Acoblament deshidrogenatiu d’alcohols i amines catalitzat per manganès per formar aldimines i H2

Abstract

Dehydrogenative coupling of alcohols and amines using a manganese based catalyst allows obtaining aldimines, which have a wide reactivity. Furthermore, as a result of this coupling, molecular hydrogen is also obtained, which is a fuel. Its combustion does not allow the generation of carbon based compounds, such as carbon dioxide or carbon monoxide, only water. Therefore, the aldimine synthesis represents an interesting reaction from all points of view. In this work, a manganese based catalyst is presented to be able to provide this synthesis, using benzene as a solvent. It has been proved that the reaction pathway proposed by Milstein and collaborators is possible by checking the energy barriers using Density Functional Theory (DFT) calculations. The formation of an aldehyde from an alcohol has a high energy barrier (81.4 kcal/mol), which decreases to 31.5 kcal/mol when using the mentioned manganese catalyst, showing that the presence of the catalyst is necessary for the first step of the synthesis. In the second stage, where the obtained aldehyde reacts with an amine to give the aldimine going through an intermediate, according to the proposed initial mechanism, the catalyst does not intervene. The formation of the intermediate before the loss of a water molecule passes through a transition state where the catalyst does not participate (it has an energy barrier of 36.0 kcal/mol), but it is assisted by a molecule of water (27.7 kcal/mol). The second part of the reaction, which consists of a hydrolysis step to give the desired aldimine also goes through a transition state assisted by water (42.0 kcal/mol). Water is not only a by-product of the formation of aldimines in the last synthesis step, but it also becomes a perfect cocatalyst that reduces the reaction barriers based on proton transfers. It has also been shown that the manganese catalyst in the initial form can lead to secondary reactions. Even though the latter reactions are kinetically favoured, they are thermodynamically less stable than the formation of the desired product