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Organic Synthesis
Published in Suresh C. Ameta, Rakshit Ameta, Garima Ameta, Sonochemistry, 2018
Chetna Ameta, Arpit Kumar Pathak, P. B. Punjabi
An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a P-hydroxyaldehyde or P-hydroxyketone, followed by dehydration to give a conjugated enone. Aldol condensations are important in organic synthesis, because they provide a smooth way to form carbon-carbon bonds (Carey and Sundberg, 1993). These reactions are usually catalysed by strong acids or bases, and a variety of different Lewis acids have been evaluated in this reaction (Reeves, 1966). Unfortunately, the presence of a strong acid or base promotes the reverse reaction (Hathaway, 1987) and this leads to the self-condensation of the reacting materials to give the corresponding byproducts in low yields (Nakano et al., 1987).
Mechanistic considerations and characterization of ammonia-based catalytic active intermediates of the green Knoevenagel reaction of various benzaldehydes*
Published in Green Chemistry Letters and Reviews, 2019
Jack van Schijndel, Dennis Molendijk, Harmen Spakman, Edward Knaven, Luiz Alberto Canalle, Jan Meuldijk
Catalytically promoted aldol condensations are important in organic synthesis as they provide an efficient way to form carbon–carbon bonds, which are the basis of organic chemistry (5). The aldol condensation can either be acid-catalyzed or base-catalyzed. A base-catalyzed aldol condensation in the presence of an amine is called a Knoevenagel condensation. In 1898 Emil Knoevenagel was the first who realized that amines were truly catalytic (“Contactsubstanz”). He isolated catalytic intermediates and as a result of this laid the fundamentals of “organocatalysis” (6,7). Unfortunately, the impact of his research has not yet been valued by everyone (8–11). Organocatalysis is the catalysis of reactions with small organic molecules and generally seen as a more environmentally friendly form of catalysis opposed to for example (toxic) transition metal catalysts (12,13).
Synthesis of bioactive heterocycles using reusable heterogeneous catalyst HClO4–SiO2 under solvent-free conditions
Published in Green Chemistry Letters and Reviews, 2018
Leimajam Vartima Chanu, Thokchom Prasanta Singh, Laishram Ronibala Devi, Okram Mukherjee Singh
On the basis of the results obtained by the control experiments, tentative mechanisms of the chromenes and dihydropyrimidine synthesis have been shown below (Scheme 2). Initially, the condensation product of β-oxodithioester 1a and o-hydroxybenzaldehyde 2a, activated by HClO4–SiO2, generates an enolate A, which facilitates in subsequent intramolecular aldol condensation to give phenyl(2-thioxo-2H-chromen-3-yl)methanone 3a. The mechanism of Biginelli reaction has been discussed in various experimental and theoretical reports, and has been a topic of much debate. A plausible mechanism for the synthesis of 5-methylmercaptothiocarbonyl-4-aryl-3,4-dihydropyrimidin-2(1H)-ones 4a is presented in (Scheme 2). For dihydropyrimidines, the first step in this reaction, the acid-catalyzed formation of an acyl imine intermediate A formed by reaction of the aldehyde with urea, is the key rate-limiting step. Interception of the iminium ion by β-oxodithioester 1a produces an open-chain ureide B that subsequently cyclizes to the dihydropyrimidinone 4a.
Acid–base bifunctional magnesium oxide catalyst prepared from a simple hydrogen peroxide treatment for highly selective synthesis of jasminaldehyde
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Temperature plays a significant role in the aldol reaction (Figure 10). At 100°C, the reaction over MgO-NO3-H2O2-300 was slow and after 5 h, only 37% conversion was obtained. Increasing the temperature to 125°C resulted in a higher rate, allowing close to 94% conversion after 5 h. The high selectivity to jasminaldehyde (87–88%) was nearly unchanged in spite of different temperatures. From the Arrhenius equation, the activation energy is calculated to be 55 kJ/mol (Figure 11). The relatively low value shows that the rate-determining step in the reaction involves surface adsorption and desorption.