Síntese e aplicações de catalisadores com propriedades magnéticas em estudos metodológicos de reações orgânicas

Abstract

The development of nanotechnology has enabled the emergence of new materials, called magnetic nanoparticles (MNPs). The high surface area combined with the easy removal from the reaction media, makes the MNPs promising immobilization supports. Thus, in this work the synthesis and characterization studies of eight materials based on magnetic nanoparticles coated with silica (core@shell structure) resulting in two classes of catalysts are described. The first class, comprised the amino-functionalization of these magnetic nanoparticles, bound to ethylenediaminetetraacetic acid (EDTA), which in turn complexed the transition metal ions (Mn, Cr, Co and Cu) and rare earth ions (Tb and Eu). The second class, involved the functionalization of the magnetic materials with the 3- chloropropyltrimethoxysilane group covalently linked to the organic molecule14- diazabicyclo[2.2.2]octane (DABCO) or the organic molecule hexamethylenetetramine (HMTA). The catalysts were characterized by absorption spectroscopy in the infrared region, X-RD, thermogravimetric analyses, elemental analysis of carbon, hydrogen and nitrogen, and magnetization measurements. These materials were employed in Knoevenagel condensation, synthesis of imininochromenes and coumarins, synthesis of isatin-β-thiosemicarbazones and Biginelli reaction. In the Knoevenagel condensation reactions, the catalyst with basic site Fe3O4@SiO2-CPTMS-DABCO proved to be more efficient, showing better yields and reaction times (15-120 min) than the other investigated catalysts. For the sequential reactions of synthesis of iminochromes and coumarins, via Knoevenagel condensation, the catalyst with acid site Fe3O4@SiO2-1N-EDTA-Tb3+ showed better performance among all catalysts with acidic and basic sites tested. Providing yields of 79-98 % for coumarins and 69-91 % for iminochromenes, in the latter the stoichiometric control allowed the synthesis of 2-(4,5- diamino-1-cyano-7-hydroxy-1,10b-dihydro-2H-chromeno[3,4-c]pyridin-2- ylidene)malononitrile. In the synthesis reactions of isatin-β-thiosemicarbazones, the Fe3O4@SiO2-CPTMS-DABCO catalyst proved to be more efficient for, converging from literature reports, which commonly describe this reaction under acid catalysis. Good isolated yields (81-97%) and reaction times (60-180 min) were evidenced. Finally, in the Biginelli reaction, once again the Fe3O4@SiO2-1N-EDTA- Tb3+ catalyst stood out, denoting greater efficiency and selectivity for the formation of the expected product. However, it is necessary to explore other reaction conditions to obtain better times. In general, the synthesized magnetic nanoparticles showed good catalytic activity, being easily recovered by magnetic separation and were reused in Knoevenagel reactions and iminochromene synthesis, with 83- 98 and 81-91 % yields, respectively.