Selective Aerobic Oxidation of Secondary C ( sp 3 )-H Bonds with NHPI/CAN Catalytic System
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Selective Aerobic Oxidation of Secondary C (sp3)‑H Bonds with NHPI/ CAN Catalytic System Lingyao Wang1 · Yuanbin Zhang1 · Haoran Yuan1 · Renfeng Du1 · Jia Yao1 · Haoran Li1,2 Received: 2 July 2020 / Accepted: 24 September 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The direct aerobic oxidation of secondarty C(sp3)-H bonds was achieved in the presence of N-hydroxyphthalimide (NHPI) and cerium ammonium nitrate (CAN) under mild conditions. Various benzylic methylenes could be oxidized to carbonyl compounds in satisfied selectivity while saturated cyclic alkanes could be further oxidized to the corresponding lactones with the catalytic system. Remarkably, 25% of isochroman was converted to corresponding ketone with a selectivity of 96%. The reaction was initiated by hydrogen atom abstraction from NHPI by cerium and nitrates under oxygen atmosphere to form PINO radicals. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) addition experiments showed that the oxidation proceeded via a complex radical chain mechanism and an ion pathway. Graphic Abstract H OH R1 H H R1
R2
R2
+
O R1
R2
NHPI / CAN O2 45 °C O Alkyl
O
Alkyl
Keywords C (sp3)–H bonds · Aerobic oxidation · N-hydroxyphthalimide · Cerium ammonium nitrate · Radical
1 Introduction Oxidation reactions are key technologies for transformation of hydrocarbons from petroleum feedstock into chemicals of higher value [1–5]. Currently, molecular oxygen (O2) has been demonstrated as an economical and green oxidant that outperforms other traditional oxidants such as H NO3, H2SO4, MnO2 and CrO3. However, the selective aerobic oxidation of sp3 C–H bonds at low temperatures and normal * Haoran Li [email protected] 1
Department of Chemistry, ZJU‑NHU United R&D Center, Zhejiang University, Hangzhou 310027, People’s Republic of China
State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
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pressures is still a long lasting challenge as O2 is a relatively mild oxidant. The cleavage of inert sp3 C–H bonds generally demand harsh conditions [6–8]. To overcome the high activation barriers, various catalysts have been applied to activate oxygen for C–H bond oxidation [9–14]. N-Hydroxyphthalimide (NHPI) is an effective organocatalyst for activation of C–H bonds via acting as a precursor of the phthalimide-N-oxyl radical (PINO) and is well known as a catalyst in aerobic oxidation of organic compounds [15–17]. The PINO radicals can be generated in the presence of transition metal salts, azo compounds, peroxides, aldehydes, enzymes and other additives [18–24]. A Co(OAc)2 and NHPI catalytic system was used for the oxidation of organic compounds such as alkylbenzenes and cyclohexane by Ishii et al. [25]. After that, NHPI and aldehyde catalytic systems were developed for the oxidation of alkanes [26, 27]. The catalytic system of NHPI/Fe(NO3)3 was adopted
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for the oxidation of ethylbenzene
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