Investigating the Diels-Alder Reaction between Trans,trans-2,4-hexadienyl acetate and N-propylmaleimide at the Oil-Water Interface using Microfluidics

Abstract

Abstract: Greener synthetic routes for producing organic molecules are desirable to reduce environmental pollution and lower manufacturing costs.2 In this context, Sharpless & co-workers reported that it is possible to achieve dramatic rate enhancements in a number of cycloaddition reactions, if they were conducted in vigorously mixed oil-water emulsions instead of bulk organic solvents.1 These interfacial reactions – which came to be known as “on-water” reactions – thus present a tantalizing prospect for green chemistry. However, despite many experimental and theoretical studies along this theme, a clear understanding of the governing factors and mechanisms remains unavailable. For instance, proposed mechanisms vary from dangling hydrogen bonds stabilizing transition states, to the specific adsorption of hydroxide ions at the water-organic interface, and the partial dissolution of reactants in water leading to products.3,4,5Additional effects include sharp variations in dielectric constants and hydration levels across the interface and hydrodynamic effects during vigorous stirring. In this thesis, we investigate a Diels-Alder reaction between two water-insoluble reactants – trans,trans-2,4-hexadienyl acetate and N-propylmaleimide– to disentangle the contributions of bulk reactions from interfacial reactions. We compare the conversion of reactants into products in the following scenarios: pure reactants (i) mixed into each other (neat condition), (ii) dissolved in hexane, (iii) dissolved in hexane and vigorously stirred with water (1:1 v/v), and (iv) dissolved in hexane and vigorously stirred in water-methanol mixtures. In addition to vigorously-stirred emulsions that produce polydisperse emulsions, we designed and developed microfluidic devices that allowed us to precisely controlled the water-organic interfacial area. With this6 device, we pin-point interfacial effects on the reaction rates.