Photoreactions in hydrophobic pockets
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Organic chemists have long recognized the important role that reaction media play in controlling rates, product distributions and stereochemistry. Recently, much effort has been directed towards the use of organized media to modify reactivity as compared to that in isotropic liquids. Judicious selection of a given organized system for a given application requires sufficient understanding of the properties of the organized media themselves and those of the substrate interactions therein. The multimolecular aggregation of hydrophobic solutes in water could prove to be of immense value to the organic chemist. The aggregation of simple olefinic systems in water, would enable photocycloaddition to compete efficiently with the various other modes of dacay of the short-lived excited state. Investigations of a few systems (dimerization of coumarins, stilbenes and alkylcinnamates), in our laboratory have been successful and they bring to light the significance of the hydrophobic effect. One of the most accepted manifestations of the hydrophobic interactions is probably the formation of micellar aggregates in aqueous solutions. Micelles provide a unique interface between aqueous and non-aqueous phases at which the non-polar solute can orient itself. While intermolecular orientation at micellar interfaces can provide selectivity in dimerization reactions, intramolecular orientation can be utilized to bring about selectivity in unimolecular photo-transformations. Such examples are presented.
KeywordsPhotoreactions association in aqueous media micellar effect selectivity in reactions hydrophobic effect
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