Singlet oxygen reactivity in water-rich solvent mixtures

The 3-methylindole (3MI) oxygenation sensitized by psoralen (PSO) has been investigated in 100%, 20% and 5% O-2-saturated water/dioxane (H2O/Dx) mixtures. The lowering of the O-1(2)*chemical rate when water (k(chem Delta)(3MI) = 1.4 x 109 M-1 s(-1)) is replaced by deuterated water (k(chem Delta)(3MI) = 1.9 x 108 M-1 s(-1)) suggests that hydrogen abstraction is involved in the rate determining step. A high dependence of the chemical rate constant on water concentration in H2O/Dx mixtures was found showing that water molecules are absolutely essential for the success of the 3MI substrate oxidation by O-1(2)*in water-rich solvent mixtures.