Role of supramolecular steric compression during photoinduced intramolecular hydrogen abstraction reactions of ketones and thioketones

Abstract

Excited state behavior of molecules is influenced by the environment in which they are present. The influence of solvents on reactions is understood in terms of the bulk properties of the solvent such as polarity, polarizability, hydrogen bonding, solvophobic interactions etc. Under these conditions, the ‘space’ surrounding the guest does not play any role in influencing the fate of an excited molecule. The current study focusses on understanding the factors that control the excited state reactivity of molecules within a confined space. In this context organic capsules assembled from two molecules of octa acid serves as the host and aryl alkyl ketones and aryl alkyl thioketones as reactive guests. One obvious prediction has been when the product is larger in size than the host capsule it would not be formed within the confined space although this may be the predominant product in solution. The authors demonstrate below that even if the product fits well within the capsule it may not be formed if the intermediate structures that connects the reactant to product does not fit within the capsule. The above hidden factor that controls photoproduct distribution has been investigated employing several aryl alkyl ketones that undergo γ-hydrogen abstraction as guests. In some of the examples investigated here, the Yang cyclization product, cyclobutanol, is suppressed within OA capsule even though it fits well within the capsule. Another factor that has come to light is ‘time’. Aryl alkyl thioketones that react from short lived second excited state (S2) fail to undergo δ-hydrogen abstraction within OA capsule although this is the only photoreaction in solution. MD simulations of the thioketones suggest that within the capsule the relevant hydrogens are not within the ideal geometry required for abstraction. Lack of reactivity is attributed to the insufficient time for the excited molecule to achieve the required conformation within the narrow space. Longer lived aryl alkyl ketones had no such problems in adopting a conformation required for hydrogen abstraction. Obviously, time becomes critical when the space is narrow. The results reported here brings out that one could consider ‘free space’ and ‘time’ as valuable tools to control product distribution while performing photochemistry in supramolecular assemblies.