Heterogeneized Brönsted base catalysts for fine chemicals production: grafted quaternary organic ammonium hydroxides as catalyst for the production of chromenes and coumarins

A solid Brönsted base catalyst with a uniform distribution of basic sites with high base strength has been obtained by anchoring tetraalkylammonium hydroxide on the surface of MCM-41. It is highly active and selective for carrying out Knoevenagel and aldol condensation as well as Michael additions under mild reaction conditions. Salicyaldehyde and diethylglutaconate react on the catalyst to give the corresponding chromene and coumarine. While the phenoxide anion is the common intermediate, the cyclisation to chromene occurs very fast on the anchored tetraalkylammonium hydroxide. A high selectivity to chromenes is then obtained, being the catalyst of potential interest for the production of chromenes for thermochromic, photochromic and pharmaceutical materials.