Solvent-free Henry and Michael reactions with nitroalkanes promoted by potassium carbonate as a versatile heterogeneous catalyst

The use of a simple weak inorganic base such as potassium carbonate facilitated the formation of carbon-carbon bonds through both the Henry and the Michael reactions with nitrocompounds. The application of this catalyst under environmentally friendly solventless heterogeneous conditions gave satisfactory to good yields of β-nitroalcohols, involving aliphatic and aromatic starting materials, as well as high to excellent yields in the formation of Michael adducts using several different Michael acceptors and nitroalkanes

One-pot multicomponent nitro-Mannich reaction using a heterogeneous catalyst under solvent-free conditions

An environmentally-friendly, one-pot multicomponent reaction of various aldehydes, amines and nitroalkanes for the synthesis of β-nitroamines is here described. Amberlyst A-21 supported CuI was found to be a highly efficient novel heterogeneous catalyst for the three-component nitro-Mannich reaction between aldehydes, amines and nitroalkanes. The developed protocol is performed in a solvent-free medium to produce a variety of β-nitroamines in good to excellent yields within short reaction times. The catalyst can be easily prepared and recovered. It has been tested up to eight times with only a minor activity loss.peer-reviewe

Aza-Michael mono-addition using acidic alumina under solventless conditions

Aza-Michael reactions between primary aliphatic and aromatic amines and various Michael acceptors have been performed under environmentally-friendly solventless conditions using acidic alumina as a heterogeneous catalyst to selectively obtain the corresponding mono-adducts in high yields. Ethyl acrylate was the main acceptor used, although others such as acrylonitrile, methyl acrylate and acrylamide were also utilized successfully. Bi-functional amines also gave the mono-adducts in good to excellent yields. Such compounds can serve as intermediates for the synthesis of anti-cancer and antibiotic drugs.peer-reviewe

Novel biopolymer-based catalyst for the multicomponent synthesis of -aryl-4-aryl-substituted dihydropyridines derived from simple and complex anilines

Although Hantzsch synthesis has been an established multicomponent reaction method for more than a decade, its derivative, whereby an aniline replaces ammonium acetate as the nitrogen source, has not been explored at great length. Recent studies have shown that the products of such a reaction, N-aryl-4-aryldihydropyridines (DHPs), have significant anticancer activity. In this study, we successfully managed to synthesize a wide range of DHPs (18 examples, 8 of which were novel) using a metal-free, mild, inexpensive, recoverable, and biopolymer-based heterogeneous catalyst, known as piperazine, which was supported in agar–agar gel. In addition, 8 further examples (3 novel) of such dihydropyridines were synthesized using isatin instead of aldehyde as a reactant, producing spiro-linked structures. Lastly, this catalyst managed to afford an unprecedented product that was derived using an innovative technique—a combination of multicomponent reactions. Essentially, the product of our previously reported aza-Friedel–Crafts multicomponent reaction could itself be used as a reactant instead of aniline in the synthesis of more complex dihydropyridines.peer-reviewe

A new two-step synthesis of 2-alkylated 1, 4-diketones and α-alkylated γ-keto esters

2-Alkylated 1,4-diketones and a-alkylated g-keto esters can be easily prepared by a two-step procedure which involves first the conjugate addition of a nitroalkane to an enedione derivative in acetonitrile with DBU as base, followed by chemoselective hydrogenation of the C–C double bond (H2, 10% Pd/C) of the Michael adduct, obtained after elimination of nitrous acid.peer-reviewe

DABCO-Amberlyst®15 : a versatile heterogeneous catalyst in the multicomponent synthesis of tetrahydronaphthalenes and tetrahydroquinolines

Bicyclic ortho-amino carbonitriles (tetrahydronaphthalenes) are crucial in their role to develop other heterocyclic compounds such as dicyanoanilines which can be used in optical electronic (optronic) devices. Tetrahydronaphthalenes can be obtained via the four-component reaction of malononitrile (2 equivalents), a cyclic ketone and an aldehyde. On the other hand, 5,6-substituted-2-amino-3- cyanopyridines (tetrahydroquinolines) have been found to be potent anti-cancer, anti-hypertensive, antimicrobial and anti-inflammatory agents. The synthesis of such compounds is directly linked by the single reactant replacement (SRR) technique because they derive from the combination of malononitrile, a cyclic ketone, an aldehyde and ammonium acetate as a nitrogen source. In this study, the activity of the heterogeneous metal-free catalyst DABCO supported on Amberlyst® 15 was explored in relation to the synthesis of both mentioned scaffolds. To the best of our knowledge, there are no instances in the literature where this has been achieved using the same catalyst, furthermore a novel successful DABCO-Amberlyst®15 catalysed one-pot sequential combination of multicomponent reactions (MCR2) was also reported.peer-reviewe

Piperazine-Amberlyst®15-catalysed synthesis of 2-amino-4H-chromenes, chromeno[2,3-b]pyridines and chromeno[2,3-d]pyrimidines

Chromenes, biologically-active scaffolds, and their variants have been often synthesized by the combination of salicylaldehyde, malononitrile, and nucleophilic species (indoles, naphthols, nitro compounds, thiols). Whereas this combination furnishes specifically 2-amino-4H-chromenes, other related compounds such as chromeno[2,3- b]pyridines and chromeno[2,3-d]pyrimidines may be also attained using similar readily available reactants: salicylaldehyde, two equivalents of malononitrile and a thiol for the former and malononitrile, two salicylaldehyde equivalents and an amine for the latter. To the best of our knowledge, there are no reported studies which have attempted to synthesize these products using the same catalyst. Hence, the aim of the below study was to find a cheap and recyclable catalyst that would be able to drive the synthesis of all three products. Positively, piperazine supported on the polymeric sulfonic acid resin Amberlyst® 15 was found to be an inexpensive and easily-prepared novel catalyst that could be used to synthesize all three derivatives (33 examples, 18–82%) in fairly good yields whilst also being recyclable and reusable (for up to four or five runs).peer-reviewe

Thia-Michael reaction under heterogeneous catalysis

Thia-Michael reactions between aliphatic and aromatic thiols and various Michael acceptors were performed under environmentally-friendly solvent-free conditions using Amberlyst® A21 as a recyclable heterogeneous catalyst to efficiently obtain the corresponding adducts in high yields. Ethyl acrylate was the main acceptor used, although others such as acrylamide, linear, and cyclic enones were also utilized successfully. Bifunctional Michael donor, 3-mercaptopropanoic acid, positively furnished the product, albeit in a lower yield and after leaving the reaction to take place for a longer time. The catalyst was easy and safe to handle and successfully recycled for five consecutive cycles.peer-reviewe

Solvent-free, one-pot, multicomponent synthesis of xanthene derivatives

An efficient and environmentally benign one-pot condensation of cyclic diketones, aldehydes and naphthols was achieved with 1,4-diazabicyclo[2-2-2]octane supported on Amberlyst-15 as a novel catalyst, producing a variety of benzoxanthenones in good to excellent yields. The advantages of this multicomponent reaction include the use of a heterogeneous catalyst, solventless conditions and a simple methodology that is atom-economical and results in low E-factor values. A total of 17 xanthene derivatives, including two novel molecules, were synthesized and then characterized. Aromatic aldehydes carrying an electron-withdrawing group provided excellent yields. Appreciable results were also obtained using aliphatic aldehydes. The catalyst is fully recyclable and can be reused up to six times.peer-reviewe

Facile imine synthesis under green conditions using Amberlyst® 15

Imines and their derivatives are of great interest to organic synthetic chemists due to their involvement as key intermediates which facilitate the construction of nitrogen heterocycles, particularly the formation of alkaloids. Imine formation by condensation of primary amines with aromatic aldehydes and cyclohexanone has been investigated under environmentally-friendly solventless heterogeneous catalysis. An array of different imines was obtained in excellent yields in appreciably short reaction times using Amberlyst®15 as a heterogeneous catalyst. The latter was used owing to its high commercial availability, recyclability, ease of separation from the reaction mixture, and versatility.peer-reviewe