Tandem Catalytic Conversion of Benzaldehyde Dimethyl Acetal in Silica Stabilized Pickering Emulsions

Abstract

Compartmentalization is a strategy adapted by nature in order to perform multiple reactions simultaneously. Due to compartmentalization these reactions do not interfere. This strategy can also be used to perform ``one-pot’’ reactions. These are reactions in which sequential steps are performed in the same reaction vessel. In case these steps require different catalytic mechanisms, these reactions are referred to as tandem reactions.

Inspired by nature, we used the compartmentalization effects of emulsion droplets in Pickering emulsions to keep an acid- and a base-catalyst separated. These catalysts were used for acid-base tandem catalytic reactions. The Pickering emulsions were prepared with different silica types as emulsifying particles: hydrophobized Aerosil 200, hydrophobized Stöber silica and hydrophobic HDK® H20. These particles had been studied with TEM, contact angle measurements, IR spectroscopy and nitrogen physisorption. Pickering Emulsions prepared with 6:2 toluene to water volume ratios stabilized by 3 wt-% HDK® H20 were the most stable emulsions of the studied ones. Confocal fluorescence microscopy with Nile red had showed that these emulsions are water-in-oil emulsions which were very polydisperse. Emulsions were also imaged using optical microscopy.

In this study, two showcase tandems were investigated: a deacetalization-Henry and a deacetalization-Knoevenagel tandem reaction of benzaldehyde dimethyl acetal. First, the reactions were studied separately in biphasic systems and Pickering emulsions. Deacetalization of benzaldehyde dimethyl acetal catalyzed by hydrochloric acid yielded more benzaldehyde in Pickering emulsions than in biphasic systems, also when both hydrochloric acid and piperidine were present in the reaction mixtures. However, when both hydrochloric acid and piperidine were present, conversions and yield were lower, indicating some mutual destruction of acid and base. Separate base-catalyzed Henry reaction of benzaldehyde with nitromethane to yield trans-β-nitrostyrene was neither successful in biphasic systems nor in Pickering emulsions. Deacetalization-Henry reaction did not yield the desired trans-β-nitrostyrene either. Base catalyzed Knoevenagel condensation of benzaldehyde with malononitrile, however, was successful. Benzylidene malononitrile had even been formed during reaction in 3 hours at room temperature in both Pickering emulsions and biphasic systems. The reaction was more successful in biphasic systems than in Pickering emulsions, probably because the whole reaction takes place in the organic phase and the organic phase is crowded with water droplets in the emulsions. Also when both acid and base were present, Knoevenagel reactions were more successful in biphasic systems. However, benzaldehyde conversions and benzylidene yields decreased compared to the Knoevenagel reactions with only base, which again indicates mutual destruction of acid and base. Furthermore, deacetalization-Knoevenagel reactions of benzaldehyde dimethyl acetal were performed successfully and were studied with varying hydrochloric acid and piperidine concentrations. In this research it was found that deacetalization-Knoevenagel reactions could not be performed in simple toluene-water biphasic systems, but it worked well in Pickering emulsions due to the fact Pickering emulsions are able to partially compartmentalize reagents and keep water soluble hydrochloric acid and oil soluble bases separate, preventing mutual destruction to some extent. Yields of benzylidene malononitrile in these tandem reactions varied between 29 and 59 % after 3 hours at room temperature. If reaction was performed over 24 hours, benzylidene malononitrile yield even reached 76 %.