Synthesis Of α-bromo Ketones Promoted By Photoredox Catalysis

α-Bromo ketones constitute an important class of intermediates as they have beenattracting increasing attention in view of high reactivity as building blocks for thepreparation of various compounds by simple chemical transformations. The classicalsynthetic pathway for α-bromo ketones based on direct bromination of thecorresponding ketones with molecular bromine or hydrogen bromide, which arehazardous and difficult to handle and could be the source of environmental pollution.Due to these disadvantages much stress is laid on research to find cleaner reagents andmilder conditions to such subjects.Recently photoredox catalysis has emerged as an effective and versatile method inpromoting synthetic transformations. Thus, we have developed a novel and simpleprocedure for the regioselective synthesis of α-bromo ketones via visible lightphotoredox catalysis. The chosen electron-rich epoxides were rapidly transformed intoα-bromo ketone products under the condition that CBr4or KBr were used asbromo-source with the help of5W blue LEDs and Ru（bpy）₃²⁺as photocatalyst. Throughoptimization of solvent, light source and oxidants, the reaction can be rapidly achievedin a wide scope, up to90%yield.Moreover, single electron transfer and oxidatively quenching process ofphotocatalysts were proposed as the possible reaction mechanism. We examined thecomponent of products after a long-time run of the reaction and less addition ofoxidative quencher. Then we proposed two possible mechanism of intermediates─benzyl cation mechanism and benzyl radical mechanism, which were further supportedby control experiments.All of the products were characterized by NMR, IR and HRMS analysis.