One-pot synthesis of multisubstituted imidazoles under solvent-free conditions and microwave irradiation using Fe₃O₄@SiO₂–imid–PMAⁿmagnetic porous nanospheres as a recyclable catalyst

Abstract

An efficient, green and eco-friendly procedure has been developed using Fe 3 O 4 @SiO 2 -Imid-PMA n as magnetic catalyst for rapid and an improved synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles under solvent-free conditions and microwave irradiation in excellent yields. The reactions in conventional heating conditions were compared with the microwave-assisted reactions. The operational simplicity, practicability and applicability of this protocol to various substrates make it an interesting alternative to previous procedures.The present approach offers several advantages such as short reaction times, high yields, simplicity of operation, easy work-up, a cleaner reaction and ease of recovery and reusability of the catalyst by magnetic field. Also, nanocatalyst can be easily recovered by a magnetic field and reused for the next reactions for at least 5 times without distinct deterioration in catalytic activity. SEM, BET, DLS and leaching of catalyst after each reaction cycle were investigated.

One-pot synthesis of multisubstituted imidazoles catalyzed by Dendrimer-PWAn nanoparticles under solvent-free conditions and ultrasonic irradiation

“…Aromatic aldehydes having electron-withdrawing groups (Table 3, entries 2, 3, 6-8, 18, 23 and 26) reacted at a faster rate compared with aromatic aldehydes substituted with electron-releasing groups (Table 3,entries 4,5,9,10,(14)(15)(16)(17)24,25,(27)(28)(29). Moreover, the neutral conditions of our methodology make it possible to use acid-sensitive aldehydes without side reactions and/or byproducts.…”

“…Then nucleophilic attack of the nitrogen of ammonia, obtained from NH 4 OAc, and amine on the protonated carbonyl group, resulted in the formation of diamine intermediate (7). In the presence of Dendrimer-PWA n , intermediate (7) condenses with benzil (8) to form intermediate (9) which in turn forms tetrasubstituted imidazoles by dehydration.…”

“…Generally, trisubstituted imidazoles are synthesized by the condensation of 1,2-diketones, an aldehyde and ammonium acetate by using various catalysts such as: InCl 3 Á3H 2 O [6], Yb(OTf) 3 [7], TBAB [8], Fe 3 O 4 @SiO 2 -Imid-PMA n [9], Nanocrystalline MgAl 2 O 4 [10], Ferric(III) nitrate supported on kieselguhr [11], SBA-15/2,2,2-trifluoroethanol [12], and HOAc [13]. 1,2,4,5-tetrasubstituted imidazoles are synthesized by the condensation of 1,2-diketones, an aldehyde, ammonium acetate and primary amine in the presence of various catalysts such as silica gel or silica gel/NaHSO 4 [14], K 5 CoW 12 O 40 .3H 2 O [15], HY zeolite [16], Fe 3 O 4 @SiO 2 -Imid-PMA n [9], trifluoroethanol [12], HClO 4 -SiO 2 [17], heteropolyacids [18], FeCl 3 .6H 2 O [19], 1-Butyl-3-methylimidazolium bromide [20], trityl chloride [21], tetrabutyl ammonium bromide [22], alumina [23], 1,4-diazabicyclo [2,2,2]octane (DABCO) [24], nano-TiCl 4 ÁSiO 2 [25], PPA-SiO 2 [26], nanocrystalline sulfated zirconia (SZ) [27] and silica-bonded propylpiperazine N-sulfamic acid (SBPPSA) [28]. However, some of these synthetic methods have limitations such as harsh reaction conditions, use of hazardous chemicals with often expensive acid catalysts, complex working and purification procedures, long reaction times, and moderate yields.…”

Organocatalyzed Solvent Free and Efficient Synthesis of 2,4,5‐Trisubstituted Imidazoles as Potential Acetylcholinesterase Inhibitors for Alzheimer's Disease

“…To obtain good yields, the reaction is generally catalyzed by the selection of different compounds such as silica gel, BF 3 ·SiO 2 , molecular iodine, zirconium (IV) acetylacetonate Zr(acac) 4 3 , silica chloride, silica bonded propyl-Nsulfamic acid nanocatalyst (NHSO 3 H-KIT-5), Ce (SO 4 ) 2 · 4H 2 O, Keggin-structured phosphotungstic acid (HPW). [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] All these new methodologies have their own advantages but most of these are associated with drawbacks like use of toxic solvents, prolong times, complicated work-up, purification difficulties, poor yields, expensive reagents, hence there is a dire need to develop such synthetic routes that are environmentally friendly and economically. Various organocatalysts are being used nowadays to accelerate chemical reaction with easy handling.…”

Palladium nanoparticles immobilized on EDTA‐modified Fe3O4@SiO2 nanospheres as an efficient and magnetically separable catalyst for Suzuki and Sonogashira cross‐coupling reactions

“…Therefore, magnetic core‐shell nanostructures, which are consist of Fe 3 O 4 magnetic nanoparticles, are being coated with a silica layer as a stabilizer to create an interesting scaffold with attractive properties such as chemically stability, unique magnetic properties, and uniform size to prevent their aggregation . Furthermore, the silica layer provides large amount of hydroxyl groups on the surface of composite particles and as a result, provides the opportunity to link different functional molecules for many particular applications …”