Direct Synthesis of α-Keto Esters from Ethylbenzenes Using 48% Aqueous HBr by Aerobic Visible Light Photooxidation

Abstract

We report that ethylbenzenes can be directly oxidized to the corresponding α-keto esters with molecular oxygen in the presence of 48% aqueous HBr under visible light irradiation. This synthetic procedure is the first example for direct preparation of the corresponding α-keto esters from ethylbenzenes.

photooxidation - HBr - aerobic - ethylbenzene - α-keto ester

References and Notes

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<A NAME="RU01411ST-15">15</A>

When 48% aq HBr (1.2 equiv) and H₂O (100 µL) were used as additive, ethylbenzene 1d was converted into α-bromo-acetophenone (3d) in 68% yield.^¹4

<A NAME="RU01411ST-16">16</A>

Typical Procedure: An anhyd EtOAc solution (5 mL) of ethylbenzenes (0.3 mmol) and 48% aq HBr (0.75 mmol) in a pyrex test tube equipped with an O₂ balloon was stirred and irradiated with four 22-W fluorescent lamps, which were set up at a distance of 65 mm, for 20 h. The temperature of the final stage of this reaction was about 40 ˚C. The reaction mixture was concentrated under reduced pressure, and the pure product was obtained by purification with preparative TLC.