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Abstract:Microwave Assisted Synthesis of Substituted Indoles Using Montmorillonite as Catalyst
?uboš JANKOVI? and Peter KOMADEL
Institute of Inorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Slovakia
Heterogeneous organic reactions have proven useful to chemists in the laboratory as well as in the industrial context. A related development that had a profound impact on these heterogeneous reactions is the use of microwave (MW) irradiation technique for the acceleration of organic reactions.
A series of acid catalysts was prepared by heating of NH4-saturated montmorillonite at 200-600ºC for 24 h. Their catalytic activity was tested in microwave assisted Fischer indole synthesis. This reaction is sufficiently sensitive to modification of the catalyst and thus suitable for testing catalytic activity of modified montmorillonites. Most of the prepared catalysts were able to catalyse the test reaction and produce indole in higher than 50% yields. The most active catalyst was obtained after heating at 300 ºC. It was only slightly less effective (84%) than commercially available acid-activated K10 catalyst (90%).
Gradual collapse of the interlayers was proved by X-ray diffraction, showing that d001 of the samples heated at 500 and 600 ºC shrank close to 9.6 ?, i.e. to the thickness of the montmorillonite layers. Acidity of the prepared materials was studied using pyridine as adsorbate. The vibrational spectrum of adsorbed pyridine is sensitive to the nature of the adsorption site. The IR spectroscopic evidence show that all samples heated up to 400oC and fresh from pyridine exhibited a peak at 1490 cm-1 that corresponded with catalytic activity and can be associated with Lewis acid sites.
The DTG curve for this pyridine-adsorbed material had three distinct desorptive zones because the base occupied three different sites as: (a) physisorbed base, (b) base bound to Lewis acid sites, and (c) protonated base. The DTG curves obtained for desorption of pyridine exhibited maxima near 50, 160, and 320°C. The intensity of the maximum near 160oC, attributed to the desorption from Lewis acid sites, partially corresponded with the catalytic activity of the samples.
