Polyenes and Polyacetylenes

These unsaturated compounds are, with few exceptions, active mainly against fungi and especially against yeasts. 

Candida albicans Candida albicans Sachcaromyces Candida albicans Microsporium ATCC 14683 

Silica gel previously treated with phosphate buffer, pH 8. 

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Yanovskaya, L.A., Stepanova, R.N., and Kucherov, V.F., Chemistry of polyene and polyacetylenic compounds. Part 23. Synthesis of some unsymmetrically a.CO-disubstituted polyenes, kv. Akad. Nauk SSSR, Ser. Khim., 1334, 1967 Bull. Acad. Sci. USSR. Div. Chem. Sci. (Engl. Transl.). 1282, 1967. 

Scherman, O.A., Rutenberg, I.M., and Grabbs, R.H. (2003) Direct synthesis of soluble, end-functionabzed polyenes and polyacetylene block copolymers. Journal of the American Chemical Society, 125,8515—8522. 

Due to their structural simplicity, polyenes and polyacetylene have been the focus of many experimental and theoretical studies dealing with the nonlinear optical response of conjugated systems [126-135]. Experimentally, third-harmonic generation (THG) measurements in polyacetylene indicate the appearance of two distinct peaks attributed to two- and three-photon resonances [127, 128]. Calculations carried out on short polyenes by Garito and co-workers in the framework of a Complete Neglect of Differential Overlap/Configuration Interaction (CNDO/CI)... 

When attempts are made to be more quantitatively close to the experimental data the limitations of the quantum chemical methods so far used are immediately and disappointingly perceived. The disagreement between calculated and experimental data even for the simplest polyconjugated systems (e.g. polyenes and Polyacetylene) for basic quantities such as energy gap, band width, ionisation potentials and molecular conformations are remarkable and indicate that the present methods need to be improved to represent satisfactorily the electronic intramolecular phenomena. 

A striking and up to date example which can provide the reader with the general aspects of the problem is that offered by the simplest and most widely studied case of polyenes and polyacetylene discussed in detail in paper I. Other cases are going to be presented in this paper. 

The possibility that the results obtained for short polyenes can be extrapolated to longer polyenes and to polyacetylene has been discussed by several authors21,24,31 37. 

Polyacetylene reacts with chlorine rapidly to give a white polymer that is equivalent to poly(l,2-dichloroethylene) and no stereospecificity has been reported on the chlorinated polymer. The addition of halogens is an important general reaction of carbon-carbon double bonds in a stereospecific and regiospecific sense. For instance, the electrophilic additions of chlorine to ethylene, buta-1,3-diene and hexa-l,3,5-triene have been shown to proceed by 1,2-trans (anti), 1,4-cis (syn) and 1,6-trans (anti) attack, respectively. This stereospecificity has been rationalized with a mixing rule of a-n orbital interaction. The reaction of a long conjugated polyene like polyacetylene with chlorine may produce an atactic chlorinated polyene, because random l,2n additions occur to result in a random addition product. ... 

The electronic structure of trans-PA can be rationalized in the following manner [52]. In the absence of dimerization, i.e., with equal C-C and C=C bond lengths, polyacetylene would be a regular polyene," and the... 

Yet, compared to the other x-systems, the acyclic linear polyenes, and the cyclic oligoenes (i.e., the annulenes), cross-conjugated molecules have lived in the shadows so far. Acyclic linear polyenes are encountered widely in fatty acid, polyketide, and terpenoid natural product domains and the ultimate polyene, polyacetylene is a conductive polymer that was the focus of the 2000 Chemistry Nobel Prize. The annulenes played crucial roles in the development of practical and theoretical organic chemistry (aromatic and antiaromatic compounds), and serve as pivotal bulk chemicals in industry (benzene, toluene, the xylenes, styrene, etc.). 

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 one-dimensional chain of hydrogen atoms is merely a model. Flowever, compounds do exist to which the same kind of considerations are applicable and have been confirmed experimentally. These include polyene chains such as poly acetylene. The p orbitals of the C atoms take the place of the lx functions of the H atoms they form one bonding and one antibonding n band. Due to the Peierls distortion the polyacetylene chain is only stable with alternate short and long C-C bonds, that is, in the sense of the valence bond formula with alternate single and double bonds ... 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

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