Cage-Catalyzed Condensation

With the help of a palladium-based cationic cage, researchers in Japan have coaxed a Knoevenagel condensation to take place under neutral conditions in water (J. Am. Chem. Soc., DOI: 10.1021/ja210068f). Knoevenagel condensations are reversible reactions involving attack of a carbanion of a 1,3-dicarbonyl compound on an aldehyde or a ketone followed by dehydration. It’s tough to do the transformation in water because the solvent pushes the equilibrium in favor of hydrated intermediates. And yet, similar condensation reactions are common in biological systems, thanks to the stabilization provided by the specialized environments of enzymes. Seeking to create a synthetic version of this stabilized environment, Makoto Fujita, Takashi Murase, and Yuki Nishijima of the University of Tokyo used a catalytic amount of a palladium cationic cage complex in several aqueous Knoevenagel condensations of aldehydes. They found that the cage-catalyzed reactions produced the desired condensation product in yields as high as 96%. The same reaction conditions without the cage produced only traces of the desired product. The researchers believe the cationic cage, with its hydrophobic core, can sequester organic substrates and stabilize ionic intermediates, much as enzymes do.