Stilbene trans isomeric forms

Most of the olefins shown so far, for which reversibility of the bromonium ion formation had been demonstrated, are particular olefins, in which either steric bulk impedes the product forming step, or ring strain in the dibromide product retards this step. In order to check the general occurrence of the reversibility during the bromination reaction, a further approach, based on the cis-trans isomerization of stilbene derivatives during the bromination of the cis isomers, was devised. 

The investigated cw-stilbene derivatives, 4-methoxy, 4,4 -dimethyl, unsubstituted, and 4,4 -bis(trifluoromethyl)stilbenes, had k2 values spanning 6-7 powers of ten both in methanol and in acetic acid. Products 2, 4, 5 and 6 were formed. Table 8 reports the results of the cis-trans isomerization test in acetic acid (ref. 29). No acid catalyzed or free radical process was found to be responsible for these isomerizations. 

It is seen that the cation-radical of stilbene, but not stilbene itself, is snbjected to acetoxylation. Stilbene in trans form yields the trans form of the cation-radical, which nndergoes farther reaction directly. Stilbene in cis form gives the cation-radical with cis strnctnre. The cis cation-radical at first acquires the trans configuration and only after this adds the acetate ion. It is the isomerization that causes the observed retardation of the total reaction. It is the absence of adsorption at the electrode surface that allows the nonacetoxylated part of di-stilbene to isomerize and tnrn into the... 

In simple olefins of the stilbene type, cis —> trans isomerization in the cation-radical state is known to proceed readily because the exocyclic double bond is weakened. In the case of the 1,2-di(aryloxy)cylobutane cation-radical, cis —> trans isomerization also takes place (see Evans et al. 1975), but the reaction inevitably should include the open form of the distonic type, ArOCH CH2— CH2CH+OAr. Free rotation in this distonic cation-radical around CH2—CH2 bond permits ensuing ring closure with the formation of the more stable trans isomer. 

Stilbenes and associated molecules provide very good examples of the formation of intermediate unstable isomers which give a chemical route for internal conversion. Upon irradiation, stilbenes undergo a cis-trans isomerization as the predominant reaction. However, under oxidative conditions phenanthrene is also formed.12 It was shown that the phenanthrene came only from c/s-stilbene (13),61 and that an intermediate unstable isomer, nms-dihydrophenanthrene (14), was the precursor of the phenanthrene.62-64 The dihydrophenarithrene was in its ground state, but vibrationally excited, and was formed by a process calculated to be endothermic by 33 10 kcal/mole-1.02 Oxygen or other oxidants converted it to phenanthrene (15), but in the absence of oxidants it was either collisionally stabilized or reverted to m-stilbene. 

Coulometric generation of the reactants in bulk provides a known number of redox events plus a quantity of ds-stilbene that can be determined chromato-graphically. Since the cis-trans isomerization yields are known from photochemical studies, one can calculate the efficiency with which 3F1 is formed. Triplet yields ranging from 2% to 80% have been recorded for various systems in this way [86,87]. 

Configuration of cis-trans Isomeric Ozonides. In symmetrically di-substituted trioxolanes, the cis isomer is a meso and the trans isomer is a racemic form. Therefore, resolution into antipodes makes it possible to determine the configuration. In 1966, Loan, Murray, and Story (4) were able to obtain an optically active form of one of the diisopropyl-trioxolanes, proving it to be the trans isomer. Their method—partial decomposition of the ozonide by brucine—could not be applied to the stilbene ozonides. 

One of the most relevant and extensively studied stilbene is trani-resveratrol (3,5,4 -trihydroxystilbene), a phytoalexin produced by grapevines in response to fungal infection, particularly Botrytis cinerea. Synthesis of resveratrol in grape berries is mainly located in the skin cells and is absent or low in the fruit flesh. In nature, resveratrol exists in two isomeric forms (cis- and frani -conflgured) in the free as well as in 6-glucoconjugated form. The 3-0-6-D-glucosides of cis- and trans-resveratrol cis-and trans-conflgured are called piceids. The chemical structures of resveratrol and further stilbenes are depicted in Fig. 9C.6. 

Stilbene is a molecule which exists in two isomeric states because of the large energy barrier associated with rotation around the central double bond (see fig. 7.13). It has been known for some time [22] that when stilbene is excited to its first singlet state by absorption of UV light, it rapidly fluoresces to the ground state but with some molecules in the other isomeric form. Thus, if c -stilbene is irradiated with ultraviolet light, one obtains approximately 50% tran -stilbene and... 

Bohning and Weiss 3°) have suggested that cis-trans isomerization of stilbenes may proceed via an intermediate addition product (vide infra) and not by energy transfer. The results obtained in the biacetyl-anthracene system are not completely clear Backstrom and Sandros reported isolation of anthracene dimer while Dubois and Behrens 45> suggested that a 1 1 adduct of unspecified structure is formed. 

Isomerization. Finally, some investigations of unstable radical anions will be discussed. Radical anions of ds-stilbene rapidly isomerize to radical anions of trans-stilbene therefore, their spectra or relative electron affinities cannot be studied by conventional techniques. Flashing a solution of perylene radical anions mixed with cis-stilbene yields cis-stilbene radical anions, which persist for 200-300 jls before the ejected electrons are recaptured by the perylene formed by the photolysis. The difference spectrum of the photolyzed solution is shown in Figure 12. The concentration of the ds-stilbene radical anions formed can be determined by the degree of bleaching the absorption peak of the perylene radical anions. The absorption spectrum of cis-stilbene radical anions can also be determined in this way (19). Similar experiments, results of which are shown in Figure 13, permit the determination of the absorption spectrum of trans-stilbene radical anions. Because this spectrum has been established by other methods, the reliability of the flash-photolytic method can be determined by comparing the differ-... 

Silver also has been demonstrated to be reactive in solution systems. Thus, silver perchlorate has been shown to influence the photochemical reactivity of stilbene in acetonitrile and methanol. The fluorescence of the stilbene is quenched on addition of the perchlorate and this is good evidence for the enhancement of the So-Ti crossing induced by the heavy ion Ag+. It seems likely that an Ag+/stilbene complex is formed. The perturbation of the system is better in methanol than in acetonitrile. However, cis.trans isomerism of the stilbene is reduced within the excited Ag+/stilbene complex since it is difficult for the geometrical isomerism to occur. Enhanced isomerism is observed with the Ag+/azobenzene system. In this complex there are steiic problems encountered in the nitrogen rehybridization process that is operative in the isomerism . Enhanced So-T crossing is also seen in the Ag+/1 1 complex with tryptophan where the fluorescence is quenched and there is a threefold increase in phosphorescence . Complexes between Ag+ and polynucleotides and DNA cause quenching of the fluorescence. Enhancement of phosphorescence and a 20-fold increase in the dimerization of thymine moieties has also been observed when silver ions are added to the reaction system . ... 

Another interesting photochemical reaction that occurs with the monolayers is dimerization. This is exempUfled by the photochemical behaviour of the SAM of 7-(10-thiodecoxy)coumarin (52) on polycrystaUine gold. Irradiation at 350 nm results in the (2 -f 2)-cycloaddition of the coumarin moieties. The photodimerization is a reversible process by irradiating at 254 nm. Better regioselectivity in the cycloaddition is obtained when the solid monolayer is irradiated rather than when it is in contact with benzene. The dimer formed is the yn-head-to-head dimer identified as 53 . Self-assembled monolayers of cis- and frani-4-cyano-4 -(10-thiodecoxy)stilbene (54) are also photochemically reactive. Irradiation of a thin film in benzene solution using A, > 350 nm results in the formation of a photostationary state with 80% of the cis-isomer present. Irradiation in the solid shows that cis.trans isomerism occurs but that trans.cis-isomerism fails. Prolonged irradiation brings about (2 - - 2)-cycloaddition of the stilbene units to afford cyclobutane adducts. Such dimerization is a well established process . The influence of irradiation at 254 nm or 350 nm of self-assembled monolayers of 10-thiodecyl 2-anthryl ether on polycrystaUine... 

The name for stilbene (1,2-diphenylethylene) was derived from the Greek word stilbos, which means shining. There are two isomeric forms of 1,2-diphenylethylene ( )-stilbene (trans-stilbene), which is not sterically hindered, and (Z)-stilbene (cis-stilbene), which is sterically hindered and therefore less stable. 

Nonempirical calculations of reaction complexes formed during anionic polymerization ofbutadiene, in the presence of stilbenes, have been made [87]. The mechanism of cis-trans isomerization in the terminal unit of the living polymer consisted in concerted rotation about the Cp bond, and the migration of Li between and Cy... 

Data on photophysics and photochemistry of intramolecular stilbene-amine exciplexes and reactions have been reported [34]. The obtained data indicated that the photophysical and photochemical behavior of a series of traws-(aminoalkyl) stilbenes in which a primary, secondary, or tertiary amine is appended to the stUbene ortho position with a Me, Et, or Pr linker. The tertiary (aminoalkyl) stilbenes formed fluorescent exciplexes and underwent trans cis isomerization but failed to undergo intramolecular N—H addition. The secondary (aminoalkyl)stilbenes did not form fluorescent exciplexes but underwent the addition to the stilbene double bond. Intramolecular reactions were highly selective, providing an efficient method for the synthesis oftetrahydrobenzazepines. Direct irradiation of the primary (aminoalkyl) stilbenes resulted only in trans-cis isomerization, while irradiation in the presence of the electron acceptor p-dicyanobenzene resulted in regioselective intramolecular N—H addition to the stilbene double bond. 

N- Aminoaziridines have been converted to alkenes by reaction with a variety of oxidizing agents (70JA1784). Usually, the deamination reaction is stereospecific. The oxidation of l-amino-2,3-diphenylaziridines with manganese dioxide, however, was not stereospecific. The trans compound gives entirely frans-stilbene, whereas the cfs-aziridine forms a mixture of 85% trans- and 15% c -aikene. cw-Stilbene is not isomerized to trans under the reaction conditions, and the results are explained in terms of an azamine intermediate which can isomerize through a tautomeric equilibrium. 

Cfs-butene should lead initially to the anti form trrms-butene should lead initially to the syn form and 1-butene should give rise initially to both. The equilibrium distribution of syn and anti forms usually differs greatly from the equilibrium distribution of cis- and frans-butene for cobalt complexes 59, 60) the syn form, precursor of irans-butene, is by far the most stable. By way of contrast for the corresponding carbanion, the cis anion seems by far the more stable. This preference for the cis carbanion is presumed to be the source of the high initial cis-to-trans ratio in the initial products of base catalyzed isomerization. In the base catalyzed isomerization of more complex cf-s-olefins (cfs-S-methyl-stilbene), the ions corresponding to syn and anti are not interconvertible and cis-trans isomeriza-... 

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-... 

Stilbenes have a 1,2-diphenylethylene as their basic structure (C6-C2-C6). Resve-ratrol, the most widely known compound, contains three hydroxyl groups in the basic structure and is called 3,4, 5-trihydroxystilbene. In plants, piceid, the glucoside of resveratrol, is the major derivative of resveratrol. Stilbenes are present in plants as cis or trans isomers. Trans forms can be isomerized to cis forms by UV radiation (Lamuela-Raventos and others 1995). 

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