The green Knoevenagel condensation: solvent-free condensation of benzaldehydes

Abstract

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

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

“…In the pursuit to develop eco-friendly Knoevenagel condensation reactions to produce ligno-phytochemicals, we reported earlier about a green Knoevenagel method with ammonium bicarbonate as a catalyst and source of ammonia (25). This method provides good to an excellent conversion of various benzaldehydes and high yields of the corresponding cinnamic acids in an efficient, rapid, scalable, and environmentally friendly way.…”

Sustainable Synthesis of p-Hydroxycinnamic Diacids through Proline-Mediated Knoevenagel Condensation in Ethanol: An Access to Potent Phenolic UV Filters and Radical Scavengers

“…The reaction mixture was then evaporated under vacuum (conversion yields were determined by 1 H NMR) and then purified by column chromatography on a C- 18 Figure 1. Synthetic pathways to p-hydroxycinnamic diacids according to Vivier et al [31] (1), Mangala et al [32] (2), Schijndel et al [33] (3), and our study (4). [ Although the reaction conditions of the above procedures are globally in line with the green chemistry principles and HPLC conversions are promising, isolated yields remain relatively low [31] or not described [32,33].…”

“…Synthetic pathways to p-hydroxycinnamic diacids according to Vivier et al [31] (1), Mangala et al [32] (2), Schijndel et al [33] (3), and our study (4). [ Although the reaction conditions of the above procedures are globally in line with the green chemistry principles and HPLC conversions are promising, isolated yields remain relatively low [31] or not described [32,33]. Additionally, Mangala et al's [32] (Figure 1, pathway 2) method uses titan derivatives (Ti-PCS) as catalyst.…”

“…Preferably, the reaction can also be carried out in Figure 1. Synthetic pathways to p-hydroxycinnamic diacids according to Vivier et al [31] (1), Mangala et al [32] (2), Schijndel et al [33] (3), and our study (4). [a] Isolated yield; [b] evaluated by HPLC.…”

Repeatable molecularly recyclable semi‐aromatic polyesters derived from lignin

“…Previously, a green, efficient, and scalable method has been developed, producing cinnamic acid analogs from 4‐hydroxybenzaldehydes 1 by condensation with malonic acid. [ 19,20 ] The calculated E‐factors in the low range of 1–2 are remarkable for a transformation that produces fine chemicals that typically have E‐factors in the range of 5–100. [ 21 ] Because the environmental impact of ammonium bicarbonate is relatively low, the unfriendliness quotient Q is probably also small but needs further investigation.…”