Mechanism study and preparing (hetero)arylidene malononitriles through a greener and scalable catalytic strategy

Abstract

One of the important concerns in the field of catalysis is to find a reliable route to reduce the transition energies to reach the corresponding product. The mechanism of Knoevenagel condensation of 2-chlorobenzaldehyde and malononitrile in the presence of imidazole, an organocatalyst, is investigated through control experiments. In this study, the interactions and intermediates in synthesizing 2-chlorobenzylidenemahmonitrile (CS) are studied by FTIR and NMR analyses. First, a screen activity of seven nitrogen-based organocatalysts for preparing 2-chlorobenzylidenemahmonitrile (CS) was conducted in ethanol, an eco-friendly medium. The conversion of 100 % and excellent yields were obtained in a short reaction time. The products could be crystallized directly from the reaction solution. After separating pure products, the residue solution was employed directly in the next run without any concentration, activation, purification, or separation. Then, the generality and substrate scope of the new strategy were investigated in the presence of imidazole as a selected nitrogen-based organocatalyst. In addition, the scale-up synthesis of 2-chlorobenzylidenemahmonitrile (CS) was carried out, afforded crystalline products with 95 ± 2 % yield in five consecutive runs. Energy efficiency, cost saving, greener conditions, the use of catalytic loading organocatalyst and its high recyclability, prevention of waste, and recycling extractant by a rotary evaporator for non-crystallized products, demonstrate the newly developed metal-free and halogen-free catalytic methodology as a more sustainable process for the synthesis of (hetero-)arylidene malononitriles in the laboratory and industrial scale.