The use of insoluble benzoylated polystyrene beads (polymeric benzophenone) in photochemical reactions

Abstract

Friedel–Crafts benzoylation of 2%-divinylbenzene (DVB) cross-linked polystyrene beads yielded insoluble polymeric analogs of benzophenone which were used as sensitizers in the |2+2| photocycloaddition of cyclohexene to maleic anhydride and in the E–Z isomerization of (E)-1,3-pentadiene and methyl (E)-2,2-dimethyl-3,5-hexadienoate. In the first reaction, the use of the polymer-anchored sensitizers resulted in the separation of polymer-attached by-products by simple filtration, although the yield of photoadducts was less than three quarters of that obtained when using benzophenone itself, and the recovered polymers could not be reused. In the photoisomerizations, the compositions of the photostationary states reached when using the polymeric sensitizers (now reusable) were shown to depend on the degree of functionalization of the polymers via steric hindrance to the energy transfer process. Intrapolymerically sensitized E–Z photoisomerization was also demonstrated on a polymer containing benzophenone and diene units. Photooxidation of secondary alcohols (isopropanol, cyclohexanol) to ketones and photooxygenation of 2,3-dimethyl-2-butene to 2,3-dimethyl-3-buten-2-yl hydroperoxide (in the presence of oxygen) were other reactions for which polymeric benzophenones could be used.