Green Chemistry: free radical oxidation addition reaction initiated by visible light without metal participation

Carbonyl is an important part of organic target and intermediate Olefins are very rich chemical raw materials, a large number of which come from petrochemical products Therefore, they have been developed as raw materials for various fine chemicals, and the oxidation of olefins has become one of the most direct ways to synthesize carbonyl compounds The important reaction of ethylene to acetaldehyde in industry is promoted by palladium copper catalytic process called Wacker reaction, which represents the milestone achievement of olefin utilization Since the discovery of this method, this Pd / Cu catalyst system has been extended to the oxidation of various end olefins (Wacker Tsuji oxidation), which provides strong support for the synthesis of methyl ketones However, the expensive palladium (II) salt is usually used as catalyst in Wacker type reaction, sometimes it needs high catalyst loading, which leads to cost and environmental problems, and limits its practical use Therefore, it is very necessary to develop metal free synthesis methods to improve the efficiency and applicability of olefin to carbonyl compounds Zhu Chunyin's team from Jiangsu University has developed a green and cost-effective method for converting olefins to ketones in a metal free environment In this reaction, the free radicals produced by the oxidation of arylhydrazine under visible light are oxidized to olefins and converted into ketones The main characteristics of the reaction include a wide range of substrates, easily available reagents and adaptability for large-scale synthesis Wacker type reactions usually depend on the interaction between palladium (II) salt and π - orbital of olefins As an alternative, visible light redox catalysis can contact olefins in the process of single electron transfer (set), resulting in the functionalization of double bond free radical sources Hydrazine is reported to produce free radicals through metal compounds However, many of these reactions are carried out in the presence of toxic transition metals The team previously reported visible light catalyzed oxidation of hydrazine to diazene, followed by the addition of Michael receptors to form polysubstituted pyrazoles They envisioned that in the presence of Photocatalyst (PC) and oxygen molecules, olefins could be converted into ketones by reacting with the free radicals produced by the oxidation of hydrazine The research team of screening reaction conditions (source: Green Chemistry) started from the model reaction of Experiments 1a and 2A After a large number of reaction conditions were tested, they determined that ketone 3AA could be obtained in good yield (81%) under the conditions of methylene blue (MB +, 2mol%), DABCO (1 equivalent) and air (bottle opening) in MeCN as solvent and 7W blue LED light at room temperature for 8 hours After a series of experiments, it is found that photocatalyst and visible light are necessary for the reaction At present, the best photocatalyst is methylene blue On the other hand, the removal of DABCO from the reaction mixture will lead to a reduction in the yield, indicating that an alkali is needed in the reaction to achieve the appropriate yield Several bases in the experiment can promote the reaction, but DABCO is the best The substitution of other solvents for MeCN will also lead to a decrease in production A series of derivatives (source: Green Chemistry) were synthesized After establishing appropriate conditions, the research team then studied the scope of application of hydrazine and olefin reaction At first, the styrene derivatives (3aa-3ap) with various functional groups on the benzene ring were studied Most of the para substituted styrene substrates undergo the required conversion, and the yield is more than 80% However, the yield decreased about 10% when the neighborhood applied space pressure The electronic properties of the substrate have little effect on the reaction efficiency It is worth noting that under the current conditions, β - methylstyrene can also be successfully reacted with a yield of 76% They then turned their attention to the range of application of the arylhydrazine fragment Different arylhydrazine is also suitable for this method, including arylhydrazine with different groups at different positions on the aromatic ring (3ba - 3BF, 70-86% yield) The stereoscopic effect was observed, so the yield of ortho substituted hydrazine was slightly lower In order to prove the synthesis effect of this method, the research team also carried out a gram level reaction of 1a and 2a, and the yield of 3AA was 78% Results of stern Volmer fluorescence quenching experiment (source: greenchemistry) in order to further understand the reaction mechanism, the research team also used stern Volmer fluorescence quenching experiment to understand the process of oxidation catalysis Based on the experimental observation, they proposed an explanation of the reaction mechanism, as shown in the figure below The reaction mechanism speculated by the research team (source: Green Chemistry) summarized that Zhu Chunyin's team developed a method of conversion of metal free alkene to ketone, which was realized by visible light initiated oxidation free radical addition of arylhydrazine The reaction avoids the use of unstable aryl diazo or diaryl iodine, thus avoiding a series of possible problems By using arylhydrazine as the source of free radicals, the by-products of this visible light initiated reaction are H 2O and N 2 Through model experiments, they determined the best reaction conditions, and explored the scope of application of the reaction, the results are very satisfactory The yield is generally about 80%, and there is potential for further optimization Finally, stern Volmer fluorescence quenching experiment was carried out to further reveal the specific mechanism of this reaction Because of its simple operation, easy access to reagents, wide range of substrate application and adaptability to large-scale synthesis, this green reaction has great development value Paper link: http://pubs.rsc.org/en/content/article landing/2017/gc/c7gc01083dා! 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