PALLADIUM COMPLEX-CATALYZED REDUCTIVE N-HETEROCYCLIZATION OF NITROARENES - NOVEL SYNTHESIS OF INDOLE AND 2H-INDAZOLE DERIVATIVES

The dichlorobis(triphenylphosphine)palladium (PdCl2(PPh(3))(2))-tin(II) chloride (SnCl2) system shows high catalytic activity for the reductive N-heterocyclization of various 2-nitrostyrene and N-(2-nitrobenzylidene)amine derivatives when employed at 100 degrees C for 16 h under 20 kg cm(-2) of initial carbon monoxide pressure, to give the corresponding indole and 2H-indazole derivatives in good yield. For example, 2-phenylindole was obtained in 75% yield from the reductive N-heterocyclization of 2-nitrostilbene. Similarly, 2-propyl-2H-indazole was readily prepared in 83% yield by the reductive N-heterocyclization of N-(2-nitrobenzylidene)propylamine. A nitrene intermediate for the present reaction is proposed on the basis of deuterium-labeling experiments and the investigation of alkyl rearrangement in the construction of the indole skeleton. Carbon monoxide effectively operates as a deoxygenating agent of the nitro group to afford a nitrene intermediate.