Study On The Cu₂O-Catalyzed CuAAC Reactions 'in Water'

Since the seminal discovery of the Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) by the groups of Sharpless and Meldal independently, this'near–perfect'bond–forming reaction has found a myriad of applications in chemistry, biology, and materials science. Aplethora of copper catalytic systems have been utilized to catalyze this transformation including the combination of copper with other elements such as Cu(II)/sacrificial reducing agent, Cu(0)/oxidizing agent or Cu(I)/auxiliary ligand, and the use of Cu(I) species alone. Despite these catalytic systems available, it still appeals as a more simple and practical catalyst for this reaction. Compared with other cuprous species, Cu₂O is the most readily available and above all an inexpensive catalyst. For a long time, the surface of Cu₂O has been assumed to be the catalytically active species in metallic Cu(0)-catalyzed AAC reactions, supported by experimental evidence. Unfortunately, reactions directly catalyzed by Cu₂O powder were usually met with incomplete conversion and poor yields even after longer reaction times. Until now, the catalytic capability of Cu₂O in the CuAAC reactions has yet been thoroughly exploited because of lacking appropriate reaction conditions.As our continued interest in developing and applying smart organic reactions to label biomolecules under bio-compatible conditions, we noted that Cu₂O has been emerging as a versatile catalyst for organic reactions such as coupling and cycloaddition reactions, some of which can be performed in water, so we envisaged that water might be a required medium for Cu₂O-AAC reactions. Further studies realized this hypothesis.Cu₂O as the catalyst in water was found to be quite robust for the azide-alkyne cycloaddition (AAC) reaction, which was verified by a wide variety of applicable azides and alkynes. Water was proved to play an essential role because of a significant rate acceleration compared with reactions using organic solvents and conducted under neat conditions. The high catalytic performance of Cu₂O/H₂O system was further argued by decreasing the catalyst loading to ppm levels.