Inverse kinetic isotope effect in Magtrieve™ mediated oxidation or deoximation of benzaldoxime: mechanistic implication

Magtrieve™ (CrO2) mediated reactions with benzaldoxime (1a) and its deuterium congener (d-1a) led to the observation of inverse deuterium kinetic isotope effect (i-DKIE) for the substrate’s oxidation (Eq. 1a) as well as deoximation (Eq. 2a) process. Disappearance of the starting material 1a and formation of the products—1,3-dipolar cycloaddition product (4a) as well as benzaldehyde (3a)—followed a typical 1st-order kinetics. The observed kD/kH values, in the range of 2–4, suggest for a strong secondary isotope effect which was further evidenced by the fact that d-labeling was retained in 3a. Therefore, the observed i-SDKIE supports our original hypothesis that aldoxime (with sp2-C) interacts with CrO2 in a rate determining step to form a tetrahedral (now with sp3-C) structure, possibly like 6, which may act as a common intermediate for both the pathways.