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Alcohols and Phenols as Hydrogen Bonding Catalysts
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
Catalysis of organic reactions using noncovalent interactions has emerged as a powerful strategy in the field of organocatalysis. Among various noncovalent interactions, hydrogen bonding has been the most frequently utilized interaction due to its strength, directionality, and predictability. In this respect, alcohol and phenol derivatives were among the first classes of hydrogen bond donors to be used as catalysts. The seminal work of Hine on biphenylenediols as dual hydrogen bond donors [13–19] and the more recent studies of Rawal on TADDOLs as chiral hydrogen bonding catalysts [23,24] represent major breakthroughs in the field that paved the way for the catalysis of a broad range of transformations using alcohol- and phenol-based organocatalysts. The relative low acidity of alcohols and the use of high catalyst loadings can be considered as the major drawbacks associated with these catalytic systems. The recent introduction of fluorinated alcohols with enhanced acidities to the field of hydrogen bonding catalysis [41,43] has the potential to offer solutions to these limitations and is expected to lead to the development of more effective catalysts.
Catalysis
Published in Aidé Sáenz-Galindo, Adali Oliva Castañeda-Facio, Raúl Rodríguez-Herrera, Green Chemistry and Applications, 2020
Fabiola N. de la Cruz, José Domingo Rivera-Ramírez, Julio López, Miguel A. Vázquez
The term organocatalysis is related to use a substoichiometric quantity exclusive of small organic compounds to accelerate a chemical reaction (List, 2007). This area has been increased exponentially not only in organic chemistry, but also to other science areas. From the 1980’s to the present day, more than 4242 records have been reported relating to organocatalysis, most of these records have focused on organic chemistry (Liu and Wang, 2017).
New Greener Developments in Direct Amidation of Carboxylic Acids
Published in Ahindra Nag, Greener Synthesis of Organic Compounds, Drugs and Natural Products, 2022
Andrea Ojeda-Porras, Diego Gamba-Sánchez
Organocatalysis has played a major role in organic chemistry, especially in the asymmetric synthesis of complex natural products. The use of environmentally friendly metal-free catalysts is highly encouraged in green chemistry due to its availability, low toxicity and low cost. As mentioned before, the direct amidation of carboxylic acids can be achieved by using light; this process can be accelerated with organic compounds (such as dyes) catalysts in several photo redox reactions, increasing yield and scope while diminishing reaction time. In 2018, Singh and coworkers reported a photoredox amidation reaction of carboxylic acids and amines in the presence of eosin Y 39 as catalyst.23 As shown in Scheme 2.9a, the desired amides were isolated in good to excellent yields in less than three hours in toluene at room temperature. Interestingly, this sustainable approach was applied to both aromatic and aliphatic carboxylic acids with aliphatic primary and secondary amines, with chemoselectivity toward the aliphatic primary amine observed in the presence of aromatic 40c or heteroaromatic amines 40b. Highly hindered amines 40a as well as further functionalized carboxylic acids 40d were also successfully synthesized. A plausible mechanism is illustrated in Scheme 2.9b. The catalytic cycle starts with the photoexcitation of eosin Y to eosin Y*. This reactive species is reduced by the carboxylate anion generating radical 41 and radical anion eosin Y. Eosin Y is regenerated by oxygen while intermediate 42 is obtained from 41 by a diradical coupling reaction. Nucleophilic attack on the carbonyl group at 42 by the amine yields one equivalent of the desired amide and one equivalent of peracid salt 43 which is also transformed in the amide. Due to the regeneration of Eosin Y, the single electron transfer that enables the reaction can be done with 2 mol% catalyst.
L-proline catalyzed green synthesis and anticancer evaluation of novel bioactive benzil bis-hydrazones under grinding technique
Published in Green Chemistry Letters and Reviews, 2021
Abdelwahed R. Sayed, Sobhi M. Gomha, Hany M. Abd El-lateef, Tariq Z. Abolibda
In recent years, organocatalysis has been a subject of great interest due to the ease of obtaining, easily stored, stable, non-toxic and inexpensive. In addition, organocatalysis is a type of catalysis that can be done without the need for inert atmosphere or anhydrous conditions in mild conditions (21, 22). Organocatalysis has now seen an exponential growth in many publications, the interest in new catalysts and reactions in which asymmetric products can be produced in classical and valuable reactions using organocatalysts, such as Diels–Alder, Mannich, Michael reactions and others (23, 24). Among organocatalysts, L-proline is a readily obtainable naturally occurring amino acid and is easy to obtain in high enantiomeric purity it has been reported as an eco-friendly catalyst for the synthesis of several heterocycles with high efficiency (25–30). In particular, L-proline has received a lot of attention because of its dual position as a ligand and catalyst (31–33).
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).
Highly enantioselective asymmetric reduction of aromatic ketimines promoted by chiral enantiomerically pure sulfoxides as organocatalysts
Published in Journal of Sulfur Chemistry, 2018
Zuzanna Wujkowska, Stanisław Leśniak, Piotr Kiełbasiński, Michał Rachwalski
Organocatalysis, which is defined as the use of small organic molecules as catalysts for organic transformations, has been included among the most successful concepts in asymmetric synthesis. In the recent years, the field of asymmetric organocatalysis has been hugely developed to become a complementary discipline to conventional metal catalysis. The vast majority of the currently available enantioselective organocatalysts is based on stereogenic carbon centres serving as the stereochemical inductors. In this way, the asymmetric formation of various bonds, among them carbon–carbon and carbon–heteroatom bonds, in a highly enantioselective manner has been realized [1,2].