Copper-Catalyzed Tandem Etherification Reactions of 2-(2,2-dibromovinyl)phenol Derivatives with Phenols

In this paper, copper-catalyzed tandem etherification reactions of 2-(2,2-dibromovinyl)phenol derivatives with phenols was developed. In the presence of copper acetate as catalyst and cesium carbonate as base, 2-(2,2-dibromovinyl)phenol derivatives reacted with phenols in N,N-dimethyl formamide to generate the corresponding 2-phenoxybenzofurans in good yields in one-pot via tandem etherification reactions under ligand-free reaction conditions. Many reaction parameters including bases, copper salts and solvents have been investigated. By screening of bases in the reaction, we found that cesium carbonate is the best one among the tested bases. Experiments shown that strong bases and weak bases, such as potassium hydroxide, potassium t-butoxide, potassium acetate and sodium acetate are unfavorable for this reaction. Some common copper catalysts were examined, and the results indicated that copper acetate exhibits the highest reactivity in the reaction. For this reaction, a divalent copper salt is better than a monovalent copper salt. However, CuO and Cu2O gave the poor yield of the product. The solvent also plays a very important role in the reaction. We found that the optimum solvent is N,N-dimethyl formamide (DMF). Dimethyl sulfoxide, N,N-dimethylacetamide, and N-methylpyrrolidone also afforded good product yield. The further investigation revealed that the reaction temperature was one of the factors affecting the yield of the reaction. Scope of the substrate was studied and 24 new 2-aryloxybenzofuran compounds were synthesized in good yields. 2-(4-tert-Butylphenoxy)-5-chlorobenzofuran has the highest reaction yield in 94%. 4-(Benzofuran-2-yloxy)benzonitrile has the lower yield. Phenol containing an electron-withdrawing group on the benzene ring is difficult to perform the reaction. On the other hand, when an electron-donating group is introduced to 2-(2,2-dibromovinyl)phenol, such as 5-methoxy-, or 4-t-butyl-2-(2,2-dibromovinyl)phenol, the reaction was failed under the present reaction conditions. This method has several advantages including commercial availability of starting materials, simple operation, easy purification of products, and in the absence of ligand. This tandem reaction has the potential applications. It is important to note that the biological activity of the obtained 2-aryloxybenzofuran compounds will be further investigated and will be reported in due course.