Stilbene Chemical Reactions

Phenyl segments of stilbene (1,2-diphenylethylene) are chemically unreactive, and only carbon-carbon double bond may be easily involved in several reactions. To activate stilbenes, it is generally necessary to introduce more reactive functional groups (Chapter 1). 

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Energy is transferred from molecules electronically excited in a chemical reaction to other molecules which emit the accepted excitation energy in the form of light alternatively the accepting molecules can undergo photochemical transformations. First examples of this photochemistry without light were described by E. H. White and coworkers 182>. Thus the trans-stilbene hydrazide 127, on oxidation, yielded small amounts of the cis- 128 beside the trans-stilbene dicarboxylate in a luminol-type reaction. 

The chemical oxidation of cis- or iranx-stilbene was also investigated (Vinogradov et al. 1976). The oxidant was cobalt or manganese acetate and, in separate experiments, thallium trifluoroac-etate. Acetic or triflnoroacetic acid was used as a solvent. The results of such chemical oxidation were considered from the geometrical standpoint of the recovered (nonreacted) part of the initial substrate and stereoisomeric composition of the products obtained. This allowed the desirable comparison of electrochemical and chemical reactions to be made. 

The addition reactions of It with amines are also presumed to occur via exciplex or radical-ion pair intermediates however, exciplex fluorescence is observed only under conditions where chemical reactions do not occur. Transfer of hydrogen from the amine a-C-H (tertiary amine) or N-H (secondary amine) bond results in the formation of a radical pair which ultimately gives rise to stilbene amine adducts and other free-radical derived products. The radical-ion pairs can also be intercepted by external electrophiles and nucleophiles, leading to formation of radical-ion-derived products. 

Hammond and co-workers have obtained evidence for chemical reactions of several triplet states in solution using triplet energy transfer. They have shown (a) triplet ethylenes and stilbenes undergo cis-trans isomerizations,41 42 (b) triplet ethyl pyruvate decomposes to give acetaldehyde and carbon monoxide,40 and that (c) triplet azomethane decomposes to give nitrogen and methylene (probably in a triplet state).47... 

Deuterium labelling studies have also been used to investigate the reaction of stilbenes and related compounds with amines (Lewis, 1979). It is known that tertiary amines form fluorescent exciplexes with stilbenes in nonpolar solvents and that polar solvents are necessary for chemical reaction to occur (Lewis and Ho, 1977). This suggests that radical ions are involved in product formation. When secondary amines are used, reaction occurs in solvents of widely differing polarity and this is presumably due to the acidity of the secondary N—H bond. N-deuteriated diethylamine reacts with 1,2-diphenyl-cyclobutene in benzene to give products [65], [66] and [67] incorporating deuterium (Scheme 6). For the reaction with unsymmetrically substituted... 

Phase separation of polystyrene/poly(vinyl methyl ether) blends was induced under spatially and temporally periodic forcing conditions by taking advantage of either photodimerization of anthracene or photoisomerization of /rt//i .9 stilbene chemically labeled on polystyrene chains. Significant mode election processes driven by these reactions were experimentally observed and analyzed for both cases. These experimental results reveal a potential method of morphology control using periodic forcing conditions. 

In conclusion, stilbenes involve in miscellaneous chemical reactions. For non-substituted stilbenes, the most chemically reactive part is double bond, which relatively easily undergoes the halogenation, epoxidation, oxidation, reduction, and addition. The chemistry of substituted stilbenes is in principle as rich as organic chemistry. Including stilbenes in dendrides, dextrins, polymers, and surfaces led to a sufficient change in their chemical, photochemical, photophysical, and mechanical properties and, therefore, establishes the basis for design of new materials. 

Time-resolved femtosecond absorption, fluorescence, IR, and Raman spectroscopy elucidate the molecular structure evolution during ultrafast chemical reactions [7-11]. The technique provides in real time direct insight into the structural dynamics of various systems including photoisomerization. In this chapter, we briefly describe modem methods of studying photochemical and photophysical processes that have been employed or can be employed in the stilbene photophysics and photochemistry. 

As a multidimensional PES for the reaction from quantum chemical calculations is not available at present, one does not know the reason for the surprismg barrier effect in excited tran.s-stilbene. One could suspect diat tran.s-stilbene possesses already a significant amount of zwitterionic character in the confomiation at the barrier top, implying a fairly Tate barrier along the reaction path towards the twisted perpendicular structure. On the other hand, it could also be possible that die effective barrier changes with viscosity as a result of a multidimensional barrier crossing process along a curved reaction path. 

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

Treatment of all four diasteromers of 72 with sodium or potassium bases yielded stilbenes with high stereoselectivity (Scheme 15) <2003T255>. Two of the diastereomers gave rise to some retro-aldol products. In two cases, hexacoordinate species were identified by the upheld 31P chemical shift (5—112 ppm). It was noted that phosphoranes that ring-closed to form hexacoordinate tricyclic species were the ones that did not undergo the retro-aldol reaction to produce aldehydes. The ring closure was disfavored for intermediates with steric repulsion between trifluoromethyl and phenyl groups. 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

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