Cis-trans isomerization photochemical

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

Fig. 17 Detection of the photochemical cis/trans isomerization of butter yeliow after UV irradiation by using the SRS technique. (A) original chromatogram — treated with hydrochloric acid vapor for better recognition (yellow then turns red) — and (B) schematic representation.

The mechanism for the direct photochemical cis-trans isomerization of stilbene has been a highly controversial subject. However, a recent review by Saltiel and co-workers greatly helps to clarify this area of research by painting a detailed and beautifully consistent picture. We will make extensive reference to this review.U)... 

A photochemical reaction in which the reactant is converted into an isomer. The most common example of such a reaction is photochemical cis-trans isomerization. Another example of photoisomerization is the effect of light on cholesta-3,5-diene. 

The photochemical cis trans isomerization is effected by direct irradiation or in the presence of a sensitiser or a catalyst. 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

Derivatives of indigo undergo photochemical cis-trans isomerization as shown generally in Scheme 13. The heteroatoms X and X can be NR, O, S and Se, but if either is NH the compound is inactive. 

Azetidinones are also formed on photolysis of cis-a-phenyl-cinnamanilide (361), but in addition a small quantity of a cis-trans mixture of 3,4-diphenyl-3,4-dihydroquinolin-2-one (362) was obtained. The yield of quinolinone was considerably increased in the photocyclization of alkyl-substituted acrylanilides.389 The anilide (363) of tiglic acid, for example, was converted into the anilide (364) of angelic acid by photochemical cis-trans isomerism, and into a mixture of cis- and [Pg.103]

Photochemical cis-trans Isomerization from the Singlet Excited State... 

Photochemical cis-trans Isomerization in the Triplet State Tatsuo Arai... 

Figure 9 Two-dimensional model surface for the photochemical cis —> trans isomerization of octatetraene.

J. Saltiel, A. A. Lamola, N. J. Turro, J. S. Bradshaw, D. O. Cowan, R. C. Counsell, V. Vogt, and C. Dalton Mechanism of Photochemical Reactions in Solution. XXII. Photochemical cis-trans Isomerization. J. Amer. chem. Soc. 86, 3197 (1964) and references cited therein. 

Another example is the thermal and photochemical cis-trans isomerization of Cp2Fe2(CO)2( -CO)( -Sip-TolH).25 In this case, both cis(H) and trans isomers can be isolated at full purity by flash chromatography. Interconversion between these isomers occurs both thermally and photochemi-cally in cyclohexane-d12, and the composition in the thermal equilibrium state (cis(H) trans = 2 98 at 25°C) is extremely different from that in the photostationary state (cis(H) trans = 70 30). Kinetics of the thermal isomerization in decalin afforded the activation parameters shown in Eq. (58). The large negative activation entropies imply that this reaction also... 

One very interesting property of arylated TEEs is their ability to undergo reversible, photochemical cis-trans isomerization [21], a property that is not exhibited by the non-arylated derivatives. The trans-to-cis isomerization is of preparative use for the synthesis of cis-TEEs that... 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

Physical details relating to the isomerization of the stilbenes (12) have been determined.A study of the photophysical properties of the styrylstiIbenes (13) has shown that quantum yields for trans-cis isomerism are low from the singlet but high from the triplet state. The photochemical isomerization of the alkene (14) in an ethanol glass affords the trans-isomer with high efficiency even at liquid helium temperatures. Photochemical cis-trans-isomerization of cis-1,2-di-l-naphthylethylene (14) has also been studied in the crystalline phase. A study of the photochemical isomerization of a series of styryl phenanthrenes has been reported. The mechanism of the reaction involved was discussed. ... 

Tung, C.-H., and Guan, J.-Q. (1996). Modification of photochemical reactivity by Nafion. Photocyclization and photochemical cis-trans isomerization of azobenzene. /. Org. Chem. 61, 9417-9421. 

The photochemical cis-trans isomerization of bisphosphine-substituted carbyne complexes [Eq. (57), Section IV,B], which was studied in our group, was postulated to proceed via pentacoordinate metal ketenyl intermediates (97). Support for this postulate was found in the low-temperature characterization of ketenyl complex 208 on irradiation of complexes 207 in the presence of PMe, [Eq. (164)]. In the presence of phenylacetylene the ketenyl alkyne tungsten complex W(/ -PhCCO)CI(PhC2H)(CO)(PMe3)2 forms at low temperatures. 

As indicated in Section 4.3.3, substituent as well as environmental effects on photochemical cis-trans isomerization can be discussed in terms of the dependence of the energy gap and consequently also of the S,— So transition rate on the electronegativity difference d. This is determined by the nature of the atoms on the double bond, by substituents, and by the solvent. If d is... 

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