Photooxygenation in polystyrene beads with covalently and non-covalently bound tetraarylporphyrin sensitizers

Two reaction protocols are described which involve the use of polymer supports as reaction media for photooxygenation processes: 1) The use of polystyrene beads (PS) loaded with tetraphenyl- (TPP) or tetratolylporphyrin (TTP), swollen with the substrate in an appropriate organic solvent and subsequent irradiation under air. Products were isolated simply by dissolution in alcoholic solvents and filtration. 2) Covalently linked tetrastyrylporphyrin in polystyrene-divinylbenzene beads were synthesized by emulsifier-free emulsion polymerization and directly used for the photooxygenation protocol described above. The latter alternative allows also the use of less polar solvents for the extraction of the oxygenation products from the polymer beads. From the sensitizer loading degree, an optimal substrate/sensitizer molar ratio of 1,000-2,000 was determined and recyclization is possible for at least five times resulting in a minimum turnover number (with respect to the sensitizer TTP) of 5 x 10(4) (after five cycles). Both approaches were applied to the ene reaction of singlet oxygen with citronellol (1), the regioisomeric pinenes 3 and 5, and the allylic alcohols 9a-c, respectively, as well as to the [4 + 2]-cycloadditions of singlet oxygen to sorbinol (7) and the chiral diene 11.