The Knoevenagel reaction: a review of the unfinished treasure map to forming carbon-carbon bonds

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.