Photosensitized isomerizations

Vkiyl aHenes (44) are rearranged with heat or metal catalysis and photosensitized isomerization to produce the vitamin D triene (156—160). 

Fig. 13.12. Energy diagram illustrating differential in energy deficit for photosensitized isomerization of cis and trans isomers.

Fujii, R. et al., Cis-to-trans isomerization of spheroidene in the triplet state as detected by time-resolved absorption spectroscopy, J. Phys. Chem. A, 106, 2410, 2002. Montenegro, M.A. et al., Model studies on the photosensitized isomerization of bixin, J. Agric. Food Chem., 52, 367, 2004. 

Figure 9.3. Variation of the stationary states in photosensitized isomerization of stilbenes with various sensitizers. [From G. S. Hammond, Kayaku to Kdgyd (Tokyo) 18, 1464 (1965).]...

For simple alkenes, Saltiel has proposed that the deviation of the decay ratio from unity is a measure of the involvement of Schenck type intermediates (22) in the cis-trans photosensitized isomerization. Table 9.4 gives pss concentration ratios for the 2-pentenes at 30°C ... 

Quadricyclane may be prepared by direct irradiation of bi-cyclop.2. l]hepta-2,5-diene3 and 2,3-diazatetracyclo[4. 3.0.0.4 8-07,9]non-2-ene6 or by photosensitized isomerization of bicyclo-... 

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)...

The photosensitized isomerization of ethyler.es and stilbenes has been studied in detail by Hammond and his group and it has been found that in each case both the cis-trans and the trans-cis isomerizations occur with equal efficiency. A common (tc, jt ) triplet state is assumed to be the intermediate in each case. This triplet state is referred to as the perpendicular triplet state and was termed a phantom triplet state by Hammond. The energy of the perpendicular triplet state is expected to be low since in this configuration the overlap between the re and rc orbitals is minimized. The potential energy of S0, Tx and Sx states as a function of twist angle is given in Figure 7.7... 

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

The first report of triplet photochemistry of three-membered rings concerned the reversibility of the photosensitized isomerization of norbornadiene to quadricyclane.270 Since no spectroscopic triplet levels of cyclopropane are known, it was felt that this reaction represents an extreme sort of nonvertical energy transfer with actual bond breaking in the acceptor. 

Bell has reported that the benzene-photosensitized isomerization of m-1,2-dimethylcyclopropane is slightly endothermic.301 The products are those which are formed by addition of triplet methylene to m-2-butene, leading Bell to suggest that the species formed by energy... 

The photoisomerization of stilbene is one of the most extensively studied photoreactions (25). Solvent effects have been thoroughly investigated for both the direct and photosensitized isomerizations, and a model has been developed which attributes these effects to solvent viscosity (26). Increased viscosity inhibits direct photoisomerization of the cis isomer, but facilitates that of trans-stilbene. As a result, the cis/trans ratio of the photostationary state increases with increasing solvent viscosity. The wide range of viscosities which are attainable by pressure manipulation of supercritical carbon dioxide provides an excellent opportunity to probe the effect of viscosity on stilbene photochemistry in the same solvent. 

Fischer, G., and E. Fischer Photosensitized Isomerization of Azobenzene. To be published. 

The chemical structures of [2]catenane 19 and the related [3]catenane 20 (Fig. 8) were conceived as an extension of their work on molecular shuttles. The larger macrocycle in 19 comprises two fumaramide stations with differing macro cycle binding affinities. In station B (red) the methyl groups on the fumaramide motif cause it to have lower affinity than the standard fumaramide station. The non-methylated fumaramide station (station A, green) is located next to a benzophenone unit. This allows selective, photosensitized isomerization of station A by irradiation at 350 nm. Station B (red) can be photoisomerized by direct irradiation at 254 nm. The third station, a succinic amide ester (station C, orange), is not photoactive and is intermediate in macrocycle binding affinity between the two fu-... 

A convenient photochemical procedure for obtaining ( )-cycloalkenes and the products derived therefrom is the photosensitized isomerization of (Z)-isomers, where more conventional instrumentations, such as low/high pressure mercury lamps and quartz/Vycor/Pyrex vessels, can be used. 

E, evaluated from that of the adduct 88 is higher than that of (Fj-cydo-octene 38E obtained under the comparable conditions, and is sensitive to the solvent polarity and reaction temperature. Upon sensitization with tetra-(—)-bornyl 1,2,4,5-benzenetetracarboxylate in hexane at — 80 °C, the ee of 36E goes up to 77% ee, which is the highest value reported for the enantiodifferentiating photosensitized isomerization to-date. 

Direct irradiation at 254 nm of 1-phenylpropene 93a in cyclohexane affords a Z/E mixture in a ratio of 65 35 [223], whilst photosensitized isomerization of 93a gives (Z/E)pss ratios of 55 45 and 88 12 upon benzophenone and chrysene sensitization, respectively (Sch. 37). Similarly, a variety of P-alkylstyrenes has been subjected to triplet sensitization [224,225]. Upon direct irradiation of ( )-P-bromo-P-nitro-styrenes, the (Z)-isomers are obtained in 40-50% yield, but the major... 

Photosensitized isomerization of fluoranthenylalkenes 94 has been studied in search of a one-way isomerization system [228-231]. 

Since so (trans) < 0.005], it is possible to rule out a photosensitized isomerization mechanism involving the establishment of thermal equilibrium between 11-cis and all-trans forms during the lifetime of the triplet state. Also excluded is the quantitative population of a common minimum along the C q-C 2 torsional coordinate. This conclusion is consistent with recent studies (169,175) which, in variance with the previous investigation (171), have reported different T-T spectra upon excitation of all-trans and 11-cis retinal. 

Mechanism (3) is consistent with the observation of two distinct T-T spectra for 11-cis and all-trans retinal (169,175). An additional argument against a "common triplet" or "equilibrated triplets" (171,182) is based on the observation (177,183) that the photosensitized isomerization of different retinal isomers (e.g., all-trans, 11-cis, and 13-cis) leads in each case to a different product distribution (13-cis, trans/9,13 dicis, and... 

The phosphorescence and the ratio phosphorescence/fluorescence depend on the exciting wavelength, indicating the contribution of step (4). (The pressure dependence of this step is uncertain.) Step (5) is supported by the results of the investigations on the acetone-photosensitized isomerization of 2-butene. The relative importance of the two steps is likely to be strongly dependent on experimental conditions. Dissipation of the vibrational energy of the triplet molecule occurs by... 

This provides a particularly comprehensive study of 5 positional isomers. The solvent dependence of the excited state reactivity of 1 -styrylisoquinoline has been examined by the direct and photosensitized isomerization of both the neutral and protonated forms24t. Other related work is on the kinetics of cis-... 

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