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

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.

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“…This effect is more dramatic for solvents with hydrogen bond donor/acceptor capabilities, such as acetonitrile, which slow down reactions. The results shown in Figure 4b, evidencing the slower kinetics in acetonitrile are supported by those findings reported earlier [33] …”

“…The reason behind this is most probably due to the hydrogen bonding capabilities of this solvent, which hinder availability of molecules for reacting. This effect was probed by us and described recently, evidencing that acetonitrile can seriously slow down reactions as already discussed above and elsewhere [33] …”

“…For instance, based on Figure 4b, methylphenyl sulfide reached 75 % conversion in dichloromethane after 1 h, while when the reaction was run in acetonitrile the substrate conversion reached 73 % after 2 h. This kinetic effect, where the solvents play an active role in catalytic chemical reactions, has been described and assessed by us recently. [33] In that work, based on neutron diffraction data and supported by quantum chemical calculations, it was found that solvents play active roles in catalytic reactions. This effect is more dramatic for solvents with hydrogen bond donor/ acceptor capabilities, such as acetonitrile, which slow down reactions.…”

“…This effect was probed by us and described recently, evidencing that acetonitrile can seriously slow down reactions as already discussed above and elsewhere. [33] The catalyst efficiency was also assessed changing the substrate:oxidant ratio to stoichiometry similarly to what was studied for the methylphenyl sulfide substrate. The obtained results demonstrated once more for these cases that the sulfoxide was the major product instead of the sulfone (Table 1, entries 12, 14, 16, 18 and 20).…”

“…Once more this could be due to the strong hydrogen bonding capabilities of the former solvent, which hinder the reaction and therefore kinetics were in a general way slower in acetonitrile as already discussed earlier in this work and found in the literature. [33] Desulfurization with hydrogen peroxide Impelled by the promising results obtained with TBHP as oxidant for both substrates, the work continued with the study of how the catalyst worked when hydrogen peroxide was used as oxidant. Its efficiency concerning the substrate:oxidant ratio was also assessed.…”