Correlation Analysis of the Substituent Electronic Effects on the Allylic H-Abstraction in Cyclohexene by Phthalimide-N-oxyl Radicals: a DFT Study

The H-abstraction by pthalimide- N -oxyl radicals is an important step in oxidation reactions catalyzed by N -hydroxyphthalimide (NHPI). Herein, substituent electronic effects on the allylic H-abstraction process by phthalimide- N -oxyl radicals are evaluated by a systemically theoretical analysis in the case of cyclohexene. The catalyst with electron-withdrawing substituent possesses larger spin density on the oxygen atom of the N−O section, which results in a larger O−H bond dissociation energy (BDE) and smaller isotropic Fermi contact coupling constants of the nitrogen atom. The BDE of O−H bond plays a very important role, determining the H-abstraction activation energy. The isotropic Fermi contact coupling is closely related to the coupling constant of the EPR spectrogram. The conjugation effect plays an important part in the aryl substituent effect. According to the results above, not all ionic-compound-supported NHPIs are good catalysts. A cation-supported NHPI is better than an anion-supported NHPI. The present theoretical study reveals the relationship between the structure and the catalytic activity of NHPI and its analogues, complementary to the previous work on NHPI, and allows for a reasonable prediction of the catalysis efficiency of NHPI analogues.