Synthesis of 1-aryl-1H-indazoles via the palladium-catalyzed cyclization of N-aryl-N′-(o-bromobenzyl)hydrazines and [N-aryl-N′-(o-bromobenzyl)-hydrazinato-N′]-triphenylphosphonium bromides

1-Aryl-1H-indazoles were synthesized by the palladium-catalyzed intramolecular amination [Pd(OAc)2/dppf/tBuONa (150 mol%)/toluene/90°C] of the corresponding N-aryl-N′-(o-bromobenzyl)hydrazines. It was further demonstrated that cyclization of [N-aryl-N′-(o-bromobenzyl)-hydrazinato-N′]-triphenylphosphonium bromides under the conditions of [Pd(OAc)2/dppf/tBuONa (250 mol%)/dioxane/90°C] also led to the formation of the corresponding 1-aryl-1H-indazoles. These methods were applied to a group of representative substrates to give indazole products in moderate to very good yields.