Singlet Oxygen Ene Reaction: Using Kinetic Isotope Effects to Probe Transient Intermediates

Abstract

Pericyclic reactions generally occur through a single concerted transition state where all bonds are being formed and broken in a synchronous fashion. Previous experimental observations in the singlet oxygen (1Δg) ene reaction provide evidence to the contrary, suggesting that this particular pericyclic reaction occurs instead through two kinetically distinguishable steps. A more complete fundamental understanding of the enigmatic mechanism of this reaction is sought. In this work, intramolecular kinetic isotope effect studies of the 1O2 ene reaction for simple, acyclic alkenes exhibit a weak temperature dependence. The low sensitivity of the product ratio on reaction temperature is interpreted as a diagnostic for a shallow perepoxide intermediate that decays with a sub-picosecond half-life to form the hydroperoxide product. However, this analysis assumes that the reaction obeys statistical rate theory. In the sub-picosecond time regime, intramolecular energy relaxation and intermediate decay can occur simultaneously, and the co-occurrence of these events can alter statistical product ratios predicted from transition state theory (TST). The experimental isotope effects measured here cannot be unambiguously reconciled with those predicted using statistical methods, which reinforces the suggestion that dynamic effects can influence the outcome of reactions that feature a formal intermediate.