Studies On Acyloin Condensation Catalyzed By Thiazolium Salts

Acyloin condensation of aldehyde was of great importance for the synthesis of intermediates, which were used as the important building blocks for the synthesis of many pharmaceuticals and fine chemicals. In the work, thiazolium salts have proved to be the alternatives to traditional cyanide for this condensation of aromatic or aliphatic aldehydes. A serial of thiazolium salts were synthesized and their catalytic performances were investigated in the reaction of acyloin condensation. Based on their excellent catalysis in the acyloin condensation reaction, a novel technique to prepare the diacetyl was designed used paraldehyde as the starting material. The acetoin was obtained through the acyloin condensation of aldehyde over the thiazolium salts catalyst in self-pressure autoclave, and then further oxidized to diacetyl. The process has emerged as an important technique to synthesize diacetyl for the characteristic of low cost, high yield, easy operation and environmentally friendly.(1) A serial of thiazolium salts were synthesized through the reaction of 5-(2-hydroxyethyl)-4-methylthiazolium with different hydrocarbon. The resultant thiazolium salts were well characterized by NMR and FT-IR.(2) The performances of the synthesized thiazolium salts were investigated in the solvent-free benzoin condensation reaction. The influence of the kind of catalyst, the amount of catalyst, reaction time, and reaction temperature on the benzoin condensation on the catalystic activities was studied systemically. It found that the 3-ethyl-5-(2-hydro-xyethyl)-4-methylthiazolium bromide was effective catalyst in the ben-zoin condensation reaction of benzaldehyde. Under the optimum reacti-on conditions,72.5% yield of benzoin was obtained.(3) The synthesized thiazolium salts were employed to catalyze acyloin condensation reaction of acetaldehyde to acetoin in the self-pressure autoclave. The influence of the type and the amount of the thiazolium salts, reaction temperature, and reaction time on the acyloin condensation was investigated. It showed that 3-ethyl-5-(2-hydroxyethyl)-4-methylt hi-azolium bromide was the excellent catalyst over the reaction. Under the optimum reaction conditions, the conversion of acetaldehyde achieved 89.4% with the actual yield was 76.0%, and the selectivity of acyloin was 96.3%. Anhydrous FeCl3 was used to further oxidize acetoin to the diacetyl, obtained the purity of 90.5%, total yield of 60.8% of the diace-tyl.(4) According to the disadvantages of traditional technology, a tentative exploration on one-step synthesis of acetoin and diacetyl was carried out, which offered an innovation method of commercial production.