Modification of face selectivity by inclusion in cyclodextrins

Article abstract of DOI:10.1021/ja00159a048

The photocycloadditions of 5-X-adamantan-2-ones (5-X-AD, where X = F, Cl, Br, OH, Ph, and t-Bu) with fumaronitrile have been studied in acetonitrile and in aqueous solutions. When X is Cl, Br, Ph, or t-Bu, irradiation of an aqueous solution containing β-cyclodextrin (β-CD) leads to a dramatic reversal in face selectivity compared to that found in acetonitrile and water; however, there is no significant change in product ratio compared to that found in aqueous solution in the presence of α- and γ-CD. The effect observed with β-CD is interpreted with the assumption that the carbonyl π face syn to the bulky 5-substituent is partially blocked by the torus of the host due to complexation of the AD and CD. ¹H NMR and concentration dependence studies provide support for this interpretation. Similar reversals of face selectivity upon complexation with β-CD are observed in the thermal reduction of these ketones by sodium borohydride. It is also noted that a much enhanced syn hydride delivery occurs in water as compared to organic solvents and that this enhancement vanishes upon cooling.