The green Knoevenagel condensation: solvent-free condensation of benzaldehydes

This paper presents a novel, green Knoevenagel procedure for the chemical transformation of benzaldehydes into their corresponding α,β-unsaturated acids. The essential part of this procedure is a solvent-free condensation which uses environmentally benign amines or ammonium salts as catalysts instead of pyridine and piperidine as used in the traditional Knoevenagel condensation. The condensation step is followed by a decarboxylation in the solid phase, resulting in high overall yields and purity. The influence of temperature and catalyst type on the yield of sinapinic acid was monitored for the reaction between syringaldehyde and malonic acid. After optimization of this reaction, its scope was explored for various types of benzaldehydes demonstrating a broad applicability of this procedure. The developed method provides good to an excellent conversion of various benzaldehydes and high selectivity to the respective α,β-unsaturated acids in an environmentally friendly and efficient way.