Mechanism of Ene Reactions of Singlet Oxygen. A Two-Step No-Intermediate Mechanism

Abstract

The mechanism of the ene reaction of singlet (¹Δ_g) oxygen with simple alkenes is investigated by a combination of experimental isotope effects and several levels of theoretical calculations. For the reaction of 2,4-dimethyl-3-isopropyl-2-pentene, the olefinic carbons exhibit small and nearly equal ¹³C isotope effects of 1.005−1.007, while the reacting methyl groups exhibit ¹³C isotope effects near unity. In a novel experiment, the ¹³C composition of the product is analyzed to determine the intramolecular ¹³C isotope effects in the ene reaction of tetramethylethylene. The new ¹³C and literature ²H isotope effects are then used to evaluate the accuracy of theoretical calculations. RHF, CASSCF(10e, 8o), and restricted and unrestricted B3LYP calculations are each applied to the ene reaction with tetramethylethylene. Each predicts a different mechanism, but none leads to reasonable predictions of the experimental isotope effects. It is concluded that none of these calculations accurately describe the reaction. A more successful approach was to use high-level, up to CCSD(T), single-point energy calculations on a grid of B3LYP geometries. The resulting energy surface is supported by its accurate predictions of the intermolecular ¹³C and ²H isotope effects and a very good prediction of the reaction barrier. This CCSD(T)//B3LYP surface features two adjacent transition states without an intervening intermediate. This is the first experimentally supported example of such a surface and the first example of a valley−ridge inflection with significant chemical consequences.