New Nonsymmetric Phenanthrolines as Very Effective Ligands in the Palladium-Catalyzed Carbonylation of Nitrobenzene

Inspired by the results of a previous mechanistic study, a series of mostly new nonsymmetric phenanthrolines were synthesized and tested as ligands in the palladium-catalyzed reductive carbonylation reaction of nitrobenzene to methyl phenylcarbamate. Very good results were obtained when the asymmetry was of an electronic nature (a donating substituent in the para position of one of the two pyridinic rings and an electron-withdrawing or no substituent on the para position of the other pyridinic ring), but steric hindrance in the ortho or meta position retarded the reaction. The TOF for the modified system is the highest ever reported for any carbonylation reaction of nitroarenes.