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Accelerating Organic Reactions in Droplets
1  The Chinese University of Hong Kong


Herein I present two works of using droplets to study the mechanism of “on water” reactions and to perform large-scale organic synthesis. A series of organic reactions such as Diels-Alder cycloadditions have been found to proceed dramatically faster in a heterogeneous mixture of the reactants and water than in the homogeneous mixture. It is not clear whether the interfacial chemical species or the hydrodynamic effects or both are responsible for the rate acceleration.[1]


In the first work, we produced droplets containing diethyl azodicarboxylate (DEAD) and quadricyclane to study the interface effect on organic reactions (Figure 1a).[2] We confined the droplets in the glass capillaries (Figure 1b) to minimize the hydrodynamic effects, and analyze the “on-water” reaction to find out which factor, the catalysis by the free OH groups at the interface, the hydrodynamic effects, or both, is responsible for the rate acceleration. The cycloaddition reaction process was recorded by a CCD camera. The results showed the reaction proceeded in three steps (Figure 2). The comparison of the reaction in droplet and the bulk emulsion (Table 1) showed that the organic-water interface effectively accelerated the “on-water” reaction to the same level as in bulk solution, indicating that the organic-water interface was the major catalysis factor, and the hydrodynamic effects were negligible during the “on-water” reaction.[3]


In the second work, we produced charged droplets for the large-scale synthesis of isoquinoline. Previously, Richard and co-workers used electrospray to generate charged droplets and analyze the reaction process by mass spectrometry [4]. They found the surface protons of the charged droplets efficiently catalyzed conversion of the reactant benzalaminoacetal to the product isoquinoline. Herein, we combined the ultrasound and electric field to continuously generate large amounts of charged droplets (Figure 3), which served as the micro-reactors for the isoquinoline synthesis. The synthesis result was evaluated by mass spectrometry (Figure 4), indicating the high efficiency of the droplet-based synthesis.