Sensitized Photoisomerization

Dye-Sensitized Photoisomerization. One technological appHcation of photoisomerization is in the synthesis of vitamin A. In a mixture of vitamin A acetate (all-trans stmcture) and the 11-cis isomer (23), sensitized photoisomerization of the 11-cis to the all-trans molecule occurs using zinc tetraphenylporphyrin, chlorophyU, hematoporphyrin, rose bengal, or erythrosin as sensitizers (73). Another photoisomerization is reported to be responsible for dye laser mode-locking (74). In this example, one metastable isomer of an oxadicarbocyanine dye was formed during flashlamp excitation, and it was the isomer that exhibited mode-locking characteristics. 

The sensitized photoisomerization of alkenes is complex, with a variety of sensitizer-olefin interactions operative.(78 83)... 

An efficient synthetic route to (10Z)- and (10 )-19-lluoro-la,25-dihydroxy vitamin D3 has been developed (488). The key feature of this pathway is the introduction of a 19-fluoromethylene group to a (5 )-19-nor-10-oxo-vitamin D derivative. The 10-oxo compound 445 has been obtained via a 1,3-dipolar cycloaddition reaction of (5 )-la,25-dihydroxyvitamin D with in situ generated nitrile oxide, followed by ring cleavage of the formed isoxazoline moiety with molybdenum hexacarbonyl. Conversion of the keto group of (5 )-19-nor-10-oxo-vitamin D to the E and Z fluoromethylene group has been achieved via a two-step sequence, involving a reaction of lithiofluoromethyl phenyl sulfone, followed by the reductive de-sulfonylation of the u-lluoro-j3-hydroxysulfone. The dye-sensitized photoisomerization of the (5 )-19-fluorovitamin D affords the desired (5Z)-19-fluorovitamin D derivatives, (10Z)- and (10 )-19-fluoro-la,25-dihydroxy-vitamin D3. 

They also reported the sensitized photoisomerization of the cis[10.10] precursor 73 to a 50 50 mixture of (Z) and ( ) isomers, and this mixture was said to be converted to virtually pure ( )[10.10] isomer on acid treatment. They attributed this facile isomerization to the thermodynamical stability of the ( ) isomers over the (Z) isomers. However, their third short communication 55c) showed that the course of this acidic isomerization had not followed the route reported in their preceding paper. 

The products obtained upon direct excitation of butadiene,291 norbornadiene,292 cyclohexadiene,293 and other dienes294 are also completely different from those obtained by photosensitization, again demonstrating quite different chemical reactivity of excited singlet and triplet dienes. However, 1,l -bicyclohexenyl295 and 1,3-cycloocta-diene298 represent exceptions, undergoing both direct and sensitized photoisomerizations to cyclobutene derivatives. 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

The study of the direct irradiation and triplet sensitized isomerization of styryl-ferrocene, reaction (46), is noteworthy.134) For either direct or benzophenone sensitized photoisomerization the photostationary state is exclusively the trans isomer. 

The enantiodifferentiating photoisomerization of (Z )-cycloheptene 36Z was also studied in some detail [163]. Low temperature irradiations of 36Z in the presence of chiral benzenepolycarboxylates as sensitizers give the labile (ii)-isomer 36E, which is subsequently trapped in the dark by either 1,3-diphenylisobenzofuran or osmium tetraoxide to afford the adduct 88 or the diol 89, respectively (Sch. 33). The observed photostationary-state EfZ ratios of 0.1 are comparable or slightly lower than that obtained for the singlet-sensitized photoisomerization of (Z)-cyclo-octene 38Z. The ee of... 

Direct and sensitized photoisomerizations of 1,2-diphenylpropene 92 have also been examined (Sch. 36). The (Z/E)pss ratio upon sensitization in benzene varies critically with the sensitizer employed, thus giving 54 46 for acetophenone, 17 83 for eosin, 89 11 for fluorenone and 90 10 for duroquinone [222]. 

In the photoisomerization of l-(2-anthryl)-2-phenylethene [232-234] and l-(2-naphthyl)-2-phenylethene [235-237] 95, the s-cis/s-trans conformational isomerism plays an important role. (Z)-l-(2-Naphthyl)-2-phenyl-ethene is isolated in 70% yield in fluorenone-sensitized isomerization of the ( )-isomer (Sch. 39) [238,239]. The photochemical behavior of related (E)-stilbenoids (96-98) has been reported (Sch. 40) [240]. The direct and sensitized photoisomerizations of (Z)-styrylferrocene in benzene have also been reported [241]. 

For general synthetic purposes, any of the direct and sensitized photoisomerization routes can be used in the preparation of (E )-stilbenes from the corresponding (Z)-isomers. However, if really pure (Z)-stilbene 90aZ is required, fluorenone sensitization is highly recommended. Otherwise, trace... 

The addition takes place exclusively in a cis fashion when silylated ynamines are used 42, l37) to give 58a having maleic acid configuration. The isomers with fumaric acid configurations are formed in 40-50% conversions upon sensitized photoisomerization in benzene solutions, 37). Methyl propiolate behaves in a like manner 42). 

The 1,3-cycloadduct 22 which is isolated (25% yield) together with the expected l,2-bis(trifluoromethyl)cyclo-octatetraene (40%) from the photoreaction of benzene and perfluorobut-2-yne is a valence isomer of the normal 1,3-adduct 23 and may arise by a simple isomerization from it, although it is thought more likely that product 22 arises by sensitized photoisomerization of the 1,4-cycloadduct (24). 

From the quantum yields of the direct cis-trans photoisomerization of 2,4-hex-adienes it is seen that a one-step conversion of the trans-trans isomer into the cis-cis isomer or vice versa does not occur. A common intermediate can therefore be excluded, and at least two different intermediates have to be assumed as indicated in Scheme 1 (Saltiel et al., 1970). In contrast to this singlet reaction with very fast return to S, the triplet reaction is characterized by intermediate lifetimes long enough to allow for interconversion. The result of the benzo-phenone-sensitized photoisomerization of 2,4-hexadienes is therefore in agreement with a common triplet intermediate for the isomerization of both double bonds (Saltiel et al., 1969). 

Methylsulfanyl-4-azatricyclo[3.3.2.0 ]deca-3,6,9-triene (17) was prepared from lactam 11 and phosphorus pentasulfide in pyridine which gave the thioamide 15 (68%), which on reaction with trimethyloxonium tetrafluoroborate gave the methylsulfanyl iminoether 16 (71%). Acetone sensitized photoisomerization of 16 produced 3-methylsulfanyl-4-azatricyclo-[3.3.2.0 ]deca-3,6,9-triene (17) in 93% yield when the photolysis of 16 was not carried out under conditions of triplet sensitization, only polymeric dark material was produced. ... 

Fig. 12.15. Schematic potential energy diagram illiistrating differential in energy reqnired for triplet-sensitized photoisomerization of cis and trans isomers.

Both direct and triplet-sensitized photoisomerization of pyridylhydrazones has been reported, and a yn-ontZ-photoisomerization of the carbon-nitrogen double bond has been shown to be responsible for the photochromism observed... 

---