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 reduct ive N-heterocyclization of various 2-nitrostyrene and N-(2-nitrobenzyl idene)amine derivatives when employed at 100 degrees C for 16 h under 20 kg cm(-2) of initial carbon monoxide pressure, to give the correspo nding indole and 2H-indazole derivatives in good yield. For example, 2 -phenylindole was obtained in 75% yield from the reductive N-heterocyc lization of 2-nitrostilbene. Similarly, 2-propyl-2H-indazole was readi ly prepared in 83% yield by the reductive N-heterocyclization of N-(2- nitrobenzylidene)propylamine. A nitrene intermediate for the present r eaction is proposed on the basis of deuterium-labeling experiments and the investigation of alkyl rearrangement in the construction of the i ndole skeleton. Carbon monoxide effectively operates as a deoxygenatin g agent of the nitro group to afford a nitrene intermediate.