The formation of carbon-carbon bonds is a quest that has required a lot of research throughout the last 125 years of history of chemistry. The creation of these bonds is key to producing a wide variety of advanced products with great societal importance. In the past, the Knoevenagel reaction entailed a big part of these reactions. All the Knoevenagel reactions are performed with nitrogen-based catalysts, categorized as tertiary amines, secondary amines, primary amines, and ammonium salts. These catalysts operate through different reaction paths, with very deviant intermediates concerning each category. Subsequently, these intermediates create new research opportunities considering their catalytic activity. Some remarks are made in the context of green chemistry, with quantitative analysis using e-factor calculations. In this perspective, the importance of Knoevenagel chemistry exemplified, which is repeatedly overlooked throughout history.

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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. ARTICLE HISTORY

This paper presents mechanistic considerations on an efficient, green, and solvent-free Knoevenagel procedure for the chemical transformation of furanic aldehydes into their corresponding α,β-unsaturated compounds. In the proposed mechanism furanic aldehydes react with ammonia, released from ammonium salts, to form a catalytically active double Schiff base. The catalytic intermediates involved in the condensation step are characterized.

(2019) Mechanistic considerations and characterization of ammonia-based catalytic active intermediates of the green Knoevenagel reaction of various benzaldehydes,