“On‐Droplet” Chemistry: The Cycloaddition of Diethyl Azodicarboxylate and Quadricyclane

Abstract

Sharpless and co-workers previously studied the [2s + 2s + 2p]c ycloaddition of diethyl azodicarboxylate (DEAD) and quadricyclane and reported that the addition of water to the neat reagents caused an acceleration in the reaction rate,g iving birth to what has been called "on-water" chemistry.W eh ave examined the same reaction in aqueous microdroplets (ca. 5 mmd iameter) and find that the cycloaddition reaction is accelerated even further (by afactor of 10 2 ) compared to that of the "on-water" reaction reported previously.T he trends of acceleration in solvents other than water demonstrated by Sharpless and colleagues were replicated in the corresponding microdroplet experiments.W ealso find that DEAD reacts with itself to form av ariety of hydrazine carboxylates and intercept intermediates of this reaction in microdroplets to validate am echanism proposed herein. We suggest that "on-droplet" chemistry,s imilar to "on-water" chemistry,may be ag eneral process of synthetic interest.Water, long the solvent of choice for biochemical reactions, has seen increased use in organic synthesis. [1] Thes eminal discovery by Breslow and co-workers [2] of the rate acceleration of Diels-Alder cycloadditions in water suggested that the poor solubility of hydrophobic reagents may promote, rather than hamper, certain classes of reactions.I ndeed, the exploration of reactions at the organic-aqueous interface has become at opic of research in the synthetic community. [3] In these reactions,a no rganic phase floats atop ab ulk water layer. Termed "on-water" chemistry by Sharpless and coworkers, [4] these reactions now encompass adiverse range of transformations including cycloadditions, [5] sigmatropic rearrangements, [6] aldol reactions, [7] and Grignard additions. [8] In almost all cases,anacceleration of several to several hundredfold has been noted relative to reactions conducted in organic solvents or even under neat conditions.M ellouli et al. examined "on-water" cycloaddition reactions in biphasic microfluidic systems and investigated the role of surface-tovolume ratios on conversion rates,d emonstrating that acceleration can be ascribed to the increased rate of conversion at the surface. [9] Water is an ideal solvent in terms of safety and cost. Additionally,m any reactions conducted "on-water" allow for the facile isolation of products,w hich often precipitate and can be collected by simple filtration rather than after an often complex and time-consuming workup. [4a] Work from our group, [10] as well as others, [11] has demonstrated that reactions in microdroplets generated through electrospray ionization (ESI) are often orders of magnitude faster than their counterparts in bulk solution. This unique reactivity in the ESI process was first proposed by Augusti et al. [12] during their study of Eberlin transacetalization reactions in which acylium ions were generated by ESI of at etramethylurea solution and gaseous acetals were subsequently introduced and allowed to react with the acylium ions. In this work, Augusti et ...

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