Aerobic oxidation of β-isophorone catalyzed by N-hydroxyphthalimide: the key features and mechanism elucidated

Abstract

Due to the insufficient understanding of the selective oxidation mechanism of α/β-isophorones (α/β-IP) to ketoisophorone (KIP), the key features in the β-IP oxidation catalyzed by N-hydroxyphthalimide (NHPI) have been explored via theoretical calculations. β-IP is more favourable to being activated by phthalimide-N-oxyl radical (PINO˙) and peroxyl radical (ROO˙) than α-IP owing to the different C-H strengths at their reactive sites, thereby exhibiting selective product distributions. It was found that NHPI accelerates β-IP activation due to the higher reactivity of PINO˙ than ROO˙ and the equilibrium reaction between them, yielding considerable hydroperoxide (ROOH) and ROO˙. In addition, the ROOH decomposition is more favourable viaα-H abstraction by radicals than its self-dehydration and thermal dissociation. The strong exothermicity of this α-H abstraction, along with that from H-abstraction by co-yielded hot HO˙, is in favor of the straightforward formation of KIP, simultaneously leading to the isomerization of a few β-IP to α-IP and production of 4-hydroxyisophorone (HIP) and water. The proposed mechanisms, consistent with the experimental observations, allow for the deeper understanding and effective design of oxidation systems involving similar substrates or NHPI analogues that are of industrial importance.

Theoretical study on the catalytic reactivity of N-hydroxyphthalimide tuned by different heterocyclic substitutions on its phenyl ring for aerobic oxidation

“…The intrinsic reaction coordinates were also used to confirm the minimum energetic pathways from transition states to the corresponding minima. The H-abstraction barriers by the nitroxy radicals of hydroxyimide catalysts from toluene were calculated at the B3LY P/6-311++G(d,p)//B3LYP/6-311+G(d) level, which could give satisfied results for these H-abstraction processes [12,17,[19][20][21]. The NOAH BDEs of hydroxyimide catalysts were computed at the B3LYP/6-311+G(d) level by using their isodesmic work reactions with phenol, as described in Refs.…”

“…Full geometry optimization and vibrational frequency calculations were performed employing hybrid functional and B3LYP method combined with 6-311+G(d) basis set, which has been widely used in the theoretical studies on the catalytic reactions promoted by hydroxyimide catalysts [12][13][14][15][16][17][18][19][20][21]. The frequency calculations were undertaken to ensure the nature of all transition states with a pure imaginary frequency and all stationary points with none imaginary frequencies.…”

“…The H-abstraction reactions by PINO Å from toluene with electron-donating substituents [17] or by the derivatives of PINO Å with electronwithdrawing substituents from cyclohexene [18] can be largely favored, which have been experimentally supported. Besides, the selective oxidation mechanisms of isophorones catalyzed by NHPI [12], the promotion effect of t-BuOCl on the generation of PINO Å from NHPI [26], and the facile decomposition routes of hydroperoxides to ketones by nitroxy radicals [27] were systemically elucidated theoretically. Furthermore, the experimentally observed higher catalytic efficiencies of multi-nitroxyl catalysts than that of NHPI have been rationalized as the stronger electron-withdrawing effects involving in their special nitroxy radicals [19,20].…”

“…1) is the most representative hydroxyimide catalyst, which serves as an efficient carbonradical-producing catalyst (CRPC). The general mechanistic profiles of their catalytic oxidations have been disclosed extensively [2][3][4][5][6][7][12][13][14][15][16][17][18][19][20][21], where the hydrogen-abstraction reactivity from the substrates by their in situ generated reactive intermediate namely nitroxy radicals, typically phthalimide-N-oxyl radical (PINO Å , Fig. 1), and their thermal decompositions into phthalimide (NH), phthalic anhydride (O) and others [4,22], can crucially influence their catalytic enhancements.…”

“…Among the proposed strategies with the awareness of environmental issues and sustainability of chemical processes, the catalytic systems comprising hydroxyimide organocatalysts with or without co-catalysts have received substantial attention recently due to their established fascinating high reactivity and selectivity in diverse transformations [2][3][4][5][6][7], such as the production of phenols or hydroperoxides from isopropyl aromatics [8][9][10], aromatic carboxylic acids from toluenes [11], and keto-isophorone from a/b-isophorone [12].…”