Computational Mechanism Study on Allylic Oxidation of cis-Internal Alkenes: Insight into the Lewis Acid-Assisted Brønsted Acid (LBA) Catalysis in Heteroene Reactions.

The catalytic allylic C-H oxidation of alkenes plays an important role in the field of medicine chemistry. Recently, Tambar et al. improved this transformation via a heteroene reaction with the assistance of a Lewis acid-assisted chiral Brønsted acid (LBA) and achieved a selective allylic oxidation of inactivated cis-internal alkenes to versatile oxidation products. By means of density functional theory (DFT) calculations, we provided a detailed investigation on the mechanism of the heteroene reaction and successfully located a new catalytic process, which is able to explain the experimental observations very well. Four different reactive pathways (pathways A, B, C, and D) for the LBA-catalyzed heteroene reaction have been designed. We found pathway D, which undergoes a protonation process for the activation of enophile benzenesulfonyl sulfurimide, has the lowest overall free energy barrier. Pathway E was put forward to lead to a minor enantiomer. The theoretical enantiomeric ratio calculated via their energy difference is consistent with the experimental report. For the heteroene reaction, we proposed a new reaction mechanism, which can assist in related transformations and the design of new LBA catalysts.