Modification of Photochemistry by Cyclodextrin Complexation: Competitive Norrish Type I and Type II Reactions of Benzoin Alkyl Ethers

The photochemical Norrish type I and type II reactions of cyclodextrin-bound benzoin methyl ether, benzoin ethyl ether, and benzoin isopropyl ether have been investigated in aqueous solution and in the solid state. Irradiation in cyclodextrin media leads to a large change in product distribution from that found in benzene and methanol. In aqueous solution type II products compete with type I, and in the solid state type II products constitute more than 90% of the product distribution. This sensitivity was interpreted as a measure of changes in the ground state distribution of reactive and nonreactive (type II) conformers brought about by cyclodextrin inclusion. Cage effects also play a significant role in altering the product distribution. ¹H NMR and X-ray powder photographic studies provide support for complexation.