Organic reactions in self-organized surfactant systems
The aim of this work has been to investigate how microemulsions and micellar systems can be used as reaction media to overcome reactant incompatibility, enhance reaction rates and especially to control product composition, i.e., introducing regioselectivity, in organic synthesis.
It was found that a significant increase in reaction rate can be obtained in properly formulated self-assembled systems as compared to the rates obtained in non-surfactant systems. The reaction rate differed between different surfactant classes and in many of the reactions studied cationic surfactant systems performed better than nonionic surfactant systems.
It was found that there can be a considerable difference in reaction rate also between self-assembled systems based on surfactants of the same class. Investigation of the rates of a substitution reaction in microemulsions based on either a fatty alcohol ethoxylate or a sugar-based amphiphile, both of which are nonionic surfactants, showed that the reaction went much faster in the system based on the ethoxylated surfactant. This was attributed to a lower dielectric constant of the surfactant headgroup layer of the fatty alcohol ethoxylate than of the sugar-based surfactant, which should result in higher reactivity of the attacking anionic nucleophile. An alternative explanation to the higher reactivity of the system based on the ethoxylated surfactant is that this microemulsion has a higher dynamics, i.e., faster rate of disintegration and reformation of the oil-water interface.
A substitution reaction between an alkyl phenol and a water soluble electrophile was studied in microemulsions and in micellar systems based on either a nonionic or a cationic surfactant. It was found that the reaction went faster in the self-assembled systems based on cationic surfactant than in those based on nonionic surfactant and that the micellar systems gave higher reaction rates than the microemulsions. The high reactivity in the systems based on the cationic surfactant was attributed to formation of a π-cation complex between the benzene ring of the alkyl phenol and the surfactant headgroup.
It was showed that the oil-water interface of microemulsions could be used as template to induce regioselectivity both of electrophilic aromatic substitutions and of nucleophilic aliphatic substitution reactions. Monosubstitution of a symmetrical bifunctional reactant constituted a particularly striking example of how a microemulsion can be used as a tool to control regioselectivity.