Efficient synthesis of 1,5-benzodiazepines catalyzed by silica supported 12-tungstophosphoric acid

Silica supported 12-tungstophosphoric acid efficiently catalyzes the reaction of o-phenylenediamine with ketones under solvent free condition to afford the corresponding 1,5-benzodiazepines in good to excellent yields. The catalyst can be recovered by simple filtration and reused.