Natural and synthetic phosphates: New and clean heterogeneous catalysts for the synthesis of 5-arylhydantoins

Natural and synthetic phosphates were used as heterogeneous catalysts to promote the Friedel–Crafts reaction between 5-bromohydantoin and aromatic compounds. In the case of phenol, ortho and para isomers were always obtained. Fluoroapatite (FAP) and natural phosphate (NP) lead to the best selectivity. However, the best overall yield of 5-(hydroxyphenyl)hydantoin was obtained when hydroxyapatite (HAP) was used. This method can also be applied to other aromatics, such as anisole and thiophene. In some cases, activation by zinc bromide (ZnBr2) was also investigated in order to increase the activity of these phosphate catalysts.