Photoinitiators As Copper(II) Reductants: A New Approach to the Copper-Catalyzed Azide–Alkyne Cycloaddition

The copper catalyzed azide–alkyne cycloaddition (CuAAC reaction) click reaction is utilized in a wide range of synthetic strategies, from dendrimer and block copolymer synthesis to bioconjugation.  Traditional methods for catalyzing the CuAAC reaction include the direct addition of Cu(I) salts, the reduction of copper sulfate via sodium ascorbate, and the comproportionation reaction of copper metal and Cu(II).  Recently, we have demonstrated that the CuAAC reaction can be catalyzed using conventional photoinitiators to reduce Cu(II).  This approach allows for spatial control of the CuAAC reaction, as demonstrated by patterning of hydrogel formation and labelling of hydrogels with features as small as 25 microns.  This photochemical approach not only simplifies patterning surfaces, compared to traditional approaches such as dip pen lithography, but also allows the CuAAC reaction to be used for spatially selective coupling reactions within a material.  Herein, we discuss recent kinetic experiments suggesting that undesirable catalyst consuming reactions, such as disproportionation, further reduction, and oxidation are suppressed, and that the ligands present play a key role in the reaction.  Accordingly, like the conventional approach of reducing Cu(II) using sodium ascorbate this process is robust and does not require purging to eliminate oxygen.  However, unlike conventional approaches this methodology readily allows for spatial control of the reaction using conventional photolithographic techniques.