Sustainable cascade reaction combining transition metal-biocatalysis and hydrophobic substrates in surfactant-free aqueous solutions

A cascade reaction consisting of a Heck reaction followed by an enzyme-catalyzed reaction is carried out in different aqueous solutions. In particular, the impact of the structuring of the reaction solvent is investigated. For this purpose, several ternary mixtures of water, isopropanol, and benzyl alcohol, including surfactant-free microemulsions, as well as binary mixtures of water and isopropanol, are taken into account. A micellar solution of the surfactant TPGS-750-M serves as a reference system. The coupling of the two reactions can be successfully performed in all 3 types of solvents, whereby the best result is achieved in a surfactant-free microemulsion. In addition, our surfactant-free system allows for reducing the temperature during the Heck reaction. The studies further reveal that the structures built up by surfactants are not necessarily the main reason allowing organic reactions to be carried out in water. Rather, it is a question of solubility and stability of the reaction components. Better solubility does not always correlate with increased reactivity. We thus provide a deeper understanding of solvent – reactivity relationship and want to advert an approach beside micellar solvents for transferring organic reactions into environmentally friendly aqueous reaction solutions.