Visible Light Photoredox Catalysis Promoting The Synthesis Of Pyrazole Derivatives

The structure of pyrazole derivatives have a wide range of applications in various fields,particularly in pharmaceutical and agricultural applications.At present,there are many methods for the synthesis of pyrazole derivatives.But the reaction conditions are harsh,such as high reaction temperature,the use of heavy metals,and hazard oxidants,which are contrary to the concept of green synthesis.In the last decade,visible light photoredox catalysis has become an important method in organic synthesis.We have tried to catalyze the synthesis of pyrazole derivatives by using visible light photoredox catalysis.In this paper,through the exploration of the reaction conditions and the study of the mechanism,an efficient and green method for the synthesis of pyrazole derivatives promoted by visible light photoredox catalysis has been developed.The main work includes the following aspects:(1)the synthesis of pyrazole derivatives through sunlight initiated radical annulation of unsaturated hydrazonesPart I: we have described a practical method for the synthesis of pyrazoles through sunlight initiated radical annulation of α,β-unsaturated hydrazones.This protocol features mild sustainable reaction conditions,and broad substrate scope for aromatic compounds.The reaction does not require visible light photocatalyst and saves the cost of the reaction,having a high economic and practical value.Based on the analysis of UV-Vis absorption of the substrate,this reaction was designed to avoid the use of external photocatalysis and proceeds via direct irradiation of N-centred anion by sunlight.Taken together with its operational simplicity,readily available reagents,and amenability to gram-scale synthesis,this green reaction will find practical applications for the synthesis of pyrazole derivatives.(2)One-pot method for the synthesis of polysubstituted pyrazole derivatives through visible light photoredox catalysisPart II: we explored a one-pot method for the synthesis of polysubstituted pyrazole derivatives.At first,1,4-diphenylbutadiyne was produced by phenylacetylene through visible light photoredox catalysis.Then,a series of polysubstituted pyrazole derivatives were generated by 1,4-diphenylbutadiyne reacting with hydrazines.The method employs very green and mild reaction conditions.Since the structure of pyrazole derivatives is widely used in medicine,agriculture and other fields,the search for a green synthetic method is a goal pursued by scientific researchers.This method provides a new approach for the synthesis of polysubstituted pyrazole derivatives.