Substrate–Solvent Crosstalk—Effects on Reaction Kinetics and Product Selectivity in Olefin Oxidation Catalysis

Rita N Sales,Pedro D Vaz,Carla D Nunes,Samantha K Callear

doi:10.3390/chemistry3030054

Abstract

In this work, we explored how solvents can affect olefin oxidation reactions catalyzed by MCM-bpy-Mo catalysts and whether their control can be made with those players. The results of this study demonstrated that polar and apolar aprotic solvents modulated the reactions in different ways. Experimental data showed that acetonitrile (aprotic polar) could largely hinder the reaction rate, whereas toluene (aprotic apolar) did not. In both cases, product selectivity at isoconversion was not affected. Further insights were obtained by means of neutron diffraction experiments, which confirmed the kinetic data and allowed for the proposal of a model based on substrate–solvent crosstalk by means of hydrogen bonding. In addition, the model was also validated in the ring-opening reaction (overoxidation) of styrene oxide to benzaldehyde, which progressed when toluene was the solvent (reaching 31% styrene oxide conversion) but was strongly hindered when acetonitrile was used instead (reaching only 7% conversion) due to the establishment of H-bonds in the latter. Although this model was confirmed and validated for olefin oxidation reactions, it can be envisaged that it may also be applied to other catalytic reaction systems where reaction control is critical, thereby widening its use.

Highlights

Developing selective reactions is an everyday struggle that mankind faces in order to replicate nature’s work, which is remarkable at all levels
We described the differences observed when a given epoxidation reaction is conducted in different solvents
The most striking effect concerns the reaction kinetics, which is largely sensitive to the solvent change

Summary

Introduction

Developing selective reactions is an everyday struggle that mankind faces in order to replicate nature’s work, which is remarkable at all levels. Further insights of solvent effects influencing catalytic oxidation reactions in terms of both substrate conversion and product selectivity modulation have been addressed, from the pioneering work of Corma in 1996 [32] to more recent studies [33,34,35]. In all of these studies, the authors attempted to provide evidence of solvent influence as an explanation for the observed effects. All reactions were monitored (substrate conversion and product yield) by sampling at regular time intervals and analyzed using a Shimadzu QP-2010 Plus GC-MS system (Kyoto, Japan)

General Procedure for Catalytic Epoxidation of Olefins

H O O tBu

Findings

Conclusions