Insights into the Mechanism of O(3P)-Mediated Oxidation of Alkenes through Kinetic Isotope Effects and Computational Modeling

Photodeoxygenation of aryl sulfoxides, such as dibenzothiophene S-oxide (DBTO), produces atomic oxygen [O(P-3)] in solution. The mechanism of alkene oxidation with O(P-3) remains uncertain. To address this, the current study utilized kinetic isotope effects (KIEs) and computational approaches to study the reaction of O(P-3) with styrene and its isotopologues. Notably, the 2 degrees CH/D KIE at the internal and terminal carbons of the reactive pi-bond was similar to 1.00 and similar to 0.87, respectively. These findings indicate a terminal addition of O(P-3) to the pi-bond, supporting a stepwise oxidation pathway. Both epoxide and aldehyde products go through the same rate-determining transition state and then diverge based on the intermediate conformation. The O-C-C-C dihedral angle (phi) on the triplet surface dictates the product distribution, where phi = 50 degrees or 310 degrees leads to epoxide formation and phi = 180 degrees leads to aldehyde formation. Computational modeling suggests that the epoxide is formed through rapid ring closure upon intersystem crossing from the triplet to the singlet ground state. Similarly, the aldehyde is generated via a 1,2-H shift immediately following intersystem crossing. This study integrates experimental and computational methods to understand the O(P-3)-mediated oxidation of alkenes, providing supporting evidence for a stepwise addition mechanism.