CATALYSIS BY PALLADIUM SALTS .13. THE REDUCTIVE CARBONYLATION OF NITROAROMATIC COMPOUNDS TO ISOCYANATES WITH PDII AND PD0 COMPLEXES AS HOMOGENEOUS CATALYSTS

A study has been made of the reductive carbonylation of 2,4-dinitrotoluene (2,4-DNT) to 2,4-diisocyanotoluene (2,4-TDI) with catalysis either by [Pd(isoquinoline)2Cl2], in the presence of Fe2O3 and MoO3 or of Fe2(MoO4)3 as cocatalysts, or by Pd0 complexes without cocatalysts. In the case of catalytic systems based upon [Pd(isoquinoline)2Cl2] the reaction can be carried out at about 200-degrees-C and under 200 atm of CO to produce 2,4-TDI with high conversions and acceptable selectivities. With Pd0 complexes as catalysts good conversions can be achieved at much lower temperatures (100-120-degrees-C) but with a low selectivity when a higher pressure of CO is used (300 atm or more). An investigation of the reductive carbonylation of nitrobenzene to phenylisocyanate as a model system, together with a study of the thermal stability of [Pd(isoquinoline)2Cl2] in the presence of CO, has provided evidence that the actual active catalyst could be a reduced (probably zerovalent) form of palladium stabilised by the nitroaromatic substrate or by some of the products formed from it as ligands.