Organic Synthesis Inside Particles in Water:  Lewis Acid−Surfactant-Combined Catalysts for Organic Reactions in Water Using Colloidal Dispersions as Reaction Media

A Lewis acid−surfactant-combined catalyst (LASC) has been developed and applied to Lewis acid-catalyzed organic reactions in water. LASCs are composed of water-stable Lewis acidic cations such as scandium and anionic surfactants such as dodecyl sulfate and dodecanesulfonate and are easily prepared. These catalysts have been successfully used for various typical carbon−carbon bond-forming reactions such as aldol, allylation, and Mannich-type reactions in water. Furthermore, the results of aldol reactions in various solvents show that water is the best solvent for the LASC-catalyzed reactions. A preliminary kinetic study of the aldol reaction showed that the initial rate in water was 1.3 × 102 times higher than that in dichloromethane. In the workup procedure, it was demonstrated that centrifugation of the reaction mixture led to phase separation without addition of any organic solvents. The LASCs was found to form stable colloidal dispersions rapidly in the presence of reaction substrates in water, even when the substrates are solid. The characterization of the colloidal particles has been carried out by means of dynamic light scattering, light microscopy, transmission electron microscopy, and atomic force microscopy. These observations revealed the size of ∼1 μm and the spherical shape of the particles. It was suggested that most of the substrates and catalyst molecules were concentrated in the spherical particles, which acted as a hydrophobic reaction environment and enabled the rapid organic reactions in water. In light of the increased demand for reduction of organic solvents in industry, the surfactant-aided Lewis acid catalysis described here may have practical consequences in organic synthesis.