"On-Droplet" Chemistry: The Cycloaddition of Diethyl Azodicarboxylate and Quadricyclane.

Sharpless and co-workers previously studied the [2σ+2σ+2π] cycloaddition of diethyl azodicarboxylate (DEAD) and quadricyclane and reported that the addition of water to the neat reagents caused an acceleration in the reaction rate, giving birth to what has been called "on-water" chemistry. We have examined the same reaction in aqueous microdroplets (ca. 5 μm diameter) and find that the cycloaddition reaction is accelerated even further (by a factor of 10(2) ) compared to that of the "on-water" reaction reported previously. The trends of acceleration in solvents other than water demonstrated by Sharpless and colleagues were replicated in the corresponding microdroplet experiments. We also find that DEAD reacts with itself to form a variety of hydrazine carboxylates and intercept intermediates of this reaction in microdroplets to validate a mechanism proposed herein. We suggest that "on-droplet" chemistry, similar to "on-water" chemistry, may be a general process of synthetic interest.