“Ene” Reactions of Singlet Oxygen at the Air–Water Interface

Prenylsurfactants [(CH₃)₂CCH­(CH₂)_<i>n</i>SO₃^– Na⁺ (<i>n</i> = 4, 6, or 8)] were designed to probe the “ene” reaction mechanism of singlet oxygen at the air–water interface. Increasing the number of carbon atoms in the hydrophobic chain caused an increase in the regioselectivity for a secondary rather than tertiary surfactant hydroperoxide, arguing for an orthogonal alkene on water. The use of water, deuterium oxide, and H₂O/D₂O mixtures helped to distinguish mechanistic alternatives to homogeneous solution conditions that include dewetting of the π bond and an unsymmetrical perepoxide transition state in the hydroperoxide-forming step. The prenylsurfactants and a photoreactor technique allowed a certain degree of interfacial control of the hydroperoxidation reaction on a liquid support, where the oxidant (airborne ¹O₂) is delivered as a gas