Stilbene irradiation

Squires, Venier, and Aida (1983) describe an experimental technique they use to study the effect of solvent viscosity on the cisitrans ratio of stilbene irradiated in supercritical CO2. They use a dynamic flow technique similar to that described in chapter 4. In their system trau5-stilbene is coated onto glass beads, which are then packed into a high-pressure column. Supercritical CO2 flows through the column and solubilizes some of the trans-stilbene. The C02-stilbene phase is continuously irradiated with ultraviolet light as it flows through a quartz photoreactor at a fixed temperature and pressure. As the solvent viscosity increases, the photoisomerization of the cis isomer is inhibited while that of the trans isomer is facilitated. We should expect to see the cisitrans ratio of stilbene vary as the density of CO2 varies. This viscosity effect is clearly shown in figure 11.11. While there is a small effect of pressure on the... 

In one of the early works [13], a photocurrent of 10 A was measured in a system containing frans-stilbene irradiated with Hg high-pressure lamp HBO 500 at 20 cm... 

Several fluorinated phenanthrenes and [4]helicenes were prepared by the oxidative photocyclization of fluorinated stilbenes. Irradiation of the compounds (18) and (19) gave 5-fluorobenzo gAz]peiylene (20), but no 2-fluoro[5]helicene. Diederich et al. tried to prepare (P)-[13]belicene by the oxidative photocyclization of an enantiomerically pure (E)-(P,R)-[6]helicene, but tbe attempt was unsuccessful. ... 

As shown in Fig. 13.10, the absorption spectra of Z- and -stilbene differ substantially, with the E isomer absorbing more strongly at A > 280 nm. Thus, the composition of the photostationaiy state can be controlled by altering the wavelength of the irradiation, as indicated by the equation on p. 766. ... 

In addition,<1,48,48) it was noted that whereas the azulene effect on the sensitized reaction is sensitive to changes in solvent viscosity, the azulene effect on the direct photoreaction was independent of solvent viscosity, as would be predicted for Forster-type energy transfer. The inescapable conclusion is that cis-trans isomerization upon direct irradiation of stilbene takes place in the singlet manifold. 

This mechanism is consistent with all of the experimental and theoretical information presented. The fact that no Pms-stilbene fluorescence is observed when riir-stilbene is irradiated would indicate that the following reaction is not important, that is, the twist singlet is much lower in energy than is the trans singlet state ... 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Stilbene-like olefins can be irradiated directly to afford cyclodimers... 

When either the cis or the trans isomer of an alkene is irradiated, a mixture of both isomers will be formed in a particular ratio, which is dependent on the wavelength of light used. For example if either trans-stilbene or cis-stilbene is irradiated at 313 nm, the final composition of the reaction mixture will consist of a mixture of 93% cis and 7% trans (Figure 8.2). 

Figure 8.2 Product of irradiation of either trans-stilbene or cis-stilbene at 313nm...

Cis-stilbene undergoes electrocyclic ring closure on irradiation ... 

The gratings can also be made in situ by holographic irradiation as was demonstrated for low molecular stilbenes in a polystyrene matrix [197]. Here, the spatial modulation of gain dominates over the refractive index modulation in its contribution to optical feedback. The principles of holographic irradiation will be described in Section VIII, which discusses photosensitive materials. 

Irradiation of o-methyl-fraws-stilbene with diethylamine in acetonitrile solution results in the formation of two regioisomeric adducts in a 1 1 ratio, together with comparable... 

Irradiation rraws-2-[3-(7V-methylamino)propyl] stilbene 89 results in the formation of 7V-methyl-l-benzyltetrahydro-2-benzazepine 90 as the only significant primary photoproduct (equation 26), which in turn undergoes secondary photochemical N-demethylation. The final mixture contains 90 (38%) and 91 (25%) at high (>95%) conversion. Intramolecular photoadditions of these (equations 24-26) secondary (aminoalkyl)-stilbenes are highly regioselective processes24. 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

This reaction gives us an opportunity to consider the roles of the salt additive, the solvent polarity, the stilbene concentration, the temperature level, and the intensity of potoirradiation. The reaction is facilitated by the replacement of a nonpolar solvent (benzene) by a polar one (acetonitrile), a rise in reaction temperature, an increase in the stilbene concentration, a decrease in the irradiation intensity, or the addition of alkali metal salts. All of these factors intensifying the process are directly related to the mechanism just described. It is substantial enough to analyze the effects of these factors on the efficiency of the photoreaction. 

An increase in the cA-stilbene concentration favors the chain propagation and decreases the probability of termination when the DCNA anion-radicals react with the stilbene cation-radicals. A decrease in the irradiation intensity has a similar effect The chain propagation is the first-order process, whereas termination of the chains is the second-order process. A temperature rise accelerates the accumulation of the stilbene cation-radicals. In this system, the free energy of electron transfer is -53- —44 kJ moD (the cation-radical generation is in fact an endothermal process). If a polar solvent is substituted for a nonpolar one, the conversion of the cii-stilbene cation-radical into the trani-stilbene cation-radical deepens. Polar solvents break ion pairs, releasing free ion-radicals. The cA-stilbene cation-radicals isomerize more easily on being released. The stilbene cation-radical not shielded with a counterion has a more positive charge, and therefore, becomes stabilized in the... 

The photolysis of donor-acceptor systems provides unique synthetic opportunities. Direct irradiation of the donor-acceptor systems, such as systems containing arene and amine components, leads to intramolecular electron transfer, that is, to amine cation-radical and arene anion-radical moieties. After generation, these moieties undergo cyclization reactions providing efficient synthetic routes to fV-heterocycles with a variety of ring sizes. Thus, direct irradiation of secondary amino-ethyl and aminopropyl stilbenes leads to benzazepines in improved yields (Hintz et al. 1996). As known, benzazepines are used in medicine as antidepressants. Scheme 7.44 illustrates ion-radical cyclization with the formation of benzazepine derivative (65% yield). 

The same catalyst 73 also worked efficiently for catalytic asymmetric ring opening of cyelohexene oxide and meso-stilbene oxide [34] with anilines under mierowave irradiation to afford P-amino alcohols in high yield (up to 95%) and good enantioseleetivities (ee up to 55%). The reaction under mierowave was found to be 10 times faster than traditional oil-bath heating without compromising enantioselectivity. Also the ee for the produet P-amino aleohols was comparable with the values obtained at room temperature (Table 12). 

Saturation of double bonds can also be achieved by catalytic transfer of hydrogen [74, 76], Styrene was quantitatively reduced to ethylbenzene by 1,4-cyclohexadiene in the presence of iodine under ultraviolet irradiation [76], and /ru j-stilbene to 1,2-diphenylethane in 90% yield by refluxing for 46 hours with cyclohexene in the presence of palladium on charcoal [74],... 

The valence tautomerism 251 257 has been proposed to account for the formation of iV-methylthiobenzamide by irradiation (2537 A) in methyl cyanide solution. The valence tautomer 257, R = Me, = Ph, has been detected spectroscopically (p , 2060 cm —N=C=S) either during photolysis at 25° or by heating at 160°. Photochemical oxidative cyclization of 4,5-diaryl derivatives (251, R = R = Ar) analogous to the formation of phenanthrene from stilbene has been reported. Thus, irradiation of the 4,5-diphenyl derivative 251, R = R = Ph, yields the tetracyclic meso-ionic compound (258). ... 

RuCl(dmso)(bpy)2]Cl and [RuCl(SOMePh)(bpy) ]Cl, called respectively rac-Ru-1 and A-Ru-2 by the authors, are made by microwave irradiation of cis-RuCljtbpy) with the the sulfoxide ligands. Trani-stilbene, styrene and R-styrene (R=p-MeO, a and P-methyl) were epoxidised by [RuCl(dmso)(bpy)3]Vaq. Ph(IOAc)2/CH3Cl3/40°C, while with [RuCl(SOMePh)(bpy)3]V TBHP/water-CH Cl fran -stilbene, fran -P-methylstyrene gave the (R,R) epoxides [947]. 

Crowns with Stilbene Fluorophores. Stilbenes undergo cis-trans isomerism on irradiation and this feature is exploited in (3.82). In the case of the frany-isomer, large ionic radius ions such as K+, and Cs, give fluorescent complexes whilst the smaller Lb and Na " cause quenching. UV irradiation to the dy-isomer causes a considerable change in the conformation of the crown ether substructure and increases its ability to extract alkali metals, including Lb and Na+, from water into benzene. 

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