SILICA-IMMOBILISED SALOPHENS FOR THE SYNTHESIS OF CYCLIC CARBONATES

Cyclic carbonates are an important class of organic molecules with both interest from both the commercial and academic world. These valuable chemicals can be produced from corresponding epoxides and CO2, catalysed by salen or salophen complexes and ligands. One drawback of this method of production is the high cost of the catalyst and purifying the product.
A green synthetic route to produce symmetrical salophens immobilised onto silica has been developed from 4-allylanisole, a cheap and naturally occurring compound. This process is split into three key phases, the preparation of a salicylaldehyde 3a, the hydrosilylation for 4a, and the salophen formation and immobilisation to produce immobilised salophen 6a. Control over the immobilisation ratio of silica to salophen has been demonstrated by varying the stoichiometric ratio of 5a to tetraethyl orthosilicate. This scheme has been developed with the 12 principles of green chemistry in mind and each step has been analysed for its environmental impact.
Once prepared these immobilised salophens have been shown to be active as catalysts for the formation of cyclic carbonates in the reaction between epoxides and gas phase CO2. Seven cyclic carbonates have been produced utilising these immobilised salophens with activity matching that of the homogeneous equivalent.