Michael Addition and Ester Aminolysis in w/o AOT-Based Microemulsions

Esperanza Fernández, Luis García-Río, J. Ramón Leis, Juan C. Mejuto, and Moisés Pérez-Lorenzo

New J. Chem., 2005, 29, 1594–1600

A kinetic study was carried out on the aminolysis of 4-nitrophenylacetate (NPA) by N-methylbenzylamine (NMB) and the Michael addition of piperazine to N-ethylmaleimide (NEM) in AOT–isooctane–water microemulsions. The experimental results show that the rate constant observed for the aminolysis of NPA increases together with the concentration of the surfactant and the water content of the system. Conversely, the values of kobs obtained for the Michael addition to NEM decrease as the surfactant concentration increases. This difference in behavior must be a consequence of the different distribution of the reagents throughout the different microenvironments of the microemulsion. The application of the formalism of the pseudophase shows that the aminolysis of NPA takes place solely in the interface of the microemulsion, and the rate constant in this pseudophase increases along with the water content of the system, due to a greater hydration of the interface. The different distribution of the reagents in the Michael addition of piperazine to NEM makes it possible for the reaction to take place simultaneously in the interface and the aqueous microdroplet, increasing the percentage of the reaction on the interface while the water content of the system decreases. Thus, the difference in rate observed for the Michael addition between the interface and the aqueous microdroplet is twelve-fold, due to the smaller polarity of the interface with regard to the aqueous medium.