The palladium-phenanthroline catalyzed carbonylation of nitroarenes to diarylureas: Effect of chloride and diphenylphosphinic acid

The application of the palladiurn-phenanthroline catalytic system to the carbonylation of nitrobenzene in the presence of anilitle to afford diphenylurea has been investigated. The reaction is best performed with equimolar amounts of the two reagents. Use of higher concentrations of either aniline or nitrobenzene or ail increase in temperature in the range 120-170 degrees C leads to the formation of higher amounts of azo- and azoxybenzene. The latter were found to contain exclusively the try moiety deriving from nitrobenzene, with no inclusion of that derived from aniline. The addition of a small amount of diphenylphosphinic acid doubles the conversion and improves the selectivity in diphenylurea, but the effect is attenuated for larger amounts of acid. Small amounts of chloride, of the order of 10-30 mol% with respect to palladium, improve both rate and selectivity, but only inhibiting effects are detected when chloride is added to the reaction Mixture for the carbonylation of 2,4-dinitrotoluene to dimethyl 2,4-tolueliedicarbamate. The data obtained and that previously reported in the literature has been analyzed in the context of a unifying mechanism and ail explanation for some apparent contradictions has been given. ((c)) 2005 Elsevier B.V. All rights reserved.