Supported trifluoromethanesulfonic acid as catalyst in the synthesis of flavone and chromone derivatives

Solid acid catalysts based on trifluoromethanesulfonic acid (triflic acid, TFMS) were synthesized using titania with different textural properties as support, obtained by increasing the calcination temperature (Ti-100, Ti-200, Ti-300, and Ti-400). They were characterized by FIF-IR, DTA-TGA, and BET. The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. According to this technique, the catalysts present very strong acid sites. Their acid strength decreases slightly in the order Ti-100 > Ti-200 > Ti-300 > Ti-400, as a result of the partial dehydroxylation that takes place during the thermal treatment of the supports. The amount of trifluoromethanesulfonic acid firmly adsorbed on the support depends on the number of -OH groups available on the titania to be protonated, which decreases with the increase in the calcination temperature. The new catalysts were tested in the cyclization of 1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione. The reaction experiments were performed using toluene as the solvent at reflux. In all cases, the product (flavone) was obtained with high selectivity. The activity of the catalysts is dependent on the textural properties of the support used to obtain the catalysts and the amount of triflic acid firmly adsorbed on it. The same reaction conditions were applied to the preparation of eight substituted flavones and chromones. Conversions up to 88% were obtained using the Ti-100 supported TFMS catalyst. (C) 2007 Elsevier B.V. All rights reserved.