Substituted stilbenes

The Pd°-catalyzed arylations using arenediazonium tetrafluoroborates are limited to those diazonium salts that can be manipulated at room temperature. The reaction can, if necessary, be performed at temperatures up to 50 °C by using a mixture of an arylamine and tert-butyl nitrite in chloroacetic acid or in a mixture of chloroacetic and acetic acid (Kikukawa et al., 1981a). Styrene reacted with fourteen arylamines in the presence of 5 mol-% Pd(dba)2 to give the corresponding substituted stilbenes in yields of 46-97%. It is important for good yields to carry out these reactions in an acidic system. Without acid the yield was low (11%), and diazo tars were also formed. 

Stimulated by extensive research activities on donor/acceptor substituted stilbenes, Mullen and Klarner have reported a donor/acceptor substituted poly(4,4 -biphenyl-diylvinylene) derivative (85) in which the NR2 donor and CN acceptor substituents are located on the vinylene unit [111]. The synthesis is based on a C-C-coupling reaction of in situ generated carbanion functions with a (pseudo)cation function, followed by a subsequent elimination of MeSH with formation of the olefinic double bond. 

Leng, W., Woo, H. Y, Vak, D., Bazan, G. C. and Kelly, A. M. (2006) Surface-enhanced resonance Raman and hyper-Raman spectroscopy of water-soluble substituted stilbene and distyrylbenzene chromophores. J. Raman Spectrosc., 37, 132-141. 

Lapouyade R, Czeschka K, Majenz W, Rettig W, Gilabert E, Rulliere C (1992) Photophysics of donor-acceptor substituted stilbenes. A time-resolved fluorescence study using selectively bridged dimethylamino cyano model compounds. J Phys Chem 96(24) 9643-9650... 

Malicki M, Guan Z, Ha SD, Heimel G, Barlow S, Rumi M, Kahn A, Marder SR (2009) Preparation and characterization of 4 -donor substituted stilbene-4-thiolate monolayers and their influence on the work function of gold. Langmuir 25(14) 7967-7975... 

B. Jiang, S.-W. Yang, R. Niver, and W.E. Jones, Jr., Metalloporphyrin polymers bridged with conjugated cyano-substituted stilbene units, Synth. Met., 94 205-210, 1998. 

Both these parameters reflect a moderate stability for BrN3 in methanol. This feature made it impossible to apply the azide clock method to determine the lifetime of the ionic intermediates arising from deactivated alkenes (for example, / ,p -bis(trifluoromethyl)stilbenes). Nevertheless, for a couple of p-methoxy substituted stilbenes and unsubstituted stilbenes the selectivity ratios reported in Table II have been found. (IT)... 

Donor-Acceptor and Donor-Donor Substituted Stilbenes... 

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

Phosphorane, (dichloromethylene) TRIPHENYL-1, 46, 33 Phosphorus pentachloride, for conversion of D,L-10-camphorsulfonic acid to acid chloride, 46,14 reaction with styrene, 46, 99 Photolysis of substituted stilbenes to phenanthrenes, 46, 91... 

The photocyclization has been found not to occur with stilbenes substituted with acetyl, dimethylamino, or nitro groups, lodo substituents are replaced by hydrogen by photolysis in cyclohexane solution. w-Substituted stilbenes give mixtures of 2- and 4-substituted phenanthrenes which generally are difficult to separate. 

The available information on the pharmacology of STS inhibitors is restricted to a few in vitro and in vivo models. From all steroidal compounds, the irreversible inhibitor EMATE was found to be the most potent STS inhibitor with an IC50 of 65 pM [ 107]. However, it has been proposed that the sulfamate moiety of EMATE irreversibly binds to the active site and releases the steroidal backbone estrone. This means that EMATE, although a potent inhibitor of STS, counteracts its own effect by releasing estrone. The most potent derivative from a series of sulfamoyloxy-substituted stilbenes inhibited the growth of MCF-7 breast cancer cells with an IC50 value of 13 nM [111]. 

A very similar situation has been observed in Similar but less well studied indications for metastable products have been obtained for the DHP s formed from the m-substituted stilbenes Some observations about time-dependent changes in the absorption spectmm of 1 could well be due to such processes ... 

The influence of steric effects on the photocyclization process is clearly discernible for the ortho and meta substituted stilbenes. In these cases steric repulsion intervenes as the two C atoms forming the new bond approach. This steric repulsion opposes the stabilizing interactions which promote the cyclization process. As a result the reactivity is decreased. Other consequences of such superimposed steric... 

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. 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

The objective of this article is to review critically the bimolecular photochemical reactions of the stilbenes. While the emphasis will be placed upon the unsubstituted stilbenes t-1 and and c-1, comparisons with substituted stilbenes and stilbene analogues will be made in some cases. In most of the reactions that will be described, the reactive excited state is the lowest singlet of t-1, - -t. Because of the short lifetime of It, unimolecular processes (fluorescence and trans-cis isomerization) can compete with bimolecular reactions under normal reaction conditions. While competing unimolecular processes are... 

The fluorescence Intensity of substituted stilbenes and stilbene analogues provides a useful indicator of photochemical reactivity. Virtually all of the reported bimolecular photochemical reactions of electronically excited stilbenes involve stilbenes which are fluorescent at room temperature in solution. The absence of fluorescence is indicative of a singlet lifetime too short (< 100 ps) to allow for efficient bimolecular quenching. 

TABLE 2. Stereochemistry of Substituted Stilbene Photodimeri zati on ... 

In parallel to this example, Lewis and coworkers have also observed inverse reactivity for electron transfer and cycloaddition efficiency in the interaction of trans-stilbene with unsaturated esters (52). This result is understandable if the exciplex, rather than the radical ion pair, arranges the olefins in the correct geometry for cycloaddition. Analogous results have also been reported in the (2+2) cross photoreaction of cyano-substituted stilbenes with dienes, where all possible regioisomers were formed, eq. 16 (53a) ... 

The kinetics and the products of bromination of several substituted stilbenes with Bu4N+Br3 have been investigated in aprotic solvents at different temperatures and concentrations. Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products the reaction followed a second-order rate law and inverse kinetic isotope effect Ap/Ap = 0.85 ( 0.05) was... 

The excited fraws-stilbene/fumaronitrile complex produces a locally excited triplet state, which is considered to be responsible for isomerization of substituted stilbenes. Similarly, the CT complexes between aromatic hydrocarbons and fumaronitrile produce the isomerization of fumaronitrile to malonitrile34. 

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