Triethylamine: a potential N-base surrogate for pyridine in Knoevenagel condensation of aromatic aldehydes and malonic acid

Cinnamic acids are intermediates with significant potential for synthesis of several industrially important chemicals. Classically, cinnamic acids are produced through Knoevenagel condensation of aromatic aldehydes and malonic acid in the presence of an organocatalyst and large amounts of carcinogenic pyridine. An alternative pyridine free reaction scheme for Knoevenagel condensation of malonic acid and aromatic aldehydes was investigated by replacing pyridine with an aliphatic tertiary amine surrogate in toluene as the reaction medium. Of the three aliphatic tertiary amines used, namely, triethyl amine (TEA), trioctyl amine (TOA) and tributyl amine (TBA), only TEA afforded cinnamic acids in comparable yields to those obtained with pyridine. Validation through a computational analysis is attempted to provide an explanation for the observed role of TEA as an aliphatic N-base instead of TBA and TOA. The use of TEA as a mild base in place of pyridine can be seen as playing the dual role of a base catalyst as well as a phase transfer agent evidenced by the in-process ATR-FITR spectroscopy. Use of the TEA-toluene system in place of pyridine can be seen as resulting in a process that affords ease of handling, separation and recycling of the solvent and the catalyst.