| Visible light-mediated reactions have many advantages over the traditional chemical reactions, especially for the increasingly highlighted environmental and energy problems. Due to the great efforts made by chemists, many visible light promoted reactions have been rapidly developed. For example, Reduction of ElectronPoor Olefins, Oxidative Removal of the PMB Group, Cascade Photoredox Cyclization Reactions, Oxidation of Alcohol and Amine, etc. α-Bromo-α,β-unsaturated ketones or aldehydes sever as important synthetic intermediates. They are widely used in many aspects, such as chemical engineering and the syntheses of pesticide and medicine. This visible light-mediated strategy offers a new synthetic method for the α-Bromination ofα,β-unsaturated ketones or aldehydes.o-methoxycinnamaldehyde and 2-biphenylvinyl ketone were chose as the standard substrates to optimize reaction conditions, manifesting that with 1W LED, 5 mol%photocatalyst fac-Ir(ppy)3 and oxidant CBr4, the α-Bromination of α,β-unsaturated ketones or aldehydes all can be best realized in high yields.With the optimized conditions in hand, we prepared 19 substrates, including 9 α,β-unsaturated aldehydes and 10 α,β-unsaturated ketones. All of them reacted smoothly with the yield up to 87%. According to the controlled experiments, we supposed that the reactions undergo an oxidative quenching cycle. Firstly, the α,β-unsaturated carbonyl compound reacted with bromide to produce β-bromo-ketone or aldehyde via a Michael addition process. Then this β-bromo-carbonyl intermediate was oxidated by Ir(III) to yield bromonium which was attacked by another bromide to generate dibromocarbonyl intermediate followed by the elimination of a molecule HBr to yield the aimed production.Compared with those traditional synthetic methods, this visible light-mediated strategy not only avoided the use of the quite hard operated bromine and toxic solvent CCl4, but also realized the reaction in one step under mild conditions. |
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