Stilbene, cyclodextrin

In the recent past separation of isomers has been attempted using aqueous liquid membranes based on p-cyclodextrin. Thus, separation of a mixture of o- and p-nitroaniline (in 80% i-octanol, 20% -heptane) has been studied, with the p-isomer showing a selectivity of 5 at 0.7 molar p-cyclodextrin. Even stereoisomers of stilbene cis and trans) were separated using a 0.02 to 0.2 M cyclodextrin solution, but the selectivity was less than 2 (Mandal et al, 1998). 

Table III Photostationary State upon Irradiation of Stilbene and Cinnamic Acid Esters in Organic Solvents and in Cyclodextrins.

Rao et al. observed that photoirradiation of tran.v-stilbene in crystalline y-cyclodextrin inclusion complexes yields a single isomer of. svn-tctraphenylcy-clobutane (119) [109] stereoselectively in high yield (70%). In contrast, the photodimerization of stilbene in solution is very inefficient, and no photodimer was observed even after prolonged irradiation of pure stilbene crystals. 

Wenz et al. have demonstrated that photoreactions are effective for the construction of polyrotaxanes (Scheme 18) [114]. In the knowledge that stil-benes undergo [2+2]photocycloaddition to yield cyclobutane derivatives by UV irradiation, they prepared a quaternary polymeric inclusion complex from /1-cyclodextrin, y-cyclodextrin, (E)-4,4-bis(dimethylaminomethyl)stil-bene (B), and (li)-stilbene polymer (A). Upon irradiation at 312 nm, the (E)-stilbene units of A underwent [2+2]photocycloaddition with B by catalysis of y-cyclodextrin to form the tetraphenylcyclobutane group, which acted as blocking group for /1-cyclodextrin. Wenz et al. claimed that this was the... 

Lewis and his co-workers have continued their investigations into DNA hairpins with a variety of stilbene diether linkages. In the present report they have examined systems based on the linkers (3) and (4). The rotaxane formed between the stilbene derivative (5) and a-cyclodextrin (CD) does not undergo ,Z-isomerization. Contrary to this the complex of (6) with the p-CD isomerizes slowly on irradiation. The behaviour of the stilbene (6) is different and irradiation in a-CD brings about isomerization with a quantum yield of 0.06 for the E,Z process. The quantum yield for the reverse isomerization is 0.71. These values are to be compared with 0.17 and 0.79 for the isomerization in solution. The authors suggest that the irradiation of (6) in the CD brings about a movement... 

A different type of environmental effect is manifested in the photoisomerization of trans-stilbene complexed to cyclodextrin. In picosecond laser studies a single decay in a-cyclodextrin shows binding occurs at one site, whilst in the (3-forra... 

Rotational reorientation of frans-stilbene in alkane solution at room temperature occurs in the 10 to 30-ps time domain [347]. Rare-gas complexes with trons-stilbene were studied by purely rotational coherence spectroscopy [51,364]. Moreover, the decay kinetics of excited trans-stil-bene-cyclodextrin complexes were examined [366], It is worth mentioning that great progress has also been made in high-resolution spectroscopy [52, 369-372], Resonance coherent Raman spectroscopy showed a large enhancement of the electronic hyperpolarizability of t with respect to ground state trons-stilbene [374]. Vibrational motions were observed with ps transient Raman spectroscopy [375]. 

Interest in reactions within constrained environments continues apace. A report has described the results of irradiating complexes of the stilbene (7) with y-cyclodextrin. Rather than c ,/ra/w-isomerization, dimerization occurs to afford (165) and (166) in 79% and 19% yield, respectively. These results are different... 

Figure 3.8 Changes to the outcome of photolysis of a stilbene derivative through complexation with either / - or y-cyclodextrin ...

W. Herrmann, S. Wehrle, G. Wenz, Supramolecular control of the photochemistry of stilbenes by cyclodextrins, J. Chem. Soc., Chem. Commun., 1997, 1709-1710. 

Figure 21 (a) Chemical structures and schematic representations of photoswitchable stilbene bi-p-cyclodextrin derivative 37 and bi-Ad guest 38. 

It was demonstrated that a complexation of trans-stilbene with cyclodextrin led to a decrease in stereoselectivity of additive bromination and to a significant yield of DL-stilbene dibromide in contrast to the formation of meso-stilbene dibromide in nonpolar solvents [98]. The authors suggested that this reversal of stereoselectivity was attributed to the polar environment provided by the secondary hydroxyl groups of... 

Several cyclodextrin ketones with a ketone attached to the secondary face of the cyclodextrin in the form of a 2,3-0-( 1,3-acetone) group (79), and some selected cyclodextrin ketones having the ketone at the primary face, were investigated for their catalysis of epoxidation of stilbenes and styrene by oxone in 1 1 acetonitrile/H20. It was found that secondary face ketones were better catalysts giving a cat uncat over 10, and more stereoselective giving up to 76% ee in (5)-styrene oxide. ... 

Photoisomerization of azobenzenes has been widely developed as photochromic compounds. Lots of papers sriU appeared in this subject from basic chemisty to applications. Some examples were described. Harada et al., reported the effect of cyclodextrin derivatives bearing photo-isomerized stilbene moiety. Single molecule conductance of E- and Z-4, 4 -(ethane-l,2-diyl)dibenzoic acids (E-l and Z-1) was determined using scanning tunnelling microscopy (STM). The dynamics of photoisomerization of E- and Z-urocanic acids (E- and Z-2) driven by Hg-free... 

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