Carbonylation of nitrobenzene to phenyl isocyanate and methyl carbamate catalyzed by palladium and rhodium activated by chelating nitrogen donor ligands

During our studies on the reductive carbonylation of nitrobenzene, we have found that palladium metal supported on alumina is an effective catalyst for the formation of phenyl isocyanate, PhNCO, when activated by ortho-phenanthroline or its derivatives, and by 2,4,6-trimethylbenzoic acid (TMBA) (T = 180 °C, P(CO) = 40 atm, in dry benzene). In similar conditions (T = 200 °C, P(CO) = 100 atm in toluene) rhodium metal showed very poor activity. However, when methanol was also present in the reaction medium, N-phenyl methyl carbamate was obtained in good yields and selectivities. The presence of TMBA was not required in the latter case. In both cases, at the end of the reaction, the catalytic solution showed the presence of metal complexes, suggesting the formation in situ of homogeneous catalytic systems. In fact, preformed Pd(O2CC6H2-2,4,6-Me3)2 and Rh4(CO)12 gave even better results when used as catalysts in the same experimental conditions.