Fluorinated polyenes

By introducing fluorine atoms to the polyenic system of retinal, the geometry, electronic properties, hydrophobicity, and absorption properties of the molecule will be modified. Thus, fluoro derivatives of retinal are useful tools to understand the interactions between retinal and opsin, especially on the level of charge and hydrophobic effects at the protein site. Moreover, fluorine atoms are probes in NMR and allow studies on model molecules of visual pigments Consequently, syntheses of mono-, di-, and trifluoro derivatives of retinal have been the subject of many investigations. 

Another general method for the preparation of 19,19,19- and 20,20,20-trifluor-oretinals involves the aldol condensation of trifluoroacetone with an ethylenic aldehyde. When performed in the presence of acetic acid and piperidine, it affords an a, S-unsaturated ketone. Applied to cyclocitral and its homologues, the method leads to polyenic trifluoromethyl ketones. From these ketones, the various tri-fluorinated retinoids are prepared by known methods (Figure 4.24). ... 

Amphotericin B (AmB) is a polyenic macrolide used in fungal infections and leishmaniasis, despite severe side effects (Figure 4.60). Fluorination of the macrolide skeleton has been performed using Selectfluor for F NMR studies on the mechanism of ion-channel formation in membranes by amphotericin B. ... 

This methodology has been adapted to the preparation of fluorinated polyenes, as... 

Similar coupling occurs with both fluorovinyl7,8 and non-fluorovinyl3,9 iodides, and is particularly useful for the preparation of fluorinated polyenes (Equations 4.4 and 4.5). 

Pristine SWCNTs and their fluorinated derivatives, F-SWCNTs, were reacted with organic peroxides to functionalize their sidewalls covalently by attachment of free radicals (Scheme 1.15). The tubes reactivity towards radical addition was compared with that of corresponding polyaromatic and conjugated polyene JT-systems [150, 151]. The characterization of the functionalized SWCNTs and F-SWCNTs was performed by Raman, FT-IR and UV/Vis/NIR spectroscopy and also by TGA/MS, TGA/FT-IR and with TEM measurements. The solution-phase UV/Vis/NIR spectra showed complete loss of the van Hove absorption band structure, typical of functionalized SWCNTs [150]. 

For reasons of symmetry the resulting acids are identical when a symmetrically substituted alkcne is oxidized. For the preparation of short-chain fluorinated acids, such as trifluoroacetic acid. this method is more economical than the oxidation of an alkene with a terminal C = C bond (see Table 1). Oxidation of polyfluorinated dienes and polyenes gives fluorinated dicar-boxylic acids. 

Influence in extension of chromophore conjugation on TPA was studied for the fluorinated polyenes 1-3. Pentafluorosubstitution reduces re-electron density at the left aromatic benzene ring. This results in an electron-deficient substituted benzene with a re-electron density comparable with that of cyanobenzene [341]. This was concluded from the reduction potentials of 1,2,4,5-tetrafluorobenzene (El = -2.4 V vs. SCE [342]) and cyanobenzene = -2.3 V vs. SCE [341]). The larger re-conjugation in 3 causes an increase of e compared to 1 but the change of A/i()] was rather modest compared to the increased size of the chromophore [224]. TPA data and representative photophysical parameters are summarized in Table 3.1. 

Another method for homologation of conjugated fluorinated polyenes is based on nucleophilic replacement of fluoride from fluoroolefins [7, 18] (Scheme 2.193). 

Scheme 2.193 Homologation of fluorinated polyenes by nucleophilic replacement of fluorine by lithium organyls [7, 19].

Only a few modifications of the polyene chain have been made, including those involving the introduction of saturation (Pawson et aL, 1977) and the replacement of vinylic hydrogen by fluorine (Machleidt and Wessendorf, 1964a,b Machleidt and Strehlke, 1965 Pawson et al., 1979) or the introduction... 

The density of the a-polymorph is 1.98 g cm amorphous PVDF has a density of 1.68 g cm . Thus, commercial samples with a density of 1.75-1.78 g cm have 45% crystaUinity. The a-polymorph melts at 170 °C however, the processed polymer, because of its polymorphism, displays no sharp melting point but melts between 150 and 190 °C. The thermal decomposition becomes significant at T > 300°C. Pyrolysis of PVDF yields hydrogen fluoride, the monomer C2H2F2 and C4F3H3 [12]. Up to 600 °C, pyrolysis also yields polyaromatic structures by cyclization of polyenic intermediates formed through HF ehmination [16]. This is a particular advantage over PTFE, which is less likely to yield carbonaceous products. Thus in obscurant applications, PVDF is preferred over PTFE as a fluorine source (see Chapter 11). 

In 2013, Gagne s group demonstrated catalytic enantioselective polyene cyclization and C3-fluorination for... 

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