Synlett DOI: 10.1055/a-2495-3429
Visible-Light-Mediated Benzylic Oxidation Using Bromo(diphenyl) methane
Ka-Mei Lee
,
Hong-Chai Fabio Wong
,
Sze-Yung Wong
,
Yung-Yin Chan
,
Ying-Lung Steve Tse∗
,
Ying-Yeung Yeung∗
We are grateful for financial support from the Research Grants Council of the Hong Kong Special Administration Region (Projects CUHK 14305621 and 14302221), The Chinese University of Hong Kong direct grants (Project 4053565, 4053505), Croucher Foundation Senior Research Fellowship (SRF22403), and the Innovation and Technology Commission to the State Key Laboratory of Synthetic Chemistry (GHP/004/16GD).
Abstract
Benzylic C(sp3 )–H oxidation is a useful process that can give aryl carbonyl compounds as valuable building blocks. Here, we report a study on the use of bromo(diphenyl)methane as a Br source for a phototriggered benzylic oxidation. The reaction conditions are mild and compatible with a variety of substrates. Mechanistic studies suggest that the reaction might involve a peroxyl radical intermediate formed by a reaction between a benzylic C radical and molecular oxygen.
Key words
bromine -
chemoselectivity -
free radicals -
ketones -
photooxidation -
bromodiphenylmethane
Supporting Information
Supporting information for this article is available online at https://doi.org/10.1055/a-2495-3429.
Supporting Information
Publication History
Received: 02 October 2024
Accepted after revision: 03 December 2024
Accepted Manuscript online: 03 December 2024
Article published online: 16 December 2024
© 2024. Thieme. All rights reserved
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Benzylic Oxidations with Bromo(diphenyl)methane; General Procedure
Bromo(diphenyl)methane (0.08 mmol, 0.8 equiv) was added to a solution of the appropriate benzylic substrate 1 (0.1 mmol, 1 equiv) in anhyd MeOH (1 mL) in a round-bottomed flask equipped with an oxygen balloon. The mixture was irradiated by a 30 W blue LED at 35 °C until the starting material was consumed, then concentrated under reduced pressure. The residue was purified by flash column chromatography [silica gel, hexane–EtOAc (9:1)].
Methyl Benzoate (2a)
Colorless oil; yield: 92%. 1 H NMR (500 MHz, CDCl3 ): δ = 8.05–8.03 (m, 2 H, ArH), 7.57–7.54 (m, 1 H, ArH), 7.45–7.42 (m, 2 H, ArH), 3.92 (s, 3 H, CH3 ). 13 C NMR (500 MHz, CDCl3 ): δ = 167.2, 132.9, 130.2, 129.6, 128.5, 128.4, 52.1. HRMS (APCI): m /z [M + H]+ calcd for C8 H9 O2 : 137.0597; found: 137.0600.
Xanthone (2j)
White solid; yield: 91%. 1 H NMR (400 MHz, CDCl3 ): δ = 8.38–8.32 (dd, J = 8.0 Hz, 2 H, Ar H), 7.76–7.70 (m, 2 H, Ar H), 7.53–7.48 (m, 2 H, Ar H), 7.41–7.36 (m, 2 H, Ar H). 13 C NMR (500 MHz, CDCl3 ): δ = 177.4, 156.4, 135.0, 126.9, 124.1, 118.1. HRMS (APCI): m /z [M + H]+ calcd for C13 H9 O2 : 197.0597; found: 197.0599.