Visible-Light-Mediated Benzylic Oxidation Using Bromo(diphenyl) methane

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

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• 13 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%. ¹H 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). ¹³C 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 C₈H₉O₂: 137.0597; found: 137.0600. Xanthone (2j) White solid; yield: 91%. ¹H 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). ¹³C 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 C₁₃H₉O₂: 197.0597; found: 197.0599.