterocycles accommodate the positive charge readily and maintain the aromatic nature of the dimethylarnino benzene group, the resulting bond alternation induces a polyene character to the dye chromophore, and the absorption is shifted accordingly to a shorter wavelength. 

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

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