Visible Light-Induced Semiconductor Catalyzed Decarboxylation Coupling Reaction For The Synthesis Of Imidazo[1,2-a] Pyridine Derivatives

One of the crucial research interest in organic synthetic chemistry is modern green chemistry.Its basic meaning is developing the green synthetic routes,abandoning and reducing the use of non-renewable resources,and maximizing reduction of environmental pollution caused by the synthesis process.Visible light is the most direct,abundant and clean energy source in nature.In the past decade,visible light was adopted as source in more and more green organic synthesis reactions to replace the traditional heating mode.Photocatalysts are usually used in energy transfer and electron transfer to convert light energy into chemical energy during photochemical conversion.Among them,polypyridine complexes of ruthenium and iridium,Ir(ppy)3 and Ru(bpy)3Cl2,are the most widely known for their excellent photocatalytic performance in many organic chemical reactions.But their applications were limited by the potential toxicity of precious metals ruthenium and iridium.In addition,there are inexpensive organic dyes that can be used as photocatalysts for organic transformations,but these photocatalysts have poor stability and low catalytic activity.In order to enhance the recycling efficiency of photocatalysts while maintain the high activity and reduce difficulty of the separation during the post-treatment in the homogeneous catalytic system,the development of heterogeneous photocatalysts is an enthusiastically pursued of current organic synthetic chemistry.At present,novel semiconductor materials represented by perovskite(e.g.leadhalide perovskites CsPbBr3)and carbon nitride(C3N4)are mainly used as heterogeneous photocatalysts for the water photolysis,photocatalytic CO2 reduction and degradation of organic pollutants.However,their applications as heterogeneous photocatalysts have been less investigated in organic synthesis,and the types of organic reactions that can be catalyzed were not abundant,especially in the synthesis of molecular skeletons with pharmacological activities,which need to be further developed.The imidazole[1,2-a]pyridine skeleton is one of the main skeletons in the development of medicinal chemistry.This medicine containing imidazole[1,2-a]pyridine skeleton has a variety of pharmacological activities,including antianxiety,anticonvulsant,anti-HIVs and so on.Therefore,the development of a new strategy for the green synthesis of imidazole[1,2-a]pyridine derivatives mediated by heterogeneous perovskite(CsPbBr3)and carbon nitride(C3N4)photocatalysts has significant practical implication.In this paper,two novel semiconductor,perovskite and carbon nitride nanolayers,are used as heterogeneous photocatalysts to promote the decarboxylation of imidazole[1,2-a]pyridine and N-phenylglycine under visible light conditions.The specific content is as follows:Two methods for decarboxylation coupling reaction of imidazole[1,2-a]pyridine mediated by heterogeneous photocatalysts:1.Visible light-induced perovskite-catalyzed synthesis of imidazole[1,2a]pyridine derivatives(Path 1):Using imidazole[1,2-a]pyridine and N-phenylglycine derivatives as reaction substrates under the catalysis of CsPbBr3 in irradiation of white light or sunlight,stirring in 1,2-dichloroethane solvent under air atmosphere for 8 h,and 27 aminomethylated products of imidazole[1,2-a]pyridine were obtained.Through a series of control experiments,the reaction mechanism was proposed:electrons(e-)and holes(h+)was formed on the surface of CsPbBr3 under the irradiation of visible light,which could be the effective candidates for single-electron donors and acceptors,respectively.N-phenylglycine was oxidized to radical 5 by the generated holes via a single electron transfer(SET)with the decarboxylation and deprotonation.Radical 5 was then selectively added to the C=C bond of imidazole[1,2-a]pyridine to afford radical 6,which subsequently are oxidized and deprotonated by oxygen in the air(Main pathway)or holes in the valence band(Minor pathway)to obtain the target product;The quantum yield(2.8%)and recycling performance of the catalyst in this reaction were investigated.The catalyst can be recycled for 5 times in the reaction without significant loss of activity.Path 1 mechanism:2.Visible light-induced g-C3N4 nanolayer catalyzed synthesis of imidazole[1,2-a]pyridine derivatives(Path 2):Using imidazole[1,2-a]pyridine and N-phenylglycine as reaction substrates,the reaction was carried out under the catalysis of the g-C3N4 nanolayer induced by blue light,and stirred for 17 h in the green solvent(dimethyl carbonate)and air atmosphere.19 aminomethylated products of imidazole heterocycles were obtained by the decarboxylation coupling reaction.A reasonable reaction mechanism was proposed as follows through a series of control experiments,the recycling performance of the catalyst was investigated,and the catalyst can be recycled at least 7 times without loss reactivity.Path 2 mechanism:...