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Stilbene, photochemical isomerization

The photochemical isomerization of E-stilbenes has been applied in the preparation of phenanthrenes, as Z-stilbenes undergo electrocyclie ring closure (cf. chapter 3.1.3) to dihydrophenanthrenes which in turn are easily oxidized to phenanthrenes (3.1) 305). This sequence has also been employed in the synthesis of benzoquinolines 306) or of benzoquinolizines (3.2) 307). [Pg.33]

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. ... [Pg.249]

This chapter deals with the properties of the excited states of 1,2-diary lethylenes in general, with the effects caused by specific variation of reaction conditions on photochemical cis trans isomerization in condensed phase, and with the reaction mechanisms. In particular, the influence of substitution on the properties of excited states involved in cis trans isomerization in solution is examined. Besides various substituted stilbenes (which have been most extensively studied), styrylpyridines (StPs, azastilbenes) including some of their positively charged derivatives (quaternary stilbazolium salts), dipyridylethylenes (DPEs), styrylnaphthalenes (StNs), their pyridine analogues (NPEs), and some related compounds, such as dinaphthylethylenes (DNEs), are surveyed. Results on photochemical cis ttrans isomerization of stilbenes and other 1,2-diarylethylenes under direct (Section II) and sensitized (Section III) irradiation conditions are summarized, as well as their photophysical excited singlet and triplet state properties (Section IV) and some selected side reactions (Section V). The mechanistic section (Section VI) describes several photochemical isomerization routes. Characteristic photophysical and photochemical aspects of specific classes of substituted stilbenes are discussed and mechanistic schemes are critically examined with reference to their experimental basis. [Pg.4]

In most cases studied, the cis isomer predominates in the photoequilibrium. There exist, however, some cases in the series of sterically hindered stilbenes where the cis isomers are thermodynamically the more stable isomers. For example, oc,/ -dichloro- and a-carboxystilbene contain larger amounts of cis than of trans isomers [12, 53, 55, 56] and only the cis isomer is known for a-nitrostilbene [58], Maciaszek and co-workers [58] were also interested in the photochemical preparation of the less stable isomer of several stilbenes bearing bulky substituents in the a-position Photochemical isomerization was found impracticable for cis-a-nitro- and ot-CONH2-4,4 -dimethylstilbene. With a-carboxy- and a-methoxycarbonyl-4,4 -dimethylstil-bene in benzene, however, a considerable amount of less stable trans isomer has been observed under steady state conditions. [Pg.13]

Photoisomerization of Stilbene Especially detailed study of the mechanism of photochemical configurational isomerism has been done on Z- and E-stilbene. The isomerization involves a twisted singlet state that can be attained from either the Z- or the E-isomer. Spectroscopic data have established the energies of the singlet and triplet states of both Z- and E-stilbene and of the twisted excited states that are formed from both isomers. Some features of the potential energy diagram are due to structural... [Pg.1085]

The interpretation of photochemical reactions is more complicated if they have to be followed spectroscopically. An example is given in Figure 36, which illustrates the photochemical isomerization of frans-stilbene to n. 5-stilbene followed... [Pg.452]

Ramamurthy and Jayaraman employed the cavity of poly(aIkyl aryl ether) dendrimers of the third generation with various n-alkyl linker (ethyl to n-pentyl) connecting the branch points (C2G3-C5G3) for photochemical isomerizations of azobenzene and stilbene. Isomerization was carried out in aqueous dendrimer solutions. Irradiation of solutions for 6 h furnished products. The results of isomerization of stilbene in the presence of various dendrimer solutions and in... [Pg.174]

Natarajan B, et al. Dynamic internal cavities of dendrimers as constrained media a study of photochemical isomerizations of stilbene and azobenzene using poly(alkyl aryl ether) dendrimers. J Org Chem 2012 77 2219-24. [Pg.199]

Shinmyozu and co-workers prepared photo-switchable chiral receptor trans- Q9 by linking two chiral 2,2 -dihydroxy-l,l -binapthyl (BINOL) groups with a stiff-stilbene core [172]. This compound could be photochemically isomerized to cis-109. The tmns- Q9 binds fluoride or chloride ions to form 1 1 complexes with... [Pg.180]

The formulation of the mechanism of the photochemical isomerization has relied heavily on spectroscopic data. Notably, isomerization in the S states has been conclusively ruled out by the fact that whereas strong fluorescence is observed from fmns-stilbene only a weak structureless fluorescence is observed from the cis isomer (Lewis et al., 1940 Dyck and McClure, 1962 Aurich et al., 1964 Lamola et al., 1964). Furthermore, analysis of the vibrational structure of the absorption spectra of the two stilbenes led to the conclusion that, contrary to expectation based on LCAO-MO calculations, the stretching frequency, and henceforth the bond strength, of the central bond is practically unaltered upon excitation of either isomer to its lowest excited singlet state (Dyck and McClure, 1962). [Pg.257]

The ground-state synthesis of imines, oximes, and azo compounds invariably produces the more stable -isomers and photochemical isomerization is practically the only means for preparing their less-stable Z-isomers, as in the case of olefins such as stilbene. For example, the pharmacologically active Z-isomer of isonicotinaldehyde was prepared by photoisomerization (Scheme 50)." A less stable E-isomer of methyl p-nitrophenylhydroximate O-methyl ether can be isolated in 38% yield by UV irradiation of the Z-isomer in benzene, followed by preparative TLC (Scheme 51)." ... [Pg.1926]

Especially detailed study of the mechanism of photochemical configurational isomerism has been done on Z- and A-stilbene." Spectroscopic data have established... [Pg.766]

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... [Pg.319]

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)... [Pg.191]

Up to about 10 percent of crs-stilbene was obtained when trimethyl-dioxetane 129 was decomposed in the presence of trans-stilbene 182) the electronic excitation energy of the excited carbonyl compounds formed in the cleavage of 129 (see Section V.) was transferred to trans-stilbene, so effecting the photochemical trans-cis isomerization. When bis (2.4-dinitrophenyl) oxalate reacted with hydrogen peroxide (see Section V. C. in the presence of o-tolyl-propane-1.2-dione 130, 2-methyl-2-... [Pg.129]

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... [Pg.331]

In the presence of an oxidizing species (oxygen, iodine, tetracyanoethylene and others) DHP is dehydrogenated into phenanthrene. This oxidation can also be thermal or photochemical. In general, meta substituted stilbenes give rise to two isomeric substituted phenanthrenes ortho-substituted stilbenes can lose the substituent on cyclization. [Pg.67]


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See also in sourсe #XX -- [ Pg.1384 , Pg.1385 ]




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