Cyclodextrins photochemical reactions

The observed differences in the reactivity of these crystals are rationalized in terms of crystal packing. Though cyclodextrins have been extensively studied, very few photochemical reactions involving molecules complexed to cyclodextrins have been examined. In this connection, the utility of cyclodextrins in bringing about selectivity in photochemical reactions through the study of excited state behavior of olefins and aryl alkyl ketones has been demonstrated in our laboratory. 

Organized and constrained media may provide cavities and surfaces, sometimes called microreactors or nanoreactors,171 that can control the selectivity of photochemical reactions of reactants. There are many types of microreactors, for example, molecular aggregates of micelles or monolayers, macrocyclic host cavities of crown ethers or cyclodextrins and microporous solid cavities and/or surfaces of zeolites, silica or... 

Cyclodextrins can be functionalized with various types of chromophores on both the primary and the secondary hydroxyl rims. A review of the prepared, selectively 0-substituted, CDs has been published [204], The modified CDs show interesting photochemical properties which have been exploited to detect photochemically inactive organic compounds in aqueous solutions [205], to induce stero- and regioselectivity in some photochemical reactions, and to mimic the energy transport from the antenna to the reaction center in the photosynthetic membrane (vide infra). 

De Keukeleire and co-workers have reported a further example of the intramolecular addition of a pyrimidone to a pendant benzene ring. " In this example the cyclobutane adduct (189) formed from (190) on irradiation could not be isolated. Instead it underwent rearrangement on attempted isolation. Cyclodextrin is often used as a template upon which specific photochemical reactions... 

Heptakis 6-0-t-butyldimethylsilyl-P-cyclodextrin, on reaction with 4-chloro-methyl-A -methyl-2-nitroaniline, affords the mono-3-substituted benzyl ether. P-Cyclodextrin carrying a 2-(naphthylmethyl) group at 0-6 exists with the aromatic rings within the cavity to an extent which is very temperature dependent. Temperature can therefore be used to control the degree of complexing of the aromatic system with a fluorescent naphthalene compound. Heptakis-[2,3-di-0-acetyl-6-deoxy-6-iodo]-P-cyclodextrin treated with 6-methoxycarbonyl-2-naphthol allowed access to the compound having naphthoic acid substituents at all of the primary positions, and this forms a very stable 1 1 complex with a merocyanine laser dye which is a mimic of the antenna function in photosynthesis and shows promise as a photochemical microreactor. Mono-[6-0-(8-qui-nolyl)]-P-cyclodextrin has been reported, and the stabilities of inclusion complexes with amino acid guests have been described. ... 

One of the most common types of reactions to be template using CB[ ] nanoreactors is photochemical cycloadditions a few examples were discussed in Section 5.4 [120-124]. A number of other examples will be discussed here [126-137]. Svoboda and Kbning reviewed template photochemical reactions back in 2006, and had a small section on such reactions in molecular flasks [126]. Although most examples at that time involved cyclodextrins as the molecular flask (a nice descriptive name for a nanoreactor), there were a few examples at that time involving CB[7] and CB[8]. Those examples and some more recent ones will be discussed here [127-137]. 

The photodimerisation of anthracene-2-sulphonate in water gives four products A, B, C, D (Scheme 124). However, if the reaction is carried out in presence of p-cyclodextrin, only the isomer A is obtained. " Some other examples of photochemical reactions are ... 

The synthesis and photochemical reactions of polyurethanes containing [CpFe(CX))2]2 molecules has been examined. Reactions of [CpFe(CX))2Rl, R = alkyl in cyclodextrins with CO and SO2 have been studied. Some cationic ruthenium nitriles, [(Cp)L2Ru(NCAryl)], L = phosphine as phosphite have been prqiared to use in Langmuir-Blodgett films. ... 

Cyclodextrins as donors. The photochemical reactions of 1,4-naphthoquinone in a-, P- and y-cyclodextrins have been studied using TR EPR by... 

Brett, T. J. and Stezowski, J. J., Structural studies of supramolecular (i-cyclodextrin complexes with butyrophenone and valerophenone an explanation for photochemical reaction modification, J. Chem. Soc., Chem. Common., 857, 2000. 

The inclusion complexation of spiropyrans in cyclodextrins has also been explored as a means to control photochromic reactions.1591 Distinct differences in complexation of sulfonic acid-modified spiropyrans to various cyclodextrins were observed and the closed spiropyran form bound to (3-cyclodextrin was stable towards photochemical ring-opening. 

Cyclodextrins form inclusion compounds in aqueous solution with various molecules which may then exhibit modified reactivity (see reference 185 above). It was reported in 1975 that the presence of p-cyclodextrin modified the ortho para ratio of hydroxyacetophenones formed photochemically from phenyl acetate,and the photoinduced rearrangements of acetanilide, ben-zanilide, and ethyl phenyl carbonate under similar conditions are now reported. In each case the para rearrangement isomer is favoured, and this is rationalized in terms of the aromatic nucleus being bonded into the cyclodextrin cavity such that the ortho and weta positions are shielded whDe the para position is accessible. This proposal is supported by observations that para but not ortho disubstituted arenes are well bonded into p-cyclodextrin cavities. The by-product formation of aniline from the amides and of phenol from the carbonate is also markedly reduced for reactions in the presence of the cyclodextrin. 

The amide (38a) is photochemically inert on irradiation in ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type II process to yield a mixture of products.The results of a study of the enantioselective photodeconjugation reactions of the lactones (39) have been published. The behaviour of the ketones (40) and (41) in the isotropic and the two solid phases of heneicosane (CcxH ) has been evaluated. The influence of the various phases on the ratio of elimination to cyclization products of the ketones was discussed. The modification of the photoreactivity of ketones (42) in cyclodextrin has been... 

Chattopadhyay, N. (1991) Effect of cyclodextrin complexation on excited state proton transfer reactions,./. Photochem. Photobiol. A Chem., 58, 31-36. 

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