Nitrogen ligands effects in the palladium-catalyzed carbonylation reaction of nitrobenzene to give N-methyl phenylcarbamate

• Several phenanthrolines and a quinoline–guanidine were synthesized.
• Ligands prepared had in part been poorly characterized in the previous literature.
• The prepared nitrogen ligands were tested in the Pd-catalyzed carbonylation of nitrobenzene.
• Non-symmetric phenanthrolines with a donor group in the 4 position gave higher activities.
• The effect of different phosphorus acid promoters on selectivity was tested.

We previously reported an enhancement in the catalytic activity of a palladium-phenanthroline system towards the carbonylation of nitroarenes to N-methyl arylcarbamates when non-symmetric phenanthrolines were used as ligands. In particular, best results were obtained when an electron donating group was present on just one of the pyridinic rings of the ligand. Here we report the effect of stronger donor groups on the catalytic activity and on the selectivity of the system, even in the presence of different phosphorus acids used as promoters. High catalytic activities were reached but the high basicity of the ligands can negatively affect the product selectivity. The presence of steric hindrance near one or both the nitrogen donor atoms was shown to be detrimental for the activity of the system both with phenanthroline ligands and a quinoline–guanidine hybrid ligand.

Palladium/phenanthroline complexes are the most active catalysts for the reductive carbonylation reaction of nitroarenes. The catalytically active species is [Pd(Phen)(COOCH3)2], which reacts with CO forming an adduct in which a nitrogen atom of the ligand has detached. Non-symmetric phenanthrolines can highly improve the catalytic activity. Other substituted ligands were also tested.Figure optionsDownload as PowerPoint slide