Singlet Oxygen Mediated Oxidation of Olefins within Zeolites: Selectivity and Complexities

Thiazine dyes such as thionin, methylene blue and methylene green have been cation exchanged within monovalent cation exchanged Y zeolites. Depending on the water content, the dye molecules exist as either monomers (‘dry’) or dimers (‘wet’). The monomeric dye, upon excitation with visible light, generates singlet oxygen, which has been utilized to oxidize alkenes to hydroperoxides. In the case of trisubstituted alkenes, hydroperoxidation within zeolites occurs with a certain amount of regioselectivity. The oxidation within zeolites is accompanied by photodecomposition of the dye and the product hydroperoxides and acid catalyzed rearrangement of the alkenes. In order to understand the observed selectivity, ab initio and DFT calculations on model systems have been performed. The calculations confirm fairly strong cation-alkene binding as well as additional geometric and orbital distortions. Computed activation energies for hydrogen abstraction suggest a significant rate retardation due to metal coordination. At both the MP2 and B3LYP levels, formation of the tertiary hydroperoxide by hydrogen abstraction from the methyl group (4-position) of 2-methyl 2-butene is calculated to be favored by a small margin. Between the gem-dimethyl units, abstraction from the syn methyl group is favored slightly compared to the anti counterpart. These predictions are not compatible with the observed regioselectivities. Further experimental and theoretical studies are underway to understand the observed regioselective oxidation within zeolites.