Site-selective benzylic C-H oxidation through mediated electrolysis

doi:10.1016/S1872-2067(24)60139-7

Abstract

Abstract:

A novel strategy for site-selective benzylic C-H oxidation has been developed through mediated electrolysis. A bulky maleimide N-oxyl radical (MINO) generated by proton-coupled electrochemical oxidation of N-hydroxymaleimide (NHMI), serves as a hydrogen atom-transfer mediator. Good-to-excellent site selectivity was observed among different substrates, providing a practical approach for site-selective benzylic C-H oxidation. Additionally, the hydrogen-atom transfer mechanism for C-H electrochemical oxidation allows the oxidation to proceed at much lower anode potentials relative to direct electrolysis and with minimal reliance on the substrate's electronic properties.

Key words: Organic electrochemistry, Hydrogen atom transfer, Site-selective oxidation, N-Hydroxymaleimide, Radical oxidation

Yi-Fan Xi, Rui-Xing Gao, Ping Fang, Ya-Ping Han, Cong Ma, Tian-Sheng Mei. Site-selective benzylic C-H oxidation through mediated electrolysis[J]. Chinese Journal of Catalysis, 2024, 67: 54-60.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(24)60139-7

https://www.cjcatal.com/EN/Y2024/V67/I12/54

Figures/Tables 7

Scheme 1. Challenges and solutions in selective benzylic oxidation.

Scheme 2. Optimization of the reaction conditions for the site selective oxidation of the substrate. Reaction conditions: substrate 1 (0.2 mmol), catalyst (0.04 mmol, 20 mol%), lutidine (0.1 mmol, 0.5 equiv.), 2,6-lutidine tetrafluoroborate (0.8 mmol, 0.1 mol L-1), DCE (8 mL), C-felt as anode, Pt as cathode, O2 (1 atm.), 30 °C, 0.5 mA, 2.5 V, 30 h. b Yields are determined by 1H NMR using CH2Br2 as internal standard. c Site-selectivity is determined by GC-FID.

Scheme 3. Correlation between time, and combined yield and ratio under standard conditions. Reaction conditions: substrate 1 (0.2 mmol), Catalyst I (0.04 mmol, 20 mol%), lutidine (0.1 mmol, 0.5 equiv.), 2,6-lutidine tetrafluoroborate (0.8 mmol, 0.1 mol L-1), DCE (8 mL), C-felt as anode, Pt as cathode, O2 (1 atm.), 30 °C, 0.5 mA, 2.5 V. Yields are determined by 1H NMR using CH2Br2 as internal standard. site-selectivities are determined by GC-FID.

Table 1 Substrate scope. a

Scheme 4. Intermolecular site-selective benzylic C-H oxidation of 16 and 17. Reaction condition: substrates (0.2 mmol), catalyst (0.04 mmol, 20 mol%), lutidine (0.1 mmol, 0.5 equiv.), 2,6-lutidine tetrafluoroborate (0.8 mmol, 0.1 mol L-1), DCE (8 mL), C-felt as the anode, Pt as cathode, O2 (1 atm.), 30 °C, 0.5 mA, 2.5 V, 30 h. Yields are determined by 1H NMR using CH2Br2 as internal standard, yields in parenthesis are separated yields of major products. Site-selectivities are determined by GC-FID.

Scheme 5. CV data of catalyst I. Scanning conditions: CVs are scanned using glassy carbon as working electrode. (a) background: 0.1 mol L-1 LiClO4 in MeCN (grey); (b) 5 mmol L-1 NHPI, 10 mmol L-1 pyridine in (a) (red); (c) 5 mmol L-1 Catalyst I, 10 mmol L-1 pyridine in (a) (light blue); (d) 5 mmol L-1 Catalyst I, 10 mmol L-1 pyridine, 8 mmol L-1 substrate 1 in (a) (blue); (e) 5 mmol L-1 Catalyst I, 10 mmol L-1 pyridine, 16 mmol L-1 substrate 1 in (a) (deep blue).

Scheme 6. Proposed mechanism for selective oxidation.

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