Reaction of cyclooctenes with singlet oxygen. Trapping of a perepoxide intermediate

Article abstract of DOI:10.1021/ja00134a005

The products and rate constants for total physical and chemical quenching (k_q + k_r) of singlet oxygen (¹O₂) by cis- and trans-cyclooctene were determined in two solvents of different polarity. Small amounts of cis-cyclooctene are produced during the reaction of the trans-isomer. In CDCl₃ and acetone-d₆, k_q + k_r for cis-cyclooctene was 1.3 × 10⁴ M^-1 s^-1 while the rate constants (k_r) for the reaction of trans-cyclooctene were 2.3 × 10⁴ and 3.4 × 10⁴ M^-1 s^-1, respectively. Competition experiments with 2-methyl-2-pentene (2M2P) suggest a substantial contribution of physical quenching for the trans-alkene while the cis-alkene removes ¹O₂ mostly by chemical reaction. The physical quenching of ¹O₂ by trans-cyclooctene is explained by a perepoxide intermediate which can open to a zwitterion that can abstract an allylic hydrogen to give the 3-hydroperoxycyclooctene ene product or isomerize and lose O₂ to form cis-cyclooctene. A perepoxide intermediate can be trapped using triphenyl phosphite with trans-but not cis-cyclooctene. trans-Cyclooctene quenches ³C₇₀ with a rate constant of 3.4 × 10⁵ M^-1 s^-1.