Studies on the solvent-free and waste-free Knoevenagel condensation

The mechanochemical reaction of malononitrile with various aldehydes was studied with the goal to achieve quantitative stoichiometric conversion of the reactants to their corresponding benzylidene-malononitriles in absence of any solvents and catalysts. Besides already known reactions in the solid state, reactions with liquid aldehydes were also attempted. These reactions were carried out in vibration and planetary ball mills as well as, for comparison, in a melt under microwave irradiation. A successful quantitative conversion depended strongly on the choice of the aldehyde, precluding a generalization of the reaction scheme. Furthermore, the results obtained from the various approaches for non-classical heating (ball mills, microwaves) were compared.