Palladium Complex-Catalyzed Reductive N-Heterocyclization of Nitroarenes: Novel Synthesis of Indole and 2H-Indazole Derivatives

Article abstract of DOI:10.1021/jo00091a026

The dichlorobis(triphenylphosphine)palladium (PdCl2(PPh3)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 deg 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 75percent yield from the reductive N-heterocyclization of 2-nitrostilbene.Similarly, 2-propyl-2H-indazole was readily prepared in 83percent 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.