Cyclodextrins supramolecular photoreactions

Cyclodextrins (CDs) are most frequently employed hosts for supramolecular photoreactions by virtue of the fact that they are readily available, UV-tianspaient, water-soluble and capable of accommodating a wide range of organic guests in their hydrophobic cavities. Organic guests can form inclusion complexes with a-, P- and y-CDs in various stoichiometries both in solution and in the soUd state. 

The observed photobehavior of the benzaldehyde-CDx complexes in the solid state is unique and completely different from that of these complexes in aqueous solution and also from that of benzaldehyde 36 in organic solvents. The substantial formation of 4-benzoylbenzaldehyde 38 upon irradiation in (3- and y-CDx cavities indicates that these medium-sized CDx s provide the radical pair within a fairly spacious supercage environment, thus allowing the para-rearrangement (Scheme 13). The formation of practically racemic 37 upon irradiation of the y-CDx complex may also be attributed to the looser orientation of benzaldehyde 36 in the y-CDx cavity than in the (3-CDx cavity. It was thus demonstrated that the chiral hydrophobic cavity of native cyclodextrins not only modifies the photoreactivity of the included guest but also functions as a chiral supramolecular environment for photochirogenesis, albeit resulting in only modest ee%. 

An elegant example of the use of supramolecular interactions (hydrophobic forces in this case) to accelerate and direct photochemical cycloaddition reactions is embodied in the use of 3-cyclodextrins as a reaction vessel in which to assemble two coumarin molecules in a geometry adequate for dimerization. In the case of 4,7-dimethylcoumarin, recent investigations by Brett et al. using x-ray crystallography to determine the structures of the inclusion complex before irradiation and of the photoproduct reveal that the latter is not the expected syn head-to-tail or anti head-to-head, but rather the anti head-to-tail photodimer. Single crystal-to-single crystal photoreaction was observed for the P-cyclodextrin-4-hydroxy-7-methylcoumarin complex and, in this case, the observed photoproduct is correctly predicted from the topochemical principles set forth by Schmidt over 30 years earlier. 

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