Modification of photochemical reactivity through the use of clathrates: the Norrish type I and type II reactions in Dianin's compound

Abstract

Dianin's compound (4-p-hydroxyphenyl-2,2,4-trimethylchroman) serves as host in a series of well-defined clathrate inclusion complexes with eleven linear, as well as branched chain, phenyl alkyl ketone guest molecules, chosen for their ability to undergo the Norrish type I and type II photochemical reactions in solution. The photochemical reactivity of the guest ketones within the clathrate cavity was determined by irradiation of the inclusion complexes in the solid state. The results were compared to the photoreactivity of the ketones in polar as well as nonpolar liquid media. In general, the inclusion complex medium brings about an enhancement of type I over type II reactivity and causes an increase in type II fragmentation compared to type II cyclization. This change in reactivity is interpreted as resulting from the relatively restricted environment of the clathrate cavity coupled with the greater motion required for the type II process (γ-hydrogen abstraction) compared to the type I reaction (α-cleavage), as well as from the greater steric requirements for type II cyclization (cyclobutanol formation) as compared to type II cleavage (1,4-hydroxybiradical scission).