Synthesis of Oxazolidinones from Epoxides and Isocyanates Catalysed by Aluminium Heteroscorpionate Complexes

The combination of an aluminium(heteroscorpionate) complex and tetrabutylammonium bromide acts as a highly efficient catalyst system for the synthesis of oxazolidinones from epoxides and isocyanates. Twenty two complexes were tested derived from a range of bispyrazole ligands and containing 1-3 aluminium atoms per complex. The optimal catalyst was found to be a bimetallic complex of a thioacetamidate ligand. Under the optimal reaction conditions (80 degrees C in toluene for 24h using 5mol% of both aluminium catalyst and tetrabutylammonium bromide co-catalyst), six epoxides were reacted with six aromatic isocyanates, giving 25 oxazolidinones in moderate to excellent yields showing broad substrate scope. The regiochemistry of the reaction (to produce 3,4- or 3,5-oxazolidinones) is controlled by the substrate with epoxide ring opening occurring preferentially at the less hindered end of the epoxide unless a substituent on the epoxide can stabilise a positive charge.