Isomerization photostationary state

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

Stilbene is known to undergo cis-trans isomerization in its triplet state to yield a photostationary state which is 60% cis and 40% irons,... 

The photostationary state composition for the benzophenone-sensitized isomerization of 2,4-hexadienes is given in Table 9.2. Table 9.3 gives the measured quantum yields for benzophenone-sensitized isomerization of 2,4-hexadienes along with the calculated quantum yields based on Eqs. (9.47)-(9.49) and the pss values given in Table 9.2. 

Similar isomerizations of clopenthixol (354) and chlorprothixene (355) were observed but, unlike flupenthixol (353), the photostationary states of these mixtures differed significantly in their isomer ratios from those of the drugs as normally supplied. It was pointed out that the photoisomerizations could affect their potencies [195]. 

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

When thioamides 24f-h were irradiated in benzene with a high-pressure mercury lamp, iV-isopropyl-P-thiolactam 26 and 1,3,5-dithiazine 28 were obtained (Scheme 15 and Table 10, entries 1,3, and 5). Considerably different photochemical behavior was observed between that in solution and in the solid state. Powdered thioamide 24f was irradiated in the solid state at 0 °C until 19% conversion, because the solid changed to amorphous at around 20% conversion. In this case, only dithiazine 28f was obtained as the sole photoproduct (entry 2). On the contrary, photolysis of 24g gave a new type of p-lactam 27g as a main product in 88% yield in addition to dithiazine 28g (12%) the P-lactam 26g was not detected at all (entry 4). Photochemical ( ,Z) isomerization of (Z)-24g was also observed in the early stage of the reaction, where the ratio of the photostationary state was ZIE=1.9. In the case of 24h, P-thiolactam 27h was ob-... 

In a photosensitized reaction, radical induced isomerization can occur if the sensitizer undergoes either homolytic decomposition or hydrogen abstraction, or if the system contains impurities which give radicals on irradiation. The result may be to shift the measured photostationary state in the direction of thermodynamic equilibrium and to give anomalously high values of the quantum yields for cis-trans isomerization. 

In the initial work135,136 on the sensitized isomerization of stilbene, it was found that quinone sensitizers gave photostationary states which were trans-rich relative to those predicted by energy transfer (Fig. 3). The original plot of sensitizer triplet energy versus isomer ratio of the photostationary state therefore showed several maxima and minima. Later work on the rates of energy transfer from various sensitizers to stilbene proved that the plot was in fact a smooth curve and that the photostationary states observed with quinones were not the true ones.137 Irradiation of benzene solutions of... 

Fig. 3. Photostationary states obtained in the photosensitized isomerization of stilbene as a function of triplet energy of the sensitizer. (From Wagner and Hammond.1)...

All the olefins involved in these studies are simple olefins to which energy transfer from ketones should be endothermic. Although the details of the mechanism of isomerization are still a matter of some debate,5,71 it is generally agreed that isomerization takes place by addition of the sensitizer to the olefin to form a new intermediate which may be of a biradical nature. Thus if energy transfer is not favored, there is another mechanism by which ketone sensitizers can induce olefin isomerization, and the observed quantum yields and photostationary states may differ sharply from those predicted by the energy transfer mechanism. 

A particularly interesting case of transfer from T2 has been noted238 in the 9,10-dichloroanthracene (DCA) sensitized isomerization of stilbene. As shown below, the DCA ground state which results from quenching of T2 by stilbene (St) can act as a quencher of stilbene triplets and thereby alter the stilbene cis. trans photostationary state. 

In an attempt to probe further into the complex photochemical system involved, the isomerization of stilbenes adsorbed on silica gel was examined.9 It was observed that the time required for establishment of the photostationary state was significantly increased (by a factor of 3) and that the composition of the photostationary state changed from 93y cis isomer in cyclohexane solution to 60% cis isomer in the silica gel matrix. Though not definitive, this evidence supports Fischer s triplet mechanism, 53 as we have previously reported.9... 

The reaction sequence of Eq. 23-37 can be slowed by lowering the temperature. Thus, at 70K illumination of rhodopsin leads to a photostationary state in which only rhodopsin, bathorhodopsin, and a third form, isorhodopsin, are present in a constant ratio.510 Isorhodopsin (maximum absorption at 483 nm)513 contains 9-ds-retinal and is not on the pathway of Eq. 23-37. Resonance Raman spectroscopy at low temperature supports a distorted all-frans structure for the retinal Schiff base in bathorhodopsin.510 The same technique suggests the trans geometry of the C = N bond shown in Eqs. 23-38 and 23-39. Simple Schiff bases of 11-cz s-retinal undergo isomerization just as rapidly as does rhodopsin.514... 

Many studies of benzene-sensitized isomerizations have been reported. One interesting example is provided by the behavior of cis,trans,trans-1,5,9-cyclododecatriene. With benzene photosensitization it is isomerized to the M-trans and the cis,cis,trans isomers, and a new, probably bicyclic, product is also formed in small yield.274 Another study employed a large number of sensitizers, many of which apparently reacted with the triene, and no particular trend was evident other than that the original isomer remained as the major geometric isomer in the photostationary state, with varying amounts of the all-trans and cis,cis,trans isomers being formed with different sensitizers.275... 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

More important, all of the 4 -substituted trans-9-styrylanthracenes 69b-k undergo photochemical trans- cis isomerization. The effect of solvent polarity is obvious from the greatly enhanced quantum yields of trans-+cis isomerization observed for most substituted 9-styrylanthracenes in acetonitrile solution. In some cases, the cis-isomers are actually favored at the photostationary state (see Table 10). 

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